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the food particle forming food vacuole. Inside the food vacuole, complex substances are broken down into simpler ones. Then diffuse into the cytoplasm. The remaining undigested material is moved to the surface of the cell and thrown out. In Paramoecium [fig-9(b)], which is also a unicellular organism the cell has a definite shape. Food is taken in at a specific spot. Food is moved to the spot by the movement of cilia which covers the entire surface of the cell, where the food is ingested (cytostome). 12 X Class Nutrition - Food supplying system Parasitic nutrition in Cuscuta Dodder (genus Cuscuta) is a leafless, twining, parasitic plant belongs to morning glory family (Convolvulaceae). The genus contains about 170 twining species that are widely distributed throughout the temperate and tropical regions of the world. The dodder contains no chlorophyll (Cuscuta reflexa has been found to have very small amount of chlorophyll) and instead absorbs food through haustoria. They are rootlike structures that penetrate the tissue of a host plant and may kill it. The slender, string like stems of the dodder may be yellow, orange, pink, or brown in colour. It’s leaves are reduced to minute scales. The dodder’s flowers, in nodule like clusters, are made up of tiny yellow or white bell-like petals. fig-10: Haustoria in cuscuta The dodder’s seed germinates, forming an anchoring root, and then sends up a slender stem that grows in a spiral fashion until it reaches a host plant. It then twines around the stem of the host plant and forms haustoria, which penetrate through it. Water is drawn through the haustoria from the host plant’s xylem, and nutrients are drawn from its phloem. Meanwhile, the root rots away after stem contact has been made with a host plant. As the dodder grows, it sends out new haustoria and establishes itself very firmly on the host plant. After growing in a few spirals around one host shoot, the dodder finds its way to another, and it continues to twine and branch until it resembles a fine, densely tangled web of thin stems enveloping the host plant. Identify plants in your surroundings which are parasitic on other plants. Nutrition in Human Beings Human digestive system is very complex in nature. Different parts are involved and perform different functions by using various digestive juices and enzymes. Let us observe the figure of digestive system and label the parts. The alimentary canal is basically a long tube extending from the mouth to the anus. We can see that this tube has different parts. Various regions are specialized to perform different functions. • What happens to the food once it enters our body? fig-11: Alimentary canal of man Free distribution by A.P. Government 13 salivary duct We eat various types of food which has to pass through the same digestive tract. It also has to be converted to substances small enough to be utilised by our body. This needs various processess that can be studied as follows. Passage of food through alimentary canal or gut salivary glands Food is cut and crushed by our teeth in the mouth and mixed with saliva to make it wet and slippery (also called as mastication). Saliva is secreted by three pairs of salivary glands. Two pairs are located at the side of the jaw and below the tongue. One pair is located in the palate. Saliva mainly contains an enzyme amylase (ptyalin) which helps in the breakdown of complex carbohydrates to simple ones. The tongue helps in mixing the food and pushing it into the next part. The lower jaw also helps in the whole process. epiglottis fig-12: Buccal cavity palate tongue We can find out the effect of salivary amylase on carbohydrates to observe what might be happening in our mouth. Activity-4 • Refer to activity - 7 action of saliva on wheat flour in the chapter Co ordination of life processes. ‘Thats the way with our body’. You can also perform the activity by using ‘Ganji’ (boiled rice water) The soft food mixed with saliva passes through oesophagus or food pipe by wave like movements called peristaltic movement to the stomach. At the stomach, food gets churned with gastric juice and HCl. Now the food is in semisolid condition. The digestion of food goes on as most proteins are broken down into smaller molecules with the help of enzyme pepsin acting on them. peristalitic wave esophagus stomach bolus fig-13: Peristaltic movement 14 X Class Food in the form of a soft slimy substance where some proteins and carbohydrates have already been broken down is called chyme. Now the food material passes from the stomach to the small intestine. Here the ring like muscles called pyloric spincters relax to open the passage into the small intestine. The spincters are responsible for regulating the opening of the passage such that only small quantities of the food material may be passed into the small intestine at a time. Nutrition - Food supplying system The small intestine is the longest part of the alimentary canal. It is the site of further digestion of carbohydrates, proteins and fats. It receives the secretion of liver and pancreas for this purpose. These juices render the internal condition of the intestine gradually to a basic or alkaline one. Fats are digested by converting them into small globule like forms by the help of the bile juice secreted from liver. This process is called emulsification. Pancreatic juice secreted from pancreas contains enzymes like trypsin for carrying on the process of digestion of proteins and lipase for fats. Walls of the small intestine secrete intestinal juice which carry this process further that is small molecules of proteins are broken down to further smaller molecules. The same is the condition with fats. Carbohydrate digestion that started in the mouth and did not occur in the stomach, resumes now as the medium gradually changes to an alkaline one and the enzymes become active for carbohydrate breakdown. Activity-5 Studying the enzymes chart Let us study the chart showing different enzymes and digestive juices and their functions. Table-1: some enzymes and juices of the gut S.No. Enzyme/Substance Secreted by Secreted into Digestive juice Acts on Products 1 2 3 4 5 6 7 8 Ptyalin (salivaryamylase) Salivary glands Buccal cavity Saliva Carbohydrates Dextrins and maltose Pepsin Stomach Stomach Gastric juice Proteins Peptones Bile (No enzymes) Liver Duodenum Bile juice Fats Emulification breaking down of largef ats into small globules Amylase Pancreas Duodenum Pancreatic juice Carbhoydrates Maltose Pancreas Duodenum Pancreatic juice Proteins Peptones Pancreas Intestinal wall Duodenum Pancreatic juice Intestinal juice Fats Peptidases Small Intestine Small Intestine Intestinal juice Peptides Sucrose Small Intestine Small Intestine Intestinal juice Sucrose (Cane Sugar) Fatty acids and glycerol Amino acids Glucose Trypsin Lipase • Name the enzymes which act on carbohydrates? Free distribution by A.P. Government 15 • Which juice contains no enzymes? • What are the enzymes that act on proteins? Transport of the products of digestion from the intestine into blood (through the wall of intestine) is called absorption. Internally, intestinal wall has a number of finger like projections called villi. The villi increase the surface area for absorption. Blood vessels and lymph vessels are present in the form of a network in the villi. Products of digestion are absorbed first into the villi and from here into the blood vessels and lymph vessels. Thus after maximum absorption of food in the small intestine the rest passes into the large intestine. Here most of the water present is taken up from this material. This material is then expelled through the anus which is the last part of the alimentary canal. This passage of undigested material from the body by the way of anus is called defecation. Food that passes out of the anus still contains considerable amount of proteins, fats and carbohydrates, roughages or fibres of either carbohydrates or proteins. We will learn some more points about the coordination about digestive system with other systems in the chapter coordination in life processess. Flow chart of human digestive system • What do you think is the process of digestion? • What are its major steps? Food Mouth Buccal cavity Pharynx Cardiac Stomach Pyloric Stomach Oesophagus Duodenum Small intestine Large intestine Anus Rectum Bile Pancreas Health aspects of the alimentary canal The human alimentary canal usually functions remarkably well considering how badly we treat it on occasions! Sometimes it rebels, and we either feel sick or have indigestion. 16 X Class Nutrition - Food supplying system Vomiting is the body’s method of ridding itself of unwanted or harmful substances from the stomach. The peristaltic movements of the stomach and oesophagus reverse their normal direction and the food is expelled. There are many causes of vomiting, but one of the most common is over eating, especially when the food contains a high proportion of fat. Vomiting also occurs when we eat something very indigestible or poisonous. When we have a greenish vomit usually called as ‘bilious’ or ‘liverish’, we get a bitter taste and it is often the result of having eaten ‘rich’ meals over several days. The liver is unable to cope with the excessive fat and we get a feeling of nausea. Indigestion is a general term used when there is difficulty in digesting food. Healthy people can usually avoid problems related to digestion by: a) having simple, well balanced meals b) eating them in a leisurely manner c) thoroughly masticating the food d) avoiding taking violent exercise soon after eating food e) Drinking plenty of water and having regular bowel movements. A more serious form of indigestion is caused by stomach and duodenal ulcers. These conditions occur more often in people who may be described as hurried or worried. Thus, ulcers occur more often in busy people who get into the habit of hurrying over meals and rushing from one activity to anot
her without sufficient rest. Those who are able to relax, who are not continually tensed up, and who live at a slower pace, seldom get ulcers. You studied about recent researches in the peptic ulcers caused by some bacteria in class IX. Proper functioning of all life processess require adequate amount of food in all living organisms. It is not just the intake of food but its assimilation and expulsion of wastes that play an important role. In take of fibre rich food avoids constipation. Diseases due to mal nutrition We know that food is the main source to maintain biological processes in a perfect manner. Our diet should be a balanced one which contains proper amount of carbohydrates, proteins, vitamins, mineral salts and fats. Two third of world population is affected by food related diseases. Some of them are suffering by consuming high calorific food. Most of them are facing various diseases due to lack of balanced diet. It is very important to discuss about food deficiency diseases. Free distribution by A.P. Government 17 Eating of food that does not have one or more than one nutrients in required amount is known as mal nutrition. Poor health, will full starvation, lack of awareness of nutritional habits, socio economic factors are all the reasons for mal nutrition in our country. Mal nutrition is of three types 1. Calory malnutrition, 2. Protein malnutrition, 3. Protein calory malnutrition. Let us observe harmfull effects of mal nutrition in children. fig-14: Kwashiorkor 1. Kwashiorkor disease: This is due to protein deficiency in diet. Body parts become swollen due to accumulation of water in the intercellular spaces. Very poor muscle development, swollen legs, fluffy face difficult to eat, diarrhoea, dry skin are the symptoms of this disease. 2. Marasmus: This is due to deficiency of both proteins and calories. Generally this disease occurs when there is an immidiate second pregnancy or repeated child births. Lean and week, swelling limbs, less devoloped muscles, dry skin, diarrhoea, etc., are the symptoms of this disease. 3. Obesity: This is due to over eating and excess of energy in take. It is a big health hazard. Obese children when grows, they will be target of many diseases like diabetes, cardio vascular, renal, gall bladder problems. Discuss about junk foods and other food habits which leads to obesity. fig-15: Marasmus Vitamin deficiency diseases Vitamins are organic substances. They are micro nutrients required in small quantities. Actually vitamins are not synthesised in the body, we do not generally suffer from vitamin deficiency. The source of vitamins to our body is through two ways. One is diet and other is bacteria present in the intestain that synthesises and supplies vitamins to the body. fig-16: Pellegra Vitamins are classified into two groups. One is Water soluble vitamins (Be-complex, vitamin C) and other is fat soluble vitamins (vitamin A, D, E and K). Let us study the following chart showing vitamins available sources and deficiency diseases. 18 X Class Nutrition - Food supplying system Vitamin Resources Deficiency diseases Symptoms Thiamine (B1) Cereals, oil seeds, vegetables, milk, meat, fish, eggs. Beri beri Riboflavin (B2) Milk, eggs, liver, kidney, green leafy vegetables. Glossitis Vomitings, fits, loss of appetite, difficulty in breathing, paralysis. Mouth cracks at corners, red and sore tongue, photophobia, scaly skin. Niacin (B3) Kidney, liver, meat, egg, fish, oil seeds. Pellagra Dermatitis, diarrhoea, loss of memory, scaly skin. Pyridoxine (B6) Cereals, oil seeds, vegetables, milk, meat, fish, eggs, liver. Anaemia Hyper irritability, nausea, vomiting, fits. Cyanocobalamine (B12) Synthesised by bacteria present in the intestine. Pernicious anaemia Lean and week, less appetite. Folic acid Liver, meat, eggs, milk, fruits, cereals, leafy vegetables. Anemia Diarrhoea, loss of leucocytes, intestinal mucus problems. Pantothenic acid Sweet potatoes, ground nuts, vegetables, liver, kidney, egg. Burning feat Walking problems, sprain. Biotin Pulses, nuts, vegetables, liver, milk, kidney. Nerves disorders Fatigue, mental depression, muscle pains. Ascorbic acid (C) Green leafy vegetables, citrus fruits, sprouts. Scurvy Delay in healing of wounds, fractures of bones. Retinol (A) Leafy vegetables, carrot, tomoto, pumpkin, papaya, mango, meat, fish, egg, liver, milk, cod liver oil, shark liver oil. Eye, skin diseases Night blindness, xeropthalmia, cornea failure, scaly skin. Calciferol (D) Liver, egg, butter, cod liver oil, shark liver oil, (morning sun rays). Rickets Improper formation of bones, Knocknees, swollen wrists, delayed dentition, week bones. Tocoferol (E) Fruits, vegetables, sprouts, meat, egg, sunflower oil. Fertility disorders Sterility in males, abortions in females. Phylloquinone (K) Green leafy vegatables, milk. Blood clotting Delay in blood clotting, over bleeding. Free distribution by A.P. Government 19 Key words Glucose, starch, cellulose, chloroplast, grana, stroma, light reaction, dark reaction, heterotrophic nutrition, parasitic nutrition, haustoria, Alimentary canal, salivary glands, peristaltic movement, amylase, ptyalin, pepsin, chyme, sphincter, digestion, pancreas, enzymes, villi, bile juice, lipase, fat, liver, emulsification. What we have learnt • Autotrophic nutrition involves the intake of simple inorganic materials like some minerals, water from the soil. Some gases from the air. By using an external energy source like the Sun to synthesis complex high energy organic material. • • Photosynthesis is the process by which living plant cells containing chlorophyll, produce food substances [glucose & starch] from Carbon dioxide and water by using light energy. Plants release oxygen as a waste product during photosynthesis. Photosynthesis process can be represented as 6CO2 + 12H2O light Chlorophyll C6H12O6+6H2O+6O2 • The materials required for photosynthesis are light: Carbon dioxide, Water, photosynthetic pigment chlorophyll. • Chloroplast are the sites of photosynthesis. Light reaction takes place in the grana region and light independent reaction takes place in the storma region. • The end products of photosynthesis are Glucose water and Oxygen. • During photosynthesis the important events which occurs in the chloroplast are a) Conversion of light energy into chemical energy b) Splitting of water molecule c) Reduction of carbon dioxide to carbohydrates • Heterotrophic Nutrition involves the intake of complex material prepared by other organisms. • The form of nutrition differs depending on the type and availability of food material as well as how it is obtained by the organism. • In single celled organisms the food may be taken in by the entire surface but as the complexity of the organism increases different parts becomes specialized to perform different functions. • The large complex food molecules such as carbohydrates, proteins , lipids, etc., are broken down in to simple molecules before they are absorbed and utilized by the animals. This process of breaking down of complex molecules into simple molecule is called digestion. • In human beings the food eaten is broken down in various steps with the help of enzymes secreted by digestive glands which are associated with the alimentary canal and the digested food is absorbed in small intestine to be sent to all cells in the body. 20 X Class Nutrition - Food supplying system • The digestive system includes the alimentary tract and several associated organs. The functions of system are as follows : a) Ingestion: b) Digestion: c) Absorption: d) Defecation: Taking of food into the body Breaking up of complex food substances into the simple substances by specific enzymes. So that they can be used by the body. The passage of digested food through the walls of alimentary tract (particulars in small intestine ) into circulatory system. The passage of undigested material from the body by the way of anus. Improve your learning 1. Write differences between (AS1) a) autotrophic nutrition - heterotrophic nutrition c) Light reaction - dark reaction b) Ingestion - digestion d) Chlorophyll - chloroplast 2. Give reasons (AS1) a) Why photosynthesis is considered as the basic energy source for most of living world? b) Why is it better to call the dark phase of photosynthesis as a light independent phase? c) Why is it necessary to destrach a plant before performing any experiment on photosynthesis? d) Why is it not possible to demonstrate respiration in green plant kept in sunlight? 3. Give examples (AS1) a) Digestive enzymes c) Vitamins b) Organisms having heterotrophic nutrition d) Food deficiency diseases 4. Where do plants get each of the raw materials required for photosynthesis?(AS1) 5. Explain the necessary conditions for autotrophic nutrition and what are its by products?(AS1) 6. With the help of chemical equation explain the process of photosynthesis in detail? (AS1) 7. Name the three end products of photosynthesis? (AS1) 8. What is the connecting substance between light reaction and dark reaction? (AS1) 9. Most leaves have the upper surface more green and shiny than the lower ones why? (AS1) 10. Explain the structure of chloroplast with a neatly labeled sketch. (AS1) 11. What is the role of acid in stomach? (AS1) 12. What is the function of digestive enzyme? (AS1) 13. How is the small intestine designed to absorb digested food, explain. (AS1) 14. How do fats digested in our bodies? Where does this process takes place? (AS1) 15. What is the role of saliva in the digestion of food? (AS1) 16. What will happen to protein digestion as the medium of intestine is gradually rendered alkaline? (AS1) 17. What is the role of roughages in the alimentary track? (AS1) 18. What is malutrition explain some nutrition deficiency diseases. (AS1) 19. How do nongreen plants such as fungi and bacteria obtain their nourishment? (AS2) 20. If we keep on increasing CO2 concentration in air what will be the rate of photosynthesis?(AS2) Free d
istribution by A.P. Government 21 21. What happens to plant if the rate of respiration becomes more than the rate of photosynthesis?(AS2) 22. Why do you think that carbohydrates are not digested in the stomach?(AS2) 23. What process you follow in your laboratory to study presence of starch in leaves?(AS3) 24. How would you demonstrate that green plant release oxygen when exposed to light?(AS3) 25. Visit a doctor and find out keeping in view of digestion. Prepare a chart and display in your classroom. (AS4) i) Under what condition does a patient need to become a drip of glucose. ii) Till when does a patient need to be given a glucose. iii) How does the glucose help the patient to recover. 26. If there were no green plants, all life on the earth would come to an end! Comment?(AS5) 27. Draw a neatly labeled diagram of chloroplast found in leaf, and it’s role in photosysthesis?(AS5) 28. Draw the label diagram of human digestive system? List out the parts where peristalasis takes place. (AS5) 29. Raheem prepared a model showing the passage of the food through different parts of the elementary canal? Observe this and label it’s parts. (AS5) 30. Observe the following diagram and write a note on light dependent, light independent reactions.(AS5) light Calvin Cycle Chloroplast photo chemical reaction thermo chemical reaction Photochemical reaction Thermochemical reaction 31. Almost all the living world depends on plants for food material. How do you appreciate the process of making food by the green plants?(AS6) 32. Even a hard solid food also becomes smooth slurry in the digestive system by the enzymes released at a perticular time. This mechanism is an amazing fact. Prepare a cartoon on it. (AS6) 33. What food habbits you are going to follow after reading this chapter? Why? (AS7) 22 X Class Nutrition - Food supplying system Fill in the blanks 1. The food synthesized by the plant is stored as ______________________. 2. ________________________ are be sites of photosynthesis. 3. Pancreatic juice contains enzymes for carrying the process of digestion of ___________________ and ________________________. 4. The finger like projections which increases the surface area in small intestine are called____________________. 5. The gastric juice contains _________________________ acid. 6. ___________________ vitamin sysnthesised by bacteria present in intestine. Choose the correct answer 7. Which of the following organisms take the food by parasitic nutrition? a) Yeast b) Mushrooms c) Cuscuta d) Leeches 8. The rate of Photosyntesis is not affected by: ( ( ) ) a) Light Intensity b) Humidity C) Temperature d) Carbon dioxide concentration 9. A plant is kept in dark cupboard for about forty eight hours before conducting any experiment on Photosynthesis in order to : ( a) Remove chorophyll from leaves b) Remove starch from leaves c) Ensure that no photosynthesis occurred d) Ensure that leaves are free from the starch 10. The digestive juice without enzyme is a) Bile b) Gastric juice c) Pancrtatic juice d) saliva 11. In single celled animals the food is taken a) By the entire body surface b) Mouth c) Teeth d) Vacuoles 12. Which part of the plant takes in carbondioxide from the air for photosynthesis a) Root hair b) Stomata c) Leaf veins d) Sepals ( ( ( Free distribution by A.P. Government ) ) ) ) 23 Chapter 2 Respiration - The energy releasing system Using food to carry out life processes is key to life for all living beings in both multicellular or unicellular. In the chapter, on nutrition we have discussed how the body draws out nutrients from the food taken in. The food provides energy for all bodily activities only after break down through the process known as respiration. Thus, respiration leads to final utilization of food. When oxygen is plentiful respiration normally takes over. Cells of the living body use food constantly to help our body to function properly. They require the presence of gas, food material and some chemicals. The term ‘respiration’ derived from Latin word ‘respire’ meaning ‘to breathe’, refers to the whole chain of processes from the inhalation of air to the use of oxygen in the cells. To begin with, we shall study the relation of gases and the process of respiration. Discovery of gases and respiration The term respiration came into use, a century after the word breathing was used, way back in the 14th century. It was used much before people knew that air is a mixture of gases. They hardly knew anything about all the life processes that took place internally in a living body. Respiration which was used as a medical term, usually referred to as a process involving passage of air and production of body heat. It was not until 18th century when Lavoisier and Priestley did a comprehensive work on properties of gases, their exchange and respiration that we came to know something about how the process of gaseous exchange goes on, in our body. You have already studied about Respiration - The energy releasing system fig-1: Lavoisier 24 X Class some of Priestley’s experiments in earlier classes (You have an account of it in the chapter on nutrition as well). Recall the concepts and answer the following. • Can it be said that Priestley’s experiment helped us to find out more about composition of air? How? Lavoisier also carried out several experiments to understand the property of gases. In his early experiments, it is clear that Lavoisier thought that the gas liberated on heating powdered charcoal in a belljar kept over water in a trough was like fixed air. In those days carbon dioxide was known as fixed air. The next series of experiments dealt with the combustion of phosphorus in a belljar. From these studies Lavoisier showed that whatever in the atmospheric air which combined with the phosphorus, was not water vapor. His final words are that the substance which combines with the phosphorus is “either air itself, or another elastic fluid present, in a certain proportion, in the air which we breathe”. This was the respirable air, a component of air that also helped in burning. • What was produced by combustion according to Lavoisier? • What did Lavoisier find out about air from his experiments? • What conclusion can be drawn from Lavoisier’s experiments? Lavoisier noted that there was a profound difference between the air in which combustion of a metal had been carried out and the one which had served for respiration. The air that we breathe out precipitated lime water while that after heating metal did not. From this he deduced that there were two processes involved in respiration, and that of these he probably knew only one. Therefore, he carried out another experiment by which he showed that about one sixth of the volume of ‘vitiated air’(a term used then to show air from which the component needed for burning had been removed) consists of chalky acid gas (fixed air). Therefore, to recreate common air from vitiated air, it was not enough merely to add the appropriate amount of air needed for burning or respirable air; the existing chalky acid gas must also be removed. He drew immediately the logical conclusion regarding the process of respiration. Either eminently respirable air is changed in the lungs to chalky acid air; or an exchange takes place, the eminently respirable air being absorbed, and an almost equal volume of chalky acid air being given up to the air from the lungs. He had to admit that there were strong grounds for believing that eminently respirable air did combine with the blood to produce the red colour. fig-2: Priestley Free distribution by A.P. Government 25 Lavoisier’s findings lead way to several other researchers. • Which gas do you think is Lavoisier talking about when he says chalky acid gas? • Which gas according to him is respirable air? • What steps in the process of respiration does Lavoisier mention as an inference of his experiments? A few lines from a textbook of Human Physiology, written by a renowned chemist, John Daper around mid-19th century goes like this.. ‘‘The chief materials which a living being receives are matter that can be burnt, water and oxygen gas; and out of the action of these upon one another, all the physical phenomena of its life arise. What the body expels out is water, oxide of carbon, phosphorous, sulphur and others.’’ Thus, we can see that the role of major compounds and elements in the process of respiration was known by mid19thcentury. The events involved were not very clearly understood, but, people believed that there was some relationship of the heat produced in the body and the process of respiration. • It is a common observation that our breath is warmer than the air around us; does respiration have anything to do with this? Let us study the events involved in respiration in human beings to figure it out. Events / Steps in Respiration There are no strict demarcations of events involved in the process of respiration. It is a very complex process of several biochemical and physical processes. But for a general understanding on what goes on, we shall study under the following heads. Breathing Gaseous exchange at lungs level Gas transport by blood Gaseous exchange at tissue level Air movement into and out of lungs Exchange of gases between alveoli and blood Transport of oxygen from blood capillaries of alveoli to body cells and return of carbon dioxide Exchanging of oxygen from blood into the cells and carbon dioxide from cells into the blood Cellular Respiration Using oxygen in cell processes to produce carbon dioxide and water, releasing energy to be used for life processes 26 X Class Respiration - The energy releasing system Breathing In the previous classes we did experiments to find out what was there in the air we breathe out. We had seen that in a set up with lime water, it turned milky white fast as we breathe out into it as compared to a similar set up in which normal air was passed with the help of a syringe or pichkari in lime water. (Experimen
tal set up to test the presence of Carbon dioxide in exhaled air). Arrange apparatus as shown in figure and try to do the experiment once again to find out what happens. lime water water • What does this experiment indicate? • Which gas turns lime water milky? • Which gas do you think might be present in greater quantities in fig-3: Respiratory gases the air we breathe out as compared to air around us? • We are also aware of the fact that water vapor deposits on a mirror if we breathe out on it. • Where does this water vapor come from in exhaled air? We shall have to study the pathway of air in our body through our respiratory system and the mechanism of breathing in respiration to find out how the exhaled air comes out (Fig showing the respiratory system/ pathway). By “respiratory system” we usually mean the passages that transport air to the lungs and to the microscopic air sacs in them, called alveoli (where gases are exchanged between them and blood vessels) and vice versa. Pathway of air Let us observe the pathway of air from nostril to alveolus. nasal cavity nostril epiglottis pharynx larynx trachea bronchi bronchioles lung capillaries in which O2 CO2 exchanges occur and alveoli fig-4: Respiratory system of man Free distribution by A.P. Government 27 Nostrils: Air usually enters the body through the nostrils Pharynx: Nasal cavity: Air is filtered. The moist surface of the lining of the nasal cavity, and the hairs growing from its sides, remove some of the tiny particles of dirt in the air. In addition, as the inhaled air passes through the nasal cavity, its temperature is brought close to that of the body, and it takes up water vapour so that it becomes more moist than before. Warming and moistening goes on in this common passage of digestive and respiratory system. Epiglottis, a flap like muscular valve controls movement of food and air towards their respective passages. This stiff box contains our vocal cords. When air passes out of the lungs and over the vocal cords, it causes them to vibrate. This produces sounds on the basis of our speech, song etc. Wind pipe channeling air to lungs. Touch your neck to feel the tube like structure. At its lower end the trachea or the wind pipe divides into two bronchi-one leading to each lung. Bronchioles: Each bronchi is further divided into smaller and smaller Bronchus: Trachea: Larynx: Alveolus: Blood: branches called bronchioles. These finally terminate in clusters of air sacs called alveoli in the lungs which are very small and numerous. Gaseous exchange takes place here as blood capillaries take up oxygen and expel carbon dioxide. Carries oxygen, collects CO2 to each and every cell of the body. The whole passage from nostrils to alveolus is moist and warm. Do you know? The interior of lung is divided into millions of small chambers, thus tremendously increasing the moist surface available for transfer of gases between air and blood. The linings of the lungs are much folded and so their total surface is enormous. If all alveoli of our lungs are spread out they will cover an area of nearly 160 m2. 28 X Class Respiration - The energy releasing system Epiglottis diverts air to lungs Epiglottis diverts food mass away from opening of larynx Think and discuss • What will happen if the respiratory tract is not moist? • Are both lungs similar in size? • Why alveolus are so small and uncountable in number? Epiglottis and passage of air From the nasal cavity, the air goes into the pharynx. There is a tricky problem here. From the pharynx there are two passages, beginning with nearly same opening and ending into separate ones, one to the lungs and one to the stomach. It is important that air goes into one and food into the other. It is also important that food does not enter the tube through which air goes into the lungs. The traffic is kept properly channeled by a flap like valve, the epiglottis that protects the tube to the lungs, arresting entry of food. Observe the following figures and discuss in your class how epiglottis works while breathing or swallowing fig-5(a): Breathing fig-5(b): Swallowing This valve is partly closed when we swallow food; it deflects food down to the stomach and keeps it out of the trachea or wind pipe which is the route to the lungs. The epiglottis opens more widely when we take a breath, and air enters the lungs. Nervous regulation is important in guiding the function of epiglottis and passage of food and air. Let us try to do an activity to feel what happens when we swallow food. • Why we are advised not to talk while eating food ? Activity-1 Keep your palm around an inch away from your nose, feel you are breathing out, do not remove it until you have finished the activity. Breathe steadily for 1-2 minutes. Now take a piece of any fruit, chew and before swallowing it keep the fingers of the other palm on your neck, now swallow it. • What did you notice? What happens to your breath as you try to swallow? Free distribution by A.P. Government 29 rib bones chest wall muscles lung diaphragm • What is helping you to swallow without deflecting it to the wind pipe? Mechanism of respiration in human beings We know that breathing is the process of inhaling and exhaling. The organs involved are mainly the lungs. You can’t see your lungs, but it’s easy to feel them in action. Put your hands on your chest and breathe in very deeply. You will feel your chest getting slightly bigger. Now breathe out the air, and feel your chest return to its regular size. You’ve just felt the power of your lungs! The lungs themselves can neither draw in air nor push it out. Instead, the chest wall muscles and another flexible flattened muscle called diaphragm helps the lungs in moving air into and out of them. See how the diaphragm works in the figure. • What is the role of diaphragm and ribs in respiration? Are both fig-6: Movement of diaphragm active in man and woman? The chest wall is made up of ribs, muscles, and the skin. The ribs are attached at an angle to the spine (if you run your finger along one of your ribs, you will notice that it extends downward from the spine). When we inhale, the chest wall moves up and expands. This causes an increase in the volume of the chest cavity. The diaphragm may be imagined as the ‘floor’ if you think of the chest cavity as a “room.” When the diaphragm is relaxed when we breath out, it is in the shape of a dome with the convex side of the dome extending into the chest cavity. When the diaphragm contracts during inhalation it flattens out a bit or the dome moves downward. As a result, the volume of the chest cavity is increased. When the diaphragm flattens and the volume of the chest cavity is increased, its internal pressure decreases and the air from the outside rushes into the lungs. This is inspiration (inhalation). Then the reverse occurs. The chest wall is lowered and moves inward, and the diaphragm relaxes and assumes its dome shape. These changes increase the pressure on the lungs; their elastic tissue contracts and squeezes the air out through the nose to the external atmosphere. This is expiration (exhalation). Respiration - The energy releasing system fig-7: Movement of rib cage during inhalation, exhalation 30 X Class Do you know? Our lungs are spongy in nature. They are not of the same size. The lung towards left is slightly smaller making space for your heart! Lungs are protected by two membranes called pleura. A fluid filled between these membranes protects the lungs from injury and also aid in the expansion of the spongy and elastic lung muscle, as they slide one over the other. You must have noticed that your own breathing is slow and shallow when you are at rest. It is deeper and faster when you exercise hard. Indeed, patterns of breathing show a great range, for they are coordinated with moment-by-moment needs of the body for supply of oxygen and removal of carbon dioxide. What other situations affect your breathing? It has been found that all movements of breathing stop at once when the nerves leading from the brain to the respiratory muscles are cut. • What can be concluded from this? • What happens during the process of breathing? • Which gas needs to be removed from our body during exhalation? Where does the extra amount of gas come from? • What is the composition of inhaled air? • When exhaled air is compared with inhaled air, is there any differ- ence in composition? Gaseous Exchange (alveoli to capillaries) Gaseous exchange takes place within the lungs by diffusion from the alveoli to blood capillaries and vice versa. The carbon dioxide in the blood is exchanged for oxygen in the alveoli. These tiny air sacs in the lungs are numerous and only one cell thick. They are surrounded by capillaries that are also only one cell thick. Blood, dark red in colour flows from the heart through these capillaries and collects oxygen from the alveoli. At the same time, carbon dioxide passes out of the capillaries and into the alveoli. When we breathe out, we get rid of this carbon dioxide. The bright red, oxygenrich blood is returned to the heart and pumped out to all parts of the body. carbon dioxide oxygen fig-8: Diffussion path way for gaseous exchange between lung and blood capillaries Free distribution by A.P. Government 31 branchiole alveolus blood cells capillary network As a result of gaseous exchange, the composition of inhaled and exhaled air is different. See the table given below. Approximate values are given in the table • Why does the amount of Oxygen vary between exhaled and inhaled air? Gas Oxygen Carbon dioxide Nitrogen % in inhaled air % in exhaled air 21 0.04 79 16 4 79 • What has raised the percentage of carbon dioxide in exhaled air? Do you know? The total lung capacity of human being is nearly 5800ml. Normally at rest who inhale or exhale approximately 500ml of air. 120ml of air remains in lungs after complete exhalation. Recall the activity of lung capacity performed by you in class VII in the ch
apter ‘Respiration in Organisms’. Transportation of gases We know that air is a mixture of gases, that fills the lungs and the alveoli when that enters our body. The relative amount of different gases in air and their combining capacity with haemoglobin and other substances in blood determine their transport via blood in the body. When oxygen present in the air is within normal limits (around 21%) then almost all of it is carried in the blood by binding to haemoglobin, a protein (quite like chlorophyll, the only major difference being it has iron in place of magnesium as in chlorophyll) present in the red blood cells. As oxygen is deffused in the blood, it rapidly combines with the haemoglobin to form oxyhaemoglobin. Not only haemoglobin can combine with oxygen, but the reverse can also happen to yield a molecule of haemoglobin and oxygen. Carbon dioxide is usually transported as bicarbonate, while some amount of it combines with haemoglobin and rest is dissolved in blood plasma. Study the following equation for better understanding. HbO2 Hb + O2 32 X Class Respiration - The energy releasing system HbO2 Hb + O2 Do you know? If haemoglobin is exposed to air at sea level, nearly every molecule combines with oxygen to form oxyhaemoglobin. At a height of 13 km (about 8 miles) above sea level, the concentration of oxygen is much lower about one fifth at sea level. Under these conditions only about half as increase the pressure on he air out through the nose h of the trachea. the hairs growing from its fig-9: Mountaneer many molecules of oxygen combine with haemoglobin to form oxyhaemoglobin. This is important, because blood cannot carry enough oxygen to the tissues if haemoglobin is combined with few oxygen molecules. In fact, human life is impossible at such an altitude without a supplementary supply of oxygen. Provision for such a supply is built into modern aircraft, which have pressurized cabins that maintain an enriched air supply. When we go deep into the sea we will face another type of problems. Gaseous exchange (capillaries to cells and back) In the capillaries over the tissues, haemoglobin meets a very different environment. The tissue cells are continually using oxygen, hence, the concentration of oxygen is quite low in them. It might be only one third of that in the lungs. As the concentration of oxygen is so low, oxyhaemoglobin releases the oxygen molecule that enters the cells. In the reactions that occur within cells in our bodies, carbon dioxide and water are produced and energy is released to be used up for different purposes. Cellular respiration The term cellular respiration refers to the pathway by which cells release energy from the chemical bonds of food molecules that enter them. It provides that energy for the essential processes of life. So living cells must carry out cellular respiration. It can be in the presence of oxygen that is ‘aerobic respiration’ or in its absence that is ‘anaerobic respiration (fermentation)’. Cellular respiration in prokaryotic cells like that of bacteria occurs within the cytoplasm. In eukaryotic cells cytoplasm and mitochondria are the sites of the reactions. The produced energy is stored in mitochondria in the form of ATP. That is why mitochondria are called “power houses of cell”. The exact chemical details of the breakdown of sugar or other foods within a living cell does not take place as a single reaction, but occurs in a series of small steps. How does this affect the energy release? As the change in the chemical nature of the molecule from one stage to the next is slight, in any step small amount of energy is released. The complete breakdown of a sugar molecule with the release of all its available energy Free distribution by A.P. Government 33 outer membrane inner membrane criste matrix fig-10: Mitochondria involves a series of different chemical reactions. From the breakdown of glucose the energy is released and stored up in a special compound, known as ATP (adenosine triphosphate). It is a small parcel of chemical energy. The energy currency of these cells is ATP an energy rich compound that is capable of supplying energy wherever needed within the cell. Each ATP molecule gives 7200 calories of energy. This energy is stored in the form of phosphate bonds. If the bond is broken the stored energy is released. • Do cells of alveoli or lungs also require oxygen to carry out cellu- lar respiration? Why/Why not? In short, at cellular level we could have the following pathways starting with glucose (It is one example, remember that there are other components of food as well). Absence or low amount of oxygen (anaerobic respiraton and fermentation) Lactic acid + Energy Ex: Bacteria Ethanol + CO2+ Energy Ex: Yeast Presence of Oxygen (aerobic respiration) CO2 + H2O + Energy Ex: Plants and animals Glucose Pyruvate (3 carbon compound) + Energy Do you know? Glucose is the most commonly used sugar for deriving energy in plants, animals and in microorganisms. In all these organisms the glucose is oxidized in two stages. In the first stage it is converted into two molecules of pyruvic acid. In the second stage if oxygen is available pyruvic acid is oxidized to CO2 and water, large amount of energy is released. If oxygen is inadequate or not utilised pyruvic acid is converted into either ethanol or lactic acid and very little amount of energy (nearly one tenth of that is produced with adequate amount of oxygen) is released. Can energy be released without oxygen? • After undergoing strenous exercise we feel pain in muscles, does adequate oxygen reach the muscles? • What is being formed in the muscles? 34 X Class Respiration - The energy releasing system When you sprint a hundred yards, you do a considerable amount of muscular work. But you do not start a race by standing on the track and panting for a few minutes to stoke up with oxygen first. In fact you can run the race with very little extra breathing. The fastest sprinters do not breathe at all when running a hundred yards. After you have reached the destiny, however, you feel very different. Depending on your state of training, and on how hard you ran, you will pant for some minutes after the race, until your breathing gradually returns to normal. fig-11: Athlete (Strenous excercise) These facts could be linked up with what we have learned so far about ATP. It might be that the race was run on the energy produced when the ATP already present in your muscles was being converted to ADP. Unfortunately this pleasantly simple idea is inadequate, because we only carry sufficient ATP in a muscle to last for about half a second when doing vigorous exercise. There must be some other explanation for the way in which we can produce energy first and then use up oxygen later. One approach in the study of this problem was to analyze the blood of a person during and after exercise. For obvious reasons the athlete taking part in the experiment had to stay still where the apparatus was. He pedaled a stationary bicycle, or ran on a tread mill (belt moving as fast backwards as the athlete moved forwards). Some results are shown in the graph. Vigorous exercise lasted for nine minutes (shown by the bar at the base of the diagram) and regular blood samples were taken and analyzed. One particular compound in the blood, lactic acid, was found to vary greatly in its concentration as you can see from the graph. Observe the graph of lactic acid accumulation in the muscles of an athelete and answer the following questions. (Graph showing varying amount of lactic acid in the muscles) x - axis = Time in minutes y - axis = Concentration of lactic acid in blood mg/cm3 Graph showing effects of vigorous excercise on the concentration of lactic acid in blood. Free distribution by A.P. Government 35 a. What was the concentration of lactic acid in the blood to start with? b. What was the greatest concentration reached during the experiment? If the trend between points C and D were to continue at the same rate, c. how long might it take for the original lactic acid level to be reached again? (Hint: extend the line CD until it reaches the starting value.) d. What does high level of lactic acid indicate about the condition of respiration? Accumulation of lactic acid results in muscular pain. If we take walk, brisk walk, slow jogging, running for same distance we feel that there an increase in pain levels this is because of lactic acid accumulation. It seems as if the lactic acid was being produced rapidly by the active muscles, and then only gradually removed from the blood after exercise. What is surprising is that the athlete needs a great length of time to recover. The simplest explanation we can produce at-this stage is that the sugar in the working muscles was being changed to lactic acid. The energy stored in lactic acid molecules is less than that in sugar molecules, and if the acid comes from the sugar then the energy released could be used to rebuild ATP from ADP and phosphate. During a 100 m race a well-trained athlete can hold his breath all the time it is not until afterwards that he pants. In this case, the muscles are using the energy released during the anaerobic breakdown of glucose. It is not until afterwards that the athlete obtains the oxygen needed in order to remove the lactic acid. Therefore, when we under-take strenuous exercise we build up what is called an oxygen debt which has to be repaid later. In a longer race athletes have to breathe all the time, so some lactic acid is removed while they are running, and they can go on for longer before becoming exhausted. The presence of lactic acid in the blood is the main cause of muscle fatigue, but if the body is rested for long enough the tiredness goes. Anaerobic respiration We have found that living things produce carbon dioxide and give out energy. If these processes are caused by an oxidation process, what happens if the oxygen supply is cut off? If human muscles can go on releasi
ng energy when they are short of oxygen, what can cells of other living organisms do? Let us find out by doing some experiments. 36 X Class Respiration - The energy releasing system Lab Activity Some experiments with yeast To test this idea we can see whether it is possible to detect any rise in temperature and the production of carbon dioxide, when living organisms are kept away from a supply of oxygen. thermometer liquid paraffin Yeast grows rapidly if it is supplied with glucose in solution. Indeed, wild yeasts are normally found growing on the skins of fruits like grapes and apples, from which they derive their food supplies. Our immediate problem is to remove the oxygen from the glucose solution and yeast. 1. You can remove dissolved oxygen from glucose solution by heating it for a minute, and then cooling it without shaking. Now put in some yeast; the supply of oxygen from the air can be cut off by pouring one centimetre layer of liquid paraffin on to the mixture. If you wish to check that the oxygen has been removed from the mixture, add a few drops of diazine green (Janus Green B) solution to the yeast suspension before you pour the liquid paraffin (wax) over it. This blue dye turns pink when oxygen is in short supply around it. 3. Arrange for any gas produced by the yeast to escape through a wash bottle containing bicarbonate/indicator solution (or lime water). We have not described any control experiments try working them out for yourself. You may prefer to carry out the ‘carbon dioxide production’ part of the experiment on a smaller scale, using test tubes. If you do, then warm them to about 37o C in order to speed up the test. • What happens when a baker prepares a dough by mixing yeast in it? 2. yeast in boiled and cooled glucose bicorbonate solution fig-12: Testing for production of heat and CO2 under anaerobic respiration Fermentation Let us recall maida dough and yeast powder activity that you performed in class VIII in the chapter ‘The story of microorganisms’. Why volume of the dough has increased? Which gas released in that reaction? If yeast and sugar solution are left to stand without oxygen for some days, they develop a characteristic smell, caused by production of new compound called ethanol, which has been manufactured by the yeast from Free distribution by A.P. Government 37 the sugar. The same type of smell you can notice from preserved idly, dosa dough at your home. But not in refrigirator. The ethanol can be separated from the yeast-glucose mixture by the process of fractional distillation since ethanol boils at a lower temperature (70°C) than the sugar solution. Collect information about fractional distillation with the help your teacher. Quite like aerobic respiration this is a process of producing energy when there is no supply of oxygen. • Respiration is an energy releasing pathway, do you agree? Justify your answer. Respiration versus combustion Lavoisier around the late 18thcentury, through a series of carefully performed experiments, came to the opinion that respiration was a process like combustion. He wrote in a compilation in 1783, “respiration is a combustion process. It is a very slow process and here oxygen is not only combines with carbon but also with hydrogen.” Robinson also stated that respiration is a type of combustion and combustion is the source of heat in animals. Activity-2 Observing changes during combustion of sugar Arrange apparatus as shown in the figure and heat it over a flame. Does it melt? What happen if you heat for somemore time? glucose test tube heat delivery tube lime water fig-13: CO2 - a by product of energy release When glucose burns, carbon dioxide and water are produced and energy is released as heat. We know that combustion of glucose gives us carbon dioxide, water and energy while from the respiratory equation we get the same products. But essentially the processes must differ due to following reasons. 1. Glucose must be burnt at high temperature in the laboratory to liberate energy, if it happened in our cells,all cells would be burnt. 2. Once glucose starts burning we can’t stop the process easily, but living cells are able to exercise control over the sort of burning of glucose in the presence of oxygen. 38 X Class Respiration - The energy releasing system 3. Water normally stops combustion from taking place while cells con- tain a lot of water and respiration still goes on. What can you conclude from this? Heat production by living organisms Heat production was a feature of burning glucose or sugar as you observed earlier. Living animals and plants usually produce energy in the form of heat. We feel warm when we wear sweater in winter season. We know that sweater prevents loss of heat energy produced by the body. Does this suggest any way in which our bodies lose heat to the surroundings? • What are the other ways in which our body loses heat? Heat is constantly lost from the body surface thus it must be continuously generated within our bodies to replace what has been lost to keep the body temperature constant. • Is the rate of heat production always the same? In the course of vigorous activity, a greater amount of heat is generated. We know that we feel hot after some form of strenuous exercise such as running. During celular respiration energy is released. Some part of energy is stored in the form of ATP. Some part of energy is utilised in our day to day life activities. And the excess amount of energy is released in the form of heat. But in case of vigorous activity like running we need more amount of energy. For this the rate of respiration is increased. So heat is also released in excess quantity. That’s why we feel warm. If the oxygen is not sufficient during vigorous excersise muscles start anerobic respiration. Hence, lactic acid is formed. We know that accumulation of lactic acid causes pain in muscle. We reach normal position after some rest. Deep breath helps us to restore energy in our body. Refer in annexure about Yoga Asanas. Evolution in gases exchanging system Exchange of gases is a common life process in all living organisms, but it is not same in all. Single celled organisms Amoeba or multicellular organisms like Hydra and Planarians obtain oxygen and expel carbon dioxide directly from the body by the process of diffusion. In other multicellular animals special organs are evolved. Animals either terrestrial or aquatic adopted to different types of respiration and possess different types of respiratory organs mostly depending on the habitat in which they live. Body size, availability of water and the type of their circulatory system Free distribution by A.P. Government 39 are some of the reasons for the animals to develop different types of respiratory organs. We will see tracheal respiratory system in insects like cockroach, grashopper etc. Tracheal respiratory system consists of series of tubes called trachea. This is divided into fine branches called tracheoles which carry air directly to the cells in the tissues. Some aquatic animals like fishes have developed special organs for respiration which are known as gills or branchiae. Blood supplied to gills through capillaries which have thin walls which gases are exchanged. This is called branchial respiration. Fish keeps its mouth open and lowers the floor of the oral cavity. As a result water from outside will be drawn into the oral cavity. Now the mouth is closed and the floor of the oral cavity is raised. Water is pushed into the pharynx and is forced to gill pouches through internal branchial apertures. Gill lamellae are bathed with water and gas exchange takes place. Respiration through skin is called cutaneous respiration. Frog an amphibian can respire through cutaneous and pulmonary respiration processes. Terrestrial animals like reptiles, birds and mammals, respire through lungs. Ask your teacher how crocodiles and dolphins respire? Respiration in Plants water film air spaces stomata fig-14: Leaf as a respiratory organ fig-15: Lenticells on stem You already know about stomata in leaf where gaseous exchange takes place in plants. There are other areas on the plant body as well through which gaseous exchange take place like surface of roots, lenticels on stem etc. (Fig showing stomata and lenticels). Some plants have specialized structures like breathing roots of mangrove plants as well as the tissue in orchids that produces oxygen is also required by plants to produce energy and carbon dioxide is released. But CO2 is required elsewhrere in the plants try to identify them. Conduction within the plant The stomatal opeinings lead to a series of spaces between the cells inside the plant. Which form a continuous network all over the plant. The spaces are very large in the leaves, much smaller in other parts of the plant. The air spaces are lined with water where the oxygen is dissolved in this and passes 40 X Class Respiration - The energy releasing system through the porous cell walls into the cytoplasm. Here the sugar is broken down into carbon dioxide and water with the liberation of the energy. The carbon dioxide passes out into the air spaces by a similar method. The whole system works by diffusion; as the oxygen is used up by the cells a gradient develops between the cells and the air in the spaces. Similarly between the air in the spaces and the air outside the stomata and lenticels, so oxygen passes in. In the same way, as more carbon dioxide is released by the cells a gradient occurs in the reverse direction and it passes out to the environment. Aeration of roots Most plants can aerate their roots by taking in the oxygen through the lenticels or through the surface of their root hairs (as their walls are very thin). They obtain oxygen from the air spaces existing between the soil particles. But, plants which have their roots in very wet places, such as ponds or marshes, are unable to obtain oxygen. They are adapted to these water-logged conditions by havin
g much larger air spaces which connect the stems with the roots, making diffusion from the upper parts much more efficient. fig-16: Aerial roots The most usual adaptation is to have a hollow stem. Next time when you are by a pond or marsh cut the stems of some of the plants which are growing there and see how many are hollow compared with a similar number of species of plants growing in normal soil. The problem of air transport is more difficult for trees and not many survive with their roots permanently in water. An exception is the mangrove tree of the tropics which forms aerial roots above the soil surface and takes in oxygen through these roots. To know more about respiration in plants we should perform the following activities. Activity-3 Take a handful of moong or bajra seeds. Soak the seeds in water a day before to perform your experiment. Keep these soaked seeds in a cloth pouch and tie with a string tightly. Keep the cloth pouch in a corner of your class room. Next day collect the sprouts/ germinated seeds from the pouch, keep it in a glass bottle/plastic bottle(around 200 ml capacity). Take a small injection bottle, fill three fourth of the bottle sprouted seeds beaker with lime water fig-17: Evolved CO2 in respiration Free distribution by A.P. Government 41 with lime water. Tie a thread to the mouth of the small bottle; insert it in the bottle carefully and let it hang by the thread. Close the plastic bottle tightly. Make a similar set with unsprouted seeds. Keep this set undisturbed for one or two days. During this time observe the color of lime water in both the sets. In which set does the color change faster? Why? Activity-4 thermometer flask germinating seeds fig-18: Heat evolved during respiration Take sprouts which were prepared for above activity in a thermos flask. Remove the lid and prepare a cork (with thermocol, or rubber or any other material) through which you can bore a hole to insert a thermometer. Take care that the bulb of the thermometer should dip in the sprouts. Close the flask with this tight fitting cork. Record the temperature every two hours. You are advised to do this for at least 24 hours. • Make a graph by using your observations. Is there any increase in temperature? • • Does the temperature increase steadily or does it abruptly increase at a time of the day? • Where does the heat come from? Photosynthesis versus Respiration Plants carry out photosynthesis, which means that they produce their own food from atmospheric CO2 using light energy from the sun. This process is a complex series of steps involving the conversion of light energy into chemical energy, which is then used to synthesize sugars from carbon dioxide. This is a process of synthesis or an anabolic process which occurs in the chloroplasts. The equation below summarizes the photosynthetic process CO2+ H2O Light energy Chlorophyll (CH2O) n+ O2 Sugar Once produced, the sugars can then be used for the process of respiration to provide energy to run all life processes. Respiration as we know is not just the exchange of gasses. It is the process of breakdown of complex food molecules or a catabolic process to produce chemical or potential energy. This can be summarized by the equation (CH2O)n+ O2 CO2+ H2O + Energy 42 X Class Respiration - The energy releasing system Photosynthesis and respiration appear to be opposing reactions, but both have very different biochemical pathways and are essential for a plant’s metabolism. Photosynthesis takes place in the chloroplast to produce sugars, starch and other carbohydrates for the plant’s metabolic needs. Cellular respiration occurs in mitochondria where these carbohydrates are “burned” to produce chemical energy to function at the cellular level. During day time, the rate of photosynthesis is usually higher than that of respiration while at night it is just reverse in most plants. Temperature, humidity, light intensity etc. seem to affect the ratio of photosynthesis and respiration in plants. Key words Aerobic respiration, Anaerobic respiration, Alveoli, Trachea, Bronchi, Bronchioles, Epiglottis, Pyruvate, Anabolic, Catabolic. What we have learnt • By “respiratory system” we usually mean the passages that transport air to the lungs and to the microscopic air sacs in them, called alveoli (where gases are exchanged) and vice versa. • The term ‘respiration’ refers to the whole chain of processes from the inhalation of air to the use of oxygen in the cells. • Lavoisier found that the air that we breathe out precipitated lime water while that after heating metal did not. He also found that something even beyond lungs occurred to produce carbon dioxide (he knew it as fixed air) and body heat. • Air passes from nostrils to nasal cavity to pharynx, larynx, trachea, and bronchi, bronchioles to alveoli and blood and back through the same route. • Gas exchange in the lungs takes place in the tiny air sacs called alveoli in the lungs. The lungs have millions of alveoli and each lies in contact with capillaries. Oxygen and carbon dioxide diffuse readily across a combination of the alveolar wall, the capillary wall and a thin layer that lies between them. • Diaphragm is a muscular tissue present at the floor of the chest cavity. • During inspiration (inhalation) the volume of the chest cavity is increased as the diaphragm contracts and dome flattens out, its internal pressure decreases and the air from the outside rushes into the lungs. Free distribution by A.P. Government 43 sides, remove some of the tiny particles of dirt in the air. In addition, as the inhaled air passes through the nasal cavity, its temperature is brought close to that of the body, and it takes up water vapor so that it becomes more moist than before. • Pharynx is a common passage of digestive and respiratory system. Epiglottis, a flap like muscular valve controls movement of air and food towards their respective passages. • Larynx is a stiff box like structure containing our vocal cords. When air passes out of the lungs and over the vocal cords, it causes them to vibrate. This produces sounds on the basis of our speech, song etc. • Trachea is the wind pipe channeling air to lungs. • At its lower end the trachea or the wind pipe divides into two bronchi-one leading to each lung. • The bronchi divide into smaller and smaller branches called bronchioles. • These finally terminate in clusters of air sacs called alveolus in the lungs which are very small and numerous. Gaseous exchange takes place here as blood capillaries take up oxygen and expel carbon dioxide here. • Aerobic respiration occurs in adequate supply of air producing a lot of energy, carbon dioxide and water. • Anaerobic respiration and fermentation occurs in inadequate supply or absence of oxygen to produce energy. • Cells may resort to the breakdown of 3 carbon compound, pyruvate, aerobically or anaerobically depending upon the availability of oxygen. Usually in multicellular organisms cells fail to carry on the process of anaerobic respiration for long. • Respiration is not essentially a process of combustion differ due to following reasons - Glucose must be burnt at high temperature in the laboratory to liberate energy, if it happened in our cells, all cells would be burnt. - Once glucose starts burning we can’t stop the process easily, but living cells are able to exercise control over the sort of burning of glucose in the presence of oxygen. - Water normally stops combustion from taking place while cells contain a lot of water and respiration still goes on. • • Photosynthesis and respiration appear to be opposing reactions, but both have very different biochemical pathways and are essential for a plant’s metabolism. Photosynthesis takes place in the chloroplast to produce sugars, starches and other carbohydrates for the plant’s metabolic needs. • Cellular respiration occurs in mitochondria where mainly these carbohydrates are “burned” to produce chemical energy to do work at the cellular level. 44 X Class Respiration - The energy releasing system Improve your learning 1. Distinguish between (AS1) a) inspiration and expiration b) aerobic and anaerobic respiration c) respiration and combustion d) photosynthesis and respiration 2. State two similarities between aerobic and anaerobic respiration.(AS1) 3. Food sometimes enters the wind pipe and causes choking. How does it happen?(AS1) 4. Why does the rate of breathing increase while walking uphill at a normal pace in the mountains? Give two reasons.(AS1) 5. Air leaves the tiny sacs in the lungs to pass into capillaries. What modification is needed in the statement?(AS1) 6. Plants photosynthesize during daytime and respire during the night. Do you agree to this statement? Why? Why not?(AS1) 7. Why does a deep sea diver carry oxygen cylinder on her back?(AS1) 8. How are alveoli designed to maximize the exchange of gases?(AS1) 9. Where will the release of energy from glucose in respiration take place? Mala writes lungs while Jiya writes muscles. Who is correct and why?(AS1) 10. What is the role of epiglottis and diaphragm in respiration?(AS1) 11. How gases exchange takes place at blood level?(AS1) 12. Explain the mechanism of gasses exchange at branchiole level.(AS1) 13. After a vigorous excercise or work we feel pain muscles. What is the relationship between pain and respiration?(AS1) 14. Raju said stem also respire along with leaves in plants. Can you support this statement? Give your reasons.(AS1) 15. What happen if diaphragm is not there in the body?(AS2) 16. If you have a chance to meet pulmonologist what questions your going to clarify about pulmonory respiration?(AS2) 17. What procedure you followed to understand anaerobic respiration in your school laboratory?(AS3) 18. What are your observations in combustion of sugar activity?(AS3) 19. Collect information about cutaneous respiration in frog. Prepare a note and display them in your classroom.(AS4) 20. Collect information about respiratory diseases
(because of pollution, tobacco) and discuss with your classmates.(AS4) 21. What is the pathway taken by air in the respiratory system? Illustrate with a labelled diagram.(AS5) 22. Draw a block diagram showing events in respiration. Write what you understood about cellular respiration.(AS5) Free distribution by A.P. Government 45 23. How you appreciate the mechanism of respiration in our body?(AS6) 24. Prepare an article on enaerobic respiration to present school symposium.(AS7) 25. Prepare a cartoon on discussion between haemoglobin and chlorophyll about respiration.(AS7) Fill in the blanks 1. Exhaled air contains _________ and _________. 2. A flap like muscular valve controls movement of air and food is ___________. 3. Energy currency of the cell is called _____________. 4. Lenticells are the respiratory organs exists in __________ part of plant. 5. Mangroove trees respire with their ____________. Choose the correct answer 1. We will find vocal cords in a) larynx b) pharynx c) nasal cavity d) trachea 2. Cluster of air sacs in lungs are called a) alveolus b) bronchi c) braonchioles d) air spaces 3. Which of the following is correct a) the diaphragm contracts - volume of chest cavity increased b) the diaphragm contracts - volume of chest cavity decreased c) the diaphragm expands - volume of chest cavity increased d) the diaphragm expands - volume of chest cavity decreased 4. Respiration is a catabolic process because of a) breakdown of complex food molecules b) conversion of light energy c) synthesis of chemical energy d) energy storage 5. Energy is stored in a) nucleus b) mitochondria c) ribosomes d) cell wall ( ( ( ( ( ) ) ) ) ) 46 X Class Respiration - The energy releasing system Annexure Pranayama - The art of breathing It is wonder to know that only human beings have to learn how to breath. Our lungs are devided into lobes. At each breath we will inhale or exhale only 500ml of air. Where as our lung capacity is approximately 5800ml. So most of the time breathing takes place in the upper lobes only. This means we are not using our lungs to their fullest capacity. Even after complete expiration approximately 1200ml of air remains in our lungs. So we can make use of 4600ml of lung capacity for breathing. The indian ayurvedic physician. Patanjali developed a scientific breathing practice called Yogabyasa. Maharshi Patanjali proposed a theory called Astanga yoga. He was introduced 195 yogic principles in eight divisions. 1. Yama (Social disipline), 2. Niyama (Individual disipline), 3. Asana (Body posture), 4. Pranayama (Expansion of vital energy), 5. Prathyahara (With drawal of senses), 6. Dharana (Consontration), 7. Dhyana (Meditation), 8. Samadi (Self rialisation). The art of breathing in Yogabyasa is called Pranayama prana means gas, ayama means journey. In Pranayama practice air is allowed to enter three lobes of lungs inorder to increase the amount of oxygen to defuse into blood. Deep breaths in Pranayama helps us to reduce breathings per minute form 20-22 to 15. Because of these deep breaths more amount of oxygen available to brain and tissues of the body will be more active. It is very important to practice Pranayama regularly to make our life healthy and active. All people irrespective of age and sex should practice Pranayama under the guidance of well trained Yoga Teacher to improve the working capacity of lungs. Free distribution by A.P. Government 47 Chapter 3 Transportation - The circulatory system All the living organisms need nutrients, gases, liquids etc., for growth and maintenance of the body. All the organisms would need to send these materials to all parts of their body whether they are unicellular organisms or multicellular. In unicellular organisms these may not have to be transported to longer distances while in multicellular forms have to be sent substances to long distances as far as say over 100 feet for the tallest plant on earth. In lower organisms like amoeba, hydra etc., all the materials are transported through a simple processes like diffusion, osmosis etc., In higher animals with trillions of cells in their body adopt the method of diffusion and osmosis only for the bulk movement of materials, would takes years. To avoid delay a separate system is needed to carry the materials much faster and more efficiently. This specialized system that is developed by organisms is called ‘the circulatory system’. We eat solids, we drink liquids, and we breathe gases. Do you think that it is possible to transport all the three types of materials, through a single system? Let us study how this circulation is carried out in our body. Have you ever observed a doctor holding the wrist of the patient and looking at his watch for a minute? What is that he is trying to find out from the watch and the wrist of the patient? You may wonder to know that he is 48 X Class Transportation - The circulatory system counting the heart beat of the patient. Don’t you think that is crazy, holding the hand to count the heart beat? Activity-1 You could try to find out for yourself, what the doctor was doing. Keep your index and middle fingers on your wrist below the thumb as shown in the fig-1. • What did you feel? You feel something pushing your fingers rhythmically up and down. Now let us count the rhythm which is called the pulse, for a minute. Now stand up and jog for one minute at the same place. Note the pulse for a minute. Take readings atleast two of your parents in the same manner and record in the following table. fig-1: Pulse S.No Name of the person Table-1 Pulse rate per minute at rest after jogging • What did you observe? Is the pulse rate same in both conditions? Activity-2 We see that pulse rate varies from person to person and situation to situation. So it is not constant, when you are afraid or excited the pulse rate goes up. Observe your pulse rhythm in other ways as well such as climbing stairs, running, etc. There is a relationship between the pulse rate and the beat of our heart. Now let us try to find out more about this relationship. fig-2: Matchstick stethoscope For this you have to make your own stethescope. Take a shirt button insert a matchstick as shown in fig-2. Place it on your wrist. Observe movements in matchstick. • What did you find? • When do you think that our pulse rate goes up? • What does the pulse rate show? Free distribution by A.P. Government 49 Do you know? Newborn (0–3 months) Infants (3-6 months) Infants (6–12 months) Children (1-10 years) Children over 10 years & adults, including senior citizens Welltrainedadults athletes 100-150 90-120 80-120 70-130 60-100 40-60 In the year 1816, Rene Laennac discovered the Stethoscope. Before the discovery of stethoscope doctors used to hear heart beat by keeping ear on the chest of the patient. Laennac found that paper tube helps to hear the heart beat perfectly. Then he used a bamboo instead of paper tube to hear heart beat. Laennac called it stethoscope. Activity-3 Let us repeat the work Laennac. Make a paper tube 10 inch long and one inch in diameter. Keep one end of it on the chest of your friend on a point one inch to the left side to the centre around 6 inches below from his or her neck. Keep your ear at the other end. Listen carefully and count the heart beats for a minute. Also count down your friend’s pulse rate. Note observations of at least 10 students of your class in the following tabular form. Table-2 S.No Name of the student Eswar 1 Heart beat at rest/min Pulse rate at rest/min 72 72 Let us plot histogram on heart beat and pulse rate of different persons as shown in the sample graph. Here blue bar indicates heart beat, red bar indicates pulse rate. • What is the relationship between the heart beat and the pulse? • Can we say, the pulse rate is always equal to the heart beat? You might have studied there is a relation between pulse rate and heart beat. Eshwar x-axis: Name of the student y-axis: Heart beat, pulse rate per minute 50 X Class Transportation - The circulatory system Now try to understand the structure and method of working of this vital organ, the heart. It is the beat of the heart which keeps us alive. Heart is located in between the lungs and protected by rib cage. The size of your heart is approximately the size of your fist. fig-3: Location of Heart fig-4: Heart Lab Activity Aim: Observation of the internal structure of the mammalian heart. Material required: Since the structure of all the mammalian hearts are similar, we take the sheep’s or goat’s heart for our observation. For this, we need following materials. Freshly collected specimen of heart of sheep or goat from the butcher. Soda straws, sharp and long blade or scalpel, tray, a jug of water, Dissection scissors, forceps. Procedure for observation: • Before coming to the class wash the heart thoroughly so that, blood is completely drained from the chambers of heart. • Take soda straws and insert them into the stumps of the blood vessels. Note your observations as you proceed. • How many layers are covering the heart? (Now remove the layers covering the heart, and observe) • What is the shape of the heart? • How many large blood vessel stumps are attached to the heart? • Which end of the heart is broader and which end is narrow? Observe the arrangement of blood vessels (coronary vessels) on the wall of the heart. (In case you don’t have a model or a goat’s heart, look at the figures given carefully for observation) Free distribution by A.P. Government 51 Internal structure of the heart • Keep the heart in the tray in such a way that a large arch like tube faces upwards. This is the ventral side. • Now take a sharp blade or scalpel and open the heart in such a way that the chambers are exposed. Take the help of the fig-6. arteries to head artery to left arm superior vena cava aorta pulmonary artery (right branch) right pulmonary veins right atrium right atrio vetricular valve pulmonary artery (left branch) left pulmonary veins left atrium semi - lu
nar valve in the pulmonary artery left atrio vetricular value left ventricle inferior vena cava right ventricle fig-5: Internal structure of heart Now observe the internal structure. Observe the wall of the heart. Is the thickness of the wall of the heart uniform throughout? • • How many chambers are there in the heart? • Are all the chambers of the same size? • What other differences could you observe between the chambers? • Are all the chambers connected to each other? • How are they connected to each other? How are they separated? You can observe white coloured structures in the lower part of the heart. Note down the size, shape and to which parts they are attached. Can you guess the function of these structures? Write a note on your observations of the heart. Compare your notes with the description given below. The heart is a pear shaped structure, triangle in outline, wider at the anterior end and narrower at the posterior end. The heart is covered by two layers of membranes. The membranes are called pericardial membranes. The space between these two layers is filled with pericardial fluid, which protects the heart from shocks. 52 X Class Transportation - The circulatory system The heart is divided into four parts by grooves. Two upper parts are called atria (auricles), and the lower ones are called ventricles. The left atrium and ventricle are smaller when compared to that of right atrium and ventricle. The blood vessels found in the walls of the heart are coronary vessels which supply blood to the muscles of the heart. The walls of the ventricles are relatively thicker than atrial walls. In our observation we found that the heart has four chambers in it. On the left side two chambers are present, one is anterior and the other is the posterior. On the right side also two chambers present, one upper (anterior), and one lower (posterior). Observe the presence o blood vessels attached to the heart. • How many blood vessels are attached to the heart? • Are all the blood vessels are rigid? How many of them are rigid? • Do you think that the stiffness/rigidity of blood vessel is something to do with circulation? The rigid vessels are called arteries which originate from the heart and supply blood to various organs in the body. The larger artery is the aorta. The relatively smaller one is pulmonary artery which carries blood from the heart to the lungs. The less rigid vessels are the veins, which bring blood from all body parts to the heart. The vein which is at the anterior end of the right side of the heart is superior venacava (precaval vein), which collects blood from anterior parts of the body. The vein which is coming from posterior part of the heart is inferior venacava (postcaval vein), collecting blood from posterior part of the body. The two atria and the two ventricles are separated from each other by muscular partitions called septa. The openings between atria and ventricles are guarded by valves. In the right atrium we can observe the openings of superior and inferior venacava. In the left atrium, we can observe the openings of pulmonary veins, that bring blood from lungs. From the upper part of the left ventricle, a thick blood vessel called aorta arises. It supplies oxygenated blood to the body parts. From the upper Free distribution by A.P. Government 53 part of the right ventricle pulmonary artery arises that supplies deoxygenated blood to the lungs. After careful examination we can observe valves in the pulmonary artery and aorta as well. The blood vessels and circulation Let us study how we came to know about the structure and functions of the blood vessels. It was not until 16th century that we really came to know how our blood vessels functioned. In 1574, an Italian doctor, Girolamo Fabrici, was studying the veins in the leg. He noticed that they had small valves in them. If the blood moved in one direction, the valves folded towards the walls of the vessel, so that the blood could pass without trouble. If the blood moved in the opposite direction, the valves closed. This meant they are one-way valves. The valves permitted the blood to move even when a person is standing upright. But not move downward. When a person moves his legs, or just tightens his leg muscles, those muscles squeeze the veins and force the blood in those veins to move upward against the pull of gravity (because that’s the only way to go). If a person keeps his leg muscles relaxed, the blood isn’t moving much, but at least it isn’t being pulled down by gravity. The valves won’t allow that. Everyone thought that the blood leaving the left ventricle always moved away from the heart for which Fabrici paid no attention. He missed the importance of his own discovery. But then, William Harvey (1578-1657), an Englishman who, after he became a doctor, went to Italy for further education and studied under Fabrici. Harvey dissected the hearts of dead people and studied the valves between each atrium and its ventricle. He noticed that they were oneway valves. They allowed the blood to flow from the atrium to the ventricle without any hinderance. When the heart contracted, however, no blood in the ventricle could flow back into the atrium. Instead, all the blood was pushed out into the arteries. Harvey began thinking about the valves his teacher, Fabrici, had discovered in the leg veins. They were one-way, and they forced the blood to move toward the heart. He checked that by tying off and blocking different veins in animals he experimented on the veins always bulged on the side of the block away fig-6: William Harvey 54 X Class Transportation - The circulatory system from the heart. As though the blood as trying to flow toward the heart and to accumulate just below the block because it simply couldn’t flow away from the heart. This was true of all veins. In the arteries, the blood bulged on the heart side of any block he put in, as though it were trying to flow away from the heart and couldn’t move in the other direction. Harvey now saw what was happening. The heart pushed blood into the arteries, and the blood returned by way of the veins. It did this for both ventricles. The blood had a double circulation. If one started from the right ventricle, it left by way of the arteries to the lungs, and returned by way of the veins to the left atrium and from there into the left Ventricle.From the left ventricle, it left by way of the arteries to the rest of the body and returned (in a “greater circulation”) by way of the veins to the right atrium and from there into the right ventricle. Then it started all over. Harvey also showed that it was impossible, suppose that the blood was used up in the body and that new blood was formed. He measured how much blood the heart pumped in one contraction and also counted the number of contractions. He found that in one hour, the heart pumped out a quantity of blood that was three times the weight of a man. The body couldn’t use up blood and form new blood at such a rate. The same blood had to circulate and be used over and over again. Harvey still had some problem. The smallest arteries and veins that could be seen had to be connected by vessels too small to see. Were they really there? In the 1650s, scientists had learned to put lenses together in such a way that objects too small to see with the naked eye could be magnified and made visible. Marcello Malpighi (1628-1694), with the microscope, he could see tiny blood vessels that were invisible with naked eye. In 1661, four years after Harvey’s death, Malpighi studied the wings of bats. He could see blood vessels in their thin membranes and, under the microscope; he could see that the smallest arteries and veins were connected by very fine blood vessels. He called these blood vessels “capillaries” from the Latin word for “hair”, because they were as thin as the finest of hairs. With the discovery of capillaries, the idea of the circulation of the blood was complete, and it has been accepted ever since. fig-7: Marcello Malpighi Free distribution by A.P. Government 55 Now, we know that blood circulates in the blood vessels. But how did the scientists find out that blood moves in blood vessels? Is it possible to demonstrate the movement of blood in vessels without damaging the vessels? Let us repeat the classical experiment to demonstrate the movement of blood in veins conducted by William Harvey in early 17th century, when there was no compound microscope or any other modern equipment. 1. Tie a tornquit just above the elbow of a person, whose blood vessels are prominent in the hand. 2. Ask him/her to hold the fist with a piece of cloth rolled in the hand. Now the blood vessels can be seen more prominently. 3. Find undivided blood vessel, where we have to work for the next few minutes. 4. At the end of the vessel farthest from the elbow apply steady pressure, so as to close its cavity. 5. Now apply pressure from elbow towards the palm slowly and observe the changes in the blood vessels. (Take the help of the figures given here.) Observe changes and discuss in your class. fig-8(a): Try like this fig-8(b): Harvey’s Arteries and veins There are two types of blood vessels called arteries and veins. Arteries carry blood from the heart to body parts. Whereas, veins carry blood from body organs to heart. Let us observe the structural and functional differences between arteries and veins. tough fibrous coat elastic fibrous coat muscle layer lumen lining cells lumen lining cells fig-9(a): T.S. of Artery fig-9(b): T.S. of Vein fig-9(c): T.S.of Blood capillary Blood capillaries Blood capillaries are the microscopic vessels made of single layer of cells. They allow diffusion of various substances. The leucocytes (WBC) can squeeze out of the capillary wall. They establish continuity between arteries and veins. 56 X Class Transportation - The circulatory system Answer the following after reading the experiment conducted by William Harvey. • In which blood vessels
valves are found? What do you think is the function of the valves in them? • Why do sub-cutaneous blood vessels bulge on the side away from the heart when the hand is tied? • The deep seated blood vessels (the arteries) bulge on the side towards the heart when tied. What do you understand from it? • There are valves in the heart between atria and ventricles. Is the purpose of valves in the veins and arteries same? After reading the experiments by Harvey fill in the following table. Use the clues/options given in the first column. Structure / Function Artery Vein Table-2 1. Thickness of walls(thick / thin) 2. Valves (present / absent) 3. Capacity to retain shape when blood is absent (can retain/collapse) 4. Direction of blood flow (heart to organs / body organs to heart) 5. Pressure in the vessel(low /high) 6. Type of blood transported (oxygenated / de-oxygenated) 7. Type of blood carried by pulmonary artery (de -oxygenated/ oxygenated) 8. Type of blood carried by pulmonary vein (oxygenated / de-oxygenated) Let us perform the following activities to observe arteries and veins. Sit on a table with one leg dangling and the other resting on it so that the back of one knee rests on the knee of the other. After a time you will see and feel the leg which is on top give a series of small movements with each heart beat. If you do it for long you will reduce the blood flow to the leg and so develop ‘pins and needles’. Swing your arm round several times to fill the veins with blood, hold the arm vertically downwards and gently press your finger along a prominent vein-stroking it in the reverse direction to the blood flow, i.e., towards the Free distribution by A.P. Government 57 hand. Can you see the swellings where you have pushed blood against the valves? Discuss with your teacher about reasons. Think and discuss • Artery walls are very strong and elastic. why? • Why we compare arteries like tree which devides into smaller and smaller branches. • The lumen size is bigger in vein when compared with artery. Why? The cardiac cycle The human heart starts beating around 21st day during the embryonic development (refer reproduction chapter). If it stops beating, it results in the death of a person. One contraction and one relaxation of atria and ventricles is called one cardiac cycle. 1. We start with the imagination that all the four chambers of the heart are in relaxed state (joint diastole). 2. Blood from venacava and pulmonary veins enters the right and left atria respectively. 3. Now the atria contract, forcing the blood to enter into the ventricles. 4. When the ventricles are filled with blood they start contracting and atria start relaxing. On ventricular contraction due to pressure the blood moves into the aorta and pulmonary artery. The aperture between the atria and ventricles is closed by valves. When the valves are closed forcibly, we can listen to the first sharp sound of the heart lub. 5. When the ventricles start relaxing the pressure in the ventricles is reduced. The blood which has entered the arteries tries to come back into the ventricles. The valves which are present in the blood vessels are closed to prevent backward flow of blood into the ventricles. Now we can listen to a dull sound of the heart dub. The atria are filled up with blood and are ready to pump the blood into the ventricles. The sequential events in the heart which are cyclically Transportation - The circulatory system 1. Imaginary relaxation of atria and ventricles. 2. Blood flows into atria. 3. Contraction of atria and flow of blood into ventricles. 4. Contraction of ventricles. A.V. Valves closed (Lub) blood flows into artries. 58 X Class repeated are called cardiac cycle. The cardiac cycle includes an active phase systole and a resting phase the diastole of atria and ventricles. The whole process is completed in approximately 0.8 second. The time needed for atrial contraction is 0.11-0.14 seconds. The time needed for ventricular contraction is 0.27-0.35 seconds. Hence, naturally the blood is pumped into the blood vessels at regular intervals. The tissues will not receive the blood continuously, but in the form of spurts. When we keep our finger at the wrist, where the artery is passing into the hand we feel the pressure of blood moving in it. This is the pulse. The rate of the pulse will be equal to the number of heart beats. Do you know? 5. Relaxation of ventricles. The closing of arterial valves (Dub). fig-10(1-5): Cardiac cycle Name of the animal Blue whale Elephant Man Coaltit (Bird) Weight of the body Weight of the heart 1,50,000 kg 3000 kg 60-70kg 8 gm 750 kg 12 - 21 kg 300 gm 0.15 gm No. of beats/min 7 46 76 1200 Single /double circulation We know that blood flows in the blood vessels. To keep the blood moving the heart pumps it continuously. The blood that is pumped by the heart reaches the body parts and comes back to the heart. But course taken by the blood is not the same in all the animals. Let us observe the fig11(a) and (b). Start from any point in the fig-11(a) and (b). Move in the direction of arrow. Note down the parts which are in the way in cyclical form. (Try to identify different parts of the body in both figures.) Free distribution by A.P. Government fig-11(a): Single circulation fig-11(b): Double circulation 59 Compare the two flow charts and answer the following. • How many times did your pointer touch body parts in fig-11(a) and (b)? • How many times did your pointer touch the heart in fig-11(a) and (b)? • How many times did your pointer touch the respiratory organs in fig-11(a) and (b)? From your observation it is clear that in fig -11(a) blood flows through heart only once to complete one circulation. If blood flows through heart only once for completing one circulation it is called single circulation. If the blood flows through the heart twice for completing one circulation it is called double circulation. Lymphatic system As blood flows through tissues some amount of fluids and certain solid materials are constantly flowing out of them at different junctions. Such materials are to be collected and sent back into blood circulation. Have you ever observed what happened to your feet after overnight journey, in sitting position without moving? We feel that our foot wear is little tight. In elders it will be clear that the lower part of the legs will be swollen. This stage is called edema. • Why do our legs swell? We know that blood circulates in the blood vessels, pushed by the heart. From the heart it flows into the arteries and finally into the capillaries. To supply nutrients to the cells (tissues), the liquid portion of the blood with nutrients flows out of the capillaries. This is called tissue fluid. The tissue fluid which is present in the tissues should be transported into the blood vessels again. Some portion of the tissue fluid enters into the venules, which in turn form the veins, which carry blood to the heart. What about the remaining tissue fluid? To transport the tissue fluid in to the main blood stream, a separate system is present. That is called lymphatic system. In latin lymph means water. Lymph is the vital link between blood and tissues by which essential substances pass from blood to cells and excretory products from cells to blood. The lymphatic system is a parallel system to venous system which collects tissue fluid from tissues and transports it to the venous system. Blood is a substance which contains solid and liquid particles. Transportation - The circulatory system fig-12: Lymphatic system 60 X Class Lymph is the substance that contains blood without solid particles. Tissue fluid is lymph present in the tissues. The liquid portion after formation of blood clot is serum. The muscles which are attached to the skeleton (skeletal muscles) act as pumps when they contract and help in pushing the lymph flowing in lymphatic vessels and the blood flowing in veins towards the heart. The valves that are present in the lymphatic vessels and veins stop the reverse flow of blood. We shall read about this as the system of lymph circulation in detail in higher classes. Evolution of the transport (circulatory) system When the unicellular organisms separated themselves from the sea with the formation of the limiting membrane, the problem of transportation arose. The nature has found the solution, by creating a microscopic ocean which has its own currents. In unicellular organisms like Amoeba the protoplasm shows natural movements. These movements are called Brownian movements, because of which the nutrients and oxygen are distributed throughout the protoplasm equally. This simplest intracellular transportation system, present in unicellular animals has been retained in multicellular animals including humans. The protoplasm of any cell in our body is mobile and protoplasmic currents exist even in the nerve cells. The multicellular animals have to develop more complicated system for transportation of materials. The parazoas like sponges, use sea water for transportation. Since the natural water currents are not reliable, the sponges create their own currents by beating of flagella that are present in their body. The cnidarians which are better evolved than sponges (e.g. Hydra and jelly fish) have developed blind sac like gastrovascular cavity, which has taken up the function of digestion and transportation of nutrients to each and every cell of the body. In platyhelmenthes (e.g. Fasciola hepatica), the digestive system is highly branched and supplies digested food to all the cells directly. In these animals the excretory system collects wastes from each cell individually. In these organisms most of the body is occupied by digestive and excretory systems. In animals belonging to Nematyhelmenthes, the pseudocoelom has taken up the function of collection and distribution of materials. Free distribution by A.P. Government 61 The Annelids, the first Eucoelomate animals have developed a pulsatile vessel, to
move the fluid and the transporting medium is blood. The Arthropods have developed a pulsatile organ to pump the blood, the heart. The blood instead of flowing in blood vessels floods the tissues, directly supplying the nutrients to the tissues. Oxygen is directly supplied to the tissues directly by the respiratory system. Such type of transportation system which supplies nutrients to the tissues directly is called open type of circulatory system.eg. Arthropods, many molluscs and lower chordates. The other type of transportation system where the blood takes the responsibility of delivering the materials, which flows in the blood vessels is called closed type of circulatory system. Such type of closed circulatory system is present in annelids echinoderms, cephalopod molluscs (e.g. Octopus) and all the higher animals. Do you know? The human circulatory system can move one ml of blood from heart to a foot and back which is approximately 2 meters, in about 60 seconds. It would take more than 60 years for the substance to move across this distance by diffusion. Blood pressure (B.P.) In class 9th we studied about blood and it’s components, blood grouping, etc., in the chapter animal tissues. Now we will discuss some other points related to blood. Generally you have heard the word B.P. What is B.P.? To move the blood through this network of vessels, a great deal of force is required. The force is provided by the heart and is at its highest when the ventricles contract, forcing the blood out of the heart and into the arteries. Then there is a drop in the pressure as the ventricles refill with blood for the next beat. B.P. is always measured in the upper arm artery. B.P. varies throughout the body, so a standard place must be used so that a person’s blood pressure can be compared over a period of time. Doctors measure the blood pressure (B.P.) with a device called sphygmomanometer. There are two pressure readings. One measures the strongest pressure during the time blood is forced out of the ventricles. This is called systolic pressure. For a healthy fig-13: Sphygmomanometer 62 X Class Transportation - The circulatory system young adult it will be 120 mm of Hg. The second reading is taken during the resting period, as the ventricles refill with blood. This is called diastolic pressure. It will be 80 mm of Hg. B.P. will change according to the activity in which the person is engaged, such as resting, walking and running. People who have high B.P. during resting period are said to have hypertension. Discuss with your teacher about low blood pressure. Coagulation of blood Another important part in the story of blood is coagulation. Only because of this character animals survive when they meet severe injuries. When there is an injury blood clots in 3-6 minutes. How does the blood clot? What chemistry involved in blood coagulation. You know that when you cut yourself, the blood flows out of the wound for only a short time. Then the cut is filled with a reddish solid material. This solid is called a blood clot. If blood did not clot, anyone with even a slight wound bleeds profusely. • When the blood flows out, the platelets release an enzyme called thrombokinase. • Thrombokinase acts on another substance present in the blood called pro-thrombin converting it into thrombin. • Thrombin acts on another substance called fibrin, that is present in dissolved state converting it into insoluble fibrin. • The blood cells entangle in the fibrin fibers forming the clot. • The fibrin fibers are attached to the edges of the wound and pull them together. This straw yellowish coloured fluid portion after formation of the clot is serum. Discuss with your teacher about vitamin K in relation to coagulation of blood. fig-14(a): Blood in the blood vessel fig-14(b): Clot formation Free distribution by A.P. Government 63 Normally the blood that oozes from a wound clots in 3-6 minutes. But in some people due to vitamin K deficiency it takes more time. Because of genetic disorder the blood may not coagulate. This type of disorder is called haemophilia. Haemophilia is common disorder in the children who have born from marriages between very close relatives. Thalassemia an inherited disorder is also related to blood. For more details see annexure. HOW MATERIALS TRANSPORT WITH IN THE PLANT There is a vast transport system in continual supply of essential nutrients and oxygen to perform metabolic activities, and to remove excretory substances which are found in each cell of animal body. Is there anything like that in plants which corresponds to circulatory system? In previous classes we studied about Van Helmont’s experiments on plants, which get water that contain minerals from soil through their roots. The water absorbed by roots and food prepared by leaves are supplied to the remaining parts of the plant by vascular bundles having xylem and phloem. In the root the xylem tissue is situated towards the exterior while in the stem it is arranged in bundles towards the center. fig-15: Transportation How is water absorbed? We know that roots absorb water with minerals from soil. • What is the mechanism behind this? • Are roots directly in contact with water? • How is water absorbed? Activity-4 Absorving root hairs To perform this activity, you need to germinate bajra or mustard seeds. Examine some mustard seedlings which have been grown on wet filter paper. Observe the mass of fine threads coming from the seed by hand lens. These are roots. They have fine microscopic structures called root hairs. These are root hairs trough which water enters the plant. Gently squash a portion of the root hair between slide and cover slip in a drop of water and examine under a microscope. Note the thinness of the walls of root hairs. It is not completely understood how the water enters the root 64 X Class Transportation - The circulatory system hairs and passes inwards from cell to cell until it gets into the xylem vessels, but there is no doubt that osmosis plays an important role. Every living cell acts as an osmotic system, the cytoplasm lining of the cell wall acts as the semipermeable membrane. Observe the following figure, notice how do roots penetrate into soil? You will find that the root hairs grow out into the spaces between the soil particles and that the hairs are surrounded by moisture. xylem vessel cells of cortex epidermal cell soil particles soil water nucleus air spaces cell wall of root hair cytoplasm vacuole fig-16: L.S of root showing relationship of root hair and soil water Note: In fig-16 Arrow marks shows the direction of flow of water. The soil water is an extremely dilute solution of salts, water molecular concentration is more dilute than that of the cell sap in the root hair; therefore water will pass into the vacuole of the root hair by osmosis. Recall the process of osmosis that you have learnt in the chapter “moving of substances through plasma membrane” in class IX. The entry of water dilutes the contents of the root hair vacuole so that it becomes more dilute than it’s neighbouring cell. So, water passes into the neighbouring cell which in turn becomes diluted, finally water enters the xylem vessels. As there are vast number of root hairs and root cells involved, a pressure in the xylem vessels develops which forces the water upwards. This total pressure is known as root pressure. Root pressure is not the main cause of movement of water in xylem but it is certainly one of the factor. The other factors are also there. You will learn about those reasons in higher classes in detail. Activity-5 What is root pressure Take a regularly watered potted plant and cut the stem portion 1 cm above the ground level. Then connect a glass tube by means of strong rubber Free distribution by A.P. Government 65 clamp glass tube water level strong rubber tubing cut stem portion soil tubing as shown in the figure. The size of glass tube should be equal to the size of the stem. Take care while joining tube and stem being bound tightly, so that water cannot escape from the tube. Now, pour some water in the glass tube until water level can be seen above the rubber tube. Mark the level of water (M1) in the tube. Keep your arrangement aside for 2 to 3 hours. Then observe and mark the water level (M2) in the tube. Is there any increase in the water level? • • What is the role of xylem in this action? The difference between M2 and M-1 indicates the level of water raised in the stem. Because of the root pressure, the water level increases in the tube. water fig-17: Root pressure The mechanism by which the water travels through the plant We have seen that there is a push from below due to root pressure on the columns of water in the xylem vessels, but this is seldom high and in some seasons it is nil. How does the water reach 180 metres high to the top of a tree like a eucalyptus? Let us recall the activity that you performed in lower classes. Why inner sides of cover become moist? Where do these water droplets or water vapour come from? We know that this type of evaporation of water through leaves is called transpiration. Water evaporates through stomata of leaves and lenticels of stem. When the leaves transpire, there is a pulling effect on the continuous columns of water in the xylem vessels. The top ends of these vessels are surrounded by the leaf’s mesophyll cells which contain cell sap, so the water is continuous from the xylem vessels to the walls of the mesophyll cells from which it evaporates into the air spaces causing the pull. The water column does not break because of its continuous molecular atraction. This is a property of water you demonstrate every time you drink through a straw. Now we have a picture of the water-conducting system of a tree. Water is absorbed by osmosis from the soil by Transportation - The circulatory system fig-18: Transpiration 66 X Class the root hairs. This is passed into the xylem vessels which form a continuous system of tubes
through root and stem into the leaves. Here the water evaporates and releases into the atmosphere. The evaporation creates the main pull of water above root pressure which gives a variable and minor push from below. This results in a continuous column of moving water, the ‘transpiration stream’. Is there any relation between transpiration and rain fall? The amount of water passing through a plant is often considerable. For example, an oak tree can transpire as much as 900 liters of water per day. It follows therefore that areas of forest significantly affect the degree of saturation of the air above them, so that when air currents bring air which is already nearly saturated to a forest area, it becomes fully saturated and comes down as rain; this is why forest areas often have a higher rainfall than areas nearby. Do you know? How much water is transpired by plants? Each fully grown maize plant transpires 15 liters per week. One acre of maize may transpires more than 13,25,000 liters of water in a hundred day growing season. A big mango tree will transpire from 750 to more than 3500 liters of water per day during growing season. Transport of mineral salts You know that mineral salts are necessary for plant nutrition (micro and macro nutrients) and that they are obtained from the soil solution through the root hairs. The salts are in the form of electrically charged ions. Sodium chloride (NaCl) is in the form of Na+ and Cl-, and Magnesium 2-. But, they are not absorbed Sulphate (MgS04) occurs as Mg2+ and S04 into the root hairs by the simple process of diffusion, but it involves the use of energy by the cytoplasm which will be discussed in later classes. Once ions are absorbed, the ions travel along with water in the xylem vessels and pass to the growing points of the plants where they are used for growth purpose. They may also pass laterally from xylem to phloem. Thus, mineral salts are one of the natural factors in plant growth phenomena. Transport of manufactured food Food such as sugar is synthesised in the green parts of plants, mainly the leaves, but this food has to be transported to all the living cells, especially to actively growing cells and the cells which stores food. Free distribution by A.P. Government 67 proboscis The veins of a leaf consist of xylem and phloem, and these tissues are continuous with the stem. The following experiments provide evidence that food is transported in the phloem cells. fig-19: Aphid extracting food material from plant aphid xylem phloem Phloem sieve tubes are extremely small and the analysis of their contents is not easy. Biologists studied about food transportation in plants with the help of aphids (greenfly). When you see aphids clustering round the young stems of plants as they feed on the plant juices. To obtain this juice an aphid pierces the plant tissues with its long needle like organ “proboscis”. It can be shown when a feeding aphid is killed and the stem carefully sectioned, the proboscis only penetrates upto a phloem sieve tube. This proboscis also provides a ready-made means of obtaining the juice for analysis! The experiment can be done in this way. An aphid is killed while in the act of feeding and the body is then carefully cut away, leaving the hollow proboscis still inserted into the phloem. It is found that because the contents of the phloem sieve tubes are under slight pressure the fluid slowly exudes from the cut end of the proboscis in the form of drops; these drops are then collected and analysed. The fluid is found to contain sugars and amino acids. Not surprisingly, aphids absorb so much sugar from the phloem that they cannot assimilate all of it and it excretes out of the out of the body as a sticky syrup called honey dew. Leaves which have been attacked by aphids often feel sticky as a result of honey dew. some growth no growth fig-20: Removing ring of bark You may notice that sometimes barks of the tree damaged more than a half, even then the tree is alive. How is this possible? Further experiments to illustrate the conduction of sugars by the phloem have been done by removing a ring of bark from a shoot to expose the wood. Remove all tissues from the center outwards, including the phloem. After a few days, when the tissues above and below the ring were analyzed it was shown that food had accumulated above the ring, but was not present below it. If it is left for some more time, the stem increases in thickness immediately above the ring, but no growth occurred 68 X Class Transportation - The circulatory system below it. So, any damage to the phloem all around the stem will prevent the food from passing down to the roots and the tree will eventually die. This is a fact of great economic importance because certain mammals scratching the bark of trees to get the food stored in the phloem, especially during hard winters when food is scarce. Voles do this to young saplings at ground level and rabbits can do much damage to older ones. Foresters find it economically worthwhile to enclose new plantations with wire netting to prevent rabbits from entering. Foresters also encourage predators such as foxes, badgers, hawks and owls as they help to keep down the population of voles and rabbits. Grey squirrels too do great damage, particularly to beech and sycamore, and for this reason, in some parts it is impossible to grow these trees as a crop. Observe barks of trees in your surroundings for evidence of bark having been gnawed off saplings and trees. Note the species of tree, the position of the damage, whether the damage is recent or old, and the size of tooth marks if these are visible. From these observations you could find out which species had caused the damage. Also look out for the effect of such damage on the tree as a whole. Key words Circulation, Right atrium, Left atrium, Right ventricle the lower right chamber of the heart, Left ventricle, Pulse, Artery, Vein, Stethoscope, Aorta, Capillary, Systole, Diastole, Cardiac cycle, Blood pressure, Lymph, Single circulation, Double circulation, Coagulation of blood, Sphygmomanometer, Prothrombin, Thrombin, Fibrinogen, Fibrin, Root hair, Radical, Root pressure, Plant nutrients, Xylem, Phloem, Vascular bundles. What we have learnt • The pulse rate is equal to heart beat. We can count the heart beat without the aid of any instrument. • Rene Lennac discovered the first stethescope. • The heart is covered with two pericardial membranes filled with pericardial fluid which protects it from shocks. • Six blood vessels are attached to the heart. The two rigid blood vessels are arteries which supply blood to body parts aorta and lungs and pulmonary artery. • The less rigid vessels are various, which bring is blood from body parts. • Heart has four chambers, two upper atria and two lower ventricles. • Atrium and ventricle of the same side are connected by atrium ventricular aperture. • Atria are separated from each other by interatrial septum, ventricles by interventricular septum. Free distribution by A.P. Government 69 • The atrioventricular apertures are guarded by valves. There are valves in the aorta and pulmonary artery also. • The right side of heart receives blood from body and sends to lungs. • The left side of the body receives blood from lungs and send it to body parts. • The arteries carry oxygenated blood except pulmonary artery. The veins carry deoxygenated blood except pulmonary veins. • One contraction and relaxation of heart is called cardiac cycle. • If the blood goes to heart only once before it reaches all the body parts. It is called single circulation. If it goes twice it is called double circulation. • Vitamin K deficiency leads to delayed coagulation of blood. • Plants absorb soil water through roots by the process of osmosis. • Water travels through xylem vessels and food material travels through phloem tissues. • There is a relation between tranportation and transpiration in plants. • Biologists studied about phloem tubes with the help of aphids. Improve your learning 1. What is transport system? How this helps to the organism?(AS1) 2. What is the relationship between blood and plasma?(AS1) 3. Which type of blood vessels carry blood away from the heart?(AS1) 4. What are the three main types of blood vessels in the body?(AS1) 5. Which is the largest artery in the body? Why is it big in size?(AS1) 6. Which blood vessel carries blood for oxygenation?(AS1) 7. Name the structures which are present in veins and lymph ducts and absent in arteries.(AS1) 8. What is the use of platelets?(AS1) 9. Write differences betweenm(AS1) a) systole - diastole b) veins - arteries c) xylem - phloem 10. Explain the way how plants get water by osmosis through root hair?(AS1) 11. What is root pressure? How it is useful to the plant?(AS1) 12. Phloem is a food source for some animals. How can you justify this statement?(AS1) 13. Read the given para and name the parts of the heart.(AS1) We have observed that the heart is divided into four chambers by muscular structure. Any structure that divides two chambers is known as septum. Now let us try to name the septa present in the heart. a) The septum that divides the two atria can be named as inter atrial septum 70 X Class Transportation - The circulatory system b) The septum that divides the two ventricles can be named as___________. c) The septum that divides the atrium and ventricle can be named as________. The holes that connect two chambers are called apertures. Let us try to name the apertures which connect the atria and ventricles. d) The aperture that is connecting the right atrium and right ventricle can be named as_______. e) The aperture that is connecting the left atrium and left ventricle can be named ___________. Any structure that closes an aperture, and allows one way movement of materials is called as valve. Now let us name the valves that are present in the chambers of the heart. f) The valve that is present between left atrium and left ventricle ca
n be named as____________. g) The valve that is present between right atrium and right ventricle can be named as ___________. 14. If the valves in veins of the legs fail to stop the flow of blood what could be the consequences of this failure?(AS2) 15. What will happen if cell sap of root hair cells contain high concentration of ions?(AS2) 16. John prepared stethoscope with paper cup and plastic tube. Write down the procedure of preparation. (AS3) 17. How can you prove that the water is transported through the xylem?(AS3) 18. What is your inference about experiments with aphids?(AS3) 19. Collect information about blood pressure of your school teachers or your nighbours prepare a report on their help problems. (AS4) 20. Draw a block diagram to explain single and double circulation. Write differences between them.(AS5) 21. Prepare a block diagram showing from water absorption by roots to transpiration by leaf . (AS5) 22. What do you want to compare with the transportation in blood vessels? (AS6) 23. How do you feel about transportation of water in huge trees? (AS6) 24. Prepare a cartoon on heart beating? (AS7) 25. After reading this lesson what precautions you would suggest to your elders about edima.(AS7) Choose the correct answer 1. The term cardiac refers to which organ in the body? a) heart b) vein c) lymph d) capillary 2. On which side of the human heart is low in oxygen? a) left vetricle b) right ventricle c) left atrium d) right atrium 3. Which structures of the heart control the flow of the blood? a) arteries b) veins c) valves d) capillaries ( ( ( Free distribution by A.P. Government ) ) ) 71 4. Which of the following opinion is correct? ( ) a) Ravi said, xylem and phloem cells arranged one upon the other to form a tube like structure. b) John said, xylem and phloem are not separate tube like structures. c) Salma said, xylem and phloem cells connect together to form a tube like structure. d) Hari said, because of its shape they said to be tube like structures 5. An aphid pierces its proboscis into the ……… to get plant juices ( ) a) Xylem b) phloem c) cambium d) vascular bundle Annexure-I The rhesus factor There is another antigen of red blood cells which is present in 85% of the people of Britain, this is known as the rhesus factor, as it was first discovered in rhesus monkeys. People who have this are said to be rhesus positive (Rh+). Those who do not have this factor are termed rhesus negative (Rh-). Normally they do not carry an antibody to this factor in their plasma. However, if Rh+ blood is transfused into the blood of a Rh- person, antibodies will be formed and these are capable of destroying Rh+ red cells. Under certain circumstances this is a potential hazard for babies. If a Rh+ man marries a Rh- woman, some of the children are likely to be Rh+. At birth there is always some mixing of blood between the circulation of mother and baby and this may occasionally happen during pregnancy. So, if a child is Rh+ some of its blood will leak into its mother’s circulation and cause antibodies to form in her blood. If the mother has more children, not all will necessarily be Rh+, but if they are, the amount of ant bodies in her blood often increases with each pregnancy, and in some instances the antibodies in her blood may pass into the baby’s blood in sufficient quantities to produce very serious anaemia and even death. Fortunately these cases are not frequent, and when they do occur, the baby is given a complete transfusion soon after birth so that that baby’s blood is replaced by blood containing no antibodies to the rhesus factor. It is now possible for this transfusion to be carrried out before birth. Another recently developed technique is for the mother to be given an injection shortly after the birth of her first child which prevents the Rh+ cells from stimulating the production of the harmful antibody. 72 X Class Transportation - The circulatory system Annexure-II Thalassemia Thalassemia is a group of inherited blood disorders characterized by mild to severe anaemia caused by haemoglobin deficiency in the red blood cells. In individuals with thalassemia, the production of the oxygen carrying blood pigment haemoglobin is abnormally low. There are two main types of thalassemia: alpha thalassemia and beta thalassemia. In each variant a different part of the haemoglobin protein is defective. Individuals with mild thalassemia may have symptoms, such as anaemia, enlarged liver and spleen, increased susceptibility to infections, slow growth, thin and brittle bones, and heart failure. 4.5% of world population (250 million) suffering with thalassemia minor. Facts about Thalassemia • Thalassemia is a serious Inherited Blood Disorder. • • There are over 35 million Indians are carriers of the abnormal Gene for Thalassemia. It is estimated that about 1,00,000 infants are born with major Haemoglobinopathies • every year in the world. 10,000 – 12,000 Thalassemic children are born every year in our country. • • Survival depends upon repeated blood transfusion and costly medicines. • Thalassemia can be prevented by awareness, pre marital or pre conceptual screening followed by antenatal diagnosis is required. Treatment Thalassemia major should be diagnosed as early as possible in order to prevent growth restriction, frail bones and infections in the first year of life. The infant’s haemoglobin levels and development should therefore be monitored closely. If Hb is less than 70% or the child shows signs of poor growth and development. Regular transfusion is the treatment of choice. According to the WHO, the aim of this treatment is to retain a median haemoglobin value of 115–120 grams per liter. This can usually be achieved by carrying out transfusions of concentrated red blood cells at intervals of every three to four weeks. Today thalassemia major can be cured by stem cell transplantation. A prerequisite is usually that the affected individual who has siblings with identical tissue type (HLA type) a transplantation of blood stem cells referred to as a “bone marrow transplant”, can be carried out. Free distribution by A.P. Government 73 Chapter 4 Excretion - The wastage disposing system There is no factory which can manufacture a product without generating any waste. This is true of our body which is a cellular factory too. And for other organisms as well. Wastes are generated at regular intervals from the bodies of most organisms. This raise questions like. • Where are the wastes produced? • How are they produced? • What are the substances present in them? • Does the composition vary in the same organism in different situations? Let us understand such kind of questions. Living beings need energy for their survival and to perform activities either building up of body material (anabolism) or its breakdown (catabolism), collectively called metabolic activities. Organisms use different substances for metabolic activities. Different products are generated as a result of these metabolic activities. Can you name different products generated by the following life processes? Table-1 Products Life processes Photosynthesis Respiration Digestion 74 X Class Excretion The wastage disposing system • What products would the organism be able to take up for other ac- tivities? • What products which would cause harm to the body, if they are not removed? • What happens if harmful products are not removed from our body every day? We have already learnt that different kinds of materials are produced out of various metabolic activities. Some of these may be harmful for the organism are removed from their body or packed and stored in some other forms. These are all the wastes produced in the body of an organism. We have already discussed how organisms get rid of gaseous wastes generated during photosynthesis or respiration. Other metabolic activities generate nitrogenous wastes have to be removed along with salts, excess water and several other materials. Excretion is the term coined for all the biological process involved in separation and removal of wastes or non useful products from the body. (In latin ex means out, crenere means shift.) Now let us study how excretion takes place in human being. Excretion in Human Beings A number of reactions take place during various metabolic activities. Many useful substances and energy are produced. At the same time many other things happen such as, toxic wastes may be produced, water content may increase, ionic balance in the body may be disturbed. The waste products include carbon dioxide, water, nitrogenous compounds like ammonia, urea, uric acid, bile pigments, excess salts etc. The most poisonous of all waste products of metabolism is Ammonia. Where are these waste material produced? How does the body manage them. Is there a way to detect their presence in our body? Now let us observe the test reports of Blood and Urine of a person given in table-2 and find out the components present in both Blood and Urine. (For 24 hours urine test urine collected for 24 hours for that 100150 ml sample will be tested.) • What are the substances present in blood? • What are the substances present in urine? • What are the substances present both in blood and urine? • Which substances are present above the normal limits both in the blood and urine? • What do you think a reading about normal limits indicates? Free distribution by A.P. Government 75 Table-2: DEPARTMENT OF BIOCHEMISTRY REPORT ON PLASMA/SERUM (BLOOD) ANALYSIS TEST/METHOD RESULT UNITS RANGE GLUCOSE FASTING SODIUM POTASSIUM CHLORIDES UREA CREATININE URIC ACID TOTAL CHOLESTEROL TRIGLYCERIDES CALCIUM PHOSPHORUS BILURUBIN(TOTAL) TOTAL PROTEINS ALBUMIN 82 137 4.10 101 29 2.8. 7.50 221 167 9.40 4.50 0.70 7.20 4.60 mg/dl mmoles/L mmoles/L mmoles/L mg/dl mg/dl mg/dl mg/dl mg/dl mg/dl mg/dl mg/dl g/dl g/dl 60-100 (GOD POD) 135-145 3.5-5.0 95-106 15-40 0.6-1.5 3.0-5.0 150-200 60-200 8.0-10.5 3-4.5 0.1-0.8 6.0-7.5 3.0-5.0 Table-3: DEPARTMENT OF BIOCHEMISTRY
REPORT ON URINE ANALYSIS TEST/METHOD RESULT 24 hrs.Protein 24 hrs Creatinine 24 hrs.Calcium 24hrs.phosphorous 24hrs.uric Acid ELECTROLYTES : Sodium potassium Osmolality (calculated) Glucose Chlorides Urea 90 2.7 305 0.8 800 140 50 180 65 128 35 UNITS mg/day mg/day mg/day mg/day mg/day mmol/L mmol/L mosm/L mg/dl mmol/L gm/day RANGE <100 mg 1-2 Up to 200 upto 1g upto 600 125-250 25-100 100-600 50-80 120-130 20-30 m moles / L means millimoles per litre, mg/dl means milligram per deci litre. 76 X Class Excretion The wastage disposing system • What are the materials needed to removed from our body? • From where do these materials removed? • What are the organs that seperate excretory materials? • How do you think the body must be remove waste substances? Studying the structure and function of our excretory system will help us to understand this better. Excretory System in Human being In human beings excretion mainly occurs through a urinary or excretory system consisting of a pair of kidneys, a pair of ureters, urinary bladder and urethra, as shown in the fig-4. Now let us observe external and internal features of a kidney in goat / sheep, which is similar to Human kidney in function. Lab Activity Aim: Studying the external and internal features of a kidney Materials required: Freshly collected specimen of sheep/goat’s kidney from the butcher or 3D Model of a kidney, sharp blade/scalpel, tray and a jug of water. Procedure for observation: Before coming to the class wash the kidney thoroughly so that, blood is completely drained from it. Put the kidney in the tray and observe it carefully. Note your observations in the observation book. With the help of sharp blade take a longitudinal section here you are advised to do this activity under the guidance of your teacher and observe the internal structure. Draw what you have observed and compare it with fig-1,2. • What is the shape of kidneys? • What is the colour of kidney? • Do you find any attachments on upper portion of kidney? • Is the Internal structures similar to fig-2 • What is the colour of the outer part in L.S of kidney? • In L.S of kidney where do you find dark colour portion? • How many tubes are coming out from kidney fissure? Don’t forget to wash your hands with antibacterial lotion after completing dissection. Now let us know the structure of human excretory system and its functions. Free distribution by A.P. Government fig-1: Kidney of goat fig-2: LS of Kidney of goat 77 Kidneys In Human being there are a pair of bean shaped, reddish brown structures in the abdominal cavity attached to dorsal body wall (fig-3) one on either side of the back bone. The right kidney is placed slightly lower than the left kidney. Think why it is so? The size of the kidney is 10 cm in length, 5-6 cm in breadth, and 4 cm in thickness. Each kidney is convex on the outer side and concave on the inner side. The position of the right kidney is lower than the left kidney due to the presence of liver above. Let us recall the last question in your lab activity. The inner side of each kidney has a fissure or hilus for the entry of a renal artery, exit of a renal vein and an ureter. Renal artery brings oxygenated blood loaded with waste products and renal vein carries deoxygenated blood. The waste products generated in various organs of the body are filtered and removed by the kidneys. posterior vena cava dorsal aorta adrenal gland renal artery left kidney renal vein left ureter opening of ureter into bladder bladder urethra external opening fig-4: Excretory system fig-3: Location of kidneys Internal structure of the kidney: Let us observe L.S of the kidney to know more about internal structure. It shows two distinct regions. Dark coloured outer zone called the cortex and pale inner zone called medulla. Each kidney is made up of approximately more than one million (1.3 to 1.8 million) microscopic and thin tubular functional units called nephrons or uriniferous tubules. 78 X Class Excretion The wastage disposing system Structure of nephron Each nephron has basically two parts. One is malpighian body and other is renal tubule. Malpighian body: It consists of a blind cup shaped broader end of nephron called Bowman’s capsule and bunch of fine blood capillaries called glomerulus. The Bowman’s capsule and glomerulus together called malpighian capsule or renal capsule. Glomerulus develops from afferent arteriole. It gives rise to an efferent arteriole. • Think why the diameter of the efferent arteriole is less than that of afferent arteriole? Because of the narrower out let (efferent arteriole) pressure exerts in the glomerulus.It functions as a filtration unit. Bowmans capsule which accommodates one glomerulus,is lined by a single layer of squamous epithelial cells called podocyte cells. There are fine pores between podocyte cells to allow passage of materials filtered out of glomerulus. cortex medulla renal artery renal vein nephron malpighian body glomerulus { ureter fig-5: Internal structure of kidney afferent arteriole efferent arteriole Bowman’s capsul first convoluted tubule second convoluted tubule collecting duct loop of Henle capillary network open to pelvis fig-6: Structure of a nephron Renal tubule: It has three parts. First or proximal convoluted tubule (PCT), loop of Henle, which is U shaped, second or distal convoluted tubule (DCT). Distal convoluted tubules open into a collecting tube. Collecting tube form pyramids and calyces which open into the pelvis. Pelvis leads into the ureter. All the parts of the renal tubule are covered by a network of peritubular (around tube) capillaries formed from efferent arteriole. The peritubular capillaries join to form renal venule, which joins the other venules to form finally the renal vein. • Why the nephron is considered to be the structural and functional unit of the kidney? Free distribution by A.P. Government 79 Mechanisms of urine formation Formation of urine involves four stages i. Glomerular filtration, ii.Tubular reabsorption, iii. Tubular secretion, iv.Concentration of urine i) Glomerular filtration Blood flows from renal artery to glomerulus through afferent arteriole. Observe the fig-7 of glomerular filtration in nephron and try to answer the following questions. • Which arteriole has more diameter, afferent or efferent? • What are the substances that are filtered into the glomerular capsule? ii) Tubular Re-absorption Filtrate from glomerular is also called primary urine which almost equal to blood in chemical composition except the presence of blood cells. It passes into proximal convoluted tubule. Useful substances in primary urine are reabsorbed into peritubular net work. • If you drink more water will you pass more urine? • What are the substances reabsorbed into peritubular net work from proximal convoluted tubule (PCT)? iii) Tubular secretion After reabsorption in PCT region, the urine travels through the loop of Henle into DCT. Here some other wastes like extra salts ions of K+ Na+ Cl – and H+ secrets from peritubular capillaries in to DCT. It occurs mostly in the distal convoluted tubule, which is also surrounded by peritubular capillaries. This maintains a proper concentration and pH of the urine. Smaller amount of tubular secretion also takes place in the area of proximal convoluted tubule. Observe tubular secretion in fig-7. • What are the substances that secretes into DCT? Do you know? After the age of 40 years the number of functioning nephrons usually decreases by about 10% in every 10 years. iv) Concentration of urine Seventy five % of water content of nephric filtrate is reabsorbed in the region of proximal convoluted tubule. 10% of water passes out of filtrate through osmosis in the area of loop of Henle. Further concentration of urine takes place in the area of collecting tubes in the presence of 80 X Class Excretion The wastage disposing system i) Glomerular filtration: Blood flows inside the glomerulus under the influence of pressure due to the narrowness of efferent arteriole. As a result it undergoes pressure filtration or ultra filtration. Waste molecules, nutrient molecules and water are filtered out and enter the Bowman’s capsule. PCT glomerolus DCT ii)Tubular reabsorption:The peritubular capillaries around PCT reabsorb all the useful components of primary urine such as glucose, amino acids, vitamin C, Potassium, Calcium, Sodium, Chlorides and 75% of Water. iii) Tubular secretion: It is the active secretion of waste products by blood capillaries into the urinary tubule. It ensures removal of all the waste products from blood, viz., urea, uric acid, creatinine, salt ions like K+, Na+ and H+ ions. This maintance proper consentration and pH of urine. fig-7: Mechanism of urine formation hormone called vasopressin. The hormone is secreted only when concentrated urine is to be passed out. Think why is it not secreted when a person drinks a lot of water? Absence of vasopressin hormone produces dilute urine. Hormone action maintains osmotic concentration of body fluids. Deficiency of vasopressin causes excessive, repeated, dilute urination called diabetes insipidus. • Why more urine is produced in winter? • What happens if reabsorption of water does not takes place? Now let us discuss remaining parts of excretory system. 2. Ureters There are a pair of whitish, narrow distensible and muscular tubes of 30cm length. Each ureter arises from hilus of the kidney. It moves downward and obliquely opens into the urinary bladder. Ureter carries urine from the kidney to the urinary bladder. The movement of urine in the ureter is through peristalsis. Free distribution by A.P. Government 81 3. Urinary bladder It is a median, pear shaped and distensible sac that occurs in the pelvic part of the abdomen. It stores urine brought by two ureters. The storage capacity of urinary bladder is 300 - 800ml. 4. Urethra It is a tube that takes urine from urinary bladder to outside. The opening of urinary bladder into urethra is guarde
d by a ring of muscles or sphincter. Urethra is 4 cm long in females and about 20cm long in males. Its opening is separate in females but is in common with the reproductive tract in males (urino-genital duct). Micturition The urine is temporarily stored in the bladder. There are two sets of circular sphincter muscles in the bladder. When the bladder is filling up both these muscles are constricted, so the exit is closed. However as the pressure of the urine increases the walls of the bladder are stretched and this triggers off an automatic reflex action which causes the upper sphincter to relax. But the lower sphincter is under the control of brain. So urine can still be retained until this muscle is relaxed too. Control of urination is not possessed by very young children but is gradually learned. Urge for micturition occurs when urinary bladder is filled with 300 400 ml of urine. The stretched bladder stimulates nerve endings to develop the reflex. However, urine can be retained in the urinary bladder till it gets filled up to the maximum capacity of 700 - 800ml. At this time the urge becomes painful and leads to voluntary micturition. Total amount of urine excreted per day is about 1.6-1.8 litres. It’s quantity increases with larger intake of fluids like water, fruit juices and decreases with lesser intake. Think and discuss • Do cells need excretion? • Why we advised to take sufficient water? • Why do some children pass urine during sleep at night until 15 or 16 years of age? Composition of urine It is a transparent fluid produced by urinary system. Urine has amber color due to presence of urochrome.Composition of normal urine varies considerably depending on several factors for instance taking a protein 82 X Class Excretion The wastage disposing system rich diet will result in more formation of urea in the urine. This is because the proteins get de-aminated in the liver with subsequent formation urea. Even sugar can appear in a normal person after a heavy intake. If other conditions are constant, a large intake of liquids or water - rich food increases the volume of water in the blood, hence more urine is excreted. Urine contains 96% of water 2.5% of organic substances (urea, uric acid, creatine, creatinine, water soluble vitamins, hormones, and oxalates etc) and 1.5% of inorganic solutes (sodium, chloride, phosphate, sulphate, magnesium, calcium, iodine). It is acidic in the beginning but becomes alkaline on standing due to decomposition of urea to form ammonia. • What happens if both kidneys fail completely? Complete and irreversible kidney failure is called end stage renal disease (ESRD). If kidneys stop working completely, our body is filled with extra water and waste products. This condition is called uremia. Our hands or feet may swell. You feel tired and weak because your body needs clean blood to function properly. Is there any solution to this problem? Let us know about artificial kidney. Dialysis (Artificial kidney) Kidneys are vital organs for survival. Several factors like infections, injury, very high blood pressure, very high blood sugar or restricted blood flow to kidneys. This leads to accumulation of poisonous wastes in the body and leads to death. Dialysis machine is used to filter the blood of a person when both kidneys are damaged. The process is called ‘haemodialysis’. In this process blood is taken out from the main artery, mixed with an anticoagulant, such as heparin, and then pumped into the apparatus called dialyzer. In this apparatus blood flows through channels or tubes. These tubes are embedded in the dialyzing fluid. The membrane separates the blood flowing inside the tube and dialyzing fluid (dialysis), which has the same composition as that of plasma, except the nitrogenous wastes. fig-8: Dialysis As nitrogenous wastes are absent in dialyzing fluids, these substances from the blood move out freely, there by cleaning the blood of its wastes. This process is called dialysis. This is similar to function of the kidney but is different as there is no reabsorption involved .The cleaned blood is pumped back to the body through a vein after adding anti-heparin. Each Free distribution by A.P. Government 83 dialysis session lasts for 3 to 6 hours. This method has been using for thousands of uremic / kidney failure patients all over the world. • Is there any long term solution for kidney failure patients? Do you know? The first kidney transplantation was performed between identical twins in 1954 by Dr. Charles Hufnagel was a surgeon at Washington, USA . In India first kidney transplantation was done on 1st December 1971 at the Christian Medical college, Vellore, Tamilnadu. Dr. Charles Hufnagel Kidney transplantation The best long term solution for kidney failure (acute renal failure) is Kidney transplantation. A functioning kidney is used in transplantation from a donor preferably a close relative. The kidney that is received by a recipient must be a good match to his body, to minimize the chances of rejection of transplanted kidney by the immune system of the recipient. Modern clinical procedures have increased the success rate of such complicated technique. • Where the transplanted kidney fixed in the body of a kidney failure patient? • What about the failure kidneys? • Can donar survive her life with single kidney without any complications? Now a days the process of organ donation helps a lot kidney failure patients. Organs are collected from brain dead patients. Then transplanted to the fig-9: Kidney transplantation recepient. Know more about organ donation see in annexure. Other pathways of excretion (accessory excretory organs) You have learnt about kidney, chief excretory organ of our body. • What are the other excretory organs of human body? Lungs, skin, liver have their own specific functions but carry out excretion as a secondary function. Lungs: In respiratory process lungs remove carbon dioxide and water. Skin: It consists of large number of sweat glands richly supplied with blood capillaries, from which they extract sweat and some metabolic wastes. fig-10: Lung, Skin 84 X Class Excretion The wastage disposing system Since the skin sends out plenty of water and small amount of salts, it serves as an excretory organ. Sebaceous glands in skin eliminate sebum which contains waxes, sterols, hydrocarbons and fatty acids. • Collect information on sebum and prepare a news bulletin, display it on bulleton board? • People in cold countries get very less/no sweat. What changes oc- cur in their skin and in other excretory organs? Liver: It produces bile pigments (bilirubin ,biliverdin and urochrome) which are metabolic wastes of haemoglobin of dead R.B.Cs. Urochrome is eliminated through urine. Bileverdin and bilerubin are excreted through bile along with cholesterol and derivatives of steroid hormones, extra drug , vitamins and alkaline salts. Liver is also involved in urea formation. Intestine: Excess salts of calcium magnesium and iron are excreted by epithelial cells of colon (large intestine) for elimination along with the faeces. Small amount of nitrogenous wastes are also eliminated through saliva and tears. Excretion in other organisms Different organisms use varied strategies in excretion. Specific excretory organs are absent in unicellular organisms. These organisms remove waste products by simple diffusion from the body surface into the surrounding water. Fresh water organisms like Amoeba, Paramoecium possess osmoregulatory organelle called contractile vacuole. It collects water and waste from the body, swells up, reaches the surface and bursts to release its content to outside. The main execretion takes place through body surface (osmosis). Table-3 fig-11: Liver, intestine Name of the phylum organism Protozoa Porifera and coelenterates Platyhelminthes and Nematoda Annelids Arthropoda Mollusca Echinodermata Reptiles, Birds and Mammals Free distribution by A.P. Government Excretory system Simple diffusion from the body surface in to the surrounding water Water bathes almost all their cells Flame cells Nephridia Green glands, Malpighian tubules Meta nephridia Water vascular system Kidneys 85 Multicellular organisms possess different excretory organs for removal of waste materials from the body. Structural and functional complexity of excretory organs increases from sponges to humans. Sponges and coelenterates do not have specific excretory organs as water bathes almost all their cells. Excretory structures appear for the first time in Flatworms (Platyhelminthes) are known as flame cells. Now let us see how this vital process takes place in plants Excretion and release of substance in plants Do plants excrete like animals? We are amazed to answer such type of questions. You are aware that a variety of end products are formed during metabolism and these nitrogenous wastes are important. Plants does not have specific organs to excrete these wastes. Plants break down waste substances at much slower rate than in animals. Hence accumulation of waste is also much slower. Green plants in darkness and plants that do not possess chlorophyll produce carbon dioxide and water as respiratory waste products. Oxygen itself can be considered as a waste product generated during photosynthesis, that exits out side through stomata of leaves and lenticels of stem. • How plants manage or send out waste products from its body? Plants can get rid of excess water by a process like transpiration and guttation. Waste products may be stored in leaves, bark, and fruits. When these dead leaves, bark, and ripe fruits fall off from the tree then waste products in them are get rid off. Waste gets stored in the fruits in the form of solid bodies called Raphides. However several compounds are synthesized by the plants for their own use especially for defense. Many plants synthesize chemicals and store them in roots, leaves, seeds, etc., for protection against herbivores. Most of the chemicals are unpleasant t
o taste. Hence, herbivores usually do not prefer to eat such plants. Some of the chemicals are toxic and may even kill the animal that eats them. Think and discuss • Why weeds and wild plants are not affected by insects and pests? Some of the plants secrete chemicals when injured. These chemicals seal the wound and help the plant to recover from an injury. Some of the plants release attractants for other organisms which will help the plants for pollination, seed dispersal or even in their nutrition. For example, plants 86 X Class Excretion The wastage disposing system having root nodules secrete chemicals to attract rhizobia into the surroundings of the roots and form a symbiotic relationship with the rhizobium. These compounds are called secondary metabolites. • Why plants shed their leaves and bark periodically? The biochemical substances produced in plants are of two types, primary metabolites and secondary metabolites. The materials like carbohydrates, fats and proteins are primary metabolites. The materials which do not require for normal growth and development are secondary metabolites. e.g.: Alkaloids, Tannins, Resins, Gums, and Latex etc. Though plants produce these chemical for their own use. Man found the usage of these chemicals for own benefits. They are generally coloured and fragrant. Alkaloids: These are nitrogenous by- products and poisonous. These are stored in different parts of the plants. Common alkaloids in plants and their uses are given below. Table-4 ALKOLOID PLANT PART USES Cinchona officinalis (Cinchona) Bark Antimalarial drug Quinine Nicotine Nicotiana tobacum (Tobacco) Leaves Morphine, Cocaine Papaver somniferum (Opium) Fruit Reserpine Rauwolfia serpentiana (Snake root) Root Caffeine Coffea Arabica (Coffee plant) Seed Insecticide Pain killer Medicine for Snake bite Central nervous system Stimulant Nimbin Azadirachta indica(neem) Seeds, Barks, Leaves. Antiseptic Scopolamine Datura stramonium Fruit, flower Sedative Pyrethroids Chrysanthemum Flower Insecticides Papaver Rauwolfia Coffea Arabica Tobacco Datura • Name the alkaloids which are harmful to us? fig-12: Plants which produce Alkoloids Free distribution by A.P. Government 87 Tannins: Tannins are carbon compounds. These are stored in different parts of the plant and are deep brown in colour. Tannins are used in tanning of leather and in medicines e.g. Cassia, Acacia. Resin: Occur mostly in Gymnosperms in specialized passages called resin passages. These are used in varnishes- e.g.Pinus. fig-13(a): Cassia fig-13(b): Acacia fig-13(c): Pinus Gums: Plants like Neem, Acacia ooges out a sticky substance called gum when branches are cut. The gum swells by absorbing water and helps in the healing of damaged parts of a plant. Gums are economically valuable and used as adhesives and binding agents in the preparation of the medicines, food, etc. Latex: Latex is a sticky, milky white substance secreted by plants. Latex is stored in latex cells or latex vessels. From the latex of Hevea braziliensis (Rubber plant) rubber is prepared. Latex from Jatropa is the source of bio-diesel. Do you know which part of jatropa used in production of bio diesel. fig-14(a): Neem fig-14(b): Jatropa fig-14(c): Rubber plant Do you know? Chewing gum is a type of gum for chewing made dates back 5000 years. Modern chewing gum originally made of chicle, natural latex from plant. Whenever pollen grains enter in our body they cause allergy due to the presence of nitrogenous substances. These allergens cause skin allergy and asthma. Ex: Parthenium. 88 X Class Excretion The wastage disposing system • Do roots secret? ‘Brugaman’ a botanist proved from his experiments that the roots not only absorb fluid from soil, but returns a portion of their peculiar secretions back into it. We can see such instances in plants like apple where a single apple crop for 4 or 5 years continuously in the same soil, that fail to produce fruits. It will not give proper yield even if you use lot of fertilizers. • Do you think is there any relation between reduction in yielding and root secretions? • Why do we get peculiar smell when you shift the potted plants? Excretion Vs Secretion Excretion and secretion are the same in nature. Since both are involved in passage or movement of materials. Both processes move and eliminate unwanted components from the body. Excretion is the removal of materials from a living being, while secretion is movement of material from one point to other point. So secretion is active while excretion is passive in nature. Humans excrete materials such as tears, urine, Carbon dioxide, and sweat while secretion on other hand, includes enzymes, hormones, and saliva. In plants too we find excretion through roots into its surroundings and falling off leaves and bark. Secretions occur in the plant body in form of latex, resins, gums etc. Key words Creatinine, tubular fluild, peritubular, podocyte, hyper-osmotic interstial fluids, glomerulus, PCT, DCT, afferent arteriole, efferent arteriole, calyces, micturation, urochrome, dialyser, heamodialysis, anticoagulant, alkoloid, biodiesel. What we have learnt • Due to metabolism several harmful excretory products are formed and process of removing toxic waste from the body is called execretion. • The human excretory system comprises kidneys, ureters, urinary bladder and urethra. • Each kidney composed of a large number of uriniferous tubules or nephrons, which are structural and functional units of kidney. • A nephron comprises glomerulus,bowman’s capsule, proximal convoluted tubule (PCT), Henle’s • loop, Distal convoluted tubule (DCT) and collecting tubule. Formation of urine involves four stages. Glomerular filteration, tubular reabsorption, tubular secretion, and concentration of urine. Free distribution by A.P. Government 89 • Kidneys remove nitrogenous waste from body, maintains water balance (osmoregulation), salt content, pH, and blood pressure in human body. • Dialysis machine is an artificial kidney which filters the blood to remove the metabolic wastes out side the body. • Kidney transplantation is a permanent solution to renal failure patients. • Different animals have different excretory organs e.g. amoeba-contractile vacuole, platyhelminthesflame cells, annelida-nephridia, arthropoda-malpighian tubule, reptiles, birds and mammals-kidney. • There are no special organs for excretion in plants. Plants store different waste materials in leaves, • bark, roots, seeds which fall of from the plants. Plant metabolites are two types i) primary metabolites eg: proteins carbohydrates and fats. ii) secondary metabolites eg: alkoloides, tannins, latex and resins. These are economically important to us. • Excretion is the removal of material from living beings where as secretion is movement of materials from one point to other. Improve your learning 1. What is meant by excretion?(AS1) 2. How are waste products excreted in amoeba?(AS1) 3. Name different excretory organs in human body and excretory material generated by them?(AS1) 4. Deepak said that ‘Nephrons are functional units of kidneys’ how will you support him?(AS1) 5. How plants manage the waste materials?(AS1) 6. Why do some people need to use a dialysis machine? Explain the principle involved in.(AS1) 7. What is meant by osmoregulation? How is it maintained in human body?(AS1) 8. Do you find any relationship between circulatory system and excretory system? What are they?(AS1) 9. Give reasons(AS1) A.Allways vasopressin is not secreted. B.When urine is discharged, in beginning it is acidic in nature later it become alkaline. C. Diameter of afferent arteriole is bigger than efferent arteriole. D.Urine is slightly thicker in summer than in winter? 10. Write differences(AS1) A. Functions of PCT and DCT C. Excretion and secretion B. Kidney and artificial kidney D. Primary metabolites and secondary metabolites 11. There is a pair of bean-shaped organs P in the human body towards the back, just above the waist. A waste product Q formed by the decomposition of unused proteins in liver is brought into organ P through blood by an artery R. The numerous tiny filters S present in organ P clean the dirty blood goes into circulation through a vein T. The waste substance Q other waste salts and excess water form a yellowish liquid U which goes from organ P into a bag like structure V through two tubes W. This liquid is then thrown out of the body through a tube X.(AS1) (a) What is (i) organ P and (ii) waste substance Q. (b) Name (i) artery R and (ii) vein T (c) What are tiny filters S known as? (d) Name (i) liquid U (ii) structure V (iii) tubes W (iv) tube X. 90 X Class Excretion The wastage disposing system 12. The organ A of a person has been damaged completely due to a poisonous waste material B has started accumulation in his blood, making it dirty. In order to save this person’s life, the blood from an artery in the person’s arm is made to flow into long tubes made of substance E which are kept in coiled form in a tank containing solution F. This solution contains three materials G,H and similar proportions to those in normal blood. As the person’s blood passes through long tubes of substance E, most if the wastes present in it go into solution. The clean blood is then put back into a vein in the person for circulation.(AS1) (a) What is organ A? (b) Name the wastes substance B. (c) What are (i) E, and (ii) F? (d) What are G, H and I? (e) What is the process described above known as? 13. Imagine what happens if waste materials are not sent out of the body from time to time?(AS2) 14. To keep your kidneys healthy for long period what questions will you ask a nephrologist/ urologist?(AS2) 15. What are the gum yielding trees in your surroundings? What procedure you should follow to collect gum from trees?(AS3) 16. Collect the information about uses of different kinds alkaloids, take help of Library? (AS4) 17. Draw a neat labeled diagram of L.S of kidney?(AS5) 18. Describe the structure of re
nal tubule with neatly labled diagram.(AS5) 19. Draw a block diagram showing the path way of excretory system in human being.(AS5) 20. If you want to explain the process of filtration in kidney what diagram you need to draw.(AS5) 21. List out the things that makes you amazing in excretory system of human being.(AS6) 22. You read about ‘Brain dead’ in this chapter. What discussions would you like to have why you think so?(AS6) 23. We people have very less awareness about organ donation, to motivate people write slogans about organ donation?(AS7) 24. After learning this chapter what habits you would like to change or follow for proper functioning of kidneys?(AS7) Fill in the blanks 1. Earthworm excreats it’s waste material through ______________. 2. The dark coloured outer zone of kidney is called _____________. 3. The process of control of water balance and ion concentration with in organism is called________. 4. Reabsorption of useful product takes place in ____________ part of nephron. 5. Gums and resins are the __________ products of the plants. 6. Bowman’s capsule and tubule taken together make a ___________. 7. The alkaloid used for malaria treatment is ________________. 8. The principle involved in dialysis is __________________. Free distribution by A.P. Government 91 9. Rubber is produced from ______________ of Heavea braziliensis. 10. ________________ invented dialysis mechine. Choose the correct answer 1. The excretory unit in the human excretory system is called (A) Neuron (b)nephron (c)nephridia (d)flame cell 2. The excretory organ in cockroch (a) malphigian tubules (b) raphids 3. Which of the following is the correct path taken by urine in our body? (c) ureters (d) nephridia ( ( ( (a) kidney ureters bladder urethra bladder (c) Kidney ureters bladder urethra 4. Malphigian tubes are excretory organs in (b) Kidney ureters bladder urethra (d) Kidney bladder ureters urethra (a) earth worm (b) house fly (c) flat worm (d) hen 5. Major component of urine is (a) urea (b) sodium (c) water (d) creatine 6. Special excretory organs are absent in (a) birds (b) amoeba (c) sponges (d) a and b 7. Which of the following hormone has direct impact on urination? (a) adrenal (b) vasopressin (c) creatine (d) estrogen 8. Amber colour to urine due to (a) urochrome ( b) bilerubine (c) bileverdine (d) chlorides 9. Sequence of urine formation in nephron is (a) Glomerular filtration, Tubular reabsorption, Tubular secretion (b) Tubular reabsorption, Tubular secretion, Glomerular filtration, (c) Tubular secretion, Glomerular filtration, Tubular reabsorption (d) Tubular reabsorption, concentration of urine, Tubular secretion 10. Part of the nephron that exists in outer zone of kidney. a) Loop of the henle b) PCT 11. After having lunch or dinner one can feel to pass urine, because of a b) solids become liquids d) spincter relaxation a) stomach pressures on bladder c) water content in food material c) DCT d) Bowman’s capsule ( ( ( ( ( ( ( ( ) ) ) ) ) ) ) ) ) ) ) 92 X Class Excretion The wastage disposing system We can live even after death Five organs of 18 year old youth donated Dc correspondent, Hyderabad, 20 June 2013 Five organs of 18 year old H.S. YASWANTH KUMAR were donated by his father H V Shiva kumar to the organ donation wing of jeevandan scheme on Thursday. Yaswanth had met an accident on June 15 while he was travelling in a shared Autorikshaw from Jagadgirigutta. He was rushed to Nizam Institute of Medical Sciences (NIMS).The Nuero surgeons at NIMS declared him brain dead. Jeevandan counsellors obtained the consent of Mr. Shiva kumar,who agreed to donate Yaswanth’s kidneys, two heart valves, liver. These organs were retrieved and sent to various Hospitals for Transplantaion. Dr. in-charge of Swarnalatha Jeevandan scheme, said in a statement. Think how great Yaswanth’s parents are? Annexure Organ donation - A gift for life So many patients are waiting for suitable organ due to failure of vital organs. In Hyderabad where there are kidney transplantation facilities minimum 25 patients per hospital are waiting for kidney donors. Daily 10 - 100 people met with accident in our State. Out of them some people get brain dead. If we collect organs from brain dead patients in time, we can save minimum 5 peoples’ life. But lack of awareness on organ donation those who are willing to donate organs and those who need organs do not get proper information even facilities are there. Medical personnels from government and private hospitals are not informing about brain dead Patients. If they inform in appropriate time it will be very useful to patients those who are waiting for organ donation. In Hyderabad, organ Transplantation facility is available only in two government hospitals (NIMS and Osmonia) and in more than 10 corporate hospitals. Other organs like cornea transplant organs like kidney, liver, heart, lungs, pancreas, skin, bone, intestines and eye (cornea) can transplanted from brain dead patients.The process of transplantation of organs from brain dead patients to another is called cadaver transplantation. If any person is willing to donate organs or in needy get organs. They must register their names in transplantation facility hospitals. Collect information about voluntary organisation for organ donation and make a report on them. There is very less awareness among people about organ donation. Society needs much awareness in organ donation, so that we can save many lives who are in need of different organs from donars for their survival. Instead of living in their memories, let us give them a chance to live in others for one more life. Free distribution by A.P. Government 93 Chapter 5 Co ordination - The linking system Sharpening of a pencil, grasping a door knob, walking or running, driving, and a few physical actions, all involve well coordinated movements made with well balanced postures. In fact, whenever we move the three basic functions, such as movement, balance, and coordination they work together to perform purposeful motions of body parts. This is actually quite a feat, because moving is a complex process for the body. Even standing upright is a difficult challenge of balancing on just two feet with a narrow base. Yet, it is common for us not only to stand upright easily and apparently, effortlessly, but also while keeping our balance to perform many other functions. • What other functions do you think needed in coordination and balance? All our functions are carried out by an effort of several systems working together. For example, while movement, we hardly ever use just the skeletal system or muscular system alone, several other systems also have their own roles to play. Even within the muscular system, several muscles work in a sequence or at once. • What triggers movement of the muscles? It is a kind of pathway involving the way that our organs, tissues and cells work. All of them pick up signals of change from their surroundings and respond to them. This process triggers different functions in our body as well as by our body. For example, it is natural to move to a side of the road when we hear or see a car approaching. 94 X Class Coordination - The liking system Responding to stimuli • What helps us to respond to such signals? • Why does the living body respond to such signals? We can think of a response as an effect of a change in the environment of the organism or signals of change or ‘stimuli’.All living organisms respond to stimuli. The cat may be running because it saw a mouse. Plants grow towards the sunshine. We start sweating when it is hot and humid. The ability to react to particular stimulus in a particular situation must be of great importance in ensuring the survival of the organism. There is a sequence of events that brings about responses. They start from detecting changes in environment (both external and internal) or stimuli, transmission of the information, processing of the same. Finally the response will detect and execute the appropriate action. Let us do the following activity to find more about response to stimuli. Activity-1 Holding a falling stick Take a long scale or stick at least around ½ meter. Keep your fingers in holding position as shown in fig-1 . Ask your friend to hold the stick / scale near the end and let the other end be suspended between your fingers. Let there be a very small gap around a centimeter between your thumb and stick/scale and the stick/scale and fore finger. Now let your friend allow it to fall. Try to hold it. • Could you hold it exactly at the point where it was suspended between your fingers? • Mark the point where you caught the stick. fig-1: Holding stic • How far up was this point from the end suspended between your fingers? • Why did this happen? • How fast do you think the process was? Responses are brought about by rapid changes in some muscles and such changes are usually related to changing stimuli. Rapidity of response indicates an efficient communication system linking those parts that pick up stimuli to those that trigger a response. • What makes this kind of communication possible? Free distribution by A.P. Government 95 fig-2: Galen Integrating pathways - Nervous coordination The Greeks believed that all functions of the body were controlled by the brain since damage to that organ produced remarkable changes in behavior. They had very little idea on how such control could be exercised though Galen, a Greek physiologist (A.D. 129-200) made one notable observation. One of his patients, having suffered a blow on the neck when falling from his chariot, complained of loss of feeling in the arm while still retaining normal muscular control of its moment. Galen concluded that nerves were of two kinds – those of sensation and those of action. According to him the blow in the neck had damaged the nerves of sensation but had not affected it’s action. • Why do you think Galen Drew such a conclusion? The functioning of nerves as integrating systems was
little known till late 18th century. Then, physiologists began to study the mechanism of nerve functioning and found how signals were transmitted by making the connection between recent work on electricity and the propositions on working of the nervous system made till then. Now we know more about how nerves from different sections of the brain and spinal cord control responses of different areas of the body. We also know the probable pathways that transmit information but we still know very little about the working mechanisms of the nerve cell. Structure of nerve cell Activity-2 Observe the permanent slide of nerve cell or neuron under microscope and try to find out its parts, compare with the following diagram. dendrites nissl’s granules nucleus cell body schwann cell axon node of ranvier myelin sheath synaptic knob axon terminal Each nerve cell consists of a cell body with a prominent nucleus. There are fine projections mainly of two types extending from the cell body of the nerve cell. The small projections are dendrites while a long one that extends to different parts of our body. The axon is surrounded by a specialized insulatory sheath called myelin sheath.This sheath is interrupted at regular intervals called nodes of ranvier.The myelin sheath is made up of schwann cells and chiefly consists of fatty material. Axons not having the sheath are non-myelinated fibers. The Coordination - The liking system fig-3: Nerve cell 96 X Class covering also forms a partition between adjacent axons. The nerve cell body lies either in our brain or spinal cord or very close to the spinal cord in a region called dorsal or ventral root ganglion. In the brain or spinal cord, it is difficult to make out the difference between dendrites and axons on the basis of their length, often, the presence of the sheath helps us to find out but several axons here do not have the sheath. We know that the nerve cell is the structural and functional unit of nervous system. Our nervous system consists of around about 10 billion of them, which communicate with each other in a specific manner. Dendrites of one nerve cell connect to the other or to the axons of the other nerve cell through connections called as a ‘synapse’. Synapse is the functional region of contact between two neurons, where information from one neuron is transmitted or relayed to another neuron. Though these are regions of minute gaps and essentially neurons do not have any protoplasmic connection between them yet information is passed from one nerve cell to the other through these gaps either in the form of chemical or electrical signals or both. These synapses are mainly found on the brain, spinal cord and around the spinal cord. Beyond these areas the axon carries the signals to respective areas in our body. fig-4: Synapse Pathways: From stimulus to response In the holding stick activity you observed that there is coordination between eye and finger. Different pathways are taken by nerves to bring about this coordinated activity. On the basis of pathways followed, nerves are classified mainly into three different types. Afferent neurons: Afferent (or ferrying towards) which carry messages towards the central nervous system (spinal cord or brain) from nerve endings on the muscles of different sense organs that sense the change in surroundings are called stimulus detectors. These are also called ‘sensory’ nerves. fig-5: Sensory neuron Free distribution by A.P. Government 97 Efferent neuron: Efferent (or ferrying away) which carry messages from the central nervous system to parts that shall carry out the response or the effectors (nerve endings). They are also called ‘motor’ nerves. fig-6: motor neuron Association nerves: Association nerves, which link together the afferent and efferent nerves. • Which organ of your body was the detector and which the effector in Activity-1? • What do you think that the information carried on the afferent and efferent nerves? Brain or spinal cord Afferent nerves Assossiation nerves Efferent nerves Muscles of the organ fig-7: Different nerve pathways Activity-3 Knee jerk reflex Activity-1 showed a response on which you had some control or it was voluntary (recall the use of the voluntary and involuntary muscles that you studied in class 9th.). We know that our body would also need to respond to certain situations on which we may not have a control. Such responses are called reflexes.A simple activity shall help us to understand this better. Cross the legs, in a seated position, so that the lower half of the uppermost leg hangs freely over the other. Strike the area below the knee cap sharply, while firmly grasping the front part of the thigh with the other hand. Note the changes in shape of the thigh muscles. Note that although we are fully conscious, we cannot prevent the thigh muscles from contracting. Such a response Coordination - The liking system fig-8: Knee jerk 98 X Class is said to be involuntary. Now the same thigh muscle can operate in a voluntary manner, as when we kick a football. Do you think most of the functions in our body go about in an involuntary manner? Why /Why not? Do you know ? The existence of the knee jerk was first noted in 1875. At first it was doubted whether a nervous reflex was involved at all. But it was discovered that if, in an anaesthetized monkey where spinal nerves supplying the limb were cut, the knee jerk reaction would not occur. Clearly a nerve pathway was involved. During actions which are involuntary and have to be carried out in very short intervals of time, the pathway that nerves follow is a short one; it does not go up to the brain while voluntary pathways are usually longer passing through the brain. Now let us see what pathways actually are. The reflex arc Not until the end of the nineteenth century the reflex was understood in terms of pathways. Picking up information of a stimulus to generate a response involves a pathway from detectors to brain or spinal cord or a set of nerve cell heads near spinal cord to the effectors. Such a single pathway going upto the spinal cord from detectors and returning to effectors is a reflex arc. spinal cord sensory neuron motor neuron effector muscle fig-9: Reflex arc If you accidentally touch a very sharp surface with your feet, several such arcs would operate to cause the muscles of the leg to withdraw the feet. Observe the fig-9, how our leg muscle responds when we step on a sharp edged object. • What other effectors would act under these circumstances? • What does this tell us about the association of nerves? In fact, you must have experienced, what happens when you do things consciously and otherwise. Say for example, when you are performing an action such as running upstairs. If you start to think about where your feet are going you often stumble. The interesting thing is that the same effectors Free distribution by A.P. Government 99 in the leg muscles can be made to perform very special movement under the control of the conscious mind (voluntarily). Hence in a football game, the muscles of the leg operate both by reflexes and voluntarily. Most actions of our body are actually controlled together by voluntary and involuntary pathways. Do you know ? Nerve transmission from stimulus to a response can occur at a maximum speed of about 100 meters per second. • Think of any action and try to make a sketch of the reflex arc. The voluntary and involuntary actions in our body are controlled by nervous system as a whole.We may study our nervous system on the basis of areas from which nerves originate and then spread out to the whole body as mainly two divisions one is the central nervous system (CNS) and the other is peripheral nervous system (PNS) Central Nervous System (CNS) Central nervous system includes brain and spinal cord. It coordinates all neural functions. Brain cerebral hemisphere Proportionate to the body size, the human brain is the largest of all animals. The brain is present in the hard bony box like structure called cranium. It is covered by three layers called the meninges. The meninges are continuous and cover the spinal cord as well. The space between the inner layers is filled with fluid called cerebro-spinal fluid. It serves as a shock-absorbing medium and protects the brain against shocks/jerks along with the meninges and cranium. Mainly the nerve cell bodies together with capillaries form a mass called grey matter while the myelinated axons or those covered by fatty sheaths form white matter. The grey matter is usually present on the periphery while white matter is present towards the center. This is mainly due to the fact that there is a small area from where the myelinated axons leave the brain. As we fig-10: Brain brain cavity spinal cord cerebellum mid brain Coordination - The liking system pituitary gland medulla 100 X Class have already studied, the function of the brain as a control center was known nearly 2000 years back by Greek physiologists. Brain has the following divisions – 1. Forebrain – cerebrum, diencephalon 2. Midbrain – optic lobes 3. Hindbrain – cerebellum, medulla. Table-1: Functions of the various parts of the brain Part of the brain Functions Cerebrum i) Seat of mental abilities, controls thinking, memory, reasoning, perception, emotions and speech. ii) Interprets sensations and responds to cold, heat, pain and pressure. Diencephalon i) Relay centre for sensory impulses, such as pain, temperature and light. ii) Reflex centre for muscular activities. iii) Centre for certain emotions such as anger. iv) Centre for water balance, blood pressure, body temperature, sleep and hunger. v) The hypothalamus controls the pituitary gland, which functions as the master gland. It relays motor impulses from the cerebral cortex to the spinal cord and relays sensory impulses from the spinal cord to the thalamus, reflexes for sight and hearing. i) Maintains posture, equilibrium and muscle tone. ii) Coordinates voluntary movements
initiated by cerebrum. Midbrain Cerebellum Medulla oblongata i) Contains centre for cardiac, respiratory and vasomotor activities. (Vasomotor refers to actions upon a blood vessel which alter its diameter) ii) Coordinates reflexes like swallowing, coughing, sneezing and vomiting. Do you know ? The brain weighs approximately 400g. Through the brain comprises little more than 2% the body’s weight, it uses 20% of the whole body energy. Free distribution by A.P. Government 101 Spinal Cord Spinal cord extends from the back of the hind brain(Medulla oblongata) to the back of the stomach or lumbar region, through the neural canal of the vertebral column. It is almost cylindrical in shape. Unlike the brain, the white matter is towards periphery while grey matter is towards the center of the spinal cord. The myelinated axons leave the spinal cord from both sides of the vertebral column. See fig-11. The role of the spinal cord in nervous control was studied largely by the experimentalists of the sixteenth and seventeenth centuries. They found that the Greeks concept of control by the brain was erroneous. Animals were shown to have the ability to respond to stimuli even when the brain was removed. ‘Leonardo da Vinci’ (1452-1519) and ‘Stephen Hales’ (1677-1771) both recorded the survival frogs those brain had been destroyed. The animal still produced muscular moments if its skin was pinched or pricked. Both observers further recorded that the animal died as soon as spinal cord was damaged by pushing a needle down it. Such evidence suggested that the spinal cord was not simply a trunk road for instructions from the brain, but might be a control center in its own right. fig-11: Spinal cord fig-12: Leonardo da vinci • According to you what would be the function of the spinal cord? • Are all functions of our body under direct control of the brain and spinal cord? Why do you think so? Do you know ? Scientists have been able to trace out the nerves that originate from brain called cranial nerves and those that originate from spinal cord called spinal nerves. There are 12 pairs of cranial nerves which arise from the brain. There are 31 pairs of spinal nerves. Peripheral nervous system Figure-13 shows you that nerves attached to the spinal cord have two types of connections or roots – One to the back or dorsal side and other to front or the ventral side of cord. The experimental work of two men, Charles 102 X Class Coordination - The liking system Bell in Scotland and Francois Magendie in France, in the early nineteenth century, showed that these roots have different functions. If the dorsal roots of an experimental animal were cut the animal made no obvious reaction. If, however, the ventral roots were even lightly touched, the muscles to which the nerve was connected switched violently. The ventral root evidently controlled muscular activity, the dorsal root did not. dorsal root dorsal horn dorsal root ganglion ventral horn ventral root spinal nerve fig-13: Periperal nerves system In 1822 they suggested that dorsal root carried messages of sensation inwards while the ventral pathway carried outwards the instruction for muscular contraction. • Which root according to you gets signals from afferent nerves? The peripheral nervous system (PNS) is a vast system of the dorsal and ventral root nerve cell heads and the network of spinal and cranial nerves that are linked to the brain and the spinal cord on one end and muscles on the other. • What do you think the end of these nerves act at the muscular end? The PNS can either involuntarily control several functions of regions like our internal organs, blood vessels, smooth and cardiac muscles.So it is called autonomous nervous system. It has voluntary control of muscles of some areas of skin and the skeletal muscle. We can take up an example to see how certain involuntary function controlled by autonmous nervous system takes place in our body. A very evident observation is the reduction and expansion of the pupil of our eye. When we enter a dark room we cannot see anything immediately. Slowly we are able to see the things around us in the room. This is because of increase in diameter of pupil, which allows more light in. When we come out of the dark room into broad day light the diameter of the pupil decreases allowing less light to enter into the eyes. Both these functions occur under the influence of the autonomous nervous system. Several functions in our body are controlled by nerves while many of them and others are controlled by other ways as well. You might have heard about people having diabetes and know that they have to take tablets or insulin injections when the level of sugar in their blood rises.Let’s find Free distribution by A.P. Government 103 out what insulin is and how we came to know about it. This would also give us an idea of controls other than nerves in our body. Do you know ? Research in the past two decades has brought out an interesting fact. Other than central nervous system and peripheral nervous system, there is a system of neurons present in our digestive tract that can function even independently of either CNS or PNS. It has been nick named as a small brain and the system is called as enteric nervous system. Coordination without nerves The Story of insulin In 1868 Paul Langerhans, Professor of Pathology at the University of Freiburg in Germany, working on the structure of the pancreas, noted certain patches of cells quite different in appearance from the normal tissue cells of the organ and richly supplied with blood vessels. They are known Islets of Langerhans (Islets stands for islands), but their function remained unknown. Many others interested in the function of pancreas and found that its removal from the body of an experimental animal would lead to the development of diseases similar to a well-known human ailment ‘sugar diabetes’. This is a condition in which the amount of free sugar in the blood and in the urine is abnormally high. It’s a cause in man was unknown but evidence pointed to the pancreas as a possible role. fig-14: Paul Langerhans fig-15: Pancreas The next stage was reached when it was found that tying up the pancreatic duct that emerged from the duodenum( a part of the small intestine) would cause the pancreas to degenerate but the Islets of Langerhans would remain normal. Moreover, an animal so treated would not develop diabetes. This was really a strong evidence that the level of blood sugar is linked with the islet cells. By 1912, workers were convinced that the islets produced a 104 X Class Coordination - The liking system secretion which directly liberated into the blood. In Latin ‘insula’ means an island. The name insulin was coined for the secretion, even though it had not been isolated. Ten years later in Toronto, Banting, Best, and Macleod finally succeeded in extracting insulin from degenerate animal pancreases whose ducts to the intestine had been tied. When given by intravenous injection to a dog with no pancreas, this substance kept it alive and healthy with a low level of blood sugar. Insulin is now produced in large quantities for the treatment of human sufferers from sugar diabetes, to whom it is administered by injection into the skin. Insulin thus is a chemical that acts as it reaches blood from the cells that produce it. Other chemical co-ordinators The evidence that events occurring in one part of the body could be affected and indeed controlled by substances circulating in the blood was now overwhelming. In 1905 the English physiologist Starling had coined the term hormone (Greek, hormao – to impel) for such secretions. The glands secreting hormones were termed ductless glands, since they have no tube or duct to carry away their products, which pass straight into the blood. In this way they different from glands such as the liver and pancreas, whose secretions pass down ducts which are connected to other organs. The human body contains many other ductless glands (endocrine glands).Glands do not produce their hormones at a steady rate. The adrenal gland, for example, normally has a low output. What will you do if a dog is after you? What will be your first reaction? Have you ever observed any change in your body when you are afraid? Nobody wants to fight with a dog. The first thing we do is running away from the place. fig-16: Cock fight Try to note the body language of humans / animals when they are fighting / scared. If we observe our body, when we are afraid, the rate of heart beat increases; the breath rate will be faster; blood pressure increases; the hair on the body becomes erect and we get goose bumps. The other things we might not observe are pupil dilation, skin becomes more sensitive, and rarely the bladder and the rectum may be emptied. We come to normal state only after we reach a safe spot. Free distribution by A.P. Government 105 We have already studied about nerve co-ordination, where nerves carry stimuli from sense organs to central nervous system and orders to effectors organs-the muscles. But, in the above situation the action of the nervous system is limited. All the changes in the body are carried out under the influence of a chemical called ‘Adrenalin’ hormone, released by Adrenal gland which is an endocrine gland. The various actions of the body are controlled by hormones and co-ordinated by nervous system. So in this type of conditions nervous system and endocrine system work together to bring about control and co-ordination. Ask your teacher why Adrenalin hormone is also called fight or flight hormone. The whole system of ductless glands is called the endocrine system. Information about a few of the endocrine glands is given in the accompanying table. Try to make a list of functions that you think are controlled both by the nervous and the endocrine system. Feedback mechanism Recall the fight or flight behavior of cat and dog. The amount of adrenalin hormone increases in the blood sharply i
n a frightening situation, getting anger or excited. • Have ever observed the duration of anger? • Why does anger come down? • What may happen if anger persists for a longer period? Anger is always short lived factor. You know that increased levels of adrenalin are responsible for anger. When the levels of adrenalin in the blood come down slowly we come to normal state. If the adrenalin levels persist for a longer period of time, regular metabolic activities are disturbed. Increase in adrenalin levels leads to anger, decrease in adrenalin levels leads to normal position. • What will happen if it is continued for longer periods of time? Similarly the sugar levels in the blood rise than normal level, they are detected by the cells of pancreas, which respond by producing more insulin into the blood. If the sugar levels come back to normal level secretion of insulin is automatically reduced. So it is necessary that the hormones are secreted by the glands in our body in precise quantities which are required for the normal functioning 106 X Class Coordination - The liking system Free distribution by A.P. Government 107 of the body. This means that there should be some mechanism to regulate the production and release of hormones in the body. The timing and amount of hormones released by endocrine glands is controlled by the feedback mechanism, which is inbuilt in our body. None of the systems, whether nervous or chemical are totally exclusive of each other. Autonomus nervous system You know that medulla oblongata is the region that regulates heartbeat, breathing etc. the system that helps to bring about such activities of internal organs is called autonomous nervous system. Normally such involuntary activities take place by the coordinated efforts of the medulla oblongata and autonomous nervous system. fig-17: Autonomous nervous system Now let us see how the autonomous nervous system influences the life activities. Observe the fig-17 and record your observations. • To which organs of the body do the nerves go from the ganglions near the vertebral column? 108 X Class Coordination - The liking system • What are the organs that receives nerves starting from the brain? • Which are the organs whose activities are influenced by the sympathetic system? • Which are the organs whose activities are influenced by the para sympathetic system? • What do you understand about the functions of para sympathetic system? • What you understand about the functions of sympathetic system? Ganglia near the vertebral column are connected to the spinal cord by nerves. The sympathetic system is formed by the chain of ganglia on either sides of the vertebral column and the associated nerves. The para sympathetic system is formed by the nerves arising from the ganglia of the brain and the posterior part of the spinal cord. These together constitute the autonomous nervous system. It is the part of the peripheral nervous system consisting of twelve pairs of cranial nerves and thirty one pairs of spinal nerves. Control mechanisms in plants How do plants respond to stimuli? So far we have studied how control mechanisms work in our body. Do plants also have control systems? Let us find out by doing a small activity. Activity-4 Touch the leaves of Mimosa pudica (athipathi, touch me not) plant and observe the response of leaves. Are they folding? If so in which direction? Try to give examples of situations where you may see plants responding to a certain stimulus. fig-18: Mimosa pudica Free distribution by A.P. Government 109 Do you know ? Mimosa pudica leaves have pad like swellings at the base. They are called pulvini. Here cells contain lot of water and large intercellular spaces. Due to water pressure pulvini hold the leaf erect. Touch me not plant shows nastic movement by touch. This is called thigmonasty. When we touch the leaves, an electrical impulse is generated. This impulse acts on plant hormone. Because of this hormone water in the pulvini cells which are closer to the leaf vein migrate to other side of the cells.Then pulvini loss its firmness hence leaves become fold. After 20 to 30 minutes water comes back pulvini attains firmness and leaves become erect. You might have observed the tendrils of plants growing towards a support. Can you imagine how is it happening? Would you think it is responding to a stimulus? Both plants and animals react to various stimuli around them. But the method of responding to stimuli is not similar in plants and animals. Higher animals respond to stimuli because they have a nervous system and an endocrine system. Plants do not have a well-defined nervous or endocrine system. They do have some mechanism of control by means of some chemicals or hormones. Plants can sense the presence of stimuli like light, heat, water, touch, pressure, chemicals, gravity etc. The hormones present in the plants called phytohormones (phyto means plant) control responses towards the stimuli mentioned above. Phytohormones coordinate the activities of the plant usually by controlling one or the other aspect of the growth of the plant. So plant hormones are also called growth substances. Some major plant hormones and their action are given in the following table. Table-3: Major plant hormones and their action Hormones Uses Abscisic acid closing of stomata; seed dormancy Auxins cell elongation and differentiation of shoots and roots Cytokinins promote cell division, promotion of sprouting of lateral buds, delaying the ageing in leaves, opening of stomata. Ethylene ripening of fruit Gibberellins germination of seeds and sprouting of buds; elongation of stems; stimulation of flowering; development of fruit, breaking the dormancy in seeds and buds. Discuss with your teacher about seed dormancy. 110 X Class Coordination - The liking system Activity-5 Take a glass jar and fill with soil. Sow a bean seed near the wall of the jar. This helps you to observe how root and shoot are growing. After 4 - 5 days you will notice seed germinatation. Keep the jar under the sun. Observe how root and shoot grows. Then tilt the glass jar and keep the plant horizontally. Observe the direction of root and shoot growth for more than a week. • Does the shoot take a horizontal tilt after a week? • Which side of the shoot may have grown more and which side less to bring about this effect? Observe the plant growing towards light and how auxins acts on bending of stem to show a response to the sunlight. More auxin collects on the light illuminated side of the stem. So cells on that side grow faster. On opposite side cells grow slow to make the stem bend. Collect bending and straight portions of tender stem. Take transverse sections of both stems, observe them under microscope. • Do you find any difference in the shape of epidermal cells? fig-19: Bending towards sun Charles Darwin and his son Francis Darwin performed some experiments on phototropism. They covered the terminal portion of the tip of stem(coleoptile) with a cylinder of metal foil. Exposed the plant to light coming from the side. The characteristic bending of the seedling did not occur. If, light was permitted to penetrate the cylinder bending occurred normally. They stated that when seedlings are freely exposed to a lateral light some ‘influence’ is transmitted from upper to the lower part causing the material to bend. In 1926, the Dutch plant physiologist F.W. Went succeeded in separating this ‘influence’ from the plant that produced it. Went cut off coleoptile tips from oat seedlings. He placed the tips on a slice of agar and left them for about an hour. He then cut the agar into small blocks and placed a block on one side each stump of the decapitated plants. They were kept in the dark during the entire experiment. Within one hour he observed a distinct bending away from the side on which the agar block was placed. Agar block that had not been in contact with coleoptile tip produced either no bending or only a slight bending toward the side on which the block had been placed. Free distribution by A.P. Government 111 Went interpreted these experiments as showing that the coleoptile tip exerted its effect by means of chemical stimulus rather than a physical stimulus such as an electrical impulse. This chemical stimulus came to be known as auxin. In this way the first plant hormone fig-20: Went experiment auxin (greek word auxein means to increase) was discovered by Went. Tropic and nastic movements in plants The above experiments show that movement of individual parts of plants is possible when they are subjected to external stimuli. This type of response is called tropism or tropic movement. Sometimes the direction of stimuli determines direction of movement, sometimes the direction of movement may not be determined by direction of stimuli. This type of response is called nastic movement. Let us observe the growth of a creeper plant near window. The shoots of creeper bend towards sunlight. Such type of response of a plant to light is called photo tropism (photo means light, tropism means movement). We know that roots always grow downwards. This means that plant respond positively for gravitational force. This is called geotropism. If we observe plant which grow near a rock or wall side. You notice that all roots are growing in one direction, away from the rock or wall where water is available in the soil. This type of response to water is called hydrotropism. A very interesting thing in plants is movement of tendrils. All plants show positive response to phototropism. But in creepers like cucumber, bitter guard, the stem is weak and thin. Hence plant cannot grow erect. Tendrils play a vital role to make the plant erect. Tendrils are thin thread like growths on the leaves or stems of climbing plant. They grow towards support and wind around them. This type of response to make contact or touch is called thigmo tropism. If you taste the carpel of a flower it is sweet. Let us recall butterflies flutteri
ng on flowers for this nectar. Ripen stigma secretes sugary substance. This chemical substance stimulates the pollen grain which falls on the stigma. Pollen grain responds to this stimulus as pollen tubes grow to reach the ovule for fertilization. This type of response to chemicals is called chemo tropism. Unequal distribution of auxins affects the root and the stem growth. High concentration of auxin stimulates stem growth and inhibit root growth. fig-21: Tendrils 112 X Class Coordination - The liking system Key words Response, stimuli, neuron, axon, synapse, afferent or sensory nerves, efferent or motor nerves, association nerves, central nervous system, brain, spinal cord, cerebrospinal fluid, peripheral nervous system, insulin, endocrine glands, hormones, feedback mechanism, plant hormones, tropic movements, nastic movements. What we have learnt • Nervous system and endocrine system are the two systems that control and coordinate various functions in the body. • The responses of the nervous system can be classified as reflex, voluntary and involuntary actions. • The human nervous system is studied under two divisions: The central nervous system and the peripheral nervous system. • The central nervous system consists of brain and the spinal cord while the peripheral nervous system is further divided into somatic nervous system and autonomus nervous system. • The autonomus nervous system has two parts – sympathetic and parasympathetic, which cause physical reactions opposite to each other. Synapse is a gap across where signals are transmitted from one neuron to the other. • Nerve cell is the structural and functional unit of nervous system. • • Hormones produced in one body would move to another part to achieve the desired effect. • A feedback mechanism regulates the action of the hormones. • Directional movements in plants in response to specific stimuli like light, chemicals etc. are called • tropic movements. Plant hormones are usually growth effectors or inhibitors. Some growth effectors are Auxins and Gibberellins while growth inhibitors are Abscisic acid. Improve your learning 1. Fill in the missing sections in the following flow chart.(AS1) Step on a sharp edged object Brain analyse information and send commands 2. Do you think body’s team work maintains functioning of our body? Justify your answer with an example.(AS1) 3. Give an example of coordination in your body where both hormonal and nervous controls function together.(AS1) 4. Consider that you are passing by a garbage disposal area and you immediately cover your nose. Arrange the events below in a logical order by marking them from 1 to 5 to trace the events that happen in the nervous system from detection of foul smell (stimulus generation) to covering your nose (response).(AS1) (i) At the end of the axon, electrical impulse releases chemicals Free distribution by A.P. Government 113 (ii) Stimulus received on the dendritic cells of a neuron sets off chemical reaction that creates an electrical impulse (iii) Electrical impulse transmitted through cell body and axon (iv) The chemicals cross the synapse and reach the next neuron. Similarly, the electrical impulse crosses several neurons (v) Finally, the impulse is delivered from neuron to the gland that helps in recognition of the foul smell and muscle cells that help in covering the nose 5. What is a synapse? How it is useful in transfer information?(AS1) 6. Distinguish between(AS1) a) Stimulus and Response c) Central nervous system and peripheral nervous system d) Receptor and effector b) Afferent and Efferent nerves 6. How does Phototropism occur in plants?(AS1) 7. Give an example and explain how plants may immediately respond to a stimulus.(AS1) 8. Suggest an experiment to show how roots grow away from light in most plants.(AS1) 9. Give an example to show how hormones can influence visible changes in your body.(AS1) 10. How does a neuron differ from an ordinary cell in structure? Write notes.(AS1) 11. Is the structure of neuron suitable for transmission of impuleses? Analyse.(AS1) 12. Man is the most intelligent animal. What could be the fact that helped us to reach such a conclutssion?(AS1) 13. The axon of nerve cell in hand is sherter than the axon of nerve cell in leg. Do you support this statement? Why?(AS1) 14. Organs respond to the external stimulus by a fraction of second. How do you feel about such controlling mechanism of human body?(AS1) 15. State whether the following actions are voluntary action, reflex action or conditioned reflex.(AS1) i) Blinking iv) Salivating when food is put in the mouth. v) We close our ears when we hear un bearble sound iii) Playing on the key board ii) Cleaning the table 16. What will happen to the potted plant kept near window in the room?(AS2) 17. What happens if all functions of the human body is controlled only by brain?(AS2) 18. If you visit a doctor what doubts you would like to clarify about pancreas?(AS2) 19. Take a small potted plant. Cover base portion of the plant tightly and hang the part upside down. Observe the plant for a week. Based on your observation how can you support phototropism.(AS3) 20. Take a cock feather touch smoothly at different parts of your body. Findout which portion of the body has high sensation. Is this similar during sleeping?(AS3) 21. What procedure you follow to understand the effect of plant growth hormones (in agar medium) in the terminal portion of the tip of stem (coleoptile)?(AS3) 22. Collect information on the actions controlled by spinal cord by using reference books from your school library.(AS4) 23. Read the following sentences and compare with endocrine glands.(AS4) Pheromones are chemical substances secreted by organisms. These act as chemical signals secreted by exocrine glands. Pheromones are used as signals by the members of same species. Honey bee secretes pheromones that attract other bees to the location of food. 24. Collect the information about cranial nerves. Spinal nerves from internet or from your school library.(AS4) 114 X Class Coordination - The liking system 25. Draw a picture representing connection between axon-axon, axon-dendrite. Why do they connect like that?(AS5) 26. Draw a neatly labelled diagram of Brain and write few points how it is protected.(AS5) 27. You are walking in the traffic suddenlyyou heard a loud sound. How coordination takes place in this situation among respected organs? Draw a block diagram to explain this situation.(AS5) 28. Make a model of neuron using suitable materials.(AS5) 29. Draw a labelled diagram of brain.(AS5) 30. Observe different actions performed by your classmate for a period of 45 minutes. Out of those action which are controled by voluntary and involuntary pathways.(AS5) 31. Its very interesting to watch a creeper entwining its tendril to the support. Is not it? How do you express your feelings in this situation?(AS6) 32. Hormones are released at a speicific place, specific time for a specific function. Prepare a cartoon on hormones with a nice caption.(AS7) Fill in the blanks 1. The largest region of the brain is ___________ 2. A point of contact between two neurons is ____________ 3. _________ phytohormon is resposible for cell elongation and differentiation of shoots and roots. 4. Thryoxin is resposible for ______________ 5. Gibberellins and auxins promote growth in plants while abscisic acid arrests the same. Some situations are discussed here, State which hormones would be needed and why? a) A gardener wants large dahlias he should use along with nutrients and other things ____________ hormone. b) In a dwarf plant the branches have to be thickened one would use ___________ hormone. c) Seeds are to be stored for a long time _____________ hormone can help. d) Cutting the apex or tip of plants so that there are several lateral buds ____________ hormone can be used. e) The part of the brain that helps you in solving puzzles is _____________. Choose the correct answer 6. A person has loss of control on emotions, which part of brain stops it’s function. a. cerebrum b. diencephalon 7. Leaf movement in mimosa helps to a. reduce photosysthesis c. releasing phyto hormones 8. Diabetes is related to this gland. b. pancreas a. Thyroid c. mid brain d. cerebellum b. protect from greazers d. regulate it’s growth c. adrinal d. pitutary Free distribution by A.P. Government ( ( ( ) ) ) 115 Chapter 6 Reproduction - The generating system In plants and animals reproduction is necessary life process for continuation of life by the production of offsprings . • Do you think reproduction occurs only for continuation of life? • How does an organism grow? How does repair of worn out parts take place? Is there any form of reproduction involved in the process? Organisms are capable of giving rise to offsprings by the process of reproduction. Some organisms may reproduce differently in different situations. For example, in favorable conditions paramoecium give rise to more of its kind from a single parent by simply spliting into two. This happens rapidly and several of them are formed. During unfavourable conditions two paramoecia come in contact exchange certain materials of their bodies and produce forms that are more to tolerant. The time required to reproduce also varies from organism to organism. Even within the organism there could be certain environmental conditions that would make faster the process of reproduction. Let us do an activity to find out how fast an organism might be reproducing Acitivity-1 Formation of bacterial colony in milk We are aware that Lactobacillus bacteria is responsible for formation of curd. Take a tea spoon full of curd and mix it throughly with around 30 tea spoon full of (half of the glass) luke warm milk in a bowl. Take another tea spoon full of curd and mix it with 30 tea spoon full of cold milk in 116 X Class Reproduction - The generating system another bowl. Cover both the bowls and note the intial time. Keep observing every hour to se
e whether curd has formed. Curdling indicates that the increase in number of bacteria. Note the time taken for formation of curd in both the bowls. • Does it take the same time to form curd in both the bowls? • What is the time taken to form nearly 30 times the size of the bacterial colony indicate? Think, how fast they are growing. During rainy season you may have wondered how swarms of insects suddenly appear. Most insects have life cycles spanning a few days to a few months. You may find great variations in period of reproduction in yeasts, bacteria, rat, cow, elephant and man. Asexual mode of reproduction Let us study modes of reproduction involving a single parent, without involving gametes. These are known as asexual modes of reproduction. Organisms can reproduce asexually in many ways. Some of them are given here. Fission Single celled organisms, such as Paramoecium and bacteria, reproduce by splitting into two or more offsprings. This usually occurs in a symmetrical manner. They split into two by binary fission. When more cell are formed it is called multiple fission. This is often the only mode of reproduction in these organisms. • How do you think bacteria were dividing to form curd? Budding A growth on the body as a bud that grows to form nearly identical copy of parent. When the bud totally grows then it separates from the parent and survives independently. Ex: Yeast. fig-1: Fission in paramoecium Fragmentation fig-2: Budding in yeast Some can grow from a separate piece of parent organism. This can be from any part of the body. This happens only in the simplest, such as some flatworms, moulds, lichens, Spirogyra etc. grow in this manner. These may also reproduce sexually. Fragmentation is a common mode of reproduction in algae, fungi and many land plants. Free distribution by A.P. Government fig-3: Fragmentation 117 Parthenogenesis Now a days we are able to develop seedless fruits like watermelon, grapes etc. This is a process of reproduction where there is a shift from sexual to asexual mode of reproduction. • How do you think this happens? This process also occurs in nature. An organism which reproduce sexually sometimes asexually. We have utilised this process of reproduction in growing organisms of our choice with more desirable characters. In this process generally the female gametes develops into zygote without fertilization. • Would it involve two parents? fig-4: Seedless fruit This strange kind of reproduction occur in bees, ants and wasps. The zygote might develop from fertilized egg or by parthenogenesis. When meiosis does occur. The parthenocarpic zygote develop into male (Monoploid). While the fertilized one developed into female (Diploid). Discuss with your teacher about plants and animals that show Parthenogenesis and prepare a bulleten. Regeneration Many organisms have the ability to give rise to new individual organisms from their body parts. That is, if the individual is somehow cut or broken up into many pieces, these pieces grow into separate individuals. This is similar to fragmentation. • Regeneration could be kown as a type of fragmentaion? Do you agree? Why? Why not? • Which type of fission would produce larger colonies in less period of time. Why? • Which mode of asexual reproduction provides maximum scope of choice of desirable characters? Vegetative propagation fig-5: Regeneration in planaria In higher plants vegetative propagation. may be natural or artificial. Natural propagation Leaves: In Bryophyllum small plants grow at the edge of leaves. Stems: Aerial weak stems like runners and stolons, when they touch the ground, give off adventitious roots. When the connection with the parent plant is broken, the stem portion with the adventitious roots develops into Reproduction - The generating system fig-6: Bryophyllam 118 X Class an independent plant. Some examples for propagation by stem are from stolons, bulbs, corms, tuber, etc. Stolons - Vallisneria, strawberry, Bulbs - Alliumcepa or onion, Corms - Colacasia, tuber - potato, Roots: Roots of Dahlia, radish, carrot etc., grow as new plants. Bulbis Tuber Artificial propagation Cutting: Stolon fig-7 Corms Root Some plants grow individually when a piece of a parent plant having bud is cut from the existing plant. The lower part of this cutting is buried in moist cell. After few days the cut parts having buds grow as an individual plant after developing roots. Ex: Rose Layering: A branch of the plant with at least one node is bent towards the ground and a part of it is covered with moist soil leaving the tip of the branch exposed above the ground. After some time, new roots develop from the part of the branch buried in the soil. The branch is then cut off from the parent plant, the part which has developed roots grows to become a new plant. Ex: Nerium. Grafting: Two plants are joined together in such a way that two stems join and grow as a single plant. One which is attached to soil is called stock and the cut stem of another plant without roots is called scion. Both stock and scion are tied with help of a twine thread and covered by a polythene cover. Grafting is used to obtain a plant with desirable characters. This techqnique is very useful in propagating improved varieties of various flower and fruits (ex: Mango, citrus, apple, rose). Free distribution by A.P. Government fig-8: Cutting fig-9: Layering fig-10: Grafting 119 If you have two varieties of fruit yielding trees in your garden. One tree has the character of giving big sized fruits but less in number. The taste of the fruit is pretty good. The other one produce more number of fruits but they are neither big in size nor tasty. • What are the characters that would like to select? • What mode of propagation would help you to produce the plants with selected characters? • Whether they reproduce by budding or fission or fragmentation, organisms formed are copies of their parents. Is it true? Why? Do you know? The cutting, layering and grafting are the traditional methods for the artificial propagation of plants. Examples of plants produced in this manner are Banana, Pineapple, Orange, Grape, Rose, etc. For commercial purposes; they are being replaced by the modern technology of artificial propagation of plants involving tissue culture. In tissue culture, few plant cells or plant tissue are placed in a growth medium with plant hormones in it and it grows into new plants.Thousands of plants can be grown in very short interval of time. By grafting a very young scion (shoot part of a plant) can be made to flower and produce fruits quite fast. Collect information from your school library or internet about advantages and disadvantages of artificial vegetative propagation and discuss in your class room. Spore formation: Generally we may notice whitish threads and blackish powdery like substance on rotten fruits, bread slices and other food materials. When you touch it, the blackish powder sticks on your fingers. These are the reproductive spores produced by a fungi. Ex: Rhizopus. You have already learnt about this in the chapter ‘The story of micro organisms’ in class VIII. Rhizopus produces hundreds of microscopic reproductive units called spores. When the spore case (also called sporangium) bursts, the spores spreads into air. These air-borne spores land on food or soil, under favourable conditions like damp and warm conditions, they germinate and produce new individuals. Most of the fungi like Rhizopus, Mucor etc., Bacteria and non-flowering plants such as ferns and mosses reproduce by the method of spore formation. 120 X Class Reproduction - The generating system Lab Activity To examine Rhizopus or common mould under the microscope, it is best to grow your own in a controlled environment. Use a soft bread that is preservative free or a roti, fruits or vegetables such as potatoes or oranges. A good sample of mould may require 4-10 days to form spores so be sure to plan ahead for this project. (Please note: this should not be done by those with allergies to mould or with severe asthma.) Rhizopus growing on bread Rhizopus under microscope Rhizopus sporongium fig-11 Leave the bread in the open air for about an hour, so it is exposed to contaminants in the air. Place the bread in a plastic bag, sprinkle water over it to have dampness then seal the bag, leaving some air inside. Place the bag in a dark, warm place. A kitchen cup board close to the stove may be one option. Or you could place it next to a window, with a bowl or plate covering it from the light. Mould will grow best in a moist environment. Mould would start growing in 2-3 days, but will take a week or more to form spores depending upon the weather. Check the piece of bread every few days, and add some water if it is drying. Avoid opening the plastic bag as much as you can. If you touch the bread, be sure to thoroughly wash your hands afterwards. When sufficient mould has formed, you can prepare a slide and examine it under the microscope. You would find whitish thread like growth with masses of black, grey and green fine dotted structures (See fig-11). The black dotted structure is that of bread mould. Take a part of the bread or roti to school in a matchbox and ask your teacher to help you to make a slide and observe under the microscope. Material required: Mould sample, plain glass slide, coverslip, water, disposable gloves. Procedure: 1. Place a drop of water in the centre of the slide. Free distribution by A.P. Government 121 2. Using a toothpick, scrape very little of the mould and place it on the drop of water. 3. Take the cover slip and set it at an angle to the slide so that one edge of it touches the water drop, then carefully lower it over the drop so that the cover slip covers the specimen without trapping air bubbles underneath. 4. Use the corner of a tissue paper or blotting paper to blot up any excess water at the edges of the cover slip. 5. View the slide with a compound microscope first observe under
low power. The common bread mould consists of fine thread like projections called hyphae and thin knob like structures called Sporangia (sporangium in singular). Each sporangium contains hundreds of minute spores. When the sporangium bursts, the tiny spores are dispersed in air. Try to give some more examples of organisms which reproduce through spore formation. Sporophyll: Ferns also produce spores. Collect a fern leaf which is called sporophyll. Observe the leaf carefully. On the lower surface of the leaf you find clusters of dot like structures called sporangia. These contain spores. Gently rupture the sporangia with a needle and observe spores by using magnifying lens. • Do you find any similarities between rhizopus and fern spores and sporongia? • What about mushrooms, how do they grow? Discuss fig-12: Fern sporophyll in your class. Sexual reproduction As you havestudied earlier, sexual reproduction is a way of reproduction where fusion of gametes takes place, by a process called fertilisation. Fertilisation may occurs either outside the body of the mother (external fertilisation) or inside the mother’s body (internal fertilisation). As a matter of fact, the eggs of land animals are fertilised inside the body of the mother. The fertilized eggs start dividing and growing into the embryo. External fertilisation is common in aquatic animals like most of the fishes and amphibians. The female lays a vast number of eggs in water and male release some millions of sperms on them in water. As the chance of fertilisation is controlled by nature which occurs externally, hence it is inevitable to give rise to vast number of eggs and sperms (gamete). 122 X Class Reproduction - The generating system Reproduction in a placental mammal - Man While talking about mammals especially human beings special reproductive organs have developed in males and females to carry out reproduction. Let us study them in detail. Male reproductive system In human males, the two testes are located in pocket like structure outside the body wall called the scrotum. The male reproductive cells, the sperms, are produced in very large numbers (hundreds of millions). Observe the fig-13 of male reproductive system and findout parts essential for the transport of the sperm cells. Each testes has several lobules and each lobule contain several seminiferous tubules. They are small, highly coiled tubes and 80cm in legnth. Vasefferentia collect spermotozoa from the tubules. Vasefferentia forms epididymis. Here sperms are stored temporarily and moved into vasdeference then to uretra of penis and expels out of the body. One prostate, two cowper glands which are accessory glands in male reproducting system secretes a fluied called semen. This provide nutrients for sperm to keep alive and helps as a medium for the movement of sperms. The sperm cell is a flagellated structure with long tail. This helps them scrotum seminal ducts seminal vesicle prostate gland penis urethra epididymis testis fig-13: Male reproductory system to move towards the ovum. The development of the male reproductive organs is regulated by the male sex harmone called testosterone. You know that secondary sexual characters are also controlled by the male sex hormones, which are secreted by the testes. The production of sperms by the testes will begin when these events occur. Men produce sperm, from the age of about 13 or 14 years, and can go on doing so most of their lives, although their power to do so decreases as they grow older. Do you know? Some bacteria and other micro organisms have been found that are capable of changing the sex of the organism in which they grow. A species of wasp has lost its sexual ability to reproduce and has reverted to asexual mode. Free distribution by A.P. Government 123 Female reproductive system The two ovaries, where ova are formed, are located deep in the abdomen of female’s body. Observe the fig-14 of female reproductive system to know how it works. fallopian tube funnel ovary uterus cervix vagina fig-14: Female reproductory system The ova develop in tiny cellular structures called follicles, which at first look like cellular bubbles in the ovary. They are called graffian follicles. As a follicle grows, it develops a cavity filled with fluid. Each follicle contains a single ovum which is formed after the process of cell division (meiosis). When an ovum is mature, the follicle ruptures at the surface of the ovary and the tiny ovum is flushed out. This release of the egg or ovum is called ovulation. Generally the ovum enters the widened funnel of an oviduct (fallopian tube), a tube that extends from the neighbourhood of an ovary to the muscular, thick-walled uterus. Fertilization occurs as the ovum passes through the oviduct thus begins a new life, fertilization with sperm would lead to for,ation of a mass that might grow to form a baby. As the egg passes from the oviduct to the uterus, we encounter one of the most marvelous control mechanisms that man and other mammals possess. The uterus at the time of fertilization is beautifully adapted to receiving the developing embryo, providing it with food, and disposing of its wastes. A few days prior to this time, the uterus is in normal condition. Then it was small, its tissues were thin, and its supply of blood vessels was poor. Now that the fertilized egg, or zygote, is about to enter, the uterus enlarges and become much larger. Its inner wall is thick, soft, and moist with fluid; its blood supply is greatly increased. It is, so to speak, just waiting for an embryonic occupant. chorion umbilicalcord amnion placenta fig-15: Human embryo Shortly we shall return to this transformation and see how it occurs and how it is timed for the arrival of the fertilized ovum. But now let us see what the transformation does for the developing embryo. The fertilized ovum undergoes division. As it moves down the oviduct and finally attaches to the soft tissues of the uterus. Once attached, the embryo 124 X Class Reproduction - The generating system sinks into the soft inner uterine wall. Then certain cells of the embryo develop into membranous structures that help to nourish, protect, and support the developing embryo. During the development of the embryo, tiny finger like projections grow from the surface of the outer membrane called chorion into the soft tissues of the uterus. Gradually, small pools of rapidly moving blood around these finger like projections in the uterine wall. These tissues of the chorion and the adjacent part of the uterine tissue make up the placenta. Placenta is a tissue formed by the cells from the embryo and the mother. It is formed at around 12 weeks of pregnancy and becomes an important structure for nourishment of the embryo. Under normal conditions there is no direct flow of blood from mother to the young. The blood systems of the two are separated by thin membranes made up of cells that allow an exchange mainly by diffusion, of oxygen, carbon dioxide, nutrients, and waste materials. Another embryonic membrane, the amnion, grows around the embryo itself. The cavity within the amnion becomes filled with fluid called amniotic fluid. The embryo develops in this fluid-filled cavity, which keeps it moist and protects it from minor mechanical injury. Another membrane called allantois, which originates from the digestive canal of the embryo forms the major part of a tube like structure called umbilical cord. It contains the very important blood vessels that connect the embryo with the placenta. Thus the embryo develops until it is ready to be born. From the third month of pregnancy the embryo is called foetus. Pregnancy lasts, on an average, 9 months, or 280 days. This period is called gestation period. Let us observe the chart showing monthwise developmental stages of human embryo Free distribution by A.P. Government 125 fig-16: Developmental stages of human embryo Do you know? The average length of pregnancy varies by species: it is about 63 days for the domestic cat and dog, 330 days for the horse, 280 days for the cow, and 20-22 days for the rat and mouse. Child birth As pregnancy progress, the foetus of an embryo with certain characters grows and the uterus increases in diameter. Usually, at about the ninth month after fertilization. The head of the foetus is turned down towards the opening of the uterus. At birth, the head usually comes out first. Sometimes the feet come first; this makes the delivery more difficult. We still do not know much about the mechanism of child birth and how it is triggered. Childbirth begins when the muscle layers of the uterus starts to a rhythemic contract and relax, these actions are felt as labour pains. At first, muscular activity of the uterus is just strong enough to move the baby slowly toward the vagina the outer canal of the female reproductive tract. Generally, at this stage, the sac (amnion) around the baby breaks, and its fluid contents are released. This is a good sign that labour is well on its way. Then the contractions of the muscles become stronger and more frequent, and the baby is pushed through the vagina and into the outer world. The umbilical cord leading from the baby to the placenta, is tied off and cut by the doctor. (The small piece of cord remaining attached to the baby shrivels and falls off within a few days. The navel marks the place where it once entered the body.) After the birth of the baby, the muscular contractions of the uterus continue until they push out the tissues of the placenta, which are commonly called the “afterbirth.” During the last part of pregnancy, a watery lymph like fluid called ‘colostrum’ accumulates in the mammary glands, which have gradually been enlarging and undergoing a transformation. For the first few days after the baby is born, the mammary glands secrete only colostrum. It is very important to feed this to the new born baby. It helps in developing the immune system of the child. umbellical cord place
nta amnoin cervix baby fluid fig-17: Shortly before birth 126 X Class Reproduction - The generating system Do you know? Need for sexual reproduction Asexual reproduction as we have studied produce organisms which are normally copies of the single parent. Sexual reproduction would require two parents and organism produced would have a combination of characters of both parents. Asexual reproduction appears to be more efficient as only one parent is required and no time or energy is spent in finding a mate. But sexual reproduction helps organisms to develop characters that would be help them to adapt better to their surroundings. Think of the paramoecium mentioned in the begining of the chapter. When compare with animals sexual reproduction is less complex in most flowering plants. Let us study how it happens in them. Sexual reproduction in plants So far we know about nearly 275,000 species of flowering plants. With a few exceptions, all of them give rise to seeds enclosed in fruits. Most of the plants you are familiar with are mostly flowering plants. Their characters are quite remarkable. The plant size range from trees weighing many tonns to tiny water plants about the size of a rice grain. A sal tree growing in the Himalyan moutains, a giant cactus in the Sahara desert, an orchid plant on the branch of a jungle tree-all are flowering plants. Now let us examine the essentials parts of sexual reproduction in flowering plants. Flower - The reproductive part The reproductive parts of flowering plants are located in the flower. You have already studied the different parts of the flower- sepals, petals, stamens and carpels. The reproductive parts of the flower which possess the sex cells or germ cells called stamens and carpels. • What function do you think is served by petals and sepals? • Draw the diagram of the flower that you collect and label the parts shown and write their functions. Flowers having either stamens or carpels are called unisexual like that of fig-18: Structure of flower Free distribution by A.P. Government 127 bottle gourd and papaya. Flowers having both the stamen and carpel are bisexual like Datura. Stamens (male portion called androecium) produce male sex cells in the pollen grain. Carpels (female portions called Gynoecium) produce female sex cells in ovules inside ovaries. Carpels have three main parts, one to receive the pollen called as stigma, one for passage of compatible male sex cells called the style and the part where fusion of male and female sex cells occur to form zygote, is the ovary. The plants having flowers where male reproductive cells of stamen of the flower fertilise the female reproductive cell of the carpel of the same flower is called self-pollination. We can see this type of pollination in plants like those of the pea family. Try to find out some other plants that are self-pollinating types. Are there any observable characters that help you to find out whether a plant is self-pollinating type or not? The illustrations given here will help you. If anthers are present below the stigma of the carpel. • How does the male reproductive cells fertilise the female reproductive cells in flowers of such plants? You have studied in earlier classes how birds and insects help plants as agents of pollination. What happens in plants that carry the female reproductive structure or the male reproductive structure borne in separate flowers? Remember the flowers of bottle gourd you studied in earlier classes. Do you know? Darwin in1876 showed that plants when isolated had the greatest tendency to self-fertilize while when surrounded by varieties of the same flower, they readily cross fertilize. In cases where male cells of flower of a plant fertilise the other female flower on the same or different plant of the same group, this type of pollination is called cross pollination. Observe fig-21 showing plant carpel with pollen on stigma and pollen tube running down. Do you know what is self-pollination? Let us now observe some smaller parts that are involved in the process of reproduction in plants. The male reproductive part or the stamen consists of some sac like structures at its head bearing small ball like structures. We can easily observe these structures called pollen with the help of hand lens. The pollen grain reach the female reproductive part and fertilize the egg to form a zygote. 128 X Class Reproduction - The generating system Activity-2 Observation of pollen grain Take a slide and put a few drops of water on it. Now take any flower like hibiscus, tridax, marrigold, etc. Tap the anther over the drop of water. You will see small dot like structures in water. These are pollen grains. Observe these first under hand lens then under a compound microscope. You may also see a permanent slide of pollen grain from your lab. Observe under microscope. Make a drawing of what you observe and compare with the given diagram. • How many cells are present in the pollen grain? pollen grain The given diagram shows two nuclei. Do you think they may have formed if we assume that pollen grain may have started as a single cell stage? The pollen grain germinates only on the stigma. What happens then? Inorder to find out the remaining process we must look into the structure of the ovule. Structure of the ovule An ovule is an egg-shaped structure attached by a stalk to the inner side of the ovary. Depending upon the species of plant involved, an ovary may have one, two, several, or even hundreds of ovules. At the center of each ovule is a microscopic embryo sac filled with food and water. The embryo sac is composed of gametophyte cells. pollen tube nucleus fig-19: Pollen grain stigma style ovary ovule gametophyte cells embryo sac fig-20: Structure of ovule The majority of flowering plants have an embryo sac consisting of seven cells and eight nuclei. Two of which are important to our discussion. One is a large central cell containing two nuclei. These are called polar nuclei. The other cell is the egg. It is located at the end of the embryo sac closest to the opening through which the pollen tube enters. Cells on the furface of the stigma secretes a sticky nutrient fluid contains sugars and other substances. This will help the pollengrain to germinate. Then it forms pollen tube. It bears to nuclei. Soon after the tip of the pollen tube enters the embryo sac, the end of the tube ruptures and releases the two sperms into the embryo sac. One of the two sperms fuses with the egg to form a zygote. By the time the egg cell has been fertilized, the two polar nuclei combine to form a single fusion nucleus. Now the second sperm deposited in the embryo Free distribution by A.P. Government 129 sac by the pollen tube moves to the center and unites with the fusion nucleus. The zygote will develop into an embryonic plant within the ovule. Fertilization of the fusion nucleus stimulates the formation of a new tissue the endosperm. In which, food materials are stored as development of the ovule proceeds. stigma pollen tube style ovary integuments ovule gametophyte cells central cell antipodals polar nuclei synergids egg cell embryo sac fig-22: Female gametophyte fig-21: Fertilisation Union of one sperm with the egg, and the second sperm with the fusion nucleus is called double fertilization. As far as we know, double fertilization occurs only in flowering plants. After double fertilization, the ovule increases in size rapidly as a result of the formation of endosperm tissue by mitosis and the development of the new embryo. The embryo consists of one or more cotyledons an epicotyl and a hypocotyl. Both the epicotyl and hypocotyl are parts of a rod like axis attached to the cotyledons. The cotyledons digest and absorb the endosperm and make the stored food available for the growth of the epicotyl and hypocotyl. The cotyledons of some flowering plants, beans for example, digest, absorb, and store the foods from the endosperm as the ovule is maturing into a seed. As a consequence, the cotyledons become greatly enlarged because of stored food and the endosperm disappears more or less completely. Many other flowering plants (such as corn or castor bean), the endosperm tissue continues to grow as the ovule matures into a seed. After fertilisation, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is generally converted into a seed. The ovary grows rapidly and ripens to form the fruit. Meanwhile the other floral parts may shrivel and fall off. 130 X Class Reproduction - The generating system • Which floral parts is may be seen in a fruit? The seed produced after fertilisation contains the future plant or embryo that develops into a seedling under appropriate conditions. The process is called germination. Activity-3 Seed germination Soak few groundnut or bengal gram (chana) seeds overnight. Drain the excess water and cover the seeds with wet cloth. Leave them for a day. Keep sprinkling water at regular intervals so that they do not dry up. Open the seeds carefully and observe the parts, compare with figure to identify the parts. • How cotyledons are usefull for the plant? cotyledons plumule radicle Fig-23: Seed germination Observe the life cycle of plant as a whole in the following diagram. mature plant fertilization pollination zygote embryo simple fruit seed seedling germination fig-24: Life cycle of flowering plant Do you know? In sexually reproducing organisms useually single fertilization gives rise to zygote. In plants there occurs a second fertilization giving rise to a nutritive tissue that provides nutrition to the baby plant which develops from the zygote. The pollen grain has two cells. In one of its cells called a tube cell, there are two nuclei. They travel down Free distribution by A.P. Government 131 through the stigma and style to the ovary. One of the nuclei fertilizes the egg to form zygote and the other nucleus fertilizes fusion cucleus to form an endospe
rm which provides food to the baby plant. This is called double fertilization. Cell division and continuation of life Continuation of life starts from cells either those of the general body or the sex cells (gametes). Virchow (1821–1902) a proponent of cell theory is given the credit for the phrase Omnis cellula de cellula, or cells arise from pre-existing cells, indicates the importance of cell division in the creation of new cells. In 1852 a German scientist, Robert Remak, published his observations on cell division, based on his observations of embryos. This was one of the first attempts to understand the mechanism of cell division. He stated that binary fission of cells was the means of reproduction of animal cells. What happens during cell division could only be understood better when scientists came to know what is present inside the nucleus of the cell. fig-25: Walther flamming In 1879 Walther Flemming (1843–1905) examined many kinds of animal and plant cells and selected those that showed division. He reported from his observations of such cells that there were string like structures in the nucleus which split longitudinally during cell division. He named such a process of division as mitosis (mitos- means fine threads) as the dividing structures resembled threads. He made a meticulous observation and made sketches and observed that there were a sequence of events in the process of division. A decade later these thread like structures were named as chromosome (coloured bodies) as repeatedly in efforts to see them scientists were trying to use dyes to stain the nucleus and found that these structures were stained most often. His most important discovery was chromosomes appear double in nature. Wilhelm Roux (1850-1924) proposed that chromosomes carried a different set of heritable elements and longitudinal splitting observed by Flemming, ensured the equal division of these elements. Combined with the rediscovery of Gregor Mendel’s 1866 paper on heritable elements in peas, these results highlighted the central role of the chromosomes in carrying heritable material (or genetic material). In cell division the cell divides into two halves with equal number of chromosomes which are similar to parent cell and are diploid in nature. But the chromosomes number always remained the same. Biologists 132 X Class Reproduction - The generating system also began to wonder about this. When cells divide, the daughter always have the same number of chromosomes as the parent cell. Let us assume that cell division is always preceded by mitosis. In case of man egg cells and sperm cells like other cells, must contain 46 chromosomes. But if this were so, then the union of egg nucleus and sperm nucleus , which takes place during fertilization would produce a total of 92 chromosomes in zygote. If it continues this would be 184, 368 and so on. But the situation is not like that. 1. 2. August Weiseman (1834-1914) a biologist hypothesised that In successive generations, individuals of the same species have the same number of chromosomes. In successive cell division the number of chromosomes always remain constant. fig-26: August weisman Do you know? August Weiseman was a scientist with poor eye sight, it was difficult for him to use a microscope to study cells. But there were other things that he could do. Advancement of science is not only possible by mere collection of data. Someone must think, analyse and interpret the data. August Weiseman’s poor eyesight forced him to spend time thinking. Think how great he was! The scheme of mitotic division was confirmed in 1904 by Theodor Boveri (1862–1915). The chemical nature of the genetic material was determined after a series of experiments over the next fifty years, bone muscle skin Two kinds of cell division in the life of an individual. The chromosome numbers 2n and n are respectively the number of chromosomes following mitosis (2n) and half the number (n) following meiosis - the type of division predicted by Weisman. nerve gland blood other cells sperm egg fertilised egg immature reproductive cell fig-27: Cell division sperm egg Free distribution by A.P. Government 133 culminating in the determination of its structure the deoxy ribonucleic acid (DNA) in 1953 by James Watson(DNA) and Francis Crick. Scientists proved that mitosis takes place in all body cells which retains same number of chromosomes. Meiotic division takes place in sex cells where the chromosome number is halved. Observe the following flow chart. Cell division in Human beings We know that cell as the structural and functional units of life of any organism. In all organisms the cell divide and form new cells. The process of cell divisoin is same in unicellular organisms and highly evolved multicellular organisms like human being. Cell division is the process that transforms a human fertilized egg into a baby in nine months and into an adult in the next 20 years. Cell division and function in a multicellular organism is highly regulated. It occurs only when there is a need for it. Cells in some organs, such as heart and brain of an individual never divide. On the other hand bone marrow cells actively divide to produce red blood cells, which have a short life span in the body. For example, if you cut your finger and bleed, soon a blood clot forms to stop the bleeding. This brings in various chemicals to the site that stimulate skin cells to divide and heal the wound. Cell division ceases as the wound is completely healed. In contrast, cancer cells do not respond to such growth regulating factors and continual dividing at the expense of normal cells, thus ultimately killing the host. So it becomes important to understand the processes involved in cell division. The cell cycle will help us understand this better. G2 (3.5 hrs) M (1 hr) M G2 S G1 S (10.5 hrs) G1 (10.5 hrs) fig-28: Interphaace Cell cycle The process of cell division is called ‘Mitosis’, which is completed in 40 to 60 minutes (this is the time of active division). The period between two cell divisions is called ‘Interphase’. This is actually the period when the genetic material makes its copy so that it is equally distributed to the daughter cells during mitosis. Interphase can be divided into three phases. G1 phase: This is the linking period between the completion of mitosis and 134 X Class Reproduction - The generating system the beginning of DNA replication (Gap 1 phase). The cell seize increase during this period. S phase: This is the period of DNA synthesis (Synthesis phase) leading duplication of chromosomes. G2 phase: This is the time between the end of DNA replication and the beginning of mitosis.(Gap 2 phase). Cell organells divide and prepare chromosome for mitosis. M phase: This is mitotic cell division phase. To understand the functional relationship between these phases, Potu Rao and Johnson (see annexure) conducted some experiments using the cell fusion technique. That is combining two cells in experimental conditions. With this cell fusion technique Johnson and Potu Rao revealed for the first time the structure of interphase(GI, S and G2) chromosomes that are not ordinarily visible under the microscope. They provided evidence on progression of cells through the cell cycle in sequential unidirectional and controlled way by a series of chemical signals that can diffuse freely between nucleus and cytoplasm. These experiments are considered to be a ‘mile stone’ in the cell cycle studies. Activity-4 Observe different stages of mitotic cell division Take permanent slides which shows different stages of mitotic cell division from your lab kit. Observe carefully under microscope. Draw diagrams what you observe, and compare your observations with the following chart. Division of cytoplasm is called Cytokinesis which finally brings about formation of two daughter cells. While observing cells in tissues undergoing division, it is not easy to differentiate different stages of division. Prophase Prophase Metaphase Anaphase Telophase fig-29: Mitosis Free distribution by A.P. Government 135 Stage 1. Prophase Table-1: Mitosis Description 1. Chromosomes contract, spiral and becomevisible even in light microscope and nucleoli become smaller (material to chromosomes). 2. Chromosomes split lengthwise to form chromatids, connected by centromeres. 3. Nuclear membrane breaks down. 4. Centrosome, containing rod-like centrioles, divide and form ends of spindle (probably animal cells only). (Note: No pairing of chromosomes as in meiosis). 2. Metaphase 1. Chromosomes move to spindle equator, centromeres attached to spindle fibres. 2. Centromeres split, separating the chromatids. 3. Anaphase 1. Spindle fibres attached to centromeres contract, pulling chromatids towards poles 4. Telophase 1. Chromatids elongate, become invisible, (replication at this stage to become chromosomes). 2. Nuclear membranes form round daughter nuclei. 3. Cell membrane pinches in to form daughter cells (animals) or new cell wall material becomes laid down across spindle equator (plants) 4. Nucleus divides into two and division of cytoplasm starts. Process of meiosis Unlike mitosis which is a continuous process for division in most cells. Meiosis occurs only during the formation of gametes in sexual reproduction. Meiosis has two phases. During the first phase of meiosis the parent cell (containing two sets of chromosomes) divides twice, though the chromosomes divide only once. The second phase of meiosis is similar to noramal mitosis, but chromosomes do not duplicate, more over the Prophase 1 Metaphse 1 fig-30: Meiosis Anaphase 1 Telophase 1 New cells 136 X Class Reproduction - The generating system chromosomes are distributed equally to each cell. Thus the four daughter cells have just half the number of chromosomes of the parent cell. These are haploid (containing only one set of chromosome). Thus this division is also called reduction division. You will learn more about this in further classes
. • What differences do you find in mitosis and meiosis? Write in a tabular form. • What would happen if the gamets do not have half the chromosome number as the skin parent? • How would it affect the progeny formed by sexual reproduction? Reproductive health • Why did the government of India fixed the legal marriage age of boys (21 years) and girls (18 years)? • Do you feel that it is a social responsibility to control birth after having one or two children? • What do you understand by the term ‘Healthy Society’? • Will you encourage child marriage? Why? As we have seen, the process of sexual maturation is gradual, and takes place while general body growth is still going on. Therefore, some degree of sexual maturation does not necessarily mean that the body or the mind is ready for sexual acts. Further, it is not fit for having and bringing up children. How do we decide if the body or the mind is ready for this major responsibility? All of us are under many different kinds of pressures about these issues. There can be pressure from our friends for participating in many activities, whether we really want or not. There can be pressure from families to get married and start having children. There can be pressure from government and voluntary organisations to avoid having children. In these situations, making right choices is important. In the lesson why do we fall ill, we learnt that the diseases can be transmitted from person to person in a variety of ways. Since the sexual act is a very intimate connection of bodies, it is not surprising that many diseases can be sexually transmitted. These include bacterial infections such as Gonorrhoea and syphilis, and viral infections such as AIDS (Acquired Immuno Deficiency Syndrome). • What is the virus which causes AIDS? These diseases spread by unsafe sexual contacts, using infected devices, infected blood transfusion, from an infected mother to child. Free distribution by A.P. Government fig-31: Red ribbon 1st December AIDS Day 137 It is very sad to say Andhra Pradesh has the highest number of HIV positive patients in the country. According to official statistics, the state had 24 lakh HIV positive patients in the country during 2011-12. Maharashtra, Karnataka are followed by Andhra Pradesh. Officials said that one in every 300 adults is suffering from HIV elsewhere. The prevalence of HIV is 1.07 percent among males and 0.73 among female in the state, which again is higher than other states. Its prevalence among adults (15-49 years) 0.90 percent, pregnant women 1.22 percent in Andhra Pradesh. Illiteracy, poor health, unemployment, migration, non-traditional sex practise, unethical contacts and trafficking are some of the factors contributing to the spread of HIV in the state, according to experts. The government established Anti Retroviral Therapy (ART centres) to supply medicine to HIV patients. Medical and health, family health departments AIDS control projects implementing various programmes like ASHA (Accredited Social Health Activist), Red Ribbon Express, etc., to create awareness in society on the risks and symptoms of AIDS. • Invite local health worker to your school and discuss about HIV and its impact on society. Social discrimination against AIDS patients is also a social evil. Can you support this? Why? If we follow the simple life styles as cited below one could avoid • many sexually transmitted diseases. • Avoid sex with unknown partners/multiple partners • Even though contraceptives are available it is better follow ethical and • healthy life practices. In case of doubt, go to a qualified doctor for early detection and get complete treatment if diagnosed with disease. Birth control methods The sexual act always has potential to lead to pregnancy. Pregnancy will make major demands on the body and the mind of the woman, and if she is not ready for it, her health will be adversely affected. Therefore, many ways have been devised to avoid pregnancy. The prevention of pregnancy in women by preventing fertilisation is called contraception. Any device or chemical (drug) which prevents pregnancy in woman is called a contraceptive. The birth control methods can be of various types and can be used by any of the partners as preferable. Physical devices such as condoms and diaphragm (cap) are used. This 138 X Class Reproduction - The generating system prevents reaching of sperms to ova for fertilisation. This device not only prevents fertilisation but also transmitting some sexually transmitted diseases (STD) like gonorrhoea, syphilis and AIDS. No other method of contraception provides protection against sexually transmitted diseases. Chemicals in the forms of pills are induced either orally or inserting into female reproductive organ vagina. It contains hormones which stop the ovaries from releasing ovum into the oviducts. Now a days pills for males are also available. These pills kill the sperms and hence are called spermicides. blood supply vasdeferens epididymis testis small incision copper - T vasectomy - cut ends of vas deferens are sealed fig-32: Birth control methods cauterised tied and cut tubectomy - cut ends of follopian tubes are sealed banded The use of intra-uterine device called copper-T, loop etc. are also very effective in preventing pregnancy. If a woman uses a copper-T as a method of contraception for avoiding unwanted pregnancies, they cannot protect her from acquiring sexually transmitted diseases. Surgical methods of birth control are available for males as well as females. In males a small portion of vas deferens (sperm ducts) is removed by surgical operation and both ends are tied properly. This method is called vasectomy. In females a small portion of oviducts (fallopian tube) is removed by surgical operation and the cut ends are tied. This prevents the ovum from entering into the oviducts. This method is called tubectomy. Fighting against social ills Teenage motherhood We have studied how complicated the process of reproduction is. Child birth is even more complicated. Understanding it and getting prepared for it needs maturity of the mind and body. Thus a girl only after 18 years of age can be said to be prepared for the same. Most of the times this age is also dangerous to the girl. According to the department of family welafare 21% of teenage mothers die during delivery. So girls below 18 years of age should not be marry. Stop female foeticide Who knows today’s girl child may become a great scientist, a famous doctor, a top class engineer, a dedicated administrative officer, a world Free distribution by A.P. Government 139 renowned economist, a wonderful teacher of an unmatched world leader of tomorrow. Stop female foeticide! Save the girl child. Due to reckless female foeticide the male female child sex ratio is declining at an alarming rate in some sections of our society. Our government has already enacted laws to ban on determination of sex of foetuses. In spite of laws it’s a social responsibility of us to prevent female foeticide. • Why doctors are prohibited to do sex determination through altrasound scanning for pregnent women? We know that if health is lost, everything is lost. It’s our responsibility to be healthy and to make others realise the importance of health. Sound body is to sound mind. To be an ideal citizen of India we should have knowledge of reproductive health not only to control high population growth but to create a healthy society. Key words Progeny, cyst, fragmentation, regeneration, vegetative propagation, artificial propagation, parthenogenesis, cutting, layering, grafting, stock, scion, desirable characters, tissue culture, amniotic fluid, placenta, umbilical cord, mitosis, meiosis, chromatid, chromosome, foeticide, HIV-AIDS, vasectomy, tubectomy. What we have learnt • Reproduction is necessary for perpetuation and continuation of life. • Reproduction is of two types keeping in view of fusion of gametes- Sexual and Asexual. • • • In sexual reproduction only half of each parent’s chromosomes are passed to the next generation. Fission, budding, fragmentation, regeneration, spore formation are the ways of asexual reproduction. Several plants may be grown from vegetative parts like stems, roots, leaves etc and is called vegetative propagation. • Vegetative propagation may be natural or man made. It has got some economic importance. • • • In grafting we can acquire desirable characters of plants. Tissue culture is a modern technique of growing plants. It helps to grow more plants in less time and place. Sexual reproduction in higher animals is through specialised organs, distinctively male and female reproductive systems. • Cells divide for growth of the individual to repair and replace the worn out cells and also for the formation of gametes. 140 X Class Reproduction - The generating system • Cell division is of two types-a) Mitosis-or somatic cell division B) meiosis-or reproductive cell division. The cell of the body may either be somatic cells that constitute the general body of the organism or germ cells that take part in formation of gametes. • • G-1, G-2, S and M are the stages in a cell cyclic which occur in a manner. The longest phase is the synthesis phase in cell cycle where duplication of genetic material takes place. • • At the end of mitosis two daughter cells are formed with the number of chromosomes same as that of their parents. It runs through Prophase, Anaphase Metaphase and Telophase. • Division of cytoplasm is called Cytokinesis. During meiosis the parent cell divides twice and four daughter cells are formed. • • Reproductive health is important to possess sound mind in a sound body. • One should be aware of the facts related to transmission of sexually transmitted diseases. There is no cure for AIDS. Prevention is the only way to avoid it. • • Now a days various methods of contraception are available to control child birth. • It is our responsibility to build a healthy society. • Determinat
ion of sex before birth is illegal. • Stop female foeticide. Improve your learning 1. Why do fish and frog produce a huge number of eggs each year?(AS1) 2. Give examples and explain what is meant by external fertilisation?(AS1) 3. Write differences between.(AS1) a) asexual reproduction - sexual reproduction b) stamen-carple 4. Explain the process of fertilisation in plants.(AS1) 5. What are the different modes of asexual reproduction? Cite them with examples.(AS1) 6. In what ways does sexual reproduction differs from asexual one? State at least three reasons.(AS1) 7. How are sperm cells adapted for their function?(AS1) 8. The menstrual cycle prepares the uterus for a fertilised egg. How long is an average menstrual cycle from start to finish?(AS1) 9. When the foetus is growing inside the uterus it needs nutrients. What provides these nutrients?(AS1) 10. What does the mother’s blood take away from the baby and into the placenta?(AS1) 11. What is the job of the amniotic sac?(AS1) 12. What are the advantages of sexual reproduction?(AS1) 13. How does reproduction help in providing stability to population of species?(AS1) 14. Write the differences between mitosis and meiosis.(AS1) 15. What happens to the wall of the uterus during menstruation?(AS2) 16. “All unicellular organisms undergo only mitotic cell division during favourable conditions”- Do you support this statement? Why?(AS2) Free distribution by A.P. Government 141 17. Vicky’s father wants to grow a single plant having two desirable characters colourful flowers and big fruits What method will you suggest him and why?(AS3) 18. Uproot an onion plant and take a thin section of its root tip. Stain it and observe under microscope. Draw as you see and identify the stages of the cell division.(AS3) 19. Visit a nearby village and collect information how farmers grow sugarcane, flowering plants like chrysanthamum, primerose and vegetables like stem tubers, plump gourd (dondakaya) etc. Make a report and submit in class.(AS4) 20. Collect information from school library or using internet what vegetative methods are followed in your district as well as in your state to propagate various plants of economic importance. Represent it in a graph.(AS4) 21. Make a flow chart to show the cell cycle and explain cell division describing different stages of mitosis.(AS4) 22. Draw neat labelled diagrams of male and female reproductive system of plant.(AS4) 23. Observe the following part of a flowering plant prepare a note.(AS5) 24. Prepare a flow chart to explain the process of sexual reproduction in plants.(AS5) 25. Draw a neatly labled diagram to explain plant fertilisation. Write few points on pollen grain.(AS5) 26. What would be the consequences if there is no meiosis in organisms that reproduce sexually?(AS6) Q.No.23 27. How will you appreciate cell division that helps in perpetuation of life? (AS6) stigma style ovary ovule gametophyte cells embryo sac 28. What precautions will you take to keep away from various sexually transmitted diseases?(AS7) Choose the correct answer 1. The part of the female reproductive system that produces the eggs? ( a) Ovary b) Epididymis c) Cervix d) Fallopian tube 2. The term that we use to describe a sperm cell fusing with an egg cell? a) Fragmentation b) Fermentation c) Fertilisation d) Fusion 3. Which part of the male reproductive system produces (human) the sperm cells? a) Vasdiference b) Epididymis c) Bladder d) Scrotum ( ( ) ) ) 4. How does the sperm break through the egg cell membrane? Choose the option you think is right. a) Tears a hole in the membrane b) Dissolves the membrane with chemicals ( c) Bites through the membrane with teeth d) Squeezes through gaps in the membrane 5. Why are egg cells larger than sperm cells? Choose the option you think is right. ( ) ) a) Egg cells have more cells in them b) Have food store to help growth after fertilisation c) Have thicker cell membranes d) Have larger nuclei 142 X Class Reproduction - The generating system 6. Which of these things will affect the way a foetus grows? Choose the option you think is right. ( ) a) Chemicals in cigarette smoke b) Alcohol c) Drugs d) All of the above 7. Which of the following is the correct sequence of steps in the human life cycle? Choose the right option. ( ) a) Babyhood, childhood, adolescence, adulthood b) Childhood, babyhood, adulthood, adolescence c) Adolescence, babyhood, adulthood, childhood d) None of these Annexure Dr. Potu Narasimha Rao, a renowned scholar and an eminent cytologist came from a poor family in Muppalla village of Guntur district. He completed his graduation in Agriculture and did his MS at IARI, New Delhi. Later, he went to USA for research. He worked on the cytogenetics of tobacco plant. During his research, a cell line called Hela, isolated from a human tumour was established in 1952 and received his PhD in 1963. He switched his attention from plant cytogenetic to the field of cancer cells. He conducted research in cell kinematics and studied extensively on the ‘triggering factor’ of cell division i.e mitosis. Dr. Potu Narasimha Rao He found that human cells either normal or cancer cells in culture media usually divide every 20 to 24 hours. But actually normal mitosis is completed in 40 to 60 minutes. The period in between two cell divisions is called interphase. The interphase further consists of 3 phases G1, S and G2 phases. To understand the functional relationship between these phases of cell cycle. Dr.N.Rao and his research associate Dr.Johnson conducted experiments on cell fusion technique. His researches revealed that the cell cycle is sequential Unidirectional and controlled by a series of chemical signals. His experiments are considered to be a milestone in the cell cycle studies.This study threw a new hope of ray for the budding scientists to carry out researches on cell division. If you want to talk to this great scientist log in with email poturao@yahoo.com He La Cell Read the poem ‘Ma Mughe Ane Do’in your Hindi book. Collect information about Rashrtriya Kishore Swasthya Karakram (RKSK) Free distribution by A.P. Government 143 Chapter 7 Coordination in life processes Human body is a wonderful machine. It is a complicated structure than it appears. Did you ever imagine the complexity of your body? Different life processes in living organisms like respiration, digestion, blood circulation, excretion, nervous system etc., are inbuilt in our body at their specific places and carry out their specific functions in a coordinated manner. We have studied each of the processes in detail nearly in isolation except in the chapter on ‘Control and Coordination’. In this chapter, we shall go a step further to experience the complexsities involved and appreciate the wonderful integration in our life processes. Let’s recall the parts of the digestive canal or gut, that are involved in the digestive process where the food is broken down at different stages. • Write down the parts of the gut where the journey of food starts from mouth to anus. • Which type of life processes would be involved in the breakdown of food in the stomach? • If any of those processes fail to function, what affect would it have on our body? Every process is dependent on other to keep the body in good condition. To understand this concept we analyze how digestive system is coordinated with other systems as an example. We shall study the digestive system from feeling hungry to utilization of food, illustrating the inter connected processes going on in our body. 144 X Class Coordination in life processess Feeling Hungry • How do we know that we need food? Activity-1 Let us observe the following table. Identify and tick those options that you think makes you feel hungry. Table-1 Smell of food Taste of food Sight of food Being tired and exhausted Need of food Thought of food • What stimulates hunger? • What would be the result of stimulation of hunger? • Which system do you think would send the signals to make us realize that we are hungry? Well, a major cause for feeling hungry lies in the physiology of blood circulation. Levels of different substances are generally maintained in the blood mainly by our digestive system. One of the major substances is glucose. When its levels in the blood fall, we get hunger pangs in stomach. This again involves production of a series of proteins, some of which are hormones like Ghrelin. Secretion of the hormone “Ghrelin” in the stomach when it goes empty. Ghrelin is secreted from certain cells in the wall of the stomach. Hunger contractions (hunger pangs) start to occur in the stomach due to hunger generating signals that reach the brain from the stomach due to the secretion of this hormone. It is believed that the Diencephalon in fore brain and vagus nerve (10th cranial nerve) plays an important role in carrying these signals to the brain. Hunger pangs continue up to 30- 45 minutes. Increase in ghrelin levels results in sensation of hunger and motivation to consume food. • What kinds of controls are exercised during sensation of hunger? Are they hormonal or neural or both? • Can you suggest any 4 systems involved in the process of generating hunger sensation? When you feel your stomach is full and there is no need of food any Free distribution by A.P. Government 145 more, another hormone leptin is secreted that suppresses hunger. Usually we take food at a particular time. Every day we usually start feeling hungry at that time. You may have experienced this in your school during lunch hour. Outcome of sensation of hunger We find that different organ systems are involved in the digestive process. Let us find more about how the organ systems are involved. Feeling hungry leads us to consume food. Sometimes you may have often experienced that stale food is out rightly rejected even before intake. • What plays a major role to identify stale food? • If you are having a tasty dish do you think the smell of it increases your appetite? Taste and
smell are closely related Taste and smell are intimately entwined. This close relationship is most apparent in how we perceive the flavors of food. Anyone with severe cough and cold can not make out the difference in tastes of certain food items. Actually, what is really being affected is the flavor of the food, or the combination of taste and smell. That’s because only the taste, not the food odors, are being detected. Taste itself is focused on distinguishing chemicals that have a sweet, salty, sour, bitter, or umami taste (umami is Japanese for ‘savory’). However, interactions between the senses of taste and smell enhance our perceptions of the foods we eat. The following activity helps us to observe how are taste affected by the sense of smell. Activity-2 Chewing, cumin (fenugreek), sounf (fennel seeds), potato and apple First close your nose with your fingers. Pop in some zeera in your mouth and chew it for some time. After that, chew some sounf. Could you recognize the taste? How long has it taken to know the taste? After some time wash your mouth and repeat the activity by chewing a piece of an apple followed by a potato(remember to close your nose). • What are your observations? To conclude, if you want to taste the food material, the food should dissolve in saliva. On the other hand, we can taste the food that is in the form of liquid only. We know that different types of taste buds are present 146 X Class Coordination in life processess on the tongue. You have also learnt about different types of papillae (taste buds) on the tongue for different tastes in ninth class. Let us recall them. Only after the dissolved food enters into the cup like taste buds, the sense of taste is carried to the brain for analysis. Then only we will know the taste of the food material. • Could you know the taste of both or did it taste villate apillae foliate papillae fungiform papillae fig-1: Papillae on tongue the same? Why? When we smell, the air borne substances get dissolved in the watery film of nasal mucus. The chemoreceptors in nose are other wise called olfactory receptors which trigger signals in the form of nerve impulses to the brain where smell is detected. Similarly as we take food into our mouth the taste buds sends signals to the brain. Picking up the slight differences in smell the food tastes are identified in our brain. • What happens when we put a food material in our mouth? • Name the parts in the mouth that help us to taste food. Let’s find out more about the role of these parts. Activity-3 Take a pinch of asafoetida powder or garlic and rub it on hand kerchief or tissue paper. Close your eyes and smell it. Then try to identify taste of different types of food materials with the help of your friend. • Does garlic have a stronger scent than apple? How do you think the stronger scent affect your sensation of taste? • How many food materials you have identified correctly? • Write a few lines on relation between smell and taste? • Have you ever felt that a particular food is tasty just by looking at it? Sometimes mouth starts watering just by hearing the name like tamarind / lime / mango etc., Now let us summarize the result of the activities with the help of your answers. In general, we prefer the food material, which is attractive to our eyes, and flavor to nose, then we taste it. Therefore, when we eat, without our knowledge, we use our sight, nose and tongue for selecting food for ingestion. Free distribution by A.P. Government 147 Russian scientist Pavlov has conducted experiments and found that even the thought of food will water your mouth (conditioned reflexes). You have discussed about Pavlov experiment in the chapter animal behavior in class 9th. • Are there any other sensation that affect taste? • What happens to your taste sensation while sipping hot milk or tea? You may also find something more tastey when they are hot. While some others are relished cool. • What do you think could be the range of temperature for us to relish food items? Taste is something connected to the tongue and the palate Let us do a simple activity to see the role of different parts of the mouth in helping us to taste. Activity-4 Sugar crystals over the tongue Place some sugar crystals on your tongue keep your mouth opened and see that your tongue doesn’t touch the palate. Record the time from the moment you placed the crystals on your tongue till you got the taste by using stop watch. palate tongue Now repeat the test by placing the sugar crystals on the tongue and pressing it against the palate. Record the time from placing sugar crystals to getting the taste .Or put a drop of sugar solution on your tongue using a dropper. fig-2: Tongue and palate • Can we taste on dry tongue? • Which way helped you taste faster ? Why? Based on the above activity we know that taste can be identified easily when the tongue is pressed against the palate. As we know the tongue is sensory in function and contains taste buds. These taste buds are tiny papillae with an opening on top. Within them there are several taste sensitive cells. Any food substance when placed on the tongue gets dissolved in the saliva secreted by salivary glands in the mouth. When the tongue is pressed against the palate the food substance is pressed against the opening of the taste bud letting it to reach the taste cells and triggering taste signals. Finally the taste is recognized in the brain. 148 X Class Coordination in life processess Taste Nose Mouth Tongue Brain Olfactory receptors Salivary glands Taste buds • What do you think would happen if the salivary glands do not function in our mouth? • Suppose your taste buds were affected what would happen to your interest in having food? Mouth - the munching machine Would you be able to comfortably munch your food if you had lost some of your teeth? Activity-5 To show break down of food by using the model of chalkpiece kept in vinegar Break a piece of chalk into two halves. Crush one half to tiny pieces leaving the other as it is. Take two small mineral water bottles (1/2 ltr bottle) cut them into two equal halves and discard the upper portion. Now we have two beakers from the lower cut portion. Fill them half with vinegar and add the crushed chalk to one beaker and the other uncrushed half chalk to the other. Obseve them after half an hour or so. • Which one dissolved faster the crushed chalk or the whole one ? The above experiment tells us the need of mechanical crushing of food. Hence the food in the mouth has to be broken down into tiny pieces to increase the surface area for action of substances that aid in digestion. • How does this process of mechanical crushing go on in the mouth? • Which parts in the mouth are involved in this? • What are the systems involved in this process? You know that teeth helps in chewing food material. Let us know about different types of teeth in our mouth and how they helps in digestive process. Free distribution by A.P. Government 149 Activity-6 } molars (3) } } premolars (2) canine (1) inscissors (2) fig-3: Dentition Observe the model or chart of jaw, how are the teeth arranged? Are all the teeth similar in shape and size? Is there any relation between shape and function of the teeth? Dental formula explains the arrangement of teeth. On the basis of the figure try to guess what could be the the function of molars? You had studied in earlier classes inscissors have sharp edges, canines have sharp and pointed edges while molars and premolars have blunt and nearly flat surface. • What do you think could be the function of inscissors? • Which set of teeth help in grinding food? • Which set helps in tearing food? • What is your dental formula? Now fill up the following table with proper information based on the figure given here. Table-2 Type of teath Number Shape Function The circular muscles of the mouth enable the food to be pushed into the oral cavity and to be moved around. As the food cannot be swallowed directly the teeth grind, chew and shred it. This process is called mastication. For this purpose the surface muscles of the jaw help in biting and chewing actions, and move the jaw up, down, forward and backward during food mastication. You may have observed your lower jaw moving up and down as you chew food. The teeth help in cutting and grinding while tongue movements evenly spread out the food and help in mixing it with saliva. The muscles of the mouth enable the food to be pushed in the oral cavity and to be moved around. The fifth cranial nerve has been found to control the movement of muscles in the jaw. 150 X Class Coordination in life processess • Does the level of saliva secretion change due to presence of food in the mouth? • Can the process of chewing go on in the absence of saliva? • Does the saliva have any other roles to play? Let us find out the role of saliva. Activity-7 Action of saliva on flour (ata) Take a test tube half filled with water and add a pinch of flour to it. Shake the test tube well till the flour gets mixed. Take a few drops of this in a watch glass and test for the presence of starch by putting a drop of diluted tincture iodine in it. A blue black color confirms the presence of starch. Now again dissolve a pinch of flour into half filled water in a test tube. Now divide the mixture into two equal halves by transferring it to another test tube. Note that both the test tubes have the same amount of solution. Add a teaspoon of saliva to one of the test tube and mark it. Do not add anything in the other test tube. After some time (45minutes) add a drop of dilute Tincture Iodine solution to test tubes containing the solution. • Do you observe any change in the solutions? Why does the change occur? • Do you think the same process goes on in the mouth when food is taken? Under the action of autonomous nervous system saliva is secreted by three pairs of the salivary glands to moisten the food to make chewing and swallowing easier. As a result of che
wing, food forms into a slurry mass called ‘bolus’ that is transported into the oesophagus by the action of swallowing with the help of the tongue. The enzyme salivary amylase in the saliva breaks down the large starch molecule into smaller subunits usually into sugars. The mechanism for swallowing is also under nervous coordination and its control center is somewhere in the brain stem (medulla oblongata and others). During mastication food size becomes convenient to swallow. • What is the use of such an increase in surface area of food? • What about the nature of medium for salivary amylase to act on food component? • If we swallow food material directly without mastication what will happen? • Do you think the pH of our mouth changes? Free distribution by A.P. Government 151 Activity-8 Testing pH of mouth at intervals of one hour. Ask your chemistry teacher to give you a strip of pH paper with a colour chart. You can do this in your school by taking a small piece of the pH paper and touching it to your tongue. Match the colour with the colour chart and note the pH first. See to it that you are able to take some readings after having your food at lunch break. Compare your readings with that of your friend. Take at least 4 readings. You have to prepare your own table to record your observations. • What is the usual range of pH of your mouth? Acidic or basic? • Did you observe any change in pH after eating? What may have caused the change? • In what kind of pH do you think salivary amylase acts well? • Does the type of food have any role to play on the pH of our mouth? Test with different types of food as you eat them and check just after you have swallowed them. Do not hurry to complete the table. Take your own time. (pH beyond 7 is alkaline, pH- below 7 is acidic, pH 7 is neutral). Based on the above tests we know that the saliva secreted causes the medium to change to alkaline as it aids in action of enzyme, salivary amylase. fig-4: pH Do you know? • Why do we salivate during a nap of daytime? You have heard about Nocturnal animals, which are active during nights, but we are active during daytime and take rest at night. All the systems of our body are active in function during the time of our activity. Hence, man is a diurnal animal. Our digestive system is also active and ready to receive the food for digestion. If we sleep during daytime saliva oozes out of our mouth and wets the pillows. This will not happen during night time. We secrete 1-1.5 liters of saliva per day. • What are the different systems that contribute to the proper functioning of digestion in the mouth? • After the digestive process in the mouth where does the food move to? 152 X Class Coordination in life processess Travel of food through oesophagus The oesophagus receives the food pushed by the swallowing action of the mouth. • What are the systems that come into play for swallowing food? The following schematic representation shows some functional and stuctural attributes of oeasophagus. Observe it and answer the following questions. Walls secrete mucus slimmy substance Long tube upper end connects to pharynx lower end connects to the stomach passes food to stomach by wave like movement of walls (peristalsis) Oesophagus muscular and elastic walls carry on wave like movement by contraction and relaxation • What does the schematic diagram tell us about the oesophagus? • What kind of the tube is oesophagus? • How does mucus help in passage of food? Activity-9 Making a model of oesophagus to observe how bolus moves forward Take a piece of waste cycle tube and insert one or two potatoes into it. Lubricate the inner side of the tube with oil. In the same way smear oil over the potatoes. Insert oil coated potatoes in the tube. Now try to push the potatoes by squeezing the tube. • How do you squeeze the tube to make the potatoes pass through? • Do you think that the muscles in the wall of the oesophagus have to do something like this? • How did oil heip you in pushing the pototoes through the pipe? Peristaltic movement in oesophagus Look at the fig-6, which shows wave like movement of wall of oesophagus and observe the position of the food bolus. fig-5: Potato in cycle tube Free distribution by A.P. Government 153 • How did the position of the bolus change? • What is the similarity of movement of food illustrated in the diagram and the activity performed by you? epiglottis up oesophageal sphincter contracted relaxed muscles contracted muscles stomach bolus sphincter relaxed The walls of the food pipe secrete a slippery substance called mucus. Mucus lubricates and protects the oesophageal walls from damage. This helps the food bolus to slide down easily just as the oiled potatoes that move in the tube. Besides this, the saliva in the bolus also aids in easy movement of food, which moves into the stomach. The wall of the oesophagus is made up of two kinds of smooth muscles. The inner layer consists of circular muscles and the outer layer of longitudinal muscles. Contraction of the circular muscles results in narrowing of the oesophagus just behind the bolus. So the food is squeezed downwards. Contraction of the longitudinal muscles infront of the bolus widen the tube, this results in shortening of that particular part of the oesophagus. Contraction and relaxation of these muscles bring in a wave like motion that propels the food bolus into the stomach by the action called as “peristalsis” (you have studied about this in the chapter on nutrition). This is involuntary and under the control of autonomous nervous system. fig-6: Peristalic movement of bolus • What makes the movement of the food bolus in the oesophagus easy? Think why people are advised not to swallow food without chewing properly or do not eat in hurry. Stomach the mixer and digester • Why do you think the stomach is structured like a bag rather than a tube like oesophagus? • What sets such processes into action? When the food is in the oral cavity, the nerves in the cheek and tongue are stimulated. These carry messages in the form of nerve impulses-to the brain. These messages are transmitted from the brain, to the wall of the stomach, and stimulate the gastric glands to produce gastric juice. The walls of stomach secrete juice containing hydrochloric acid (HCl) Most of us may have experienced belching and the burning sensation after it. What do you think may have caused the burning sensation? These secretions are stimulated by the nervous system. The contraction of the 154 X Class Coordination in life processess stomach muscles squeeze and mix the food with the acids and juices of the stomach. These digestive juices turns the food into a smooth porridge like consistency called chyme. Some large protein molecules are also broken down here. • What stimulates stomach muscle into action? • What causes the stomach to churn and mix the food? As the process of digestion in the stomach nears completion, the contractions of the stomach decrease. What would be the reason? Which substance present in blood regulates contraction of stomach? This prompts the muscles, called as pyloric sphincter at the opening of the stomach and the first part of the small intestine or duodenum, to relax. This opens the pathway into duodenum releasing the partially digested food (chyme) in small quantities into the duodenum. • Why should only a small quantity of food be passed from stomach to duodenum? pyloric valve closed pyloric valve closed pyloric valve slightly opened. Propulsion: Peristaltic waves move food from one part to the other. Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. Retropulsion: Small amounts of chyme is pushed into the duodenum, simultaneously forcing most of it back into the stomach. fig-7: Peristalic movement in stomach Peristalsis involves the contraction of the muscle behind the food and the relaxation of the muscle in front of the food giving rise to a thrust that pushes the food forward through the digestive canal. A wave of contraction followed by relaxation in muscles helps in forward movement of food. • What is involved in bringing about peristalsis? • What is the direction of peristalsis (which end of the gut does it begin)? • What happens if the direction of peristalsis is reversed? Have you observed a ruminating cow/ buffalo under a tree or somewhere else? Carefully observe its neck and throat. Do you see something moving from its throat to mouth? After that, the cow or buffalo starts chewing. It is the bolus moving from a part near the stomach of the animal to its mouth. It is reverse peristalsis. Though it is a common process Free distribution by A.P. Government 155 in ruminants such as the cow, buffalo etc. that have an extra pouch in the stomach to store quickly swallowed food, in human beings it is mainly a protective mechanism to expell unwanted substances from the food canal. We observe that digestion of food starts from the mouth. While the food passes through the gut the food settles for some time for digestion at certain locations. So food does not move uniformly through the digestive system. Let us observe the time period. Table-3 Percentage 50% Total 100% Emptying of stomach Emptying of small intestine 2.5 to 3 hours 4 to 5 hours 2 .5 hours 30 to 40 hours (Transit through colon) (These are only averages. The movement of materials varies among individuals and time after different meals.) Our stomach is not like a bag with specific volume. It is like a pouch which is elastic in nature. the size of the stomach increases based on the food that we intake. Digestive juices are produced depending on the quantity of food material. If the stomach would produce same amount of digestive juices irrespective of the food quantity the walls of stomach would be destroyed. We also know that stomach secretes strong acids during digestion. The HCl secreted by the walls of the stomach is strong enough to digest the hard bones as well..Then how is th
e stomach protected from the secretions of its own acids. To understand this we will perform the following experiment. Lab Activity Take two similar green leaves. Grease one leaf with petroleum jelly leave the other free. Add 1 or 2 drops of some weak acid on both the leaves .Observe them after half an hour or so and write your observation in your note book. • Which leaf was affected by the acid? • What kind of change did you observe in the leaves? • What saved the other leaf from the effect of acid? Mucus secreted by some cells present in the walls of the stomach 156 X Class Coordination in life processess form a thin lining on the walls of the stomach. This counters the action of acid. The function of petroleum jelly can be compared to that of mucus lining the stomach walls. Hence the stomach is protected from damage being caused by the secretion of its own acids. Travel of food from the stomach to the intestine Now food is a soup like mixture when it leaves the stomach and enters the small intestine. When the food enters the intestine the acidic nature of the chyme initiates the production of hormones like secretin and cholecystokinin which stimulate pancreas, liver and walls of small intestine to secrete pancreatic juice, bile juice and succus entericus. The absorption of nutrients by villi in the small intestine is a very selective process. The walls of the intestine allows only tiny nutrient particles to pass through. • Why do you think small intestine is long and coiled? • What process is involved in this process of absorption? Activity-10 Paper tube and folded papers Take 1020cm size chart paper fold the chart paper and join both ends with gum to make a tube. Now take 2020cm chart paper and make a tube as mentioned the same above try to insert the big tube in small one. Can you? It is not possible. Now take another 2020cm chart paper fold the paper as many folds as possible. Now join both ends to make a folded paper tube. Try to insert folded tube in first tube. Can you? How is it possible? • Compare the area of the folded papers with that of the roll. Do you find any increase in the area ?If so try to find out the reasons? The inner surface of the small intestine contains thousands of finger like projections called villi. These villi increase the surface area so that the food retained in the folds can remain longer thereby enhancing absorption. • What systems do you think are working together here? fig-8: Paper tube epithelium network of blood mucus mucus gland lacteal (part of • Do you think those systems work together in the whole length of the digestive canal? Why/ Why not? fig-9: Schematic diagram of a villus Free distribution by A.P. Government 157 The digestive tract is unique among internal organs because it is exposed to a large variety of physiochemical stimuli from the external world in the form of ingested food. As a consequence, the intestine has developed a rich store of co ordinated movements of its muscular apparatus along with neural apparatus to ensure the appropriate mixing and propulsion of contents during digestion, absorption, and excretion. The neural apparatus of our digestive tract comprises of such a vast and complicated network of neurons that it has been nicknamed by scientists as the second brain! Research in this area is currently investigating how the second brain mediates the body’s immune response; after all, at least 70 percent of our immune system is aimed at the gut to expel and kill foreign invaders. Scientists are also working to find out how trillions of bacteria in the gut ‘communicate’ with the cells of gut nervous system. A deeper understanding of this mass of neural tissue, filled with important neurotransmitters, is revealing that it does much more than merely handle digestion or inflict the occasional nervous pang of hunger. The little brain in our inner yards, in connection with the big one in our skull, partly determines our mental state and plays key roles in certain diseases throughout the body. • Often you may have experienced that if you have tension for some reason you start having loose motions. What does this show us? Although its influence is far-reaching, the second brain is not the seat of any conscious thoughts or decision-making. Technically known as the enteric nervous system, the second brain consists of sheaths of neurons embedded in the walls of the long tube of our gut, or alimentary canal, which measures about nine meters end to end from the oesophagus to the anus. The second brain contains some 100 million neurons, more than in either the spinal cord or the peripheral nervous system. This multitude of neurons in the enteric nervous system enables us to “feel” the inner world of our gut and its contents. Stimulating and coordinating the breaking down of food, absorbing nutrients, and expelling of waste requires chemical processing, mechanical mixing and rhythmic muscle contractions that move everything down the line. Thus equipped with its own reflexes and senses, the second brain can control several gut functions often independently of the brain. Several scientists also believe that the system is a way too complicated to have evolved only to make sure things move through and out of our gut smoothly. 158 X Class Coordination in life processess Expulsion of wastes Explusion of wastes via blood through the kidneys, skin etc., are mainly salts, water and urea. Undigested food matter is expelled in the form of stool. • What moves out of the gut? • Two major pathways of waste expulsion are shown above. Which of the two do you think happens exclusively through the gut? Imagine you made a roll by wrapping a hand full of left over tea leaves in a tissue paper. Later you press the roll gently and open it. What did you observe? You find the tissue paper had absorbed the water from the tea leaves. Similarly when the unwanted waste material (stools / faeces) reach the large intestine. The peristaltic waves move the stool into the rectum. The left side of the colon acts like a storage tank of faeces. Water gets reabsorbed and the remaining wastes usually hard mass gets stored in the last part (Rectum) of the large intestine. This smelly yellowish faecal mass usually called as stool is later expelled out of the body through the anus. • What controls the exit of stools from the body? • Do you think the control is voluntary? Why / why not? There are two muscular layers helping the exit of stool. One that is under involuntary control and the other is under voluntary control. These muscular structures help in opening and closing of the aperture of the canal which are called as anal sphincter. rectum sphincter anus fig-10: Anal spincter • Did we have a sphincter in any other part of the digestive cannal? Where was it? Suppose a person has consumed more fluids than what the body actually need. How do think the extra fluid will be removed from the body? We have so far seen how several systems work together to bring about the process of digestion. Where does this process draw energy from to run smoothly? Free distribution by A.P. Government 159 • What is the fate of the digested substances that move into blood from the intestine? If energy has to be obtained from food it has to be oxidised. For this purpose respiration has to go on. During inhalation oxygen moves across the walls of the alveoli and enters the blood. From here it enters the red blood cells and gets distributed throughout the cells of our body. At the same time carbon dioxide from the blood moves into the alveoli of the lungs and breathed out during exhalation. Nutrients in the cells get oxidized and energy is released. • Where is the energy stored? • Which system do you think will remove the excess salts from our body? • What would be the path of salt removal from gut to the out side of our body? During respiration we breathe continually by inhaling and exhaling air. This is an involuntary process controlled by the medulla oblongata of the autonomous nerves system (ANS). During respiration the movement of inter costal muscles/diaphragm moves the ribs cage inflating and deflating the lungs. Air containing more of oxygen enters the blood stream through lungs. If the oxygen has to reach the tissues it has to be circulated through blood. How does this process go on? Hence the process of digestion which is a complex process that involves many organs and organ systems. Though digestion occurs in the food canal, co-ordination of respiration and blood circulation is necessary otherwise oxidation of food and transport of substances which is vital for energy releasing process will not take place. This may lead to the shut down of systems interdependent on each other. Key words Ghrelin, Leptin, Gustatory, Chemoreceptors, Papillae, Food bolus, Peristalsis, Chyme, Pyloric Sphincter, Villi, Medulla oblongata , Brain stem. What we have learnt • The food taken by us it has to be broken down into constituent substances for proper digestion, assimilation and energy releasing processes. 160 X Class Coordination in life processess • The human digestive system involves both the muscular and nervous systems. • A special nervous system that exists in the gut consist of nearly 100 billion nerves that coordinates the muscular activity ,blood flow,digestion and absorption of nutrients and other activities of the food canal (gastro intestinal tract). • The hormone Ghrelin secreted in the stomach is responsible for hunger generating sensations. Another hormone leptin that gets secreted suppresses hunger. • Taste can be identified easily only when the tongue is pressed against the palate. • Taste and smell are closely related. The chemoreceptors receptors present in the nose and and the tongue trigger signals in the form of nerve impulses to the brain where the smell and taste is detected. • The saliva secreted maintains an alkaline medium that aids in digestion of starch. Our mouth secretes acid as well ,this gives prote
ction to our mouth from harmful bacteria etc., Under the action of autonomous nervous system Saliva released by the salivary glands moistens the food to make chewing and swallowing easier. • The muscular and sensory organ in the oral cavity is the tongue which is not only gustatory in function but also performs different functions including, shifting and mixing the food in the oral cavity and swallowing. • The mechanism for swallowing is coordinated by the swallowing centre in the brain stem. • Contraction and relaxation of the muscles in the gut brings in a wave like motion that propels the food forward and is called peristalsis. This is a muscular wave that travels the entire length of the food canal. This is involuntary and under the control of autonomous nervous system as well as gut nervous system. • The muscular contractions of the stomach churns the food into a semiliquid substance known as chyme. Entry of chyme into the duodenum is regulated by a muscle called as the pyloric sphincter. • The strong acid (HCl) renders the pH in stomach acidic causing the protein digesting enzymes to swing into function. Juices secreted in the stomach breaks down the food into a smooth mixture called chyme. • • The mucus lining of the stomach protects it from damage by its own acids. • The coordination among the processes of digestion, respiration and circulation is necessary for utilization and oxidation of food and transport of the nutrients. Muscular and nervous control helps to carry out the processes in a regulated manner. Improve your learning 1. What do you meanby hungerpangs?(AS1) 2. What are the organ systems involved in digestion of food which we eat?(AS1) 3. Rafi said smell also increase our appetite can you support this statement. how?(AS1) 4. Write a note on peristalsis and sphincter function in stomach.(AS1) 5. Observe the given part of the digestive system. What is it? What is it’s role during digestion?(AS1) 6. Give reasons.(AS1) a) If we press tongue against the palate we can recognise taste easily. b) We can’t identify taste when food is very hot. Free distribution by A.P. Government Q.No: 5 161 c) If glucose level falls in blood we feel hungry. d) Small intestine is similar to a coiled pipe. e) Urination increases when we take lot of fluids f) The process of digestion goes on in a person whose central nervous system has been largerly affected 7. Write difference between the following.(AS1) a) bolus - chyme c) mastication - rumination b) small intestine - large intestine d) propulsion - retropulsion 8. How can you say that mouth is a munching machine?(AS1) 9. What is mastication? Explain the role of different sets of teeth in this process.(AS1) 10. During the journey of food from mouth to stomach through oesophagus. How muscular system coordinate in this process?(AS1) 11. Is there any reason for the intestine to be coiled with many folds.In what way it is helpful during the process of digestion?(AS1) 12. What is the function of peristalsis in these parts?(AS1) b) stomach a) oesophagus 13. How can you justify the enteric nervous system as the second brain of the gut?(AS1) 14. Rajesh feels hungry upon seeing food.Sheela says no to food as she is not hungry.What makes c) small intestine d) large intestine Rajesh hungry and what suppresses Sheelas hunger?(AS1) 15. How are taste and smell related?(AS1) 16. List out the sphincter muscles of the food canal you have observed and give a brief description?(AS1) 17. What experiment should you perform to understand action of saliva on flour? Explain it’s procedure and opparatus that you followed.(AS3) 18. What happens if salivary ducts are closed?(AS2) 19. If size and shape of small intestine is like oesophagus what will happen?(AS2) 20. Prepare a questionnaire to understand nervous coordination in digestion process.(AS2) 21. Suggest a simple experiment to prove the role of palate in recognizing taste.(AS3) 22. Collect information related to feeling hunger from your school library and prepare a note on it.(AS4) 23. Draw the block diagram showing sensation of taste from food material to brain.(AS5) 24. Draw a neatly labled diagram showing a peristaltic movement in oesophagus. Explain the importance of mucus on the walls of food pipe.(AS5) 25. Draw a schematic diagram of villus in small intestine. Explain how digestive system coordinate with circulatory system.(AS5) 26. The mere smell or sight of food stimulates hunger .Describe the process through a neat diagram?(AS5) 27. With the help of a diagram show the movement of food from mouth to the stomach.What muscles and nerves are involved in the movement of food and what is this action called as?(AS5) 28. Prepare a cartoon on Pavlov’s experiment with a suitable caption.(AS6) 29. How do you appreciate stomach as a churning machine .How does this coordination go on?(AS6) 30. There is a great variety in diversified life processes, express your feelings in the form a poem.(AS7) 31. Suggest any two important habitual actions to your friend while eating food, keeping in view of this chapter.(AS7) 162 X Class Coordination in life processess Fill in the blanks 1. 3:2:1:2 is the ratio of our dentition. Here 1 represents ____________ 2. Large protein molecule are broken down in _________ of digestive track. 3. __________ is the strong acid which is secreated during digestion. 4. Olfactory receptors present in ___________ trigger signals to brain. 5. pH of saliva is _________ in nature. 6. Fill in the blanks with suitable words given below. Fluctuation s of hormone (i)_____________ levels results in sensation of hunger and motivation of consuming food. When you feel your stomach is full and there is no need of food any more. Another hormone (ii)_____________ that gets secreted suppresses hunger. When we take food into the mouth it has to be chewed thoroughly. For this purpose the (iii)_____________ muscles help in chewing actions, while the (iv)_____________ muscles of the jaw moves the jaw up,down ,forward and backward during food mastication. The (v)_____________ nerve controls the muscles of the jaw. . Under the action of (vi)_____________ nervous system Saliva is released by the salivary glands moistens the food to make chewing and swallowing easier. The salivary (vii)_____________ in the saliva breaks down the starch into sugars. As a result of chewing the food is transported into the oesophagus by the action of swallowing which is coordinated by the swallowing centre in the (viii)_____________ and the (ix)_____________. The tongue which is gustatory recognizes the taste and (x)_____________ nerve plays an important role in sensation of taste. Choose the right ones. 1) leptin, grehlin gastrin secretin. 2) ghrelin leptin secretin gastrin. 3) deep muscles ,surface muscles ,circular muscles, striated muscles. 4) surface muscles, deep muscles, neck muscles, long muscle. 5) fifth cranial nerve ,second cranial nerve,fifth facial nerve, spinal nerve. 6) central nervous system, peripheral nervous system autonomous nervous system. 7) lipase, sucrase, galactase,amylase. 8) medulla oblongata, cerebrum , 8th spinal nerve,cranial nerve.7th cranial nerve. 9) Pons varoli, brain stem ,medulla oblongata, mid brain. 10)6th cranial nerve, 5th cranial nrve, 10th cranial nerve, optic nerve. Choose the correct answer 1. In which of the following situations you can taste quickly. a) Put sugar crystals on tongue c) Press the tongue slowly against the palate d) Swallow directly without grainding and shreding b) Put sugar solution on tongue. ( ) 2. Peristalsis is because of a) Contraction of longitudinal muscles. c) Under control of autonomous nervous system d) Digestive secretions. Free distribution by A.P. Government ( b) Contraction of circular muscles ) 163 3. Sphincter a) Cardiac that helps in opening of stomach into duodenum b) Pyloric c) Anal d) Gastric 4. Glucose and amino acids are absorbed through the following part of villus. a) epithelial cells b) blood capillary c) lymphatic vessel d) all 5. The regian in brain portion that controls hunger signals ( ( ( a) medulla b) diencephalon c) cerebrum d) mid brain 6. Human organism is an internal combustion machine because of ) ) ) ) ( b) liberate CO2 during respiration d) secrete powerful digestive juices a) assimillation of energy from food c) expel waste food at the end state digestion Annexure Historical evidence of human digestion that led to discovery of other truths The man with a window in his stomach. One fine morning at Fort Mackinac on the upper Michigan peninsula a 19 year Voyageur Alex St.Martin had a gun wound in his stomach that was fired accidentally. The wound perforated the abdominal wall and stomach with profuse bleeding. Dr.Beaumont the army surgeon was called on to attend the wounded man. Dr. Beaumont cleaned the wound and pushed the protruding portions of lungs and stomach back into the cavity and dressed the wound. Dr. Beaumont was surprised to see St. Martin alive the next day as he never expected so. With his medical expertise Dr. Beaumont treated the wound and did his best to extend his life. When the wound got healed completely, the stomach had fused with the body wall leaving a hole. Part of the wound formed a small flap that resembled a natural valve. This allowed Dr.Beaumont to draw out fluids from Martin’s stomach for testing. Dr.Beaumont turned St.Martin to the left side depressing the flap he inserted a 5-6 inch tube into the stomach gathered gastric juice had its components identified. He introduced food through the hole of the stomach with a string attached to it so that he could retrieve the partially digested food for further examination. He conducted many experiments on food digestion to know the function of stomach which had not been done before. He discovered many things that were new to science. For centuries stomach was thought to cook food by producing heat. Also theStomach was viewed as a mill,a fermenting vat or a stew pan. Through his experiments Dr.B
eaumonts experiments revolutioned the concepts of digestion. June -on16,1822 became the beginning for the the most pioneering experiments in medicine. He recounted many of his observations and experiments in his journal which says “I consider myself but a humble experimentor “ in which the information provided still obeyed scientific method basing all the inferences on direct experimentation. 164 X Class Coordination in life processess Some of the discoveries of Dr.Beaumont were. 1) He measured the temperature of the stomach during digestion .To his surprise he found there was no change or alteration in temperature. He found the temperature being maintained constant ( 100F/38C ). 2) He found out that pure gastric juice contains large amounts of HCl ,contrary to the previous opinions that gastric was nothing but contains simply water. As suggested by some authors as the most general solvent in nature and of the alimentary canal. Even the hardest bone cannot with stand its action .Even outside the stomach its capable of effecting digestion. Based on the evidences he concluded that HCl as chemical agent that aids in chemical reaction. 3) He found Gastric juice is not stored in the stomach. But ,is secreted when the food is taken. When the food enters the stomach it exites the vessels to discharge its contents immediately for digestion. 4) He understood that digestion begins immediately when the food enters the stomach. He tested the contents of the stomach exactly 20mts after taking food (dinner containing ordinary food of boiled and salted beef, potatoes ,bread, beef and turnips) by collecting the fluids from the flap and found digestion had commenced and was progressing well at that time. 5) He also discovered that food in the stomach satisfies hunger even though its not eaten. (food reaching the stomach without passing the mouth and oesophagus). To confirm his assumptions he made St. martin fast from breakfast time till 4’0 Clock and then introduced food into the stomach through the flap. The sensation of hunger subsided. Though it was fortuitous experiments in medicine connected to digestion raised many questions. • What is the cause of hunger? • How does the brain know the happenings of the stomach? • What causes the gastric juices to secrete? • How and what makes the food to be mixed with the digestive juices? • Does the process of digestion occur independently or involves other systems like nervous and muscular as well ? Free distribution by A.P. Government 165 Chapter 8 Heredity - From parent to progeny When we observe our world and its myriad forms of life, we are struck by two seemingly opposite observations, the fantastic variety of life and the similarity between them. As we shall see, we would need to understand these two characteristics of life in order to understand how life evolves. When we say that something evolves, we mean not only that it changes, but that there is also some component of direction in that change. But, how does evolution take place? Does it occur in a slow and steady manner or in quick jumps? Is it just about change and producing something new and different? In the chapter on reproduction we had studied that reproductive processes usually give rise to individuals that have some new characters in spite of the similarity that they share with their parents. Often such new characters give rise to observable changes in life forms. • How are new characters produced? • Are they inherited? • Do they have any role in the process of evolution? In this chapter we shall try to explore several such questions. New characters and variations Think of your own family, what similarities do you share with your father and your mother? Draw a table to represent the similarities of some characters like colour of eye(cornea), colour of hair, shape of nose, shape of face, type of ear lobe(attached or free), inner thumb markings etc. Write your characters and that of your parents in two separate other columns. 166 X Class Heredity - From parent to progeny • How many similar characteristics do you find among you and your • parents? Is there any character in you that is neither like your father nor like your mother? • Where do you think you got such a character from? Let us do an activity to find out more about this. Activity-1 Compare your traits with the traits of your parents and grand parents by drawing a table as given below in your notebook. Characters In me In my Mother/ Father In my grandma / grandpa Table-1 • Is there any character in you similar to that of your mother as well as your grandma? Is there any character in you similar only to that of your grandma? • • How do you think these characters may have been inherited by you • from grandma? Is there any character that is not present in grandma but present in your mother and you? • Think where from your mother got that character? Activity-2 Observe some of your friends and note their characters in the following table. Fill in yours as well. Table-2 Name of your friend Colour of skin Ear lobes Free/ attached Marking on inner side of thumb Length of fore head Colour of eyes (Cornea) Any other features Free distribution by A.P. Government 167 • Compare your characters to that of any one of your friend. How many characters did you find were similar among you and your friend? • Do you share more similar characters with your parents or with your friends? • Do you think that your differences from parents are same as differ- ences from friends? Why /why not? fig-1: Variations in organs Differences in characters within very closely related groups of organisms are referred to as variations. Often a new character in a group may lead to variations that are also inherited. • Is variation all about apparent differences? Is it about some subtle differences as well that we most often overlook? (Remember looking for two similar neem /doob grass plants in the chapter on diversity and classification in class IX) Activity-3 Observe seeds in a pea or bean pod. You may observe several parts to arrive at a generalisation. • Can you find two similar seeds there? • What makes them vary? (Hint: You know that seeds are formed from ovules) • Why variations are important? How are variations useful for an organism or a population? Over centuries variations and their role in nature have been studied by naturalists. During early 19th century, a lot of work was done by several scientists. Some of these studies will help us to understand how variations occur and are transferred from one generation to the next. We shall study a detailed account of experimental evidences provided by Mendel in the early 19th century who is known as ‘father of genetics’. In 1857 Gregor Johann Mendel started working on the problem of how variations were passed from one generation to the other. Mendel did 168 X Class Heredity - From parent to progeny fig-2: Gregor Johann Mendel not do his experimental work either in a University or in a Laboratory. As he was a monk in a monastery he simply did his experiments in the monastery garden. He worked for over seven years after which he presented the conclusions from his experimental data in the form of a detailed research paper. Mendel made many careful observations of plants and found that pea plants would be most suitable to carry on futher experimentation. Then he planned and designed the experiments to find out the answers to questions that came to his mind. He had worked on nearly 10,000 pea plants of 34 different varieties. Observing pea plants carefully, Mendel noted that they differ from one another in many ways. For example plants were tall or dwarf, seed shape round or wrinckled, seed cover (cotyledon) colour yellow or green. Thus, Mendel had chosen 7 pairs of contrasting characters for his study as shown in the table-2. 1. The difference in the form of the ripe seeds. These are either round or deeply wrinkled. 2. The difference in the color of the seed albumen (endosperm). The albumen of the ripe seeds is either pale yellow, bright yellow and orange coloured, or it possesses a more or less intense green tint. This difference of colour is easily seen in the seeds as their coats are transparent. 3. The difference in the colour of the seed coat. This is either white, with the character of white flowers are constantly correlated, or it is grey, grey-brown, leather-brown, with or without violet spotting. 4. The difference in the form of the ripe pods. These are either simply inflated, not constricted in places, or they are deeply constricted between the seeds and more or less wrinkled. 5. The difference in the colour of the unripe pods. They are either light to dark green, or vividly yellow. 6. The difference in the position of the flowers. They are either axial, that is, distributed along the main stem, or they are terminal, that is, bunched at the tip of the stem. 7. The difference in the length of the stem. The length of the stem is varied in some forms. In experiments with this character, in order to discriminate with certainty, the long axis of 6 to 7 feet. was always crossed with the short one of 3/4 to 11/2 feet. (Popularly called the tall and dwarf varieties.) Free distribution by A.P. Government 169 Table-3: The results of Mendel’s F1 crosses for seven characters in pea plants Character Dominant Trait Recessive Trait F2 Generation Dominant: Recessive Ratio Purple White Flower colour 705:224 3.15:1 Flower position Axial Terminal Yellow Green 651:207 3.14:1 Seed colour 6022:2001 3.01:1 Round Wrinkled Seed shape 5474:1850 2.96:1 Inflated Constricted Pod shape 882:299 2.95:1 Green Yellow Pod colour 428:152 2.82:1 Tall Dwarf Stem length 787:277 2.84:1 170 X Class Heredity - From parent to progeny Mendel hypothesized that characters were carried as traits and an organism always carried a pair of factors for a character. He also hypothesized that distinguishing traits of the same character were present in the population of an organism. He assumed
that the traits shown by the pea plants must be in the seeds that produced them. The seeds must have obtained these traits from the parent plants. How do parent plants pass on their traits to the seeds? Will the seeds from tall plants always produce new tall plants? Mendel carried out several experiments to find out answers to such type of questions. Examples of experiments performed by Mendel The following section shows number of experiments performed, number of fertilizations carried out and the number of plants involved in the study. 1. 1st experiment 60 fertilizations on 15 plants. 2. 2nd experiment 58 fertilizations on 10 plants. 3. 3rd experiment 35 fertilizations on 10 plants. 4. 4th experiment 40 fertilizations on 10 plants. 5. 5th experiment 23 fertilizations on 5 plants. 6. 6th experiment 34 fertilizations on 10 plants. 7. 7th experiment 37 fertilizations on 10 plants. • Why Mendel had choosen garden pea as material for his experiments? Because it has following advantages. 1.Well defind characters, 2.Bisexual flowers, 3.Predominently self fertilization, 4.Early hybridization Mendel selected such kinds of plants that expresses a selected character over several generations. Such plants according to him were pure breed for that character. Mendel did the experiment with two pure breeds of peas for seed colour contrasting characters yellow and green, and they are represented as yellow with ‘Y’ and green with ‘y’. He started cross fertilizing pure breeds having contrasting characters. Mendel started with two pure breeds of peas with different properties. We here take as an example a characteristic colour of the pea seeds yellow and green. Do you know? Pea is an annual plant, with a life cycle of one year. It is a cool season crop grown in many parts of the world. Peas were present in Afghanistan in 2000 BC, in Harappa, Pakistan, and in northwest India in 2250– 1750 BC. In the second half of the 2nd millennium BC, this pulse crop appeared in the Gangetic basin and southern India. Pea contains vitamin ‘A, C, E, K & B’ and minerals like Ca, Fe, Mg, Mn, P, S & Zn. Free distribution by A.P. Government 171 Cross pollinating a pure breed of yellow and green gave First Filial (F1) generation (Mendel called it as first filial or progeny of first generation parents. Filial means progeny.) All pea seeds were Yellow. These pea plants on self pollination gave Second Filial (F2) generation [About 75% yellow (seeds) peas and about 25% green (seeds) peas.] Third Filial (F3) generation (Mendel self pollinated these pea plants too, and found out that 1 2 3 1 2 3 1. A set of peas (about 25%) gave only yellow seed giving pea plants. 2. Rest of the yellow seed giving pea plants gave about 75% yellow and about 25% green seed giving pea plants. 3. The set of green peas gave only green seed giving pea plants. Mendel made some assumptions by which he could explain his observations. Assumption 1: Every pea plant has two ‘factors’ which are responsible for producing a particular character or trait. The determining agent responsible for each trait is called a factor. Mendel carefully choose the plants which did not produce a mixed result (pure). In our example of yellow and green peas, a pure breed (parental stage) will have both the ‘factors’ of the same type. A pure breed (parental) yellow seed giving pea will have both the ‘factors’ of the same type. Let us denote them by ‘Y’. A pure breed (parental) green seed giving pea will have both the ‘factors’ of the same type. Let us denote them by ‘y’. 172 X Class Heredity - From parent to progeny Assumption-2 : During reproduction one ‘factor’ from each parent is taken to form a new pair in the progeny. Assumption-3 : One of these will always dominate the other if mixed together. The trait expressed in F1 generation was called dominant. While the other which did not express was called recessive. Assume that ‘Y’ (the one causing yellow colour) is a dominating ‘factor’. That means if ‘Y’ and ‘y’ come together ‘Y’ will dominate. Then the pea seeds will be always yellow in colour. From assumption-2, the breed after cross pollination will have one factor from pure breed yellow (Y) and one from the pure breed green (y). That is, all the peas will have the paired factor ‘Yy’ and by assumption-3 all the peas will be Yellow as ‘Y’ factor is dominant. Parental generation - Cross pollination y y Y Yy Yy Y Yy Yy Yy ............... (yellow) Yy ............... (Yellow) Yy ............... (yellow) Yy ............... (Yellow) All the pea plants are yellow (F1-Generation). The trait expressed in F1 is dominent, unexpressed is recessive. This is the law of dominenance. Self pollination in F1-Generation On self pollinating these peas (ones with Yy factor), the new breed have any combinations of ‘Y’ and ‘y’. y y Yy YY yY yy Y y It can be YY, Yy, yY or yy. All of them are in equal ratio. So in this heap we will get approximately equal number of YY, Yy, yY and yy peas. But any pea that has a Y factor will be yellow. Any pea that has both yy will be green. Since all combinations are equally likely: 1. YY will be approximately 25% and is yellow. 2. yY will be approximately 25% and is yellow, Yy will be approximately 25 % and is yellow 3. yy will be approximately 25% and is green. Some seeds appear yellow in colour in F1 generation. When these Free distribution by A.P. Government 173 seeds were sown some of the plants produced green coloured seeds. So we can’t determine internal character based on external visible character. Phenotype Thus in F1 generation we can clearly observe that 75 percent are yellow seed producing pea plants and 25 percent are green ones. This is known as ‘Phenotype’(externally visible characters) and this ratio is called ‘phenotypic ratio’ is 3:1. Genotype Genetically, in 75 percent yellow seed producing pea plants only 25 percent pea plants produce yellow seeds that are pure breeds (YY) and are ‘homozygous’ that is to have the same factors for representing a character. Remaining 50 percent yellow seed producing pea plants are (Yy) heterozygous . The remaining 25 percent green seed producing pea plants are pure (yy) homozygous type. The constitution of pea plants as shown by the representative letters Y and y to show the probable nature of factors is known as genotype. This ratio is known as genotypic ratio is 1:2:1 On self-pollinating these peas of F2 generation we get, Y Y YYY YY Y YY YY 1 Y y YYY Yy y yY yy 2 y y y yy yy y yy yy 3 1. The YY peas will on self pollination give only yellow (YY) peas. This was explained with the experimental result that this set gave 100% yellow peas. 2. The Yy or yY peas on self pollination give about 75% yellow peas and about 25% green peas. This situation is same as step 2 ratio 3:1. 3. The green peas that contain yy factors will give only green peas. In nature there are many factors responsible for different properties. • Can we test our hypothesis with more than one factor? How can this be applied to Mendel’s experiment? This can be done 174 X Class Heredity - From parent to progeny together when two pairs of contrasting characters are taken into consideration. 1. Colour of peas-yellow and green symbolically indicated as ‘Y’, ‘y’. 2. Shape of peas-Round and wrinkled symbolically indicated as ‘R’, ‘r’. The plants with yellow and round seeds (pure) were crossed with those having green and wrinkled seeds (pure). All pea seeds that were yellow and round skin. Each pea will have factors ‘YyRr’. Since Yellow colour (Y factor) and round skin (R factor) are dominant traits. All the pea seeds will be round and yellow (F1 generation). He got in F2 generation some seeds were round and yellow (YyRr or YYRR), some seeds were round and green (yyRR or yyRr), some seeds were wrinkled and yellow (Yyrr or Yyrr), and some seeds were wrinkled and green (yyrr). YYRR yyrr On cross pollination YyRr Self pollination • What should be the percentage of each type? Mendel explained the process of inheritance of more than one pair of characters. This law is known as law of independent assortment. We will learn more about this in further classes, for the basic understanding refer in annexure. Mendel propounded that, among a pair of closely related ‘alleles’ or factors for a character, only one expresses itself in the first generation as one of the allele is dominant over the other. This is so evident that it came to be called as Mendels’ Law of Dominance. He also stated that, every individual possesses a pair of alleles (assuming only a pair is present) for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to an offspring. The offspring then receives its own pair of alleles for that trait one each from both parents. This is what Mendel called ‘segregation’ and it is the Law of Segregation. Free distribution by A.P. Government 175 ’ B ‘ G A B Traits that may be passed on from one generation to the next are called as heritable traits. We have studied some of them for the pea plant, in the experiments conducted by Mendel. Activity- Let us do the following activity to understand the Mendelian principles of Heredity. BAG ‘A’ 1 5 9 13 2 6 10 14 3 7 11 15 4 8 12 16 Materials required : a) 3 cm length and 1cm breadth chart pieces- 16 b) 2 cm length and 1cm breadth chart pieces- 16 c) Red buttons - 16 d) White buttons - 16 e) Chart, scale, sketch pen, pencil, 2 bags. Method: Prepare a chart with 44 boxes along with number and Symbol as shown in the figure Game 1: Monohybrid cross (starting with hybrid parents) To start with take 1,2 or 3,4 . In case you start 1,2 pick all the 16 long and short pieces and prepare such pairs in each of which you have a long and short piece. Take 8 pairs each of long and short strips and put them in two separate bags. Now each bag contains 16 strips (8 long and 8 short).One bag say ‘A’ represents male and the bag ‘B’ represents female. Now randomly pick one strip each from bag A and B and put them togethe
r in the 1 on the chart. Keep picking out the strips and arrange them in the same manner till your bags are empty. Sametime your boxes in the chart are filled with pairs of strips. You might have got the following combinations, two long strips, one long and one short strip, two short strips. • What is the number of long strip pairs? • What is the number of one long and one short pairs? • What is the number of short strips pairs? • What is the percentage of each type? also find their ratios? • What can you conclude from this game? You may play this game by taking buttons instead. Compare your second game results with that of first game. What do you understand. Try to play another game mentioned in annexure then discuss with your classmates. 176 X Class Heredity - From parent to progeny Parent to progeny A person resembles his grandfather, a girl seems to be a photocopy of her aunt, generally we hear such comments. These similarities are the result of inherited traits transmitted from parent to progeny. Let us do the following activity to understand inherited traits in human beings. According to Mendel traits like the colour of seed, seed coat, length of stem etc. were heritable traits from parental generation. Transmission or passing of characters or traits from parent to offsprings is called ‘Heredity’. The process in which Traits are passed from one generation to another generation is called ‘Inheritance’. How do traits get expressed? Mendel hypothesised that each character or trait is expressed due to a pair of factors or ‘alleles’ (contrasting expressions of the same trait), as he named them. Now we know that these are known as ‘genes’. Gene is a segment of a nucleic acid called ‘DNA’ which is present in the nucleus of every cell. It controls the expression of a trait or character. In vireses RNA can also be controlling the expression of a character. Do you know? In 1953 the detailed structure of DNA was finally worked out at Cambridge by Francis Crick and James Watson. They discovered that DNA molecule looks rather like a spiral stair case, having a shape known as a double helix. The framework of stair case consists of alternate sugar and phosphate groups and the steps which join the framework together are the pairs of chemical compounds called bases. They are adenine, guanine, thymine and cytosine. Watson and Crick were awarded Nobel prize jointly with Franklin and Maurice Wilkins. Traits are determined by the chemical nature of DNA and a slight change in it leads to variations. Colour of the hair, the skin etc. are examples of trait. Slight inheritable changes in the chemical structure of DNA may lead to change in the characteristic or trait of offspring of an organism, which leads to ‘Variations’. Watson DNA fig-3: Crick Free distribution by A.P. Government 177 Sex determination in human beings mother’s sex chromosomes father’s sex chromosomes female child Father Mother male child Parents 44+XY 44+XX 22 +X Gyno Sperm 22 +Y Andro Sperm 22+X 22+X Gamates Eggs Offsping Baby girl Baby boy Baby girl Baby boy (44+XX) (44+XY) (44+XX) (44+XY) fig-4: We inherited our traits from our parents. Let us see how sex is determined in human beings. Each human cell contains 23 pairs (46) of chromosomes. Out of 23 pairs 22 pairs of chromosomes are autosomes. Chromosomes whose number and morphology do not differ between males and females of a species are called autosomes. The remaining pair is called allosomes or sex chromosomes. These are two types, one is ‘X’ and the other is ‘Y’. These two chromosomes determine the sex of an individual. Females have two ‘X’ chromosomes in their cells (XX). Males have one ‘X’ and one ‘Y’ chromosomes in their cells (XY). All the gametes (ova) produced by a woman have only X chromosomes. The gametes (sperm) produced by a man are of two types one with X chromosome and other Y chromosome. If the sperm carries Y chromosome and fertilizes the ovum (X chromosome). Then the baby will have XY condition. So the baby will be a boy. • What will happen if the sperm containing Y chromosomes fertilizes the ovum? • Who decides the sex of the baby – mother or father? • Is the sex also a character or trait? Does it follow Mendels’ law of dominance? • Were all your traits similar to that of your parents? Do you know? Discovery of the sex chromosomes Walter Setton and Thomas Hunt Morgan in the year 1910 studied on a small fruit fly (Drosophila melanogaster) at Columbia University. The discovery of sex linked traits in Drosophila indicated that genes are on chromosomes. They worked out the details of inheritance in Drosophila. 178 X Class Heredity - From parent to progeny Evolution Variations develop during reproduction in organisms. Sexual reproduction and errors in DNA copying leads to variations in offsprings in a population. Let us try to study the consequences of variations in the population of an insect in an environment. Activity-3 Variations in beetle population Observe the below diagram showing variation in beetle population and it its impact. fig-5: Variation in population Let us consider a group of twelve beetles. They live in bushes on green leaves. Their population will grow by sexual reproduction. So they were able to generate variations in population. Let us assume crows eat these red beetles. If the crows eat more Red beetles their population slowly reduced. Let us think of different situations. Situation-1: In this situation a colour variation arises during reproduction. So that there appears one beetle that is green in colour instead of red. fig-6: Red and green beetles Free distribution by A.P. Government 179 More over this green coloured beetle passes it’s colour to it’s off spring (Progeny). So that all its progeny are green. Crows cannot see the green coloured beetles on green leaves of the bushes and therefore crows cannot eat them. But crows can see the red beetles and eat them. As a result there are more and more green beetles than red ones which decrease in their number. The variation of colour in beetle ‘green’ gave a survival advantage to ‘green beetles’ than red beetles. In other words it was naturally selected. We can see that the ‘natural selection’ was exerted by the crows. The more crows there are, the more red beetles would be eaten and the more number of green beetles in the population would be. Thus the natural selection is directing evolution in the beetle population. It results in adaptation in the beetle population to fit in their environment better. Let us think of another situation. Situation-2: In this situation a colour variation occurs again in its progeny during reproduction, but now it results in ‘Blue’ colour beetles instead of ‘red’ colour beetle. This blue colour beetle can pass its colour to its progeny. So that all its progeny are blue. fig-7: Blue and red beetle Crows can see blue coloured beetles on the green leaves of the bushes and the red ones as well. And therefore crows can eat both red and blue coloured beetles. In this case there is no survival advantage for blue coloured beetles as we have seen in case of green coloured beetles. What happens initially in the population, there are a few blue beetles, but most are red. Imagine at this point an elephant comes by and stamps on the bushes where the beetles live. This kills most of the beetles. By chance the few beetles survived are mostly blue. Again the beetle population slowly increases. But in the beetle population most of them are in blue colour. Thus sometimes accidents may also result in changes in certain characters of the a population. Characters as we know are governed by genes. Thus there is change in the frequency of genes in small populations. This is known as “Genetic drift’, which provides diversity in the population. 180 X Class Heredity - From parent to progeny Let us think of another situation: Situation-3: fig-8: Poorly nourished beetles In this case beetles population is increasing, but suddenly bushes were affected by a plant disease in which leaf material were destroyed or in which leaves are affected by this beetles got less food material. So beetles are poorly nourished. So the weight of beetles decrease but no changes take place in their genetic material (DNA). After a few years the plant disease are eliminated. Bushes are healthy with plenty of leaves. • What do you think will be condition of the beetles? Acquired and Inherited Characters and Evolution We discussed the idea that the germ cells of sexually reproducing population are formed in specialised reproductive tissue. If the weight of the beetles is reduced because of starvation, that will not change the DNA of the germ cells. Therefore, low weight is not a trait that can be inherited by progeny of a starving beetle. Therefore even if some generations of beetles lose their weight because of starvation, that is not an example of evolution, since the change is not inherited over generations. Change in non reproductive tissues cannot be passed on to the DNA of the germ cells. Therefore the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution. Lamarckism In the olden days people believed that all the organisms on the earth had not undergone any change. Jean Baptist Lamarck was the first person to propose the theory of evolution. He thought that at some point of time in the history the size of giraffe was equal to that of deer. Due to shortage of food material on the ground and to reach the lower branches of trees giraffes started stretching their necks. Because of continuous stretching of neck, after several generations giraffes developed long necks. Such characters that are developed during the lifetime of an organism are called ‘acquired characters’. Lamarck proposed that these acquired fig-9: Jean Baptist Lamarck (1774-1829) Free distribution by A.P. Government 181 characters are passed on to its offsprings i.e. to next generation and proposed the theory of ‘Inher
itance of acquired characters’. For example elongation of neck and forelimbs in giraffe. But Augustus Weismann, tested this theory by an experiment on rats. He removed tails of parental rats. He observed that their offspring’s are normal with tails. He has done it again and again for twenty two generations but still offsprings are normal with tails. He proved that the bodily changes are not inherited. So they won’t be passed to it’s offsprings. fig-10: Giraffee Darwinism Charles Darwin proposed ‘Natural selection’ the famous ‘theory of evolution’. Charles Darwin (1809-1882) was born in England. He voyaged for five years, just when he was 22 years old. In the world survey ship HMS Beagle. He visited a number of places including Galapagos Islands. He keenly observed the flora and fauna of these places. He gathered a lot of information and evidences. Darwin observed a small group of related birds which are exhibiting diversity in structure in the Galapagos islands. These birds are Finch birds. Observe the fig-12. How do the beaks help them. He was influenced by the book ‘Principles of geology’ written by Sir Charles Lyell. He suggested that geological changes occured fig-11: Charles Darwin (1809 – 1882) Large ground finch (seeds) Cactus ground finch Vegetarian finch (buds) Wood pecker finch (insects) fig-12: Some Darwin finches in a uniform rate, Darwin did not agree to this idea. He felt that large changes occured due to accumulation of small changes. Darwin was also influenced by the famous ‘Malthus theory’. This was written in ‘An essay on the principles of population’. Malthus observed that population grows in geometrical progression (1, 2, 4, 8, ......) where as food sources increases in arithmetic progression (1, 2, 3, 4, 5, .......). 182 X Class Heredity - From parent to progeny Based on these ideas Darwin proposed the theory of “Natural selection”, which means that the nature only selects or decides which organism should survive or perish in nature. This is the meaning of survival of the fittest. The organisms with useful traits will survive. If traits are not usefull to organisms then they are going to be perished or eliminated from its environment. Alfred Russel Wallace also independently concluded that natural selection contributed to origen of new species. For example we have seen in the case of red beetles which were seen and eaten by crows. So, the population of red beetles gradually got eliminated or perished from its environment. But at the same time the beetles which are green in colour which are present on the green leaves were not noticed by crows. So the green beetles survived in the environment and their population have gradually increased. This is nothing but “natural selection”. Think and discuss In a forest there are two types of deers, in which one type of deer can run very fast. Where as second type of deer can not run as fast as the first one. Lions, Tigers haunt deers for their food. Imagine which type of deers are going to survive in the forest and which type of deers population is going to be eliminated? And why? Variations which are useful to an individual are retained, while those which are not useful are lost. In a population when there is a struggle for the existence the ‘fittest’ will be survived. Nature favours only useful variations. Each species tend to produce large number of offsprings. They compete with each other for food, space, mating and other needs. In this struggle for existence, only the fittest can survive. This is called ‘survival of the fittest’. Over a long period of time this leads to the formation of new species. You may observe in your surroundings some seedilings and some of the animal kids only survive. Discuss in your class based on those examples to understand survival of the fittest. Darwin’s theory of evolution in a nutshell 1. Any group of population of an organism developes variations and all members of group are not identical. 2. Variations are passed from parent to offspring through heredity. 3. The natural selection over abundance of offspring leads to a constant struggle for their survival in any population. Free distribution by A.P. Government 183 4. Individuals with variations that help them to survive and reproduce tend to live longer and have more offsprings than organisms with less useful features. 5. The offsprings of survivors inherit the useful variations, and the same process happens with every new generation until the variation becomes a common feature. 6. As the environment changes, the organism within the environment adapt and changes to the new living conditions. 7. Over a long period of time, each species of organism can accumulate so many changes that it becomes a new species, similar to but distinctly different from the original species. All species on the earth arise in this way. 8. Evolution is a slow and continuous process. There are some limitations and objections to the Darwin theory. Many new theories like synthetic theory, mutation theory are put forward. Do you know? Identical thoughts of Charles Darwin and Alfred Russel Wallace. When Charles Darwin was formulating the theory of evolution in his mind, he received a letter with an article sent by Alfred Russel Wallace about his studies in the Indonesian island. The article was about Natural selection. Darwin was surprised about same theory in his mind. Later in the same year Charles Darwin and Alfred Wallace jointly published an article in the ‘Journal of Linnaean Society’ about natural selection. It was only after this Darwin published his famous book, “The origin of Species” in 1859. However their thoughts gained criticism at that time because they did not explain how variations are inherited. After the discovery of mitosis and meiosis it was understood properly. Alfred Russel Wallace Speciation How new species are evolved? We have seen variations in a population of species, where the organism contain the traits that helped to adapt to the environment. These organisms are going to survive more efficiently. But in the same population the organism which contains the non benificial traits may not be adapted in the environment. They are going to perish or eliminated slowly, like red and blue beetles in a population which we have discussed earlier in this chapter. These small changes within the species for example colour of 184 X Class Heredity - From parent to progeny beetles red and green is known as micro evolution. Now we are going to discuss how new species are formed. This is known as speciation, which is also known as Macroevolution. We have seen red and green beetles can mate each other and can have offsprings. But let us imagine that red and green beetles are separated by some cause (for example while eating beetles crows dropped some beetles accidently in the long distance far away places) for long years. There might be a lot of variations taken place in these years in the red and green beetle population. Now even though they may meet accidentally, they cannot mate and produce new offsprings. They can only mate in their population either red or green and can reproduce its off spring. Thus new species have been formed. Evidences of evolution How does the evolution of organisms taken place? Whatever scientists propose they require evidences or proofs. In the same way evolution of organisms requires evidences. Let us examine some of them. Homologous and analogous organs When we try to understand evolutionary relationships, we identify that some traits have common ancestors. These traits in different organisms would be similar because they are inherited from a common ancestor. You may be surprise to know that the internal structure of forelimb of a whale (swimmer) wing of a bat (flyer), leg of a cheetah (runner), claw of a mole (digger) and hand of a man (grasping). If we carefully observe the anatomy of all these animals, they show a common pattern in the arrangement of bones, even though their external form and functions are different. It indicates that all the vertebrates have evolved from a common ancestor and these organs are called homologous organs. This type of evolution is called divergent evolution. However, all similarities simply in organ shape are not necessarily to have a common ancestry. What would we think about the wings of birds and bats, for example (fig-). Birds and bats have wings, but squirrels and lizards do not. So birds and bats are more closely related to each other than squirrels or lizards. fig-13: Homologus organs Free distribution by A.P. Government 185 Before we jump into this conclusion, let us look at the wings of birds and bats more closely. When we observe, we find that the wings of bats have skin folds (patagium) stretched between elongated fingers. But the wings of birds have a feathery covering all along the arm. The designs of the two wings, their structure and components are different. They look similar because they have a common use for flying, but their origin is not common. This gives the ‘analogous’ characteristics (Traits). As the above mentioned organs which are structurally different but functionally similar are known as ‘Analogous organs’. This type of evolution is called convergent evolution. Evidences from embryology Activity-5 Let us observe different stages of development of vertebrate embryos. Try to find out similarities and differences and discuss with your friends. Embryology is the study of the development of an organism from egg to adult stage. Tadpole of a frog resembles fish more than the frog. What does this indicates? Does it indicate that frogs have evolved from ancestors of fish? There are remarkable similarities in the embryos of different animals from fish to man. The resemblance is so close that at an early stage even an experienced embryologist would find difficulty to distinguish one embryo from the other. What does it indicate? Does it indicate that life history of every individual, exhibits the struct
ural features of their ancestors? This strengthens the view of the existence of a common ancestor from which all these have evolved. Pig Calf Human Rabbit fish Salamander Tortoise Chick fig-14: Embryological evidences Evidences from fossils We know some species which existed million years ago, but we may not find them now. They might be extinct and some of them may be found in the form of fossils. For example we know Dinosaurs the biggest animal on land which were present long time ago but now they are extinct. The scientists got evidences of presence of Dinosaurs like animals in the form of fossils. 186 X Class Heredity - From parent to progeny What are fossils? Fossils are evidences of ancient life forms or ancient habitats which have been preserved by natural processes. Fossil evidence is typically preserved within the sediments deposited beneath water and land. They can be actual remains of once lived such as bones or seeds or even traces of past event such as dinosarus foot print or ripple marks on a pre-historic shore. Usually when organisms die, their bodies will be decomposed and lost. Sometimes the body or some parts of the body do not decompose completely. For example if a dead insect get caught in mud, it will not decompose quickly and the mud will eventually harden and retain the impression of the body parts of insect. All such preserved traces of living organisms are called fossils. fig-15: Fossil Geologists can tell the age of a fossil. The study of fossil is called ‘Palaeontology’. Palaeontologists determine the age of fossil by using carbon dating method. The breakdown of radioactive isotopes of certain elements such as Carbon, Uranium and Potassium takes place at a known rate. So the age of rock or mineral containing isotopes can be calculated. • Collect information about carbon dating method and radioactive isotopes and discuss with your teacher or from library display your collections in your class. A rare and magnificient fossil of the dinosaurs, ketosaurs belonging to the lower Jurassic age going back to about 160 million years were collected from Yamanapalli in Adilabad district of Andhra Pradesh. This fossil has 14 metres length and 5 metres height. This fossil is preserved at BM Birla Science Centre in Hyderabad. fig-16: Dinosarus Do you know? See the picture of Archeopteryx. Does it resemble a bird? Or a reptile? Or both? The organisms which bear the characters of two different groups are called connecting links. Archeopteryx has some avian characters and some reptelian characters. Hence it is recognised as connecting link between aves and reptiles. Archeopteryx Free distribution by A.P. Government 187 Human evolution Human evolution is the evolutionary process leading up to the appearance of a modern human being. We the present human beings are also have an evolutionary history like plant and other animals. Early man like forms appeared about 7 lakhs 50 thousand years ago. The first sure fossil of our own species of man the Homosapiens, indicate that true man appeared on the earth 2 lakhs 50 thousand years ago. Evolution of man through ages: Homo habilus lived between 1.6 - 2.5 million years ago. Homo erectus lived between 1 - 1.8 million years ago. Homo sapiens neanderthalensis lived between 2,30,000 - 3,00,000 thousands years ago. Homo sapiens (present man) appeared about 40 thousand years ago. There is a great diversity in human forms and features across the planet. So that for a long time, people used to talk about human ‘races’. Skin colour used to be the commonest way of identifying the so called races. Some were called black, some white or brown. A major question debated for long time was, have these apparent groups evolved differently? Over recent years, the evidence has become very clear. The answer is that there is no biological basis to the notion of human races. All humans are a single species with a common ancestor. fig-17: Human evolution Not only that, regardless of where we have lived for the past few thousand years, we all come from Africa. The earliest members of the human species, Homo sapiens, can be traced there. Our genetic footprints can be traced back to our African roots. A couple of hundred thousand years ago, some of our ancestors left Africa while others stayed on, while the residents spread across Africa. The migrants slowly spread across the planet –from Africa to west Asia, Then to Central Asia, Eurasia, South Asia, East Asia. They travelled down the island of Indonesia and the Philippines to Australia, and they crossed the Bering land bridge to the Americas. They did not go in a single line, so were not travelling for the sake of travelling, obviously. They went forwards and backwards, with groups sometimes separating from each other, even moving in and out of Africa. Like all other species on planet, they had come into being as an accident of evolution, and were trying to live their lives the best they could. 188 X Class Heredity - From parent to progeny Human being - a moving museum During the course of evolution some organs remain in the organisms. You have studied about appendix in the digestive system. In human beings it has no role to play in the process of digestion. But in herbivores like rabbit appendix plays an important role. Such type of organs which are not useful in animal are called vestigial organs. • Think why did ancient human beings traveled from one place to other and how did they traveled? There are nearly 180 vestigial organs in human beings. For example pinna, hair on skin, mammary glands in human, etc. That’s why human being is said to be a moving museum of vestigial organs. Key words Variations, offsprings, traits, phenotype, genotype, heterozygous, homozygous, independent assortment, allele, heredity, autosomes, allosomes, natural selection, analogous organs, embryological evidences, Human evolution. What we have learnt • Variations are quite apparent among closely related groups of organisms. • In about 1857 Gregor Johann Mendel started working on the problem of how variations were passed from one generation to other. • Mendel had choosen seven distinguishing traits: flower colour, position, seed colour, shape, pod • • • colour, pod shape, stem length. In monohybrid experiment F1 generation all pea seeds were yellow. In F2 generation about 75% seeds were yellow and about 25% seeds were green. This is called phenotype and the ratio is 3:1. In F2 generation out of 75%, 25% were pure yellow seeds 50% were yellow seeds but green as a recessive factor. Remaining 25% were pure green. This is called genotype and the ratio 1:2:1. • Every pea plant has two ‘factors’ which are responsible for producing a particular property or trait called allele. • The factors for each pair of characters assorts independently of the other pairs. This is known as “Law of independent assortment”. • Crossing yellow and green seeds produced all yellow seeds. Because yellow is dominant factor. • Each parent passes randomly selected copy (allele) of only one of these to its offspring. • The process of acquiring characters or traits from parents is called ‘Heredity’. • Each human cell contains 23 pairs of chromosomes. Out of these 22 pairs are called autosomes and one pair is called allosomes. Free distribution by A.P. Government 189 • Lamarck proposed that the acquired characters are passed to the offspring in the next generation. • Each species tend to produce large number of offsprings, but only the fittest can survive. • Homologous, analogous organs and embryological evidences explain evolutionary relationships. Some traits in different organisms would be similar because they are inherited from a common • ancestor. Fossils are evidences of ancient life forms or ancient habitats which have been preserved by natural processes. • Improve your learning 1. What are variations? How do they help organisms?(AS1) 2. One student (researcher) wants to cross pure tall plant (TT) with pure dwarf (tt) plant, what would be the F1 and F2 generations? Explain.(AS1) 3. One experimenter cut the tails of parent rats , what could be the the traits in offsprings? Do the 4. daughter rats contain tails or not? Explain your argument.(AS1) In a mango garden a farmer saw one mango tree with full of mango fruits but with a lot of pests. he also saw another mango tree without pests but with few mangoes. But the farmer wants the mango tree with full of mango fruits and pest free. Is it possible to create new mango tree which the farmer wants? Can you explain how it is possible?(AS1) 5. Explain monohybrid experiment with an example, which law of inheritance can we understand? Explain.(AS1) 6. What is the law of independent assortment? Explain with an example?(AS1) 7. How sex determination takes place in human? Explain with example.(AS1) 8. Explain the Darwin’s theory of evolution ‘Natural selection’ with an example?(AS1) 9. What are variations? Explain with a suitable example.(AS1) 10. What variations generally have you observed in the species of cow?(AS1) 11. What are the characters Mendel selected for his experiments on pea plant?(AS1) 12. In what way Mendel used the word ‘Traits’- explain with an example.(AS1) 13. What differences Mendel identified between parent and F2 generation.(AS1) 14. Male is responsible for sex determination of baby – do you agree? If so write your answer with a flow chart.(AS1) 15. Write a brief note on analogous organs.(AS1) 16. How do scientists utilise the information about fossils?(AS1) 17. Mendel selected a pea plant for his experiments. Mention the reasons in your point of view.(AS2) 18. If the theory of inheritance of acquired characters proposed by Lamark was true how will the world be?(AS2) 19. Collect information on the inherited traits in your family members and write a note on it.(AS4) 20. With the help of given information write your comment on evidences of evolution.(AS4) Mammals have four limbs as do birds, reptiles and amphibians. The basic
structure of the limbs is similar, though it has been modified to perform different functions. 21. Collect information about carbon dating method. Discuss with your physical science teacher.(AS4) 190 X Class Heredity - From parent to progeny 22. Draw a checker board show the law of independent assortment with a flow chart and explain the ratio.(AS5) 23. Explain the process to understand monohybrid cross of Mendel experiment with a checker board.(AS5) 24. Prepare a chart showing evolution of man through ages.(AS5) 25. Nature selects only desirable characters. Prepare a cartoon.(AS6) 26. What is your understanding about survival of the fittest. Give some situations or examples that you observe in your surroundings?(AS7) 27. Write a monologue on evolution of a man to perform a stage show on the theatre day in your school.(AS7) Fill in the blanks 1. The process of acquiring change is called ______________. 2. Mendel’s experiment of stands for ______________. 3. The four characters observed in the experiments on law of independent assessment are __________. If we cross pollinate red flower plant with white flower we will get ________ percent of mixed 4. colour plants. 5. TT or YY, Tt or Yy are responsible for a _________________ character. 6. Female baby having 23 pairs of autosomes at the age of 18 years she has __________ pair autosomes and _______ of sex chromosomes. 7. The population grows in ________________ progression where as food sources grow in _______________ progression. 8. A goat which walks properly can’t live for a long time. According to Darwin this represents _______________. 9. Forelimb of whale is for swimming where as in horse it is used for _______________. 10. The study of fossils is called _______________. Choose the correct answer 11. Which of the following is not a variation in rose plant. a) Coloured petals b) Spines c) Tendrils d) Leaf margin 12. According to Mendel alleles have the following of a character. ( ( a) Pair of genes c) production of Gametes 13. Natural selection means a) Nature selects desirable characters c) Nature reacts with an organism 14. Palaeontologists deal with a) Embryological evidences c) Vestigial organ evidences Free distribution by A.P. Government b) Responsible for character d) Recessive factors ( b) Nature rejects undesirable characters d) a, b ( b) Fossil evidences d) all ) ) ) ) 191 Annexure-I Mendel’s laws of independent assortment Till now we have discussed about the Mendel’s hypothesis with monohybrid cross. Mendel’s also tried to understand the inheritance of two pairs of characters together. In this dihybrid cross, parents produce offsprings containing the factors for Characters (traits) of Yellow (YY), Round (RR), and wrinkled (rr), green (yy). These characters appeared independently without mixing with each other in F2 generation, which were produced by self pollination in F1 generation. Observe thc checker board given here carefully and note down different combinations of characters resulted from dihybrid cross. 1. RRYY, 2. RRYy, 3. RrYY, 4. RrYy, 5. RRYy, 6. RrYY, 7. RrYy, 8. RrYy, 9. RrYy are having Round and Yellow seeds. 1. RRyy, 2. Rryy, 3. Rryy have Round and Green seeds 1. Rryy, 2. rrYy, 3. rrYy have Wrinkled and Yellow seeds 1. rryy have Wrinkled and Green seeds R Y R y r y r Y RR YY RR Yy Rr Yy Rr Yy RR Yy RR yy Rr yy Rr Yy Rr Yy Rr yy rr yy rr Yy Rr YY Rr Yy rr Yy rr YY 3 :3 :1 Round yellow Wrinkled, yellow Round, green Wrinkled, green From the above results it can be concluded that the factors for each character or trait remain independent and maintain their identity in the gametes. The factors are independent to each other and passes to the offsprings (through gametes). In the inheritance of more than one pair of characters (traits), the factors for each pair of characters assorted independently of the other pair. This is known as “Law of independent assortment”. Mendel believed that every character or trait is controlled or responsible by a pair of factors. The factors which are responsible for a character or trait of an organism, now named as ‘genes’. The pair of genes which are responsible for a character are called as ‘alleles’. Alleles are of two types one is homozygous type (YY or TT) and the other is heterozygous (Yy or Tt). 192 X Class Heredity - From parent to progeny Chapter 9 Our environment - Our concern Everyone is familiar with one’s own surrounding. It plays an important role in the survival of all organisms. The sum of physical and biological factors along with their chemical interactions that affect an organism is called environment. The living organisms maintain a balance with each other and to its biotic and abiotic factors. All components of biosphere interact in an organized manner with the organisms. This interaction assures an organism to survive that may result in gradual evolution of the organisms in the biosphere. The physical factors refers to abiotic factors (land, air, water sun light etc) and biological factors to biotic factors. The place where an organisms lives is called biosphere. One organism cannot completely defy the balance to suit one’s need. It would in some way or the other affect the balance in such a way that the survival of the organism affecting the damage would be at stake. You have understood the relationships between organisms and their food in earlier classes with the help of food chains and food webs. Food chains are interconnected and when we try to observe these connections among a number of food chains then it becomes a food web. As you know a food chain shows who eats what in a particular habitat. The arrows between each organism in the chain always point from the food to the feeder. If we want to show a food chain consisting of grass, snake, rabbit and hawk then connect the given fig-1: Food relationship Free distribution by A.P. Government 193 organisms by putting arrows and make a food chain. • Name the producer and consumers in the above food chain. • Try to guess what does the arrows marked by you indicate? • Identify at least four other food chains in your surroundings. Name the producers and different levels of consumers in those food chains. While identifying different food chains in your surroundings you will find that most of the food chains are quite short and they rarely consists of not more than four steps. You will also notice that as we move from producer to consumers (primary, secondary & tertiary) in a food chain the numbers of organisms at each level decreases. What type of relationships exist among the biotic components? In an ecosystem the energy rich food passes from producers to consumers stepwise, with respect to their (food) relationships. Producer Primary Consumer Secondary Consumer Tertiary Consumer Top Carnivore Examples Grass Grass Grass grasshopper Rabbit Goat frog Fox Man snake Wolf Hawk • Why do most of the food chains consist of four steps? • Why do the number of organisms get decreased as we move form producer to different level of consumers? To get answers for the above question we have to recall some of the things which have been discussed in the earlier classes. In chapter 7 “Different Ecosystems” of class 8th it was mentioned that all organisms in an ecosystem derive energy from food to live and sunlight is the main source of the energy. Food chain shows that how the energy is passed from one organism to another. At each transfer a large proportion (80 to 90 percent) of energy is dissipated as heat produced during the process of respiration and other ways. Thus above three steps in a food chain very little energy is still available for living organisms to use. Within the biosphere there are a number of major ecosystems, the terrestrial ones being determined largely by the variations in climatic conditions between the Poles and Equator .In a similar way, if you climb a mountain such as Kilimanjaro in Equatorial Africa or Himalayan mountains in our country. You quickly go through a comparable system of ecosystems, 194 X Class Our environment - Our concern starting with tropical rain forest at the base and ending with perpetual snow and ice at the summit. The main climatic influences which determine these ecosystems are rainfall, temperature and the availability of light from the sun. For instance, forests are usually associated with high rainfall, but the type is influenced by temperature and light; the same applies to deserts which occur in regions where rainfall is extremely low. But these links are never as simple or rigid as the word ‘chain’ suggests. For example, aphids are eaten by many insectivorous birds in addition to warblers, and also ladybirds and other insects; hawks, on the other hand, prey upon a considerable variety of birds and small mammal-So the term food web is often a better one to use when being precise, as it suggests a far greater number of possible links and reflects the fact that the whole community is a complex inter-connected unit. Thus the original energy from the sun flows through the whole ecosystem from one tropic level to another. Let us observe the diagram (fig-2) which shows some of the feeding relationships amongst organisms living in deciduous woodland. You will see from the diagram that animals fit into special positions within the food web; each is described as its niche. For instance, there is a niche for insects such as aphids which suck up the juices of leaves. Another niche for insects such as caterpillars which have strong jaws for biting off pieces of a leaf and a niche for relatively large animals such as deer which browse on the vegetation. All these animals feed on leaves but they differ both in size and in the manner in which they feed. So the term ‘niche’ denotes not only the animal’s position in the food web and what it eats, but also its mode of life. Just as a habitat is the place where an animal lives, so a niche describes its occupation the way it ‘goes about its business and earns its livings. f
ig-2: Food relationships Ecological pyramids Apart from the food chains pyramids are another type of representations which show flow of energy from one organism to another. You may have heard about pyramids of Egypt. The ecologists also used this idea of pyramid to show relationship among organisms in an existing Free distribution by A.P. Government 195 food chain. In short we can say that graphic representation of the feeding level (tropic level) of an ecosystem by taking the shape of a pyramid is called “Ecological pyramid”. It was first introduced by a British ecologist Charles Elton in 1927. In the ecological pyramid the producers (first trophic level) are represented at the base; and other successive trophic levels (primary, secondary and tertiary consumers) are represented one above the other with top carnivores at the tip. There are three types of pyramids; pyramid of biomass, pyramid of number, pyramid of energy. In this chapter we will try to discuss about the pyramid of number, biomass and energy. fig-3: Pyramid of Egypt Do you know? A pyramid is a structure whose shape is roughly that of a pyramid in the geometric sense; that is, its outer surfaces are triangular and converge to a single point at the top. The base of a pyramid can be trilateral, quadrilateral or polygonal shape. The square pyramids, with square base and four triangular outer surfaces, is a common version. Pyramid of numbers Biologists are not only interested in studying the food relationships which exists between living things, but also in comparing the numbers of organisms at each link in the chain. Here is an example of food web to make estimates of the comparative numbers of organisms present at each stage of chain. The comparison needs to involve the use of such terms as most, many, several, few and scarce. Is there any relationship between the numbers?Is there any comparison that could be made about the sizes of the organisms involved at each stage. Tertiary consumers Secondary consumers Primary consumers Producers fig-4: Pyramid of numbers The number of organisms in a food chain can be represented graphically in a pyramid. Each bar represents the number of individuals at each trophic level in the food chain. At each link in a food chain, from the first-order consumers to the large carnivores, there is 196 X Class Our environment - Our concern normally an increase in size, but decrease in number. Let us observe fig-5, for example in a forest, the aphids are very small and occur in astronomical numbers, the ladybirds which feed on them are distinctly larger and not so numerous, the insectivorous birds which feed on the ladybirds are larger still and are only present in small numbers, and there may only be a single pair of hawks of much larger size than the insectivorous birds on which they prey. This relationship is best shown as a pyramid, which is upright. fig-5: Pyramid of numbers • Draw the pyramid of number for the following food chains (i) Banyan insects Woodpecker (ii) Grass rabbit wolf • • Are the pyramid of numbers having same structure in both of the above two cases as compare to the example given in the earlier paragraph? If there is a difference then what is it? Sometimes the pyramid of numbers does not look like a pyramid at all. This could happen if the producer is a large plant such as tree or if one of the organisms at any trophic level is very small. So keep one thing always in mind that whatever the situation, the producers still goes at the bottom of the pyramid. Pyramid of Biomass What is biomass? Biomass is organic material of biological origin that is ultimately derived from the fixation of carbon dioxide by trapping solar energy during photosynthesis. This includes trees, shrubs, crops, grasses, algae, aquatic plants, agricultural and forest residues and all forms of human, animal and plant waste. Any type of plant or animal material that can be converted into energy is called biomass. When these materials are used for energy production. They are known as bio fuels. The pyramid of biomass represents the relationships that exist between the quantity of living matter (biomass) at different trophic levels. In Free distribution by A.P. Government 197 terrestrial ecosystems, the biomass progressively decreases from producers to top carnivores. • Think why are the pyramids always upright? In an aquatic ecosystem, the biomass of phytoplankton is quite negligible as compared to that of the crustaceans and small herbivorous fish that feed on these producers. The biomass of large carnivorous fish living on small fishes is still greater. This makes the pyramid of biomass inverted. It is found that 10 - 20 % of the biomass is transferred from one tropic level to the next in a food chain. A more accurate idea of food relationship may be obtained if the above pyramid of numbers is converted into a pyramid of biomass. This indicates the mass of plant matter which is used by the aphids to produce the mass of the of aphids population, the total mass of the ladybird population that could be supported by the aphids and so on through out the chain. In short we can say that biomass is food for the next trophic level in a food chain. Do you know? To reduce our dependence on fossil fuels (fuels formed by natural processes such as anaerobic decomposition of buried dead organisms, like coal, petrol etc.), and to help reduce air pollution, biomass can also be used as a source energy. Using biomass as fuel still puts carbon dioxide back into the atmosphere, but it is the same carbon dioxide taken from the air as the biomass was produced. The biomass in each trophic level is always less than the tropic level below. This is because biomass is a measure of the amount of food available. When animals eat, only a small proportion of their food is converted into new tissue, which inturn forms the food for the next trophic level. Most of the biomass that animals eat is either not digested, or used to provide the energy needed for staying alive. The biomass pyramid shows that animals are relatively inefficient in converting food into body tissues, the remaining part of the food being undigested is passed out as waste, or broken down in respiration to supply energy for such activities as feeding. Many animals convert not more than 10% of their food into their body fig-6: Pyramid of biomass 198 X Class Our environment - Our concern Man (1kg) Fish (10kg) Zoo plankton (100kg) Phyto plankton (1000kg) tissues, some herbivores even less. Let us take an example of a food chain which has been worked out in some detail- one in which we are involved when we eat fish. In this chain the plant plankton on the surface of waters of sea are food producers. They trap energy from sunlight. The animal plankton feed on the microscopic plants and the fish in turn feed on the animal plankton; we are at the end of the chain when we eat the fish. The pyramid of biomass for this particular food chain will be as follows. In this particular food chain roughly 90% of the food is lost at each step. So it allows that it would take 1000 kg of plant plankton to produce 100 kg of animal plankton to form 10 kg of fish to produce 1 kg of human tissues, with a corresponding loss of the original plant potential energy that came from the sun. Thus we can conclude that the nearer an animal species is to the original plant source in a food chain the greater the amount of energy is available to the population of that species. In other words, the fewer the steps in the food chain, the more energy will be for the species at the top. Pyramid of Energy Food is the source of energy for organisms that are used in the growth and rebuilding of the parts of the body; that are constantly wearing out. The food by its nature is the chemical energy and by in its stored form, it is the potential energy. There are several mechanisms in organisms for continuous absorption of materials for the production of organic material, and for the release and conversion of organic material into inorganic form. Plants absorb the minerals from the soil. They are absorbed into the plant along with through roots. Photosynthesis is an essential process for the life. The energy of sunlight, carbon dioxide, and the water, which ofcourse needed by all living things, belong to nonliving things. As a result of photosynthesis, these can be made available in a suitable form of energy the food to the world of living things the animals or consumers, only by the green plants the producers. The food chains and food webs help in the transfer of the food and energy from the producers to different consumers. Animals obtain the minerals from the plant or animal food or both. Thus the mineral matter is constantly being removed from the earth to become a part of the plant, which may become a part of animal body. Curd that we eat is processed from milk, which comes from a cow, which in turn eats grass. The grass carries out photosynthesis and prepares food. In every case, the origin of food materials can be traced back to green plants. Free distribution by A.P. Government 199 Once the food is eaten, its energy follows a variety of pattern through the organisms. Not all the food can be fully digested and assimilated. Hair, feathers, insect exoskeletons, cartilage and bone in animal foods, cellulose and lignin in plant foods cannot be digested by most animals. These materials are either egected by defaecation or regurgitated in pellets of indigested remains. Assimilated energy (that is not lost through respiration or excretion) is available for the synthesis of new biomass through growth and reproduction. Organisms lose some biomass by death, disease or annual leaf-drop, where they enter the detritus pathways of the food chain i.e., after the death and decomposition of organisms the materials flow back into the environment. The remaining biomass is eventually consumed by herbivores or predators and its energy there by enters the next
higher trophic level in the ecosystem. Solar energy Producers (Chemical energy) Consumers (Chemical energy) Heat Heat The materials keep on cycling i.e. entering the living beings and through death and decay returning to the soil and atmosphere Such a flow of materials between organisms and their environment is called Cycling of materials or mineral circulation or Biogeochemical cycles (You have learnt in class IX ). Energy enters the producers in the ecosystem from the sun in the form of solar energy or solar radiation. No other organisms except green plants and Photosynthetic bacteria due to the presence of chlorophyll are capable of absorbing solar energy and converting it into chemical energy. From the producers, the chemical energy passes to the consumers from one tropic level to the next through food. At each tropic level, organisms use most of the food energy that they assimilate into their bodies to fulfill their metabolic requirements- performance of work, growth and reproduction. Because biological energy transformations are inefficient, a substantial proportion of metabolized food energy is lost, unused as heat. Only a small fraction goes to the eater at next trophic level. Organisms are no different from man-made machines in this respect. Most of the energy in gasoline is lost as heat in a car’s engine rather than being transformed into the energy of motion. In natural communities, energy 200 X Class Our environment - Our concern used to perform work or dissipated as heat cannot be consumed by other organism and is forever lost to the ecosystem. The effects of human activities on ecosystems In the earlier classes we have studied about different types of pollution as a result of human interventions in ecosystem. In this section we will try to understand that when we cut forest to grow food crops, how this activity brings harmful changes in ecosystem and affects organisms of each tropic level. Let us study a pond ecosystem to understand the components of environment, their interactions and effects of human interventions in the following story. Story of Kolleru Lake Fresh water lakes provide the nutritional requirements of the world’s poorest people. One such lake is Kolleru which is no ordinary wetland. It is one of the largest fresh water lakes in India, existing between West Godavari and Krishna districts of the state of Andhra Pradesh. The catchment of the lake extends up to 6121 km2. The lake Kolleru discharges its excess water in to the Bay of Bengal through the twisty channel called Upputeru, which is about 65 km long. Nevertheless, the Kolleru wetland receives huge quantity of nutrient rich sediments from the flood plains of these rivers. In November 1999, the Government of Andhra Pradesh had declared the lake as Bird Sanctuary. This lake is hosting 193 species of birds and a variety of flora and fauna, including medicinal plants. It attracts migratory birds from northern Asia and Eastern Europe between the months of October and March and it is estimated as 20,00,000 birds per year. The lake was also an important habitat for an estimated 20 million residents. This largest sweet water lake has not only shrunk in size but faced great threat due to pollution in the last three decades as revealed by satellite pictures. The decrease in water area and muddy ground in the lake in flooding resulted fig-7: Kolleru Lake Free distribution by A.P. Government 201 Area in 2004 (Km2) 62.65 47.45 15.20 0 99.74 16.62 1.37 Classes Lake –water spread area Lake with sparse weed Lake with dense weed Table-1 Area in 1967 (Km2) 70.70 0 0 Lake-liable to flood in rainy season 100.97 Aquaculture ponds Rice fields Settlements Total 0 8.40 0.31 180.38 180.38 In which year lake-water spread area is more? Why? problems in the lake area. Observe the data given in the following table. • • How do you think weeds are more in the lake? • What are the reasons for decrease in lake area? • How do the above reasons lead to pollution? • How was the threat to the lake due to pollution discovered? • What could be the reason for the migration of birds to this lake? Being a profitable business, Aquaculture in Kolleru was started extensively in the eighties which later spread to other areas in the Krishna Godavari delta and attracted a large number of investors to the area. In 1996, almost entire lake was brought under cultivation and bunds were constructed to keep water out to protect the crops. This divertion affected the natural flow system of the lake. The water holding capacity of the lake is also found significantly reduced. In due course of time activities such as agriculture and industries came along in ever growing intensity in the catchment area of the lake. Consequently, the drains and rivulets carry substantial quantity of various types of pollutants into the lake. The major sources of pollution are agricultural runoff containing residues of several agrochemicals, fertilizers, fish tank discharges industrial effluents containing chemical residues and different types of organic substances, municipal and domestic sewage. Excessive nutrient addition, especially from anthropogenic sources, led to explosive weed growth. Ex: Eichornia 202 X Class Our environment - Our concern As a result, the water of the lake turned more alkaline in nature, turbid, nutrient rich, low in dissolved oxygen (DO) and high in biochemical oxygen demand (BOD). Water borne diseases like diarrhoea, typhoid, amoebiasis and others are said to be common among the local inhabitants who are unaware of the state of pollution in the lake water. Vector borne diseases also increased. Prawn and fish have been found to be affected by diseases leading to some farms being abandoned. The lands thus abandoned are useless for agriculture too. Let us observe the following table showing different activities in the lake and their influence. Table-2 Agricultural Practices Aquaculture Industrial activities practices Human activities Problem Biological 1. Decreased Migratory birds 2. Population loss of flora and fauna 3. Pathogens Chemical 1. Eutrophication 2. Toxic contamination Physical 1. Siltation 2. Flooding - - + + + + + + - + + + + - - + - Legend: (+) means has influence on the mentioned problem (-) means has no influence on the mentioned problem • What are the factors that affected the number of migratory birds to decrease? • Do you find any relationship between biological and physical problems? • What are the reasons for chemical problems? • What happens if the dissolved oxygen reduce in lake water? • Is BOD of turbid and nutrient rich water high or low? What are it’s consequences? • People living in catchment area of Kolleru faced so many problems. Why? Free distribution by A.P. Government - + + - + 203 The Ministry of Environment and Forest (MoEF), Government of India (GoI) constituted a committee “Operation Kolleru” to protect the lake. The objective of the programe is to bring back the ecological balance of Kolleru Lake which is a Gift of nature. Activity-1 Observe any water ecosystem in your surroundings and identify the different food chains and food web operating in this ecosystem. Write the following details in your notebook. Work Sheet 1. Names of the students in a group: _____________________ Date: _____________ 2. Name of the ecosystem: _____________________________________________ 3. Topography: ______________________________________________________ 4. Names / Number of plants (producers) identified: _________________________ 5. Names / Number of animals identified: __________________________________ 6. Identify the different types consumers and name them & mention their number below: Herbivores (Primary consumers): ___________________________________ Carnivores (Secondary consumers): __________________________________ Top carnivores(Tertiary): _________________________________________ 7. Food relationships among them: food habits / preferences: ___________________ 8. Show / draw the different food chains: __________________________________ 9. Showcase the food web: _____________________________________________ 10. List out all abiotic factors existing in the ecosystem: ______________________ ( A check list can be given, and asked to tick) 11. Is there any threat to the ecosystem? Yes/No _____________________________ If yes, what? and how? _____________________________________________ Suggest few remedial measures _______________________________________ When a forest is cut down and a food crop is grown in its place, a natural established ecosystem with its vast number of species in a state of dynamic equilibrium is replaced by a monoculture i.e. an unnatural concentration of a single crop of various kinds grown in different fields to provide cereals or roots, others grass for domestic animals. When we grow crops in large concentrations we also get food in abundant quantities. This situation is optimum for pest, parasites like fungi to grow on this food material. If the quantities of food are larger then 204 X Class Our environment - Our concern multiplication of pests and parasites is rapid and the resulting damage would be great. To avoid such happening we have tried to eliminate these competitors for the crops by using toxic chemicals (pesticides, herbicides, and fungicides). Many of them have been very effective, but their use has also created new problems. The perfect pesticide is the one which destroys a particular pest and is completely harmless to each and every other form of life, no such pesticide exists or likely to be. • Name any two pesticides you have heard about? • How are the food grains and cereals being stored in your house and how do you protected them from pests and fungus? Pesticides are often indiscriminate in their action and vast number of other animals may be destroyed. Some of these may be predators which naturally feed on these pests, others may be the prey for other animals. Thus causing unpredictable changes in food chains and
upsetting the balance within the ecosystem. A further danger is that some have a cumulative effect. Pesticides vary in their length of “life” as toxic substances. Some of the pesticides as well as herbicides are degradable. They are broken down into harmless substances in a comparatively short time, usually a year. Others are non degradable, and include those which contain mercury, arsenic or lead. These non degradable pesticides are potentially dangerous as they accumulate in the bodies of animals and pass right through food web. Being further concentrated at each step until animals at the top of pyramid may receive enough to do considerable harm. The process of entry of pollutants into a food chain is known as Bioaccumulation, whereas the tendency of pollutants to concentrate as they move from one trophic level to the next is known as Biomagnification. Let us observe the following research study on the effects of bio accumulation on human health. Seasonal Bioaccumulation of heavy metals in fish (cyprinus carpio) of Edulabad Water Reservoir (EBWR), Andhra Pradesh, India. The aquatic bodies such as water reservoirs and rivers surrounding the urban areas in India, pose a serious risk for survival of aquatic organisms due to water quality deterioration through excessive nutrient inputs, acidification, heavy metal contamination and organic pollution. The aquatic biota is being contaminated with heavy metals due to industrialization and anthropogenic activities. Free distribution by A.P. Government 205 Recently, fish are considered to be the bio indicators of metal contamination in environmental monitoring because fish species are strongly respond to stress conditions. fig-8: Edulabad Water Reservoir A study was undertaken to assess the enrichment of heavy metal such as Lead (Pb), Cadmium (Cd), Chromium (Cr), Manganese (Mn), Nickel (Ni) and Ferrum (Fe) contamination in Edulabad Water Reservoir (EBWR) which is located in urban areas of Rangareddy district of Andhra Pradesh, highly polluted with industrial and agricultural effluents. Cyprinus carpio (common scale carp) is a cheap and high proteinaceous fish used as food for human beings, living in polluted EBWR was chosen for the study. Heavy metals in water samples and its bioaccumulation in various tissues including liver, kidney and gill of the fish growing in the reservoir were analyzed along with glycogen and lipids contents. A parallel study was conducted in water samples and fishes collected from Bibi Nagar, Nalgonda district fresh water reservoir because it is less polluted, located 30 km near to EBWR in Andhra Pradesh. The results obtained in present study revealed that higher bioaccumulation and lower glycogen and lipid content in the fish of EBWR when compared to Bibi Nagar fresh water reservoir. The water and fish samples were collected in three seasons namely pre-monsoon (February-May), monsoon (June-September) and post monsoon (October-January) in each year. Three water samples were collected in three station thrice in each season from each tank in cleaned polythene bottles and tightly stopped and used for heavy metal analysis from June 2005 to May 2007. The metal concentrations in EBWR reservoir were found to be higher than Indian standard limits and exhibiting the following sequence, Fe > Pb > Cr > Ni > Cd. The heavy metals could find their way into the human food chain, we analyzed bioaccumulation of these metals in the fish tissues. The bioaccumulation of these metals in fish tissues were of the following trend, Cd > Cr > Fe > Ni > Pb. Higher bioaccumulation factors were found for Cd in liver, gill and kidney indicated the sensitivity of fish to this metal even at low concentrations. 206 X Class Our environment - Our concern It is found that the bioaccumulation was lesser in monsoon season than pre and post monsoon seasons. The heavy metals could find their way into human beings through food chain. This bio accumulation cause various physiological disorders such as hypertension, sporadic fever, renal damage, nausea, etc. It is concluded that unplanned urban settlements, combined with the proliferation of unorganized small-scale industries and the sewage lead to the contamination of the EBWR. Such increased bioaccumulation of heavy metals in fishes not only disturbs aquatic life but also increases health risk in human beings through food chain. • Where from pollutants enter to the water sources? • How can you say fishes living in water having heavy metals in their bodies? • Researchers found that pollution levels increase during monsoon season. Why they found so? • Why did people also suffer from various diseases after consuming fishes living in local water reservoir? In many areas man has changed the natural ecosystems to a great extent by damming rivers, draining marshes, re-claiming land from the sea, cutting down forests, plough-ing up land and growing crops, and by building towns, cities, canals and motorways. These changes have greatly altered the communities of plants and animals living there. Consider the development of a large town, for example. There will be three kinds of change: a) Some plants and animal species will die out. b) Some will adapt to the new conditions sufficiently to survive in re- duced numbers. c) Some will benefit by the new conditions and will increase in numbers. Do you know? Minamata disease was first discovered in Minamata city in Kumamoto prefecture, Japan, in 1956. It was caused by the release of methyl mercury in the industrial wastewater from the Chisso corporation’s chemical factory, which continued from 1932 to 1968. This highly toxic chemical bioaccumulated in shellfish and fish in Minimata Bay and the Shiranui Sea, which, when eaten by the local populace, resulted in mercury poisoning. While cat, dog, pig and humans deaths continued for 36 years. Let us read the following story to know how cruel the human actvities are against the nature. Free distribution by A.P. Government 207 Sparrow campaign Any living organism can’t avoid crises since they are a normal part of life. However, none have ever encountered a disaster on the level of that which fell upon the Chinese Sparrows in 1958. The environmental crisis in question was not a natural one rather, it was manmade. In the entire history of sparrows around the world, they have never been hunted down as they were in China in 1958. fig-9: Sparrow in danger A radical campaign to rapidly increase China’s industrial output by mobilizing the country’s vast rural peasantry took place at this time. It was set in motion by the government with the intention to achieve rapid increase in industrial production that China would catch up with the rest of the civilized world. China had an agrarian society then. One of the most famous initiatives then was to form co-operatives or collectives up to 5,000 families and this initially yielded double the amount of crops grown. This initial success led to ambitious goals for the following year, but the weather didn’t cooperate. Even though fewer crops were harvested, rural officials overstated the amount of grain for fear of not meeting their quotas. This over-reporting led to an imbalance between the demand and the supply. The sparrows were accused of pecking away at the supplies in warehouses at an officially estimated rate of four pounds of grain per sparrow per year. In the cities and the outskirts, almost half of the labour force was mobilized into the anti-sparrow army. fig-10: Sparrow campaign People started trapping, poisoning and killing sparrows in large numbers. Several free-fire zones were set up for shooting the sparrows. People would beat drums to scare the birds from landing, so the sparrows were forced to keep flying until they dropped dead from fatigue. Sparrow nests were torn down, eggs were broken, and nestlings were killed. Non-material rewards and recognition were offered to schools, work units and government agencies in accordance with the number of sparrows killed. Later some scientists who cut open the digestive systems of dead sparrows found that three-fourth of the contents were of insects harmful for crops and only one-fourth contained grains. The scientific findings 208 X Class Our environment - Our concern showed that sparrows were basically a beneficial bird for humans. Rather than being increased, crop yields after the campaign were substantially decreased. Though the campaign against sparrows ended it was too late. With no sparrows to eat the locust populations, the country was soon swarmed. Locusts coupled with bad weather led to the Great Chinese Famine. Use of pesticides against locust population further degraded the land. Instead of working in the fields, millions of farmers had to leave their villages to work for industries. Very small area was left under agriculture and food shortages became everyday occurrence. • What is the food chain that has been discussed in the above case? • How did the campaign disturb the food chain in the fields? • How did these disturbances affect the environment? • Is it right to eradicate a living organism in an ecosystem ?How is it harmful? • Were the sparrows really responsible? What was the reason for the fall in crop production? • What was the impact of human activities on the environment? • What do you suggest for such incidents not to occur? • Read the poem “Manavi” in class VIII biology textbook and dis- cuss in your class. Steps towards prevention If we think about the ways through which we can prevent ourselves and other living beings from the harmful effects of the use of toxic material as pesticides, then the instanct reaction may be to Ban the pesticides. It is easy to say “Ban all pesticides” but the pests still have to be kept in check. After using pesticides then also we are having a significant amount of loss of food material because of pests. Now you can imagine if pesticides were totally banned what would happen to the diseases? Are they really con
trolling the pests that effect the crops we vitally need for our growing population? The long term solution to this problem is to find other effective methods of controlling pests which have far less harmful effects and are based on sound biological principles. Here are some of the important methods used. Rotation of crops: Growing different crops on a particular piece of land in successive years reduces the occurance of pests and damage to the crops from year to year in that area. Free distribution by A.P. Government 209 Studying the life histories of the pests: When this is done it is sometimes possible to sow the crops at a time when least damage will be caused. Biological control: Introducing natural predator or parasite of the pest. Sterility :Rendering the males of a pest species sterile Genetic strains: The development of genetic strains (genetically modified plants) which are resistant to certain pest. Environmental ethics: This is concerned with morality of human activities as they affect the environment. People need to know besides laws regarding environment there are some basic ethics what is is right and what is wrong in view of environment. So one should have awareness about our environment. Protect nature Protect yourselves.Read the poem “Or will the dreamer wake” given in unit-VI in your english reader of class-10. Courtesy-1: Research paper on status of Kolleru Lake between 1967 and 2004 by Marappan et.al., 2006. Courtesy-2: International Jornal of Life Sciences Biotechnology and Pharma Research. This research was done by Vidya Sagar Gummadavelli, Ravi Shankar Piska, Srinu Noothi and Pavan K. Manikonda. Key words Food chain, Food web, Niche, Ecological Pyramid, Biomass, Pesticides, Bioaccumulation, Biomagnification, Ecofriendly activities, Ecofriendly activities, Environmental ethics. What we have learnt Food chain shows that how energy passed from one organism to another. • • The arrows between each item in the food chain always point from the food to feeder. • Pyramid of numbers and pyramid of biomass are other ways to show food relationship and flow of energy among living things • A pyramid is a structure whose shape is roughly that of a pyramid in geometric sense. • • Pyramid of number shows the population of organisms at each trophic level in a food chain. Pyramid of biomass represents the available food as a source of energy at each trophic level in the food chain. • Biomass can also be used as a bio fuel. • Toxic material used to prevent the pest, fungus and other disease away from the food crop and grains do harms in many ways to ecosystem. • Bioaccumulation is the entering of pollutants in the food chain. • The tendency of pollutants to concentrate as they move from one trophic level to the next is known as Biomagnification. 210 X Class Our environment - Our concern • There are several alternatives for pesticides through which we can get more yields with less damage like rotation of crops, biological control, development of genetic resistant strains etc. Improve your learning 1. What happens to the amount of energy transferred from one step to the next in a food chain?(AS1) 2. What do pyramids and food chain indicate in an ecosystem?(AS1) 3. Write a short note on pyramid of number for any food chain? What can we conclude from this pyramid of numbers?(AS1) (ii) insect (i) tree (iii) woodpecker 4. What is biomass? Draw a pyramid of biomass for the given food chain- (AS1) (iv) hawk (i) grass leaves (ii) herbivores (iii) predators 5. How is using of toxic material affecting the ecosystem? Write a short note on bioaccumulation and biomagnifications.(AS1) 6. Should we use pesticides as they prevent our crop and food from pests or should we think of alternatives? Write your view about this issue and give sound reason for your answer.(AS1) If you want to know more about flow of energy in an ecosystem, what questions do you ask?(AS2) 7. What is a trophic level? What does it represent in an ecological pyramid?(AS1) 8. 9. What will happen if we remove predators from food web?(AS2) 10. Observe a plant in your kitchen garden, and write a note on producer- consumer relationship.(AS3) 11. What type of information do you require to explain pyramid of biomass? (AS4) 12. Draw a pyramid of numbers considering yourself as a top level consumer.(AS5) 13. Prepare slogans to promote awareness in your classmates about ecofriendly activities.(AS7) 14. Suggest any three programmes for prevention of soil pollution in view of avoiding pesticides.(AS7) Choose the correct answer 1. What does a food chain always start with (a) The herbivore (b) The carnivore (c) The producer 2. Which of the following do plants not compete for? (b) Food (a) Water (c) space ( (d) none of them ( (d) all above 3. Ban all pesticides, this means that a) Control on usage of pesticides c) promote eco friendly agricultural practices 4. According to Charles Elton b) prevention of pesticides d) stop bio chemical factories ( ( ) ) ) ) a) carnivores at the top of the pyramid c) No producers at the top of the pyramid d) a and c b) energy trapping is high at the top of the pyramid Free distribution by A.P. Government 211 Chapter 10 Natural Resources We learnt about natural resources like water, soil, forests, flora, fauna etc. and how to conserve them, in previous classes. We also learnt about the pollution of natural resources as a result of human activities. Natural resources are present in abundance, but do we really manage them properly? We shall study about human interventions affecting them and efforts that are being made to sustain and save them. Try to make an exhaustive list of natural resources in your locality. Try to find out about a particular resource especially one that is scarce in detail. Some questions below will help you to find out more about the resources. • Which resource in your locality is scarce? How does it affect you? • Was the resource present in abundance earlier? • How did it become scarce over the years? • What can you do as a step towards saving a resource? Let us study about two villages of Andhra Pradesh to make a study of an important resource and see what happens when it becomes scarce. Case I: Situation in two villages Vanaparthy and Vaddicherla of Warangal District A survey was conducted in two villages,Vanaparthy and Vaddicherla of Warangal District of Telangana region -the first with no-scarcity (good), and the second with scarce groundwater.Well census was carried out in the villages in order to get a complete picture of well irrigation and its status as well as availability of water. Basic information on well irrigation 212 X Class Natural Resource was collected using a small questionnaire from all the well owners in the sample villages. Detailed information regarding various socio-economic aspects was collected using a detailed questionnaire from a sample of 25 households owning wells. Families in both the villages were asked to narrate the changes in groundwater situation during the last five years. There are no alternative sources of supply as against wells in Vaddicherla, whereas there is an existing tank that has been converted into a percolation tank, so that the water situation is much better in Vanaparthy. Do you know Percolation tanks are normally earthen dams with masonry structures where water may overflow. Construction materials consist of a mixture of soil, silt, loam, clay, sand, gravel, suitably mixed and laid in layers at the base or bed and sides. It is properly compacted to achieve stability. Outlets for surface irrigation are made and a cut-off trench is made below the earthen bund or dam with depth limited to one fourth of the height between bed level and full storage level. Percolation tank Basic features of the villages Vanaparthy and Vaddicherla are almost similar in terms of occupational pattern, cropping pattern, infrastructure and social services. In both the villages small farmers are in majority. Vanaparthy has the higher average household income. The main livelihood activity in these villages is cultivation and the primary source of irrigation is well. Household income is dependent on the status of groundwater. Vanaparthy has a higher proportion of its area under irrigation. The cropping pattern which influences average household income in these villages differs substantially.Though rain has not been consistent for a few years, farmers in these villages prefer growing paddy. Village Total Area (acres) Table-1: Area under irrigation Percentage Area Irrigated Number of Wells Sample Size Vanaparthy Vaddicherla 3791 2970 25 15 155 175 25 25 • What is the total irrigated area in acres, in Vanaparthy? Free distribution by A.P. Government 213 • If one needs to irrigate all the land in Vanaparthy, how many wells would be required? • Though the number of wells is less in Vanaparthy, the area under irrigation is more as compared to Vaddicherla. How is this possible? • Do you think the area under irrigation will change due to rise in population? The change in area under cultivation, percentage change in number of wells and cropping pattern in 5 years as narrated by the people has been presented in table-2. The population in the villages have also gone up in a period of 5 years by nearly 10%. Table 2: Status after five years Village Percentage change in area under irrigation Percentage decline in number of wells Vanaparthy Vaddicherla -14 -30 -39 -68 Percentage change in area under crops Paddy Cotton Gingelly All Crops K R K R 11 -17 -17 -17 163 86 -22 -50 27 138 -05 -50 K stands for Kharif while R stands for Rabi. Negative values indicate loss/ decline, while positive ones show gain/rise. If the number of wells is 155 now, what was it 5 years back? • • What do you think ‘decline in number of wells’ represents? • How would crops be affected due to decline in the number of wells? • Compare table 1and2 and state what they tell us about the area under irrigation in both the villages? • Which
village do you think is more affected? • What is the change in types of crops grown in the villages? Of late, most of the open dug wells were converted into bore wells that could reach greater depths of ground water zones and would also reduce loss of water by surface evaporation.Most of the open wells have dried up and water tables have gone down substantially during the last 5 years. During this time, 85 percent of the wells, mostly open, have dried-up in Vaddicherla while 45 percent of the wells dried-up in Vanaparthy. • If 45 percent of wells have dried up in Vanaparthy, and there is 39 percent decline in number of wells, what percentage of wells do you think have been converted to bore wells? 214 X Class Natural Resource • By comparing the two villages, find out where greater number of wells dried up? What methods would have saved the wells in other village? • Which type of farmers, those having small land holdings or those with large land holdings are most affected when wells dry up? If water resource becomes scarce, will it affect the nature of soil in an area? • • As wells dry up, how are people in the area dependent on the well affected? • Why do you think water became scarce mainly in Vaddicherla? Water is usually pumped out of wells and bore wells using electricity. Farmers with small land holdings or small farmers tend to spend more money per well in terms of installation of pump sets and pipeline connections to farms(or capital expenditure) as well as running costs towards maintenance, electric charges etc. On per acre basis, both capital and running costs are the lowest in Vanaparthy (no-scarcity village) and highest in Vaddicherla (scarcity village). Table 3: Annual expenditure on well irrigation for small and large farmers(2002) Village Type of Farmer Depth of bore well (in feet) Percentage area irrigated per well Vanaparthy Large Small Vaddicherla Large 130 - 200 110 - 180 90 - 300 Kharif Rabi 2.50 1.24 2.00 1.13 1.53 0.87 Small 60 - 200 0.99 0.46 Total cost of well irrigation per acre per year in rupees 25000- 70000 25000- 65000 22000- 50000 20000- 45000 Is the availability of water resource same for a small and a large farmer? • • Do you think the availability should be same for everyone in an area? • A well irrigates more area in Kharif season as compared to Rabi, how is it possible? • How should a farmer utilize such a condition? • If a well can irrigate 2.5 percent of cultivable land, how many wells would irrigate whole of the land? • Which factor has a greater effect on expenditure, number of wells or depth of a well? • What is the total expenditure on a whole cultivable land owned by a small farmer in Vaddicherla?How do you think a small farmer meets this expenditure? • What could help the small farmer reduce expenditure? (Hint: think of crops that require less water) Free distribution by A.P. Government 215 • Do you think increasing the depth of bore well is a good solution for increasing total land area under irrigation? Why/Why not? Do you know? Because of varying monsoon behaviour in recent years, there is a pressure on groundwater utilization. Indiscriminate tapping of groundwater in the State by too much drilling and construction of deep tube wells and bore wells, have resulted in over exploitation and depletion of groundwater resources in certain areas. Average fall of water level was around 3 meters in the State during the period of 1998- 2002. Table 4: Income on crops Village Type of Farmer Net income per acre in Rupees Paddy Paddy Cotton Gingelly Kharif Rabi Total income per acre year in rupees Vanaparthy Large Small Vaddicherla Large Small 8200 8700 4900 7046 8490 10889 10698 5970 4000 9128 7380 3031 3300 3110 3595 2650 25100 29535 24263 22189 • Which crop is most profitable for a small farmer in Vaddicherla? • What is the difference between a small farmer in Vanaparthy and Vaddicherla? • Which crop could replace paddy and be profitable as well for a small farmer in Vanaparthy? • Though we know that paddy consumes maximum water, why do you think farmers still like to grow paddy? • What is the impact of a depleting resource upon the farmers? • Do you think the income of a small farmer in Vaddicherla is sufficient enough to meet his expenditure? • What are the major causes of pitiable condition of small farmers at Vaddicherla? • Do you think farming as an occupation is profitable for the small farmer in Vaddicherla? • Would the farmer have to look for other kind of occupations to meet his ends? • How did the availability of water affect a small farmer at Vaddicherla? A project of the Centre for world solidarity(Secundrabad, A.P) that addresses sustainability of ground water intervened to help in recharging wells that were drying up in the villages. 216 X Class Natural Resource They encouraged more water sharing among farmers. They formed groups of farmers including large and small ones who would use the same water resource. Farmers were also motivated to use irrigation techniques like drip irrigation, sprinklers etc.(collectively called as micro irrigation techniques). Construction of soak pits to tap rainwater optimally was carried out as community efforts. Soakpits helped in recharging dried up bore wells. Dykes or barriers, nearly 30 cm thick of brick-cement or stone cement barrier, extending down to the compact bedrock, with mud or clay fillings were built in underground streams to tap ground water optimally. • How can wells be recharged? • How would recharging dried up wells help farmers of Vaddicherla? • What does the case tell us about a water resource and its effect on farmers? Water for all Out of all the water on Earth, salt water in oceans, seas and saline groundwater make up about 97% of it. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers, ice and snow(nearly two thirds of the available freshwater), 0.7–0.8% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers. Though it is a meagre portion of the whole, if used judiciously, shall last for a long time. • How do you think we can use water judiciously? • Why were farmers at Vanaparthy at a better state than those at Vaddicherla? • How did farmers of Vaddicherla and Vannaparthy recharge their ground water resources? Do you know? In ancient times, village boundaries were decided upon on a watershed(land between water sources usually of two rivers or streams) basis fixed at the common point of the drainage system in between two villages by the expert farmers in the village. Such boundaries were socially acceptable to all the members of the system. Case II: A Study of Kothapally Village, an example of water management effort This tells us how people in the village through proper guidance could make optimum use of available water in the village. Free distribution by A.P. Government 217 A survey of Kothapally village indicated that initially: (i) dry land areas were more extensive than irrigated land; (ii) literacy was low; (iii) labour was scarce; (iv) more fertilizers/pesticides were used on small farms (v) crop yields were low, (vi) there was not even a single water harvesting structure in the village. Interventions to enhance productivity and income (Soil and water conservation measures) International Crop Research Institute for Semi-Arid Tropics( ICRISAT) educated villagers by large and provided technical support for cost-efficient water storage and soil conservation structures. The measures were community as well as individual farmer-based. These helped to restore some resources and conserve others so that they may never be depleted. Thus sustainable management was carried out. What is ICRISAT, where is it? What are it functions? Discuss with your teacher and prepare on it. Community-based interventions Fourteen water storage structures (one earthen and 13 masonry dams) with water storage capacity of 300 to 2 000 m3were constructed. 60 minipercolation pits and field bunding on 38 hectare were completed. fig-1: Communitybased masonry dam fig-2: Contour field bunding Do you know? Sri Rama Sagar Project also known as the Pochampadu Project is a project on the Godavari River. It is a “lifeline for a large part of Telangana”. It is mainly an irrigation project to serve needs in Karimnagar, Warangal, Adilabad, Nalgonda, and Khammam districts. But all is not well with the project as most of the water is retained before reaching Andhra Pradesh due to the dams built on river Godavari in another State.As of August 2013, the project has an estimated capacity of 80.66 TMC. Sri ram sagar project 218 X Class Natural Resource Twenty-eight dry open wells, near nalla or the Lakshmi canal (sourced from the Sri Rama Sagar project reservoir) were recharged by building dykes or barriers in the nalla and retaining the runoff rainwaterin it. A users group was formed for each water storage structure, and the water collected in the storage structures was exclusively used for recharging the groundwater to the dried wells. Farmer-based interventions Farmer-based soil and water conservation measures implemented in individual fields were broad bed furrow (BBF) landform and contour planting. These are all useful to conserve soil and water,fertilizer application and weeding operations, field bunding of 38 hectare, around boundaries in rectangular or in contours to conserve rain water. Planting Gliricidia (Madri, a leguminous plant adapted to grow in dry areas) on field bunds to strengthen them and make the soil nitrogen-rich.Farmers were encouraged to use water resource jointly and irrigate land using micro irrigation techniques. fig-3: Broad bed furrow fig-4: Plantation of Gliricidia on bunds Farmers obtained 250 kg more pigeon pea and 50 kg more maize per hectare using broad bed furrows and micro irrigation techniques. Drip irrigation (a type of micro irrigation) can reduce water consumption by 70% but unfortunately only 2% of cultivable land around the world is irriga
ted in this manner. Wasteland development and tree plantation Saplings of useful species were planted along the roads, field bunds and nalas. Contour trenches at 10 m intervals with a 0.3 m height of bund were laid out. Custard apple plantation along with other useful species in trenches and Gliricidia saplings on bunds fig-5: Tree plantation on wasteland Free distribution by A.P. Government 219 was undertaken by the farmers. 2500 fruit trees and teak plants were planted. • What other ways of restoration of a resource does the Kothapally case tell us about? • What are some common means of restoration and conservation of water resource that we came across in the cases studied so far? According to a survey conducted in the year 2004 Total amount of water availablein Andhra Pradesh - 3814 thousand - million cubic feet (TMC) Total amount utilised Irrigation Domestic use Industries Power generation Amount required for utilisation by 2025 is 3989TMC of which 3,814TMC is for irrigation, 122TMC for domestic use, 51TMC for industries and 2TMCis for power generation. • What do you think will happen if we do not take care of the sources 2300 TMC of which 2268 TMC 21 TMC 10 TMC 1 TMC of water? • How do you think we will meet our requirements in future? • Do you think we would have to depend upon other states or perhaps other countries as well? • Could the amount of water used for irrigation in Andhra be reduced? How? • Does cropping pattern have any role to play in reduction of water utilisation? (Hint:Think of the case of Vaddicherla and Vanaparthy). • Do you think one needs laws for distribution of water and its use? Why/Why not? Source of irrigation water in Andhra Pradesh • How much per cent of area do you think is irrigated by other sources of water? We have seen that major consumption of water is in the farming sector. In spite of some major rivers like Godavari and Krishna, the major source of irrigation is groundwater. • Why is it important to recharge the ground water sources? • Why do the rivers fail to benefit the state to an extent they should have? 220 X Class Natural Resource • Since ground water resources are getting depleted at a fast pace what are the alternatives? • River Godavari fails to provide water for the projects like Sri Rama Sagar in our State due to over usage of water by some other state. How should and countries work to provide enough for all? states Other sources 5% Tanks 15% Cannals 37% Ground water 43% fig-6: Sources vs area under irrigation It is comforting to think water is a renewable resource but we must know what limitless exploitation of a resource can lead to. According to United Nations Development Programme, “Water resource in an area, where annual water supply drops below 1700 m3 per person, is becoming scarce.” The food and Agriculture Organisation of the United Nations has predicted that by 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity. Activity-1 Study the different ways in which water is used, misused and recycled in the area where you stay. Prepare a questionnaire with the help of your friends and teacher and study at least five households in your locality for the same. Also explore and discuss ways to provide water for all. Natural resources around us The Earth’s natural resources include air, water, soil, minerals, fuels, plants, and animals. Conservation is the practice of caring for these resources so all living things can benefit from them now and in the future. All the things we need to survive, such as food, water, air, and shelter, come from natural resources. Some of these resources can be replaced after they are used and are called renewable resources. Other resources, such as fossil fuels, cannot be replaced at all. Once they are used up, they are gone forever. This is because it takes a long time for their formation while their consumption occurs very quickly. These are nonrenewable resources. People often waste natural resources. Animals are overhunted. Forests Free distribution by A.P. Government 221 are cleared, exposing land to wind and water damage. Fertile soil is exhausted and lost to erosion because of poor farming practices. Fuel supplies are depleted. Water and air are polluted. If resources are carelessly managed, many will be used up. If used wisely and efficiently, however, renewable resources will last much longer. Through conservation, people can reduce waste and manage natural resources wisely. Water use restrictions are in place in many regions of the world. In Australia, in response to chronic shortage resulting from drought, restrictions are imposed on activities like, watering lawns by using sprinkler systems, washing vehicles, using hose pipes to clean paved areas, and refilling swimming pools. The population of human beings has grown enormously in the past two centuries. Billions of people use up resources quickly as they eat food, build houses, produce goods and burn fuel for transportation and electricity. The continuation of life as we know depends on the careful use of natural resources. The need to conserve resources often conflicts with other needs. For some people, a forest area may be a good place to put a farm. A timber company may want to harvest the area’s trees for construction materials. A business company may want to build a factory or shopping mall on the land. All these needs are valid for us, but sometimes the plants and animals that live in the area are forgotten. The benefits of development need to be weighed against the harm to animals that may be forced to find new habitats, the depletion of resources we may want in the future (such as water or timber), or damage to resources we use today. • Think of any resource from your surrounding other than water that you cannot do without and write a short account on its sources, availability and condition. Development and conservation can coexist in harmony. When we use the environment in ways that ensure we have resources for the future, it is called sustainable development. There are many different resources we need to manage and conserve and main order to live sustainably. • What would you do to motivate others to manage an important resource in your locality? • How did the villagers in Kothapally resort to sustainable management? fig-7: Sustaibable development 222 X Class Natural Resource Forest: an important renewable resource Why do you think forests are important? Every continent except Antarctica has forests. Which are rich habitat for plants and animals. Forests serve as a lung for the world and a bed of nutrients for new life to prosper. They provide us innumerable products and in an urge to extract them we indiscriminately destroy them. People clear forests to use the wood, or to make way for farming or development. Each year, the Earth loses about 36 million acres of forest to deforestation an area about half the size of our state. Deforestation destroys wildlife habitats and increases soil erosion. It also releases greenhouse gases into the atmosphere, contributing to global warming. Deforestation accounts for 15 percent of the world’s greenhouse gas emissions. Deforestation also harms the people who rely on forests for their survival, hunting and gathering, harvesting forest products, or using the timber for firewood. Sustainable forestry practices are critical for ensuring resources well into the future. Perhaps the bishnoi’s of Rajasthan could tell us how. As we recall brave Amrita Devi and her daughters, followed by villagers who clung to trees in the forest surrounding their village and laid down their lives to save them, we are faced with a realization about how great a movement towards conservation can be. They were protesting against the Kings’ order to collect wood for the construction of his palace and defending the pledge of peaceful coexistence taken by them as a bishnoi. It is a set of 29 rules to conserve natures’ resources that every bishnoi vows to protect. You have also studied about the Chenchu tribe of our state and are aware of how carefully they extract resources from nature and help revive it. Some sustainable forestry methods include using low-impact logging practices, harvesting with natural regeneration in mind, and avoiding certain logging techniques, such as removing all the high-value trees or all the largest trees from a forest. Trees can also be conserved if consumers recycle. People in China and Mexico, for example, reuse much of their wastepaper, including writing paper, wrapping paper, and cardboard. If half the world’s paper were recycled, much of the worldwide demand for new paper would be fulfilled, saving many of the Earth’s trees. We can also replace some wood products with alternatives like bamboo, which is actually a type of grass. Free distribution by A.P. Government 223 Soil Soil is vital to food production. We need high-quality soil to grow the crops that we need. Soil is also important to plants that grow in the wild. Many other types of conservation efforts, such as plant conservation and animal conservation, depend on soil conservation. Poor farming methods, such as repeatedly planting the same type of crop in the same place, deplete nutrients in the soil. Soil erosion by water and wind increases when farmers plough up and down hills. One soil conservation method is called contour strip cropping. Several crops, such as corn, wheat, and clover, are planted in alternating strips across a slope or across the path of the prevailing wind. Different crops, with different root systems and leaves, help to prevent soil erosion. The practice of removing individual plants or small groups of plants leaves other plants standing to anchor the soil is called selective harvesting. Biodiversity Biodiversity is the variety of living things that populate the Earth. The products and benefits we get from nature rely on biodiversity. We need a rich mixture of living things to provide foods, buildi
ng materials, and medicines, as well as to maintain a clean and healthy landscape. When a species becomes extinct, it is lost to the world forever. Scientists estimate that the current rate of extinction is 1,000 times the natural rate. Through hunting, pollution, habitat destruction, people are speeding up the loss of biodiversity at an alarming rate. It’s hard to know how many species are going extinct because the total number of species is unknown. Scientists discover thousands of new species every year. For example, after looking at just 19 trees in Panama, scientists found 1,200 different species of beetles 80 percent of them unknown to science at the time. Based on various estimates of the number of species on Earth, we could be losing anywhere from 200 to 100,000 species each year. We need to protect biodiversity to ensure plentiful and varied food sources. Biodiversity is important for more than just food. For instance, we use between 50,000 to 70,000 plant species for medicines worldwide. A lawn in a colony is usually a pleasant sight but do you know that a lot of species of plants are completely destroyed to grow the type of grass on the lawn? Moreover the grass usually grown has been brought from other countries. 224 X Class Natural Resource • Observe a lawn in your area (if you have one) and see how it is maintained. Find out from the gardener the different types of plants that he removes from time to time. • Do you think a farmer does the same thing on his farm? Activity-2 Find out how many types of insects are present in and around your house. Do you find the same type of insects in all seasons? Make a chart of insect types(in case you don’t know their names give them one or take the help of your elders) and note their occurrence for at least a week in each season. Repeat for other seasons.Study for a year and find out when you have highest varieties of them. Study them for subsequent years to see if they have disappeared. Some governments have established parks and reserves to protect wildlife and their habitats. They are also working to abolish hunting and fishing practices that may cause the extinction of some species. Fossil Fuels Fossil fuels were produced from the remains of ancient plants and animals. They include coal, petroleum (oil), and natural gas. • What do fossil fuels provide us? • Why do we need to conserve them? Apart from its use in vehicles, many of the products we use today are made from petroleum. These include plastics, synthetic rubber, fabrics like nylon, medicines, cosmetics, waxes, cleaning products, medical devices etc. Natural gas 7% Oil 24% Nuclier 1% Waste 24% Other 2% Coal 42% fig-8: Percentage consumption of some resources in India We need to conserve fossil fuels so we don’t run out of them. However, there are other good reasons the pollution caused by them when burnt, to limit our fossil fuel use. Scientists are exploring alternatives to fossil fuels. They are trying to produce renewable biofuels to power cars and trucks. They have successfully produced electricity using the sun, wind, water etc. Free distribution by A.P. Government 225 Do you know? Seeds from the Jatropha curcas plant are used for the production of bio-fuel, a crucial part of India’s plan to attain energy sustainability.Andhra Pradesh has entered into a formal agreement with Reliance Industries for Jatropha planting. The company has selected 200 acresof land at Kakinada to grow Jatropha for high quality biodiesel. Jatropa Jatropa seed Everyone can help conserve fossil fuels by using them carefully. • Turn off lights and other electronics when you are not using them. • Purchase energy-efficient appliances. • Walk, ride a bicycle and use public transportation whenever possible. It is better to prefer public transport system like bus or train instead • of travel in personnel vehicles. Discuss in your class how this helps to the society. • Collect information about solar, wind, tidal and water power and prepare a scrap book. • Ask your teacher about nuclear energy and its impacts on ecosystem. Minerals Earth’s supply of raw mineral resources is in danger. Many mineral deposits that have been located and mapped have been depleted. As the ores for minerals like aluminum and iron become harder to find and extract, their prices go up. This makes tools and machinery more expensive to purchase and operate. Many mining methods, such as mountaintop removal mining (MTR), devastate the environment. They destroy soil, plants, and animal habitats. Many mining methods also pollute water and air, as toxic chemicals leak into the surrounding ecosystem. • Think why distaters like Uttarakhand happend? Activity-3 Now a days people are revolt against mining. Collect any such incident of our state or neighbouring states from your school library or news papers and conduct a seminar on it’s impact. 226 X Class Natural Resource Less wasteful mining methods and the recycling of materials will help conserve mineral resources. In Japan, for example, car manufacturers recycle many raw materials used in making automobiles. In the United States, nearly one-third of the iron produced comes from recycled automobiles. Conservation- A vital concern “The interest in conservation is not a sentimental one but the discovery of a truth well known to our ancient sages. The Indian tradition teaches us that all forms of life - human, animal and plant are so closelyinter-linked that disturbance of one gives rise to imbalance in the other”. (By Srimati Indira Gandhi, while launching the world conservation strategy in India on6th March 1980). In the 1960s most countries lived within their ecological resources. But the latest figure shows that today three-quarters of the human population live in countries which consume more than they can replenish. The issue of replenishment is large yet we have our individual roles. Small steps could become great efforts at conservation. What does the Kothapally experience tell us about step towards management and conservation? You may have already come across the three R’s to save the environment. They are Reduce: That is useless if you can afford to say water, by repairing leaky taps and avoiding a shower or switching off unnecessary lights and fans. Think of other things that you could reduce usage of. • Do you think it is necessary to have a lot of lighting for decoration during celebrations? Reuse: things that you often tend to throw away, like paper and wrapping papers. This would save plants and minimise pollution. • What other things could you reuse to save our resources? Recycle: may not always be a very good option as recycling plastic is a tricky process and can cause havoc. The chief problem lies in plastics’ complexity. There are as many types of plastic as their uses. Since each type can only be recycled with its own kind, plastics need to be carefully sorted before they can be processed. • Why should one sort wastes carefully before discarding them from home? Free distribution by A.P. Government fig-9: Bag from waste material fig-10: Recycling logo 227 • Often we keep a plastic bag in our dustbins to discard waste, is it a good practise? Conservation Groups Governments enact laws defining how land should be used and which areas should be set aside as parks and wildlife preserves. Governments also enforce laws designed to protect the environment from pollution, such as requiring factories to install pollution-control devices. Finally, governments often provide incentives for conserving resources. Many international organizations are also dedicated to conservation. Members support causes such as saving rain forests, protecting threatened animals, and cleaning up the air. The International Union for the Conservation of Nature (IUCN) is an alliance of governments and private groups founded in 1948. The IUCN works to protect wildlife and habitats. In 1980, the group proposed a world conservation strategy. Many governments have used the IUCN model to develop their own conservation plans. In addition, the IUCN monitors the status of endangered wildlife, threatened national parks and preserves, and other environments around the world. Take Kothapally village as an example; discuss the role of organizations involved in conservation and that of the villagers. • Can international, national or state organizations alone manage a resource? Who are all involved in the whole process of management? Suggest some ways in which you and your friends would like to manage a resource? • • Are we also an important resource of nature? How? • Find out the usage of water in Litres per day in your home? Do you actually require that much water and how much water is enough in accordance with National standards? It is time to know our responsibilities to protect natural resources for future generations. Key words Percolation tank Micro-irrigation Borewells Sustainabledevelopment, Biofuels Contour strip farming, Dyke Management. 228 X Class Natural Resource What we have learnt • Management of resources is essential for their conservation and restoration. • Resources are usually local specific and local people need to have control over them. • People need to be motivated to reduce pressure on the environment by reducing utilization of resources and reusing some of them. • We must use our resources judiciously especially fossil fuels, coal and petroleum as they will be ultimately exhausted. • Interstate and intercountry disputes should not hamper availability of a resource. Improve your learning 1. The BP statistical Review of World Energy in June measured total global oil at 188.8 million tonnes, from proved oil resources at the end of 2010. This is only enough for oil to last for the next 46.2 years. What measures should be taken to conserve oil? What will happen if we do not conserve it?(AS1) 2. Here is a news strip, read it carefully and answer the following questions Villagers oppose sand mining project Santhabommali (Srik
akulam): People of more than 20 villages in two mandals of Srikakulamhave raised a banner of revolt against the proposed beach sand mining project by a private company and threatened to intensify their agitation if the government does not cancel the project. The sand mining is being taken up to extract rich minerals from the area. The villages are located around the forest belt were mining was initiated.(AS1) i) Do you think the villagers are doing a right thing to agitate? Why? ii) What resources are the villagers trying to save by their agitation? iii) Will the villagers be benefitted by the rich minerals extracted from sand? iv) Why does the private company want to carry out mining in the area? v) Does the government have any role to play? vi) How will mining in that piece of land affect people of the area? 3. What is sustainable development?How is it useful in natural resource management?(AS1) 4. Write a detailed note on management of a natural resource.(AS1) 5. Suggest some ways of reusing a resource in your locality?(AS1) 6. Why should we conserve forests and wild life?(AS1) 7. Suggest some approaches towards the conservation of forests.(AS1) 8. Natural resources are decreased more rapidly. Guess what will be the consequences?(AS2) 9. Prepare a questionnaire to conduct interview it petrol filling station personnel about consuption of fossil fuels?(AS2) 10. Prepare model for rain water harvesting or energy saving are soil management. That reflect your innovative thoughts. (AS3) 11. List out names of villages farmers and procedure followed for restoration of any natural resource in your area.(AS4) Free distribution by A.P. Government 229 12. You might have heard the Natural Gas drillings near Kakinada by ONGC(Oil and Natural Gas Corporation). Collect information and prepare a note on the status of Gas production at the basin.(AS4) 13. Does your village or nearest city have a mechanism in place for recycling these waste materials? Find out how it is done and write in detail.(AS4) 14. Collect any graph that shows oil (petroleum) consumption in India.(AS4) i) Does the production meet consumption in India? ii) During which period of time shows highest increase in consumption rate? iii) Why will you say happened to production from past ten years, for example 2004 to 2014? iv) Suggest some ways to bring down consumption of petroleum. 15. Proper utilisation of natural resources is the way to show gratitude to our nation. Can you support this statement? Give your argument.(AS6) 16. Crop selection and cultivation should be based on availability of water. Prepare a slogun to make aware of farmers about this?(AS7) Fill in the blanks ................... plants are used for production of bio fuel. 1. 2. Bio diversity is important for more than just food and for ................ also. 3. Example for non renevable resource is .......................... 4. 5. Cultivation of paddy is suitable for ................ areas. Choose the correct answer ........................ is the alternative method to prevent ground water depeletion. 6. Percolation tanks helps to ( ) (a) supply water for agriculture (c) preserve rain water (b) increase ground water level (d) prevent overflow of water from tanks during rainy season 7. Which of the following practice is suitable to farmer with less water resources ( ) (i) select shortterm crops, (ii) cultivate comercial crops, (iii) adapt drip system, (iv) crop holiday (c) i, iv (a) i, ii (b) i, ii, iii (d) iii, iv 8. Which of the fossil fuel reserves decrease more rapidly in India (a) natural gas (d) all 9. Huge amount of toxic chemicals leak into the surrounding eco system because of (c) petrolium (b) coal ( ( (a) industries (b) mining (c) pestisides (d) modern technology 10. Sustainable development means (a) prevention of wastage (c) development without damaging (d) high yieldings in less time (b) stable growth ( Courtesy- Case 1: V. Rantha Reddy, Centre for Economics and Social Studies, Hyderabad. Case 2: Paper on water management in Andhra Pradesh by Dr. M.D. Reddy, Water Technology Centre ANGAR Agriculture Univeristy, Hyderabad. ) ) ) 230 X Class Natural Resource
Community College Robert Wise, The University of Wisconsin Oshkosh Vladimir Jurukovski, Suffolk County Community College Jean DeSaix, The University of North Carolina at Chapel Hill Jung Choi, Georgia Institute of Technology Yael Avissar, Rhode Island College Curriculum Framework for AP® Biology Big Idea 1: The process of evolution drives the diversity and unity of life. Enduring understanding 1.A. Change in the genetic makeup of a population over time is evolution. 1.A.1. Natural selection is a major mechanism of evolution. Chapter/Key Concepts 5.3, 18.1, 18.2, 19.1, 19.2, 19.3, 21.2, 23.5 1.A.2. Natural selection acts on phenotypic variations in populations. 7.3, 7.6, 18.2, 19.2, 19.3, 36.5 1.A.3. Evolutionary change is also driven by random processes. 19.1, 19.2 1.A.4. Biological evolution is supported by scientific evidence from many disciplines, including mathematics. 2.1, 5.2, 8.2, 11.1, 14.1, 17.1 18.1, 19.3 Enduring understanding 1.B. Organisms are linked by lines of descent from common ancestry. Chapter/Key Concepts 1.B.1. Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. 3.4, 4.3, 4.6, 8.2, 15.3, 13.2, 14.1, 15.5, 18.1, 20.1, 20.2 1.B.2. Phylogenetic trees and cladograms are graphical representations (models) of evolutionary history that can be tested. 14.4, 20.1, 20.2, 20.3 Enduring understanding 1.C. Life continues to evolve within a changing environment. Chapter/Key Concepts 1.C.1. Speciation and extinction have occurred throughout the Earth's history. 14.4, 18.2, 20.1, 38.1 1.C.2. Speciation may occur when two populations become reproductively isolated from each other. 18.2, 19.2, 23.5 1.C.3. Populations of organisms continue to evolve. 7.3, 7.6, 18.1, 18.3, 19.1, 19.2, 20.1, 20.2, 23.5 Enduring understanding 1.D. The origin of living systems is explained by natural processes. Chapter/Key Concepts 1.D.1. There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. 8.2, 18.1, 20.1, 21.1, 20.3 1.D.2. Scientific evidence from many different disciplines supports models of the origin of life. 8.2, 18.1, 20.2, 28.1 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Preface 5 Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis. Enduring understanding 2.A. Growth, reproduction and maintenance of the organization of living systems require free energy and matter. Chapter/Key Concepts 2.A.1. All living systems require constant input of free energy 2.A.2. Organisms capture and store free energy for use in biological processes. 6.1, 6.2, 6.3, 6.4, 6.7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 8.2, 23.1, 23.5, 36.3, 37.2 3.2, 4.3, 6.1, 6.4, 6.5, 7.1, 7.2, 7.3, 7.4, 7.4, 7.5, 7.6, 8.1, 8.2, 8.3, 9.2, 22.1, 22.2, 23.1, 23.5, 37.2 2.A.3. Organisms must exchange matter with the environment to grow, reproduce and maintain organization. 2.1, 2.2, 3.3, 4.2, 4.6, 6.1, 6.8, 22.4, 22.5, 23.5, 25.8, 37.3 Enduring understanding 2.B. Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. Chapter/Key Concepts 2.B.1. Cell membranes are selectively permeable due to their structure. 3.2, 3.3, 5.1, 5.2, 5.3, 5.4, 8.3 2.B.2. Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes. 2.3, 3.3, 5.2, 5.3, 5.4 2.B.3. Eukaryotic cells maintain internal membranes that partition the cell into specialized regions. 3.3, 4.2, 4.3, 4.4 Enduring understanding 2.C. Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. Chapter/Key Concepts 2.C.1. Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. 2.C.2. Organisms respond to changes in their external environments. 5.2, 5.3, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 8.2, 10.1, 11.1, 21.1, 23.5, 24.3, 28.3 2.1, 6.4, 7.5, 7.6, 22.5, 23.5, 26.3, 26.5 Enduring understanding 2.D. Growth and dynamic homeostasis of a biological system are influenced by changes in the system's environment. Chapter/Key Concepts 2.D.1. All biological systems from cells and organisms to populations, communities and ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and free energy. 2.1, 2.2, 7.1, 7.4, 7.5, 7.6, 8.2, 15.2, 15.3, 17.3, 21.1, 22.4, 35.1, 37.1 2.D.2. Homeostatic mechanisms reflect both common ancestry and divergence due to adaptation in different environments. 4.3, 5.2, 6.1, 18.2, 21.1, 25.1, 32.1, 32.3, 34.1 2.D.3. Biological systems are affected by disruptions to their dynamic homeostasis. 3.2, 22.3, 22.5, 23.1, 28.3, 38.2 2.D.4. Plants and animals have a variety of chemical defenses against infections that affect dynamic homeostasis. 23.6, 33.1, 33.2 Enduring understanding 2.E. Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination. Chapter/Key Concepts 2.E.1. Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms. 10.2, 10.3, 14.3, 23.5, 30.1, 32.3, 34.1, 34.6 2.E.2. Timing and coordination of physiological events are regulated by multiple mechanisms. 6.8, 10.1, 10.2, 15.3, 22.3, 23.2, 24.1, 30.6, 36.1, 36.2, 36.3, 36.4, 36.5, 43.6, 43.7 6 Preface Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis. 2.E.3. Timing and coordination of behavior are regulated by various mechanisms and are important in natural selection. 11.1, 21.2, 23.5, 30.6, 35.2, 45.7 Big Idea 3: Living systems store, retrieve, transmit and respond to information essential to life processes. Enduring understanding 3.A. Heritable information provides for continuity of life. Chapter/Key Concepts 3.A.1. DNA, and in some cases RNA, is the primary source of heritable information. 3.5, 10.3, 13.1, 13.2, 14.1, 14.2, 14.3, 14.5, 15.1, 15.2, 15.3, 15.4, 15.5, 16.1, 16.2, 16.3, 17.1, 17.3, 21.1, 21.2, 22.4 3.A.2. In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization. 3.A.3. The chromosomal basis of inheritance provides an understanding of the pattern of passage (transmission) of genes from parent to offspring. 10.1, 10.2, 10.3, 11.1, 11.2, 13.1 11.2, 12.1, 12.2, 13.1, 14.2, 17.1, 17.4 3.A.4. The inheritance pattern of many traits cannot be explained by simple Mendelian genetics. 4.3, 12.2, 13.1 Enduring understanding 3.B. Expression of genetic information involves cellular and molecular mechanisms. Chapter/Key Concepts 3.B.1. Gene regulation results in differential gene expression, leading to cell specialization. 7.3, 7.6, 16.1, 16.2, 16.3, 16.4, 16.5, 17.3 3.B.2. A variety of intercellular and intracellular signal transmissions mediate gene expression. 9.1, 9.2, 9.3, 15.3, 17.1 Enduring understanding 3.C. The processing of genetic information is imperfect and is a source of genetic variation. Chapter/Key Concepts 3.C.1. Changes in genotype can result in changes in phenotype. 5.3, 11.2, 13.1, 13.2, 14.6, 15.1, 17.1, 18.1, 19.1, 19.3 3.C.2. Biological systems have multiple processes that increase genetic variation. 11.2, 13.1, 14.1, 14.6, 15.2, 17.1, 20.3, 21.2, 22.4 3.C.3. Viral replication results in genetic variation, and viral infection can introduce genetic variation into the hosts. 21.1, 21.2 Enduring understanding 3.D. Cells communicate by generating, transmitting and receiving chemical signals Chapter/Key Concepts 3.D.1. Cell communication processes share common features that reflect a shared evolutionary history. 4.6, 9.1, 9.2, 9.3, 9.4, 10.4, 37.2, 37.3 3.D.2. Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling. 9.1, 9.3 3.D.3. Signal transduction pathways link signal reception with cellular response. 9.1, 9.2 3.D.4. Changes in signal transduction pathways can alter cellular response. 9.2, 9.3, 9.4 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Preface 7 Big Idea 3: Living systems store, retrieve, transmit and respond to information essential to life processes. Enduring understanding 3.E. Transmission of information results in changes within and between biological systems. 3.E.1. Individuals can act on information and communicate it to others. 3.E.2. Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. Chapter/Key Concepts 9.2, 9.4, 21.2, 36.1, 36.2, 36.3, 36.4, 36.5 6.1, 35.1, 35.2, 35.3, 35.4 Big Idea 4: Biological systems interact, and these systems and their interactions possess complex properties. Enduring understanding 4.A Interactions within biological systems lead to complex properties. Chapter/Key Concepts 4.A.1. The subcomponents of biological molecules and their sequence determine the properties of that molecule. 3.1, 3.2, 3.3, 3.4, 3.5, 5.2, 6.2, 14.1, 14.3, 14.4, 17.1 4.A.2. The structure and function of subcellular components, and their interactions, provide essential cellular processes. 3.4, 4.3, 4.4, 4.6, 10.3, 15.3 4.A.3. Interactions between external stimuli and regulated gene expression result in specialization of cells, tissues and organs. 16.1, 22.3, 43.6, 43.7 4.A.4. Organisms exhibit complex properties due to interactions between their constituent parts. 15.2, 17.1, 18.1, 22.3, 22.5, 30.5, 33.3, 34.3 4.A.5. Communities are composed of populations of organisms that interact in complex ways. 22.5, 23.5, 45.5, 45.6 4.A.6. Interactions among living systems and with their environment result in the movement of matter and energy. Enduring understanding 4.B Competition and
cooperation are important aspects of biological systems. 3.2, 3.3, 6.2, 6.3, 6.6, 7.5, 7.6, 8.2, 10.3, 18.1, 23.1, 22.4, 45.2, 45.6, 46.2, 47.3 Chapter/Key Concepts 4.B.1. Interactions between molecules affect their structure and function. 3.5, 5.2, 6.2, 6.5, 8.3 4.B.2. Cooperative interactions within organisms promote efficiency in the use of energy and matter. 4.3, 7.3, 7.6, 45.6 4.B.3. Interactions between and within populations influence patterns of species distribution and abundance. 45.4, 45.6 4.B.4. Distribution of local and global ecosystems changes over time. 22.4, 23.1, 46.1, 47.1, 47.3 Enduring understanding 4.C. Naturally occurring diversity among and between components within biological systems affects interactions with the environment. Chapter/Key Concepts 4.C.1. Variation in molecular units provides cells with a wider range of functions. 3.4, 9.2, 10.3, 13.1, 15.5, 42.2, 49.1 4.C.2. Environmental factors influence the expression of the genotype in an organism. 14.2, 19.3, 22.3, 30.4, 43.1 4.C.3. The level of variation in a population affects population dynamics. 7.5, 7.6, 19.1, 45.6, 47.1 8 Preface Big Idea 4: Biological systems interact, and these systems and their interactions possess complex properties. 4.C.4. The diversity of species within an ecosystem may influence the stability of the ecosystem. 45.6, 46.1 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 9 1 \| THE STUDY OF LIFE Figure 1.1 This NASA image is a composite of several satellite-based views of Earth. To make the whole-Earth image, NASA scientists combine observations of different parts of the planet. (credit: NASA/GSFC/NOAA/USGS) Chapter Outline 1.1: The Science of Biology 1.2: Themes and Concepts of Biology Introduction Viewed from space, Earth offers no clues about the diversity of life it harbors. The first forms of life on Earth are thought be microorganisms that existed for billions of years in the ocean before plants and animals appeared. The mammals, birds, and flowers that we see in modern times are mostly “recent” species, originating 130 to 200 million years ago. In fact, only in the last 200,000 years have humans started looking like we do today. Organisms evolve in response to each other. One of the best examples is disease causing organisms, which have to adapt to overcome the defenses of the organisms they infect. One such organism that has evolved to specialize in infection in humans is Plasmodium, the organism that causes malaria. Biologists use the process of science to learn about the world and the organisms living in it. For example, people have suspected for quite some time that people with blood type O are less likely to die from severe malaria. Now, a team of scientists have been able to explain why. By examining data from several experiments, and by using both inductive and deductive reasoning, the scientists concluded that A and B type blood reacts with a protein excreted by Plasmodium. This reaction causes severe illness. However, type O blood does not react with the protein. You can read more (http://openstaxcollege.org/l/32plasmodium) about the response of type A and B blood groups to infection by Plasmodium. 1.1 \| The Science of Biology In this section, you will explore the following questions: • What are the characteristics shared by the natural sciences? • What are the steps of the scientific method? 10 Chapter 1 \| The Study of Life Connection for AP® courses Biology is the science that studies living organisms and their interactions with one another and with their environment. The process of science attempts to describe and understand the nature of the universe by rational means. Science has many fields; those fields related to the physical world, including biology, are considered natural sciences. All of the natural sciences follow the laws of chemistry and physics. For example, when studying biology, you must remember living organisms obey the laws of thermodynamics while using free energy and matter from the environment to carry out life processes that are explored in later chapters, such as metabolism and reproduction. Two types of logical reasoning are used in science: inductive reasoning and deductive reasoning. Inductive reasoning uses particular results to produce general scientific principles. Deductive reasoning uses logical thinking to predict results by applying scientific principles or practices. The scientific method is a step-by-step process that consists of: making observations, defining a problem, posing hypotheses, testing these hypotheses by designing and conducting investigations, and drawing conclusions from data and results. Scientists then communicate their results to the scientific community. Scientific theories are subject to revision as new information is collected. The content presented in this section supports the Learning Objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The Learning Objectives merge Essential Knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP® Exam questions. Big Idea 2 Enduring Understanding 2.A Essential Knowledge Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. 2.A.1 All living systems require constant input of free energy. Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models Learning Objectives 2.3 The student is able to predict how changes in free energy availability affect organisms, populations and ecosystems. (a) (b) Figure 1.2 Formerly called blue-green algae, these (a) cyanobacteria, shown here at 300x magnification under a light microscope, are some of Earth’s oldest life forms. These (b) stromatolites along the shores of Lake Thetis in Western Australia are ancient structures formed by the layering of cyanobacteria in shallow waters. (credit a: modification of work by NASA; credit b: modification of work by Ruth Ellison; scale-bar data from Matt Russell) What is biology? In simple terms, biology is the study of living organisms and their interactions with one another and their environments. This is a very broad definition because the scope of biology is vast. Biologists may study anything from the microscopic or submicroscopic view of a cell to ecosystems and the whole living planet (Figure 1.2). Listening to the daily news, you will quickly realize how many aspects of biology are discussed every day. For example, recent news topics This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 11 include Escherichia coli (Figure 1.3) outbreaks in spinach and Salmonella contamination in peanut butter. On a global scale, many researchers are committed to finding ways to protect the planet, solve environmental issues, and reduce the effects of climate change. All of these diverse endeavors are related to different facets of the discipline of biology. Figure 1.3 Escherichia coli (E. coli) bacteria, seen in this scanning electron micrograph, are normal residents of our digestive tracts that aid in the absorption of vitamin K and other nutrients. However, virulent strains are sometimes responsible for disease outbreaks. (credit: Eric Erbe, digital colorization by Christopher Pooley, both of USDA, ARS, EMU) The Process of Science Biology is a science, but what exactly is science? What does the study of biology share with other scientific disciplines? Science (from the Latin scientia, meaning “knowledge”) can be defined as knowledge that covers general truths or the operation of general laws, especially when acquired and tested by the scientific method. It becomes clear from this definition that the application of the scientific method plays a major role in science. The scientific method is a method of research with defined steps that include experiments and careful observation. The steps of the scientific method will be examined in detail later, but one of the most important aspects of this method is the testing of hypotheses by means of repeatable experiments. A hypothesis is a suggested explanation for an event, which can be tested. Although using the scientific method is inherent to science, it is inadequate in determining what science is. This is because it is relatively easy to apply the scientific method to disciplines such as physics and chemistry, but when it comes to disciplines like archaeology, psychology, and geology, the scientific method becomes less applicable as it becomes more difficult to repeat experiments. These areas of study are still sciences, however. Consider archaeology—even though one cannot perform repeatable experiments, hypotheses may still be supported. For instance, an archaeologist can hypothesize that an ancient culture existed based on finding a piece of pottery. Further hypotheses could be made about various characteristics of this culture, and these hypotheses may be found to be correct or false through continued support or contradictions from other findings. A hypothesis may become a verified theory. A theory is a tested and confirmed explanation for observations or phenomena. Science may be better defined as fields of study that attempt to comprehend the nature of the universe. Natural Sciences What would you expect to see in a museum of natural sciences? Frogs? Plants? Dinosaur skeletons? Exhibits about how the brain functions? A planetarium? Gems and minerals? Or, maybe all of the above? Science includes such diverse fields as astronomy, biology, computer sciences, geology, logic, physics, chemistry, and mathematics (Figure 1.4). However, those fields of science
related to the physical world and its phenomena and processes are considered natural sciences. Thus, a museum of natural sciences might contain any of the items listed above. 12 Chapter 1 \| The Study of Life Figure 1.4 The diversity of scientific fields includes astronomy, biology, computer science, geology, logic, physics, chemistry, mathematics, and many other fields. (credit: “Image Editor”/Flickr) There is no complete agreement when it comes to defining what the natural sciences include, however. For some experts, the natural sciences are astronomy, biology, chemistry, earth science, and physics. Other scholars choose to divide natural sciences into life sciences, which study living things and include biology, and physical sciences, which study nonliving matter and include astronomy, geology, physics, and chemistry. Some disciplines such as biophysics and biochemistry build on both life and physical sciences and are interdisciplinary. Natural sciences are sometimes referred to as “hard science” because they rely on the use of quantitative data; social sciences that study society and human behavior are more likely to use qualitative assessments to drive investigations and findings. Not surprisingly, the natural science of biology has many branches or subdisciplines. Cell biologists study cell structure and function, while biologists who study anatomy investigate the structure of an entire organism. Those biologists studying physiology, however, focus on the internal functioning of an organism. Some areas of biology focus on only particular types of living things. For example, botanists explore plants, while zoologists specialize in animals. Scientific Reasoning One thing is common to all forms of science: an ultimate goal “to know.” Curiosity and inquiry are the driving forces for the development of science. Scientists seek to understand the world and the way it operates. To do this, they use two methods of logical thinking: inductive reasoning and deductive reasoning. Inductive reasoning is a form of logical thinking that uses related observations to arrive at a general conclusion. This type of reasoning is common in descriptive science. A life scientist such as a biologist makes observations and records them. These data can be qualitative or quantitative, and the raw data can be supplemented with drawings, pictures, photos, or videos. From many observations, the scientist can infer conclusions (inductions) based on evidence. Inductive reasoning involves formulating generalizations inferred from careful observation and the analysis of a large amount of data. Brain studies provide an example. In this type of research, many live brains are observed while people are doing a specific activity, such as viewing images of food. The part of the brain that “lights up” during this activity is then predicted to be the part controlling the response to the selected stimulus, in this case, images of food. The “lighting up” of the various areas of the brain is caused by excess absorption of radioactive sugar derivatives by active areas of the brain. The resultant increase in radioactivity is observed by a scanner. Then, researchers can stimulate that part of the brain to see if similar responses result. Deductive reasoning or deduction is the type of logic used in hypothesis-based science. In deductive reason, the pattern of thinking moves in the opposite direction as compared to inductive reasoning. Deductive reasoning is a form of logical This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 13 thinking that uses a general principle or law to forecast specific results. From those general principles, a scientist can extrapolate and predict the specific results that would be valid as long as the general principles are valid. Studies in climate change can illustrate this type of reasoning. For example, scientists may predict that if the climate becomes warmer in a particular region, then the distribution of plants and animals should change. These predictions have been made and tested, and many such changes have been found, such as the modification of arable areas for agriculture, with change based on temperature averages. Both types of logical thinking are related to the two main pathways of scientific study: descriptive science and hypothesisbased science. Descriptive (or discovery) science, which is usually inductive, aims to observe, explore, and discover, while hypothesis-based science, which is usually deductive, begins with a specific question or problem and a potential answer or solution that can be tested. The boundary between these two forms of study is often blurred, and most scientific endeavors combine both approaches. The fuzzy boundary becomes apparent when thinking about how easily observation can lead to specific questions. For example, a gentleman in the 1940s observed that the burr seeds that stuck to his clothes and his dog’s fur had a tiny hook structure. On closer inspection, he discovered that the burrs’ gripping device was more reliable than a zipper. He eventually developed a company and produced the hook-and-loop fastener often used on lace-less sneakers and athletic braces. Descriptive science and hypothesis-based science are in continuous dialogue. The Scientific Method Biologists study the living world by posing questions about it and seeking science-based responses. This approach is common to other sciences as well and is often referred to as the scientific method. The scientific method was used even in ancient times, but it was first documented by England’s Sir Francis Bacon (1561–1626) (Figure 1.5), who set up inductive methods for scientific inquiry. The scientific method is not exclusively used by biologists but can be applied to almost all fields of study as a logical, rational problem-solving method. Figure 1.5 Sir Francis Bacon (1561–1626) is credited with being the first to define the scientific method. (credit: Paul van Somer) The scientific process typically starts with an observation (often a problem to be solved) that leads to a question. Let’s think about a simple problem that starts with an observation and apply the scientific method to solve the problem. One Monday morning, a student arrives at class and quickly discovers that the classroom is too warm. That is an observation that also describes a problem: the classroom is too warm. The student then asks a question: “Why is the classroom so warm?” Proposing a Hypothesis Recall that a hypothesis is a suggested explanation that can be tested. To solve a problem, several hypotheses may be proposed. For example, one hypothesis might be, “The classroom is warm because no one turned on the air conditioning.” But there could be other responses to the question, and therefore other hypotheses may be proposed. A second hypothesis might be, “The classroom is warm because there is a power failure, and so the air conditioning doesn’t work.” 14 Chapter 1 \| The Study of Life Once a hypothesis has been selected, the student can make a prediction. A prediction is similar to a hypothesis but it typically has the format “If . . . then . . . .” For example, the prediction for the first hypothesis might be, “If the student turns on the air conditioning, then the classroom will no longer be too warm.” Testing a Hypothesis A valid hypothesis must be testable. It should also be falsifiable, meaning that it can be disproven by experimental results. Importantly, science does not claim to “prove” anything because scientific understandings are always subject to modification with further information. This step—openness to disproving ideas—is what distinguishes sciences from nonsciences. The presence of the supernatural, for instance, is neither testable nor falsifiable. To test a hypothesis, a researcher will conduct one or more experiments designed to eliminate one or more of the hypotheses. Each experiment will have one or more variables and one or more controls. A variable is any part of the experiment that can vary or change during the experiment. The control group contains every feature of the experimental group except it is not given the manipulation that is hypothesized about. Therefore, if the results of the experimental group differ from the control group, the difference must be due to the hypothesized manipulation, rather than some outside factor. Look for the variables and controls in the examples that follow. To test the first hypothesis, the student would find out if the air conditioning is on. If the air conditioning is turned on but does not work, there should be another reason, and this hypothesis should be rejected. To test the second hypothesis, the student could check if the lights in the classroom are functional. If so, there is no power failure and this hypothesis should be rejected. Each hypothesis should be tested by carrying out appropriate experiments. Be aware that rejecting one hypothesis does not determine whether or not the other hypotheses can be accepted; it simply eliminates one hypothesis that is not valid (see this figure). Using the scientific method, the hypotheses that are inconsistent with experimental data are rejected. While this “warm classroom” example is based on observational results, other hypotheses and experiments might have clearer controls. For instance, a student might attend class on Monday and realize she had difficulty concentrating on the lecture. One observation to explain this occurrence might be, “When I eat breakfast before class, I am better able to pay attention.” The student could then design an experiment with a control to test this hypothesis. In hypothesis-based science, specific results are predicted from a general premise. This type of reasoning is called deductive reasoning: deduction proceeds from the general to the particular. But the reverse of the process is also possible: sometimes, scientists reach a general conclusion from a nu
mber of specific observations. This type of reasoning is called inductive reasoning, and it proceeds from the particular to the general. Inductive and deductive reasoning are often used in tandem to advance scientific knowledge (see this figure) Think About It Almost all plants use water, carbon dioxide, and energy from the sun to make sugars. Think about what would happen to plants that don’t have sunlight as an energy source or sufficient water. What would happen to organisms that depend on those plants for their own survival? Make a prediction about what would happen to the organisms living in a rain forest if 50% of its trees were destroyed. How would you test your prediction? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 15 Figure 1.6 The scientific method consists of a series of well-defined steps. If a hypothesis is not supported by experimental data, a new hypothesis can be proposed. In the example below, the scientific method is used to solve an everyday problem. Order the scientific method steps (numbered items) with the process of solving the everyday problem (lettered items). Based on the results of the experiment, is the hypothesis correct? If it is incorrect, propose some alternative hypotheses. Scientific Method 1 Observation 2 Question A B Everyday process There is something wrong with the electrical outlet. If something is wrong with the outlet, my coffeemaker also won’t work when plugged into it. 3 Hypothesis (answer) C My toaster doesn’t toast my bread. 4 5 Prediction D I plug my coffee maker into the outlet. Experiment E My coffeemaker works. 16 Chapter 1 \| The Study of Life Scientific Method Everyday process 6 Result F What is preventing my toaster from working? a. The original hypothesis is correct. There is something wrong with the electrical outlet and therefore the toaster doesn’t work. b. The original hypothesis is incorrect. Alternative hypothesis includes that toaster wasn’t turned on. c. The original hypothesis is correct. The coffee maker and the toaster do not work when plugged into the outlet. d. The original hypothesis is incorrect. Alternative hypotheses includes that both coffee maker and toaster were broken. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 17 Figure 1.7 Scientists use two types of reasoning, inductive and deductive reasoning, to advance scientific knowledge. As is the case in this example, the conclusion from inductive reasoning can often become the premise for deductive reasoning. Decide if each of the following is an example of inductive or deductive reasoning. 1. All flying birds and insects have wings. Birds and insects flap their wings as they move through the air. Therefore, wings enable flight. 2. Insects generally survive mild winters better than harsh ones. Therefore, insect pests will become more problematic if global temperatures increase. 3. Chromosomes, the carriers of DNA, separate into daughter cells during cell division. Therefore, DNA is the genetic material. 4. Animals as diverse as insects and wolves all exhibit social behavior. Therefore, social behavior must have an evolutionary advantage for humans. a. 1- Inductive, 2- Deductive, 3- Deductive, 4- Inductive b. 1- Deductive, 2- Inductive, 3- Deductive, 4- Inductive c. 1- Inductive, 2- Deductive, 3- Inductive, 4- Deductive d. 1- Inductive, 2-Inductive, 3- Inductive, 4- Deductive The scientific method may seem too rigid and structured. It is important to keep in mind that, although scientists often follow this sequence, there is flexibility. Sometimes an experiment leads to conclusions that favor a change in approach; often, an experiment brings entirely new scientific questions to the puzzle. Many times, science does not operate in a linear fashion; instead, scientists continually draw inferences and make generalizations, finding patterns as their research proceeds. Scientific reasoning is more complex than the scientific method alone suggests. Notice, too, that the scientific method can be applied to solving problems that aren’t necessarily scientific in nature. 18 Chapter 1 \| The Study of Life Two Types of Science: Basic Science and Applied Science The scientific community has been debating for the last few decades about the value of different types of science. Is it valuable to pursue science for the sake of simply gaining knowledge, or does scientific knowledge only have worth if we can apply it to solving a specific problem or to bettering our lives? This question focuses on the differences between two types of science: basic science and applied science. Basic science or “pure” science seeks to expand knowledge regardless of the short-term application of that knowledge. It is not focused on developing a product or a service of immediate public or commercial value. The immediate goal of basic science is knowledge for knowledge’s sake, though this does not mean that, in the end, it may not result in a practical application. In contrast, applied science or “technology,” aims to use science to solve real-world problems, making it possible, for example, to improve a crop yield, find a cure for a particular disease, or save animals threatened by a natural disaster (Figure 1.8). In applied science, the problem is usually defined for the researcher. Figure 1.8 After Hurricane Ike struck the Gulf Coast in 2008, the U.S. Fish and Wildlife Service rescued this brown pelican. Thanks to applied science, scientists knew how to rehabilitate the bird. (credit: FEMA) Some individuals may perceive applied science as “useful” and basic science as “useless.” A question these people might pose to a scientist advocating knowledge acquisition would be, “What for?” A careful look at the history of science, however, reveals that basic knowledge has resulted in many remarkable applications of great value. Many scientists think that a basic understanding of science is necessary before an application is developed; therefore, applied science relies on the results generated through basic science. Other scientists think that it is time to move on from basic science and instead to find solutions to actual problems. Both approaches are valid. It is true that there are problems that demand immediate attention; however, few solutions would be found without the help of the wide knowledge foundation generated through basic science. One example of how basic and applied science can work together to solve practical problems occurred after the discovery of DNA structure led to an understanding of the molecular mechanisms governing DNA replication. Strands of DNA, unique in every human, are found in our cells, where they provide the instructions necessary for life. During DNA replication, DNA makes new copies of itself, shortly before a cell divides. Understanding the mechanisms of DNA replication enabled scientists to develop laboratory techniques that are now used to identify genetic diseases. Without basic science, it is unlikely that applied science would exist. Another example of the link between basic and applied research is the Human Genome Project, a study in which each human chromosome was analyzed and mapped to determine the precise sequence of DNA subunits and the exact location of each gene. (The gene is the basic unit of heredity; an individual’s complete collection of genes is his or her genome.) Other less complex organisms have also been studied as part of this project in order to gain a better understanding of human chromosomes. The Human Genome Project (Figure 1.9) relied on basic research carried out with simple organisms and, later, with the human genome. An important end goal eventually became using the data for applied research, seeking cures and early diagnoses for genetically related diseases. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 19 Figure 1.9 The Human Genome Project was a 13-year collaborative effort among researchers working in several different fields of science. The project, which sequenced the entire human genome, was completed in 2003. (credit: the U.S. Department of Energy Genome Programs (http://genomics.energy.gov)) While research efforts in both basic science and applied science are usually carefully planned, it is important to note that some discoveries are made by serendipity, that is, by means of a fortunate accident or a lucky surprise. Penicillin was discovered when biologist Alexander Fleming accidentally left a petri dish of Staphylococcus bacteria open. An unwanted mold grew on the dish, killing the bacteria. The mold turned out to be Penicillium, and a new antibiotic was discovered. Even in the highly organized world of science, luck—when combined with an observant, curious mind—can lead to unexpected breakthroughs. Reporting Scientific Work Whether scientific research is basic science or applied science, scientists must share their findings in order for other researchers to expand and build upon their discoveries. Collaboration with other scientists—when planning, conducting, and analyzing results—are all important for scientific research. For this reason, important aspects of a scientist’s work are communicating with peers and disseminating results to peers. Scientists can share results by presenting them at a scientific meeting or conference, but this approach can reach only the select few who are present. Instead, most scientists present their results in peer-reviewed manuscripts that are published in scientific journals. Peer-reviewed manuscripts are scientific papers that are reviewed by a scientist’s colleagues, or peers. These colleagues are qualified individuals, often experts in the same research area, who judge whether or not the scientist’s work is suitable for publication. The process of peer review helps to ensure that the research described in a scientific paper or grant p
roposal is original, significant, logical, and thorough. Grant proposals, which are requests for research funding, are also subject to peer review. Scientists publish their work so other scientists can reproduce their experiments under similar or different conditions to expand on the findings. The experimental results must be consistent with the findings of other scientists. A scientific paper is very different from creative writing. Although creativity is required to design experiments, there are fixed guidelines when it comes to presenting scientific results. First, scientific writing must be brief, concise, and accurate. A scientific paper needs to be succinct but detailed enough to allow peers to reproduce the experiments. The scientific paper consists of several specific sections—introduction, materials and methods, results, and discussion. This structure is sometimes called the “IMRaD” format. There are usually acknowledgment and reference sections as well as an abstract (a concise summary) at the beginning of the paper. There might be additional sections depending on the type of paper and the journal where it will be published; for example, some review papers require an outline. The introduction starts with brief, but broad, background information about what is known in the field. A good introduction also gives the rationale of the work; it justifies the work carried out and also briefly mentions the end of the paper, where the hypothesis or research question driving the research will be presented. The introduction refers to the published scientific work of others and therefore requires citations following the style of the journal. Using the work or ideas of others without proper citation is considered plagiarism. The materials and methods section includes a complete and accurate description of the substances used, and the method 20 Chapter 1 \| The Study of Life and techniques used by the researchers to gather data. The description should be thorough enough to allow another researcher to repeat the experiment and obtain similar results, but it does not have to be verbose. This section will also include information on how measurements were made and what types of calculations and statistical analyses were used to examine raw data. Although the materials and methods section gives an accurate description of the experiments, it does not discuss them. Some journals require a results section followed by a discussion section, but it is more common to combine both. If the journal does not allow the combination of both sections, the results section simply narrates the findings without any further interpretation. The results are presented by means of tables or graphs, but no duplicate information should be presented. In the discussion section, the researcher will interpret the results, describe how variables may be related, and attempt to explain the observations. It is indispensable to conduct an extensive literature search to put the results in the context of previously published scientific research. Therefore, proper citations are included in this section as well. Finally, the conclusion section summarizes the importance of the experimental findings. While the scientific paper almost certainly answered one or more scientific questions that were stated, any good research should lead to more questions. Therefore, a well-done scientific paper leaves doors open for the researcher and others to continue and expand on the findings. Review articles do not follow the IMRAD format because they do not present original scientific findings, or primary literature; instead, they summarize and comment on findings that were published as primary literature and typically include extensive reference sections. 1.2 \| Themes and Concepts of Biology By the end of this section, you will be able to: • Identify and describe the properties of life • Describe the levels of organization among living things • Recognize and interpret a phylogenetic tree Connection for AP® Courses The AP® Biology curriculum is organized around four major themes called the Big Ideas that apply to all levels of biological organization—from molecules and cells to populations and ecosystems. Each Big Idea identifies key concepts called Enduring Understandings, and Essential Knowledges, along with supporting examples. Simple descriptions define the focus of each Big Idea: Big Idea 1, Evolution; Big Idea 2, Energy and Homeostasis; Big Idea 3, Information and Communication; and Big Idea 4, Systems and Interactions. Evolution explains both the unity and diversity of life, Big Idea 1, and all organisms require energy and molecules to carry out life functions, such as growth and reproduction, Big Idea 2. Living systems also store, transmit, and respond to information, from DNA sequences to nerve impulses and behaviors, Big Idea 3. All biological systems interact, and these interactions result in emergent properties and characteristics unique to life, Big Idea 4. The redesigned AP® Biology course also emphasizes the investigative practices that students should master. Scientific inquiry usually uses a series of steps to gain new knowledge. The scientific method begins with an observation and follows with a hypothesis to explain the observation; then experiments are conducted to test the hypothesis, gather results, and draw conclusions from data. The AP® program has identified seven major categories of Science Practices, which can be described by short phrases: using representations and models to communicate information and solve problems; using mathematics appropriately; engaging in questioning; planning and implementing data collection strategies; analyzing and evaluating data; justifying scientific explanations; and connecting concepts. A Learning Objective merges content with one or more of the seven Science Practices. The information presented and the examples highlighted in this section support concepts and Learning Objectives outlined in Big Idea 1 of the AP® Biology Curriculum. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 21 Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice Learning Objective 3.1 The student can pose scientific questions. 1.14 The student is able to pose scientific questions that correctly identify essential properties of share, core life processes that provide insights into the history of life on Earth. Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice Learning Objective 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 1.18 The student is able to evaluate evidence provided by a data set in conjunction with a phylogenetic tree or simply cladogram to determine evolutionary history and speciation. Biology is the science that studies life, but what exactly is life? This may sound like a silly question with an obvious response, but it is not always easy to define life. For example, a branch of biology called virology studies viruses, which exhibit some of the characteristics of living entities but lack others. It turns out that although viruses can attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use to define life. Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the early molecular evolution that gave rise to life; since the events that preceded life are not biological events, these scientists are also excluded from biology in the strict sense of the term. From its earliest beginnings, biology has wrestled with three questions: What are the shared properties that make something “alive”? And once we know something is alive, how do we find meaningful levels of organization in its structure? And, finally, when faced with the remarkable diversity of life, how do we organize the different kinds of organisms so that we can better understand them? As new organisms are discovered every day, biologists continue to seek answers to these and other questions. Properties of Life All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, adaptation, growth and development, regulation, homeostasis, energy processing, and evolution. When viewed together, these nine characteristics serve to define life. Order Figure 1.10 A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems. (credit: “Ivengo”/Wikimedia Commons) 22 Chapter 1 \| The Study of Life Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules; these in turn make up cell organelles and other cellular inclusions. In multicellular organisms (Figure 1.10), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems. Sensitivity or Response to Stimuli Figure 1.11 The leaves of this sensitive plant (Mimosa pudica) will instantly droop and fold when touched. After a few minutes, the plant returns to normal. (credit: Alex Lomas) Organism
s respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 1.11). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response. Watch this video (http://openstaxcollege.org/l/movement_plants) to see how plants respond to a stimulus—from opening to light, to wrapping a tendril around a branch, to capturing prey. Which example most clearly shows a way that humans can respond directly to a change in the environment? a. We shiver when we are cold and sweat when we are hot. b. We walk by putting our front leg forward and pushing off with our back leg. c. We are able to breath in and out unconsciously. d. Our hair and fingernails grow at a constant rate over time. Reproduction Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. When reproduction occurs, genes containing DNA are passed along to an organism’s offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape. Growth and Development Organisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 23 will direct cellular growth and development, ensuring that a species’ young (Figure 1.12) will grow up to exhibit many of the same characteristics as its parents. Figure 1.12 Although no two look alike, these kittens have inherited genes from both parents and share many of the same characteristics. (credit: Rocky Mountain Feline Rescue) Regulation Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body. Homeostasis Figure 1.13 Polar bears (Ursus maritimus) and other mammals living in ice-covered regions maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin. (credit: “longhorndave”/Flickr) In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through homeostasis (literally, “steady state”). For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 1.13), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat. 24 Chapter 1 \| The Study of Life Energy Processing Figure 1.14 The California condor (Gymnogyps californianus) uses chemical energy derived from food to power flight. California condors are an endangered species; this bird has a wing tag that helps biologists identify the individual. (credit: Pacific Southwest Region U.S. Fish and Wildlife Service) All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food; others use chemical energy in molecules they take in as food (Figure 1.14). Activity Select an ecosystem of your choice, such as a tropical rainforest, desert, or coral reef, and create a representation to show how several organisms found in the ecosystem interact with each other and the environment. Then, using similarities and differences among the organisms make a hypothesis about their relatedness. Consider the levels of organization of the biological world and create a diagram to place these items in order from the smallest level of organization to the most encompassing: skin cell, planet Earth, elephant, tropical rainforest, water molecule, liver, wolf pack, and oxygen atom. Justify the reason why you placed the items in the hierarchy that you did. Think About It Homeostasis—the ability to “stay the same”—is a feature shared by all living organisms. You go for a long walk on a hot day. Describe how homeostasis keeps your body healthy even though you are sweating profusely. Then describe an example of an adaptation that evolved in a desert plant or animal that allows them to survive in extreme temperatures. Levels of Organization of Living Things Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 25 than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 1.15), which contains the instructions for the structure and functioning of all living organisms. Figure 1.15 All molecules, including this DNA molecule, are composed of atoms. (credit: “brian0918”/Wikimedia Commons) Watch this video (http://openstaxcollege.org/l/rotating_DNA) that animates the three-dimensional structure of the DNA molecule shown in this figure. The word helix means spiral. What does this tell you about the structure of DNA, which is a double helix macromolecule? a. The nucleotides of the two strands bond together with spiral bonds. b. A double-stranded DNA molecule has two spiral stands bound together. c. DNA is a double helix because it has two spiral strands held together like a spiral staircase. d. Nucleotides are spiral-shaped molecules that bond together to form DNA. 26 Chapter 1 \| The Study of Life Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membranebound nuclei; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus. In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms. All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many pine trees. All of these pine trees represent the population of pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (see this figure), the biosphere is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 27 Fi
gure 1.16 The biological levels of organization of living things are shown. From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy. (credit “organelles”: modification of work by Umberto Salvagnin; credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer/ National Cancer Institute; credit “tissues”: modification of work by Kilbad; Fama Clamosa; Mikael Häggström; credit “organs”: modification of work by Mariana Ruiz Villareal; credit “organisms”: modification of work by "Crystal"/Flickr; credit “ecosystems”: 28 Chapter 1 \| The Study of Life modification of work by US Fish and Wildlife Service Headquarters; credit “biosphere”: modification of work by NASA) Which of the following statements is false? a. Tissues exist within organs which exist within organ systems. b. Communities exist within populations which exist within ecosystems. c. Organelles exist within cells which exist within tissues. d. Communities exist within ecosystems which exist in the biosphere. The Diversity of Life The fact that biology, as a science, has such a broad scope has to do with the tremendous diversity of life on earth. The source of this diversity is evolution, the process of gradual change during which new species arise from older species. Evolutionary biologists study the evolution of living things in everything from the microscopic world to ecosystems. The evolution of various life forms on Earth can be summarized in a phylogenetic tree (Figure 1.17). A phylogenetic tree is a diagram showing the evolutionary relationships among biological species based on similarities and differences in genetic or physical traits or both. A phylogenetic tree is composed of nodes and branches. The internal nodes represent ancestors and are points in evolution when, based on scientific evidence, an ancestor is thought to have diverged to form two new species. The length of each branch is proportional to the time elapsed since the split. Figure 1.17 This phylogenetic tree was constructed by microbiologist Carl Woese using data obtained from sequencing ribosomal RNA genes. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes, single-celled organisms lacking intracellular organelles. (credit: Eric Gaba; NASA Astrobiology Institute) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 29 Carl Woese and the Phylogenetic Tree In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and bacteria. The organizational scheme was based mainly on physical features, as opposed to physiology, biochemistry, or molecular biology, all of which are used by modern systematics. The pioneering work of American microbiologist Carl Woese in the early 1970s has shown, however, that life on Earth has evolved along three lineages, now called domains—Bacteria, Archaea, and Eukarya. The first two are prokaryotic cells with microbes that lack membrane-enclosed nuclei and organelles. The third domain contains the eukaryotes and includes unicellular microorganisms together with the four original kingdoms (excluding bacteria). Woese defined Archaea as a new domain, and this resulted in a new taxonomic tree (see this figure). Many organisms belonging to the Archaea domain live under extreme conditions and are called extremophiles. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape). Woese’s tree was constructed from comparative sequencing of the genes that are universally distributed, present in every organism, and conserved (meaning that these genes have remained essentially unchanged throughout evolution). Woese’s approach was revolutionary because comparisons of physical features are insufficient to differentiate between the prokaryotes that appear fairly similar in spite of their tremendous biochemical diversity and genetic variability (Figure 1.18). The comparison of homologous DNA and RNA sequences provided Woese with a sensitive device that revealed the extensive variability of prokaryotes, and which justified the separation of the prokaryotes into two domains: bacteria and archaea. Figure 1.18 These images represent different domains. The (a) bacteria in this micrograph belong to Domain Bacteria, while the (b) extremophiles (not visible) living in this hot vent belong to Domain Archaea. Both the (c) sunflower and (d) lion are part of Domain Eukarya. (credit a: modification of work by Drew March; credit b: modification of work by Steve Jurvetson; credit c: modification of work by Michael Arrighi; credit d: modification of work by Leszek Leszcynski) 30 Chapter 1 \| The Study of Life In which domain would a fish be classified? Why? a. Archaea, because fish are multicellular. b. Eukarya, because fish are multicellular. c. Archaea, because fish are single-celled. d. Eukarya because fish are single-celled. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 31 Phylogenetic trees can represent traits that are derived or lost due to evolution. One example is the absence of legs in some sea mammals. For example, Cetaceans are marine mammals that include toothed whales, such as dolphins and killer whales, and baleen whales, such as humpback whales. Cetaceans are descended from even-toed ungulates and share a common ancestry with the hippopotamus, cow, sheep, camel, and pig. Figure 1.19 32 Chapter 1 \| The Study of Life Phylogenetic trees can represent traits that are derived or lost due to evolution. One example is the absence of legs in some marine mammals. One such group is the Cetaceans, which includes toothed whales, such as dolphins and killer whales, and baleen whales, such as humpback whales. Cetaceans are descended from even-toed ungulates and share a common ancestry with the hippopotamus, cows, sheep, camel, and pig. Based on this phylogenetic tree, which of the following animal is the most closely related to a horse? a. an armadillo b. a camel c. a bat d. a cat Branches of Biological Study The scope of biology is broad and therefore contains many branches and subdisciplines. Biologists may pursue one of those subdisciplines and work in a more focused field. For instance, molecular biology and biochemistry study biological processes at the molecular and chemical level, including interactions among molecules such as DNA, RNA, and proteins, as well as the way they are regulated. Microbiology, the study of microorganisms, is the study of the structure and function of organisms that cannot be seen with the naked eye. It is quite a broad branch itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists, among others. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 33 Forensic science is the application of science to answer questions related to the law. Biologists as well as chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for use in courts, and their job involves examining trace materials associated with crimes. Interest in forensic science has increased in the last few years, possibly because of popular television shows that feature forensic scientists on the job. Also, the development of molecular techniques and the establishment of DNA databases have expanded the types of work that forensic scientists can do. Their work involves analyzing samples such as hair, blood, and other body fluids and also processing DNA (Figure 1.20) found in many different environments and materials. Forensic scientists also analyze other biological evidence left at crime scenes, such as insect larvae or pollen grains. Students who want to pursue careers in forensic science will most likely be required to take chemistry and biology courses as well as some intensive math courses. Figure 1.20 This forensic scientist works in a DNA extraction room at the U.S. Army Criminal Investigation Laboratory at Fort Gillem, GA. (credit: United States Army CID Command Public Affairs) Another field of biological study, neurobiology, studies the biology of the nervous system, and although it is considered a branch of biology, it is also recognized as an interdisciplinary field of study known as neuroscience. Because of its interdisciplinary nature, this subdiscipline studies different functions of the nervous system using molecular, cellular, developmental, medical, and computational approaches. Figure 1.21 Researchers work on excavating dinosaur fossils at a site in Castellón, Spain. (credit: Mario Modesto) Paleontology, another branch of biology, uses fossils to study life’s history (Figure 1.21). Zoology and botany are the study of animals and plants, respectively. Biologists can also specialize as biotechnologists, ecologists, or physiologists, to name just a few areas. This is just a small sample of the many fields that biologists can pursue. 34 Chapter 1 \| The Study of Life Biology is the culmination of the achievements of the natural sciences from their inception to today. Excitingly, it is the cradle of emerging sciences, such as the biology of brain activity, genetic engineering of custom organisms, and the biology of evolution that uses the laboratory tools of molecular biology to retrace the earliest stages of life on earth. A scan of news headlines—whether reporting on immunizations, a newly discovered species, sports doping, or a genetically-modified food—demonstrates the way biology is active in and important to our everyday world. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life KEY TERMS 35 abstract opening section of a scientific paper that summarizes the research and conclusions applied science form of science t
hat aims to solve real-world problems atom smallest and most fundamental unit of matter basic science knowledge science that seeks to expand knowledge and understanding regardless of the short-term application of that biochemistry study of the chemistry of biological organisms biology the study of living organisms and their interactions with one another and their environments biosphere collection of all the ecosystems on Earth botany study of plants cell smallest fundamental unit of structure and function in living things community set of populations inhabiting a particular area conclusion section of a scientific paper that summarizes the importance of the experimental findings control part of an experiment that does not change during the experiment deductive reasoning form of logical thinking that uses a general inclusive statement to forecast specific results descriptive science (also, discovery science) form of science that aims to observe, explore, and investigate discussion section of a scientific paper in which the author interprets experimental results, describes how variables may be related, and attempts to explain the phenomenon in question ecosystem all the living things in a particular area together with the abiotic, nonliving parts of that environment eukaryote organism with cells that have nuclei and membrane-bound organelles evolution process of gradual change during which new species arise from older species and some species become extinct falsifiable able to be disproven by experimental results homeostasis ability of an organism to maintain constant internal conditions hypothesis suggested explanation for an observation, which can be tested hypothesis-based science form of science that begins with a specific question and potential testable answers inductive reasoning form of logical thinking that uses related observations to arrive at a general conclusion introduction opening section of a scientific paper, which provides background information about what was known in the field prior to the research reported in the paper life science field of science, such as biology, that studies living things macromolecule large molecule, typically formed by the joining of smaller molecules materials and methods section of a scientific paper that includes a complete description of the substances, methods, and techniques used by the researchers to gather data microbiology study of the structure and function of microorganisms molecular biology study of biological processes and their regulation at the molecular level, including interactions among 36 Chapter 1 \| The Study of Life molecules such as DNA, RNA, and proteins molecule chemical structure consisting of at least two atoms held together by one or more chemical bonds natural science field of science that is related to the physical world and its phenomena and processes neurobiology study of the biology of the nervous system organ collection of related tissues grouped together performing a common function organ system level of organization that consists of functionally related interacting organs organelle small structures that exist within cells and carry out cellular functions organism individual living entity paleontology study of life’s history by means of fossils peer-reviewed manuscript scientific paper that is reviewed by a scientist’s colleagues who are experts in the field of study phylogenetic tree diagram showing the evolutionary relationships among various biological species based on similarities and differences in genetic or physical traits or both; in essence, a hypothesis concerning evolutionary connections physical science field of science, such as geology, astronomy, physics, and chemistry, that studies nonliving matter plagiarism using other people’s work or ideas without proper citation, creating the false impression that those are the author’s original ideas population all of the individuals of a species living within a specific area prokaryote single-celled organism that lacks organelles and does not have nuclei surrounded by a nuclear membrane results section of a scientific paper in which the author narrates the experimental findings and presents relevant figures, pictures, diagrams, graphs, and tables, without any further interpretation review article paper that summarizes and comments on findings that were published as primary literature science knowledge that covers general truths or the operation of general laws, especially when acquired and tested by the scientific method scientific method method of research with defined steps that include observation, formulation of a hypothesis, testing, and confirming or falsifying the hypothesis serendipity fortunate accident or a lucky surprise theory tested and confirmed explanation for observations or phenomena tissue group of similar cells carrying out related functions variable part of an experiment that the experimenter can vary or change zoology study of animals CHAPTER SUMMARY 1.1 The Science of Biology Biology is the science that studies living organisms and their interactions with one another and their environments. Science attempts to describe and understand the nature of the universe in whole or in part by rational means. Science has many fields; those fields related to the physical world and its phenomena are considered natural sciences. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 37 Science can be basic or applied. The main goal of basic science is to expand knowledge without any expectation of shortterm practical application of that knowledge. The primary goal of applied research, however, is to solve practical problems. Two types of logical reasoning are used in science. Inductive reasoning uses particular results to produce general scientific principles. Deductive reasoning is a form of logical thinking that predicts results by applying general principles. The common thread throughout scientific research is the use of the scientific method, a step-based process that consists of making observations, defining a problem, posing hypotheses, testing these hypotheses, and drawing one or more conclusions. The testing uses proper controls. Scientists present their results in peer-reviewed scientific papers published in scientific journals. A scientific research paper consists of several well-defined sections: introduction, materials and methods, results, and, finally, a concluding discussion. Review papers summarize the research done in a particular field over a period of time. 1.2 Themes and Concepts of Biology Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing. Living things are highly organized parts of a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere. The great diversity of life today evolved from less-diverse ancestral organisms over billions of years. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms. Biology is very broad and includes many branches and subdisciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others. REVIEW QUESTIONS 1. What is a suggested and testable explanation for an event called? a. discovery b. hypothesis c. d. scientific method theory 2. Which of the following sciences is not considered a natural science? a. Astronomy b. Biology c. Computer science d. Physics 3. What is the name for the formal process through which scientific research is checked for originality, significance, and quality before being accepted into scientific literature? a. publication b. public speaking c. peer review d. the scientific method 4. What are two topics that are likely to be studied by biologists and two areas of scientific study that would fall outside the realm of biology? a. diseases affecting humans, pollution affecting species habitat, calculating surface area of rectangular ground, functioning of planetary orbitals b. calculating surface area of rectangular ground, functioning of planetary orbitals, formation of metamorphic rocks, galaxy formation and evolution c. plant responses to external stimuli, functioning of planetary orbitals, formation of metamorphic rocks, galaxy formation and evolution d. plant responses to external stimuli, study of the shape and motion of physical objects, formation of metamorphic rocks, galaxy formation and evolution 5. Which of the following is an example of deductive reasoning? a. Most swimming animals use fins; therefore, fins are an adaptation to swimming. b. Mitochondria are inherited from the mother; therefore, maternally inherited traits are encoded by mitochondrial DNA c. Small animals lose more heat than larger animals. One would not expect to find wild mice in the poles. d. Water conservation is a major requirement to survive in the desert. Long leaves increase loss of water by evaporation. Therefore, desert plants should have smaller leaves. 38 Chapter 1 \| The Study of Life 6. Why are viruses not considered living? a. biosphere, ecosystem, community, population, a. They are not made of cells. b. Viruses do not have genetic material. c. Viruses have DNA and RNA. d. Viruses are obligate parasites and require a host. 7. The presence of a membrane-enclosed nucleus is a characteristic of what? a. bacteria b. eukaryotic cells c. all living organisms d. prokaryotic cells 8. What is a group of individuals of the same species living in the same area called? a. a community b. an ecosystem c. a family d. a population 9. Which of the following sequences represents the hierarchy of biological organization from the most inclusive to the least complex level? CRITICAL THINKING QUEST
IONS 12. Is mathematics a natural science? Explain your reasoning. a. No, it is not a natural science because it is not used in the study of the natural world. b. No, it is not a natural science. Mathematics focuses on understanding mathematical relations and calculations, which is useful in natural sciences but which is distinct. c. Yes, it is a natural science. Mathematics deals with verifying the experimental data. d. Yes, it is a natural science. It uses chemical and physical measurements. 13. Although the scientific method is used by most of the sciences, it can also be applied to everyday situations. A situation is given below. Using the scientific method try to arrange the given steps in the correct order. Situation: 1. If the car doesn’t start the problem might be in the battery. 2. Car doesn’t start. 3. After changing the battery. Car starts working. 4. The car should start after charging the battery or changing the battery. 5. The car doesn’t start because the battery is dead. 6. The car doesn’t start even after charging the battery, the battery must have stopped working. organism b. organelle, tissue, biosphere, ecosystem, population c. organism, organ, tissue, organelle, molecule d. organism, community, biosphere, molecule, tissue, organ 10. Where in a phylogenetic tree would you expect to find the organism that had evolved most recently? a. at the base b. at the nodes c. at the branch tips d. within the branches 11. What is a characteristic that is not present in all living things? a. homeostasis and regulation b. metabolism c. nucleus containing DNA d. reproduction a. 1, 2, 3, 4, 5, 6 b. 2, 1, 3, 4, 5, 6 c. 2, 1, 5, 4, 6, 3 d. 2, 1, 5, 6, 3, 4 14. Read the following questions. Does the statement lend itself to investigation using the scientific method? In other words, is the hypothesis falsifiable (can be proven false)? 1. Is macaroni and cheese tastier than broccoli soup? 2. Are hummingbirds attracted to the color red? 3. 4. Is the moon made out of green cheese? Is plagiarism dishonest? a. Questions 1 and 2 are subjective and cannot be disproven using scientific method. Questions 3 and 4 can be tested using scientific method. b. Questions 3 and 4 are subjective and cannot be disproven using scientific method. Questions 1 and 2 can be tested using scientific method. c. Questions 1 and 3 are subjective and cannot be disproven using scientific method. Questions 2 and 4 can be tested using scientific method. d. Questions 1 and 4 are subjective and cannot be disproven using scientific method. Questions 2 and 3 can be tested using scientific method. 15. Consider the levels of organization of the biological This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 1 \| The Study of Life 39 world and place each of these items in order from smallest level of organization to most encompassing: skin cell, elephant, water molecule, planet Earth, tropical rainforest, hydrogen atom, wolf pack, liver. a. hydrogen atom, water molecule, skin cell, liver, elephant, wolf pack, tropical rainforest, planet Earth b. hydrogen atom, skin cell, water molecule, liver, elephant, wolf pack, tropical rainforest, planet Earth c. hydrogen atom, skin cell, water molecule, liver, wolf pack, elephant, tropical rainforest, planet Earth d. water molecule, hydrogen atom, skin cell, liver, elephant, wolf pack, tropical rainforest, planet Earth 16. What scientific evidence was used by Carl Woese to determine there should be a separate domain for Archaea? a. a sequence of DNA b. a sequence of rRNA c. a sequence of mRNA. d. a sequence of tRNA. 17. Both astronomy and astrology study the stars. Which one is considered a natural science? Explain your reasoning. a. Astrology is a natural science as it indirectly influences human affairs and the natural world. b. Astronomy is a natural science as it deals with observations and prediction of events in the sky, which is based on the laws of physics. c. Astrology is a natural science as it deals with observations and prediction of events in the sky, influences human affairs and the natural world. d. Astrology is a natural science as it deals with the study of asteroids and comets, which is based on the laws of natural sciences. TEST PREP FOR AP® COURSES 18. Which of the following structures is conserved in all living organisms and points to a common origin? a. All living organisms have mitochondria that produce energy. b. All living organisms store genetic material in DNA/RNA. distantly related to 2? a. 1 b. 3 c. 4 d. 5 c. All living organisms use the energy from 21. sunlight d. All living organisms have a nucleus. 19. Which of the following statements is the strongest argument in favor of two organisms, A and B, being closely related evolutionarily? a. A and B look alike. b. A and B live in the same ecosystem. c. A and B use the same metabolic pathways. d. The DNA sequences of A and B are highly homologous. 20. In the phylogenetic tree shown, which organism is most In the diagram shown which is the most recent common ancestor of 1 and 3? a. A b. B c. C d. D 22. The French scientist Jacques Monod famously said, “Anything found to be true of E. coli must also be true of elephants.” How is this statement based on the notion that living organisms share a common ancestor? 40 Chapter 1 \| The Study of Life a. E. coli is a eukaryote and share similarities with line of evidence that has led to this reclassification? most of the living organisms. b. E. coli is a prokaryote. The various metabolic processes and core functions in E. coli share homology with higher organisms. c. E. coli contains a nucleus and membrane bound cell organelles that are shared by all the living organisms. d. E. coli is a prokaryote and reproduces through binary fission which is common to most of the living organisms. 23. Birds have been reclassified as reptiles. What is one a. Archeopteryx is the connecting link between birds and reptiles which shows that birds and reptiles are related. b. Birds have scales, having the same origin as that of reptiles. c. Birds and reptiles have the same circulatory and excretory systems and both are egg laying animals. d. Birds and reptiles have similar anatomical and morphological features. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 41 2 \| THE CHEMICAL FOUNDATION OF LIFE Figure 2.1 Atoms are the building blocks of molecules found in the universe—air, soil, water, rocks . . . and also the cells of all living organisms. In this model of an organic molecule, the atoms of carbon (black), hydrogen (white), nitrogen (blue), oxygen (red), and sulfur (yellow) are shown in proportional atomic size. The silver rods indicate chemical bonds. (credit: modification of work by Christian Guthier) Chapter Outline 2.1: Atoms, Isotopes, Ions, and Molecules: The Building Blocks 2.2: Water 2.3: Carbon Introduction All matter, including living things, is made up of various combinations of elements. Some of the most abundant elements in living organisms include carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. These elements form the major biological molecules—nucleic acids, proteins, carbohydrates, and lipids—that are the fundamental components of living matter. Biologists study these important molecules to understand their unique structures which determine their specialized functions. All biological processes follow the laws of physics and chemistry. Therefore, in order to understand how biological systems work, it is important to understand the underlying physics and chemistry. For example, the flow of blood within the circulatory system follows the laws of physics regulating the modes of fluid flow. Chemical laws dictate the breakdown of large, complex food molecules into smaller molecules as well as their conversion to energy stored in adenosine triphosphate (ATP). Polar molecules, the formation of hydrogen bonds, and the resulting properties of water are key to understanding living processes. Recognizing the properties of acids and bases is important to understand various biological processes such as digestion. Therefore, the fundamentals of physics and chemistry are the foundation for gaining insight into biological processes. An example of how understanding of chemical processes can give insight to a biological process is recent research on seasonal affective disorder (SAD). This form of depression affects up to 10% of the population in the fall and winter. Symptoms include a tendency to overeat, oversleep, lack of energy, and difficulty concentrating on tasks. Now scientists 42 Chapter 2 \| The Chemical Foundation of Life have found out that not only may SAD be caused by a deficiency in vitamin D, but that it is more common in individuals with darker skin pigmentation. You can read more about it here (http://openstaxcollege.org/l/32vitdsad) . 2.1 \| Atoms, Isotopes, Ions, and Molecules: The Building Blocks In this section, you will explore the following questions: • How does atomic structure determine the properties of elements, molecules, and matter? • What are the differences among ionic bonds, covalent bonds, polar covalent bonds, and hydrogen bonds? Connection for AP® Courses Living systems obey the laws of chemistry and physics. Matter is anything that occupies space and mass. The 92 naturally occurring elements have unique properties, and various combinations of them create molecules, which combine to form organelles, cells, tissues, organ system, and organisms. Atoms, which consist of protons, neutrons, and electrons, are the smallest units of matter that retain all their characteristics and are most stable when their outermost or valence electron shells contain the maximum number of electrons. Electrons can be transferred, shared, or cause charge disparities between atoms to create bonds, including ionic, covalent, and hydrogen bonds, as well as van del Waals interactions. Isotopes
are different forms of an element that have different numbers of neutrons while retaining the same number of protons; many isotopes, such as carbon-14, are radioactive. The information presented and examples highlighted in this section support concepts and Learning Objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 2 Enduring Understanding 2.A Essential Knowledge Science Practice Science Practice Science Practice Learning Objective Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. 2.A.1 All living systems require constant input of free energy. 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 2.8 The student is able to justify the selection of data regarding the types of molecules that an animal, plant, or bacterium will take up as necessary building blocks and excrete as waste products. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1.12] [APLO 2.9] [APLO 2.42] [APLO 2.22] At its most fundamental level, life is made up of matter. Matter is any substance that occupies space and has mass. Elements are unique forms of matter with specific chemical and physical properties that cannot be broken down into smaller This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 43 substances by ordinary chemical reactions. There are 118 elements, but only 98 occur naturally. The remaining elements are synthesized in laboratories and are unstable. Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is already “taken” by another element, a combination of two letters. Some elements follow the English term for the element, such as C for carbon and Ca for calcium. Other elements’ chemical symbols derive from their Latin names; for example, the symbol for sodium is Na, referring to natrium, the Latin word for sodium. The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). In the nonliving world, elements are found in different proportions, and some elements common to living organisms are relatively rare on the earth as a whole, as shown in Table 2.1. For example, the atmosphere is rich in nitrogen and oxygen but contains little carbon and hydrogen, while the earth’s crust, although it contains oxygen and a small amount of hydrogen, has little nitrogen and carbon. In spite of their differences in abundance, all elements and the chemical reactions between them obey the same chemical and physical laws regardless of whether they are a part of the living or non-living world. Approximate Percentage of Elements in Living Organisms (Humans) Compared to the Non-living World Element Life (Humans) Atmosphere Earth’s Crust Oxygen (O) Carbon (C) Hydrogen (H) Nitrogen (N) Table 2.1 65% 18% 10% 3% 21% trace trace 78% 46% trace 0.1% trace The Structure of the Atom To understand how elements come together, we must first discuss the smallest component or building block of an element, the atom. An atom is the smallest unit of matter that retains all of the chemical properties of an element. For example, one gold atom has all of the properties of gold in that it is a solid metal at room temperature. A gold coin is simply a very large number of gold atoms molded into the shape of a coin and containing small amounts of other elements known as impurities. Gold atoms cannot be broken down into anything smaller while still retaining the properties of gold. An atom is composed of two regions: the nucleus, which is in the center of the atom and contains protons and neutrons, and the outermost region of the atom which holds its electrons in orbit around the nucleus, as illustrated in Figure 2.2. Atoms contain protons, electrons, and neutrons, among other subatomic particles. The only exception is hydrogen (H), which is made of one proton and one electron with no neutrons. Figure 2.2 Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus. Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists arbitrarily define this amount of mass as one atomic mass unit (amu) or one Dalton, as shown in Table 2.2. Although similar in mass, protons and neutrons 44 Chapter 2 \| The Chemical Foundation of Life differ in their electric charge. A proton is positively charged whereas a neutron is uncharged. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge. Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. Hence, they do not contribute much to an element’s overall atomic mass. Therefore, when considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass based on the number of protons and neutrons alone. Although not significant contributors to mass, electrons do contribute greatly to the atom’s charge, as each electron has a negative charge equal to the positive charge of a proton. In uncharged, neutral atoms, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus. In these atoms, the positive and negative charges cancel each other out, leading to an atom with no net charge. Accounting for the sizes of protons, neutrons, and electrons, most of the volume of an atom—greater than 99 percent—is, in fact, empty space. With all this empty space, one might ask why so-called solid objects do not just pass through one another. The reason they do not is that the electrons that surround all atoms are negatively charged and negative charges repel each other. Protons, Neutrons, and Electrons Charge Mass (amu) Location Proton +1 Neutron 0 Electron –1 Table 2.2 1 1 0 nucleus nucleus orbitals Atomic Number and Mass Atoms of each element contain a characteristic number of protons and electrons. The number of protons determines an element’s atomic number and is used to distinguish one element from another. The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess. Together, the number of protons and the number of neutrons determine an element’s mass number, as illustrated in this figure. Note that the small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Since an element’s isotopes will have slightly different mass numbers, scientists also determine the atomic mass, which is the calculated mean of the mass number for its naturally occurring isotopes. Often, the resulting number contains a fraction. For example, the atomic mass of chlorine (Cl) is 35.45 because chlorine is composed of several isotopes, some (the majority) with atomic mass 35 (17 protons and 18 neutrons) and some with atomic mass 37 (17 protons and 20 neutrons). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 45 Figure 2.3 Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively. Its relative atomic mass is 12.011. How many neutrons do carbon-12 and carbon-13 have, respectively? a. Carbon-12 contains 6 neutrons while carbon-13 contains 7 neutrons. b. Carbon-12 contains 7 neutrons while carbon-13 contains 6 neutrons. c. Carbon-12 contains 12 neutrons while carbon-13 contains 13 neutrons. d. Carbon-12 contains 13 neutrons while carbon-13 contains 12 neutrons. Isotopes Isotopes are different forms of an element that have the same number of protons but a different number of neutrons. Some elements—such as carbon, potassium, and uranium—have naturally occurring isotopes. Carbon-12 contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 (six protons and six neutrons). Carbon-14 contains six protons, eight neutrons, and six electrons; its atomic mass is 14 (six protons and eight neutrons). These two alternate forms of carbon are isotopes. Some isotopes may emit neutrons, protons, and electrons, and attain a more stable atomic configuration (lower level of potential energy); these are radioactive isotopes, or radioisotopes. Radioactive decay (carbon-14 losing neutrons to eventually become nitrogen-14) describes the energy loss that occurs when an unstable atom’s nucleus releases radiation. 46 Chapter 2 \| The Chemical Foundation of Life Carbon Dating Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and methane. Carbon-14 (14C) is a naturally occurring radioisotope that is created in the atmosphere from atmospheric 14N (nitrogen) by the addition of a neutron and the loss of a proton because of cosmic rays. This is a continuous process, so more 14C is always being created. A
s a living organism incorporates 14C initially as carbon dioxide fixed in the process of photosynthesis, the relative amount of 14C in its body is equal to the concentration of 14C in the atmosphere. When an organism dies, it is no longer ingesting 14C, so the ratio between 14C and 12C will decline as 14C decays gradually to 14N by a process called beta decay—the emission of electrons or positrons. This decay gives off energy in a slow process. After approximately 5,730 years, half of the starting concentration of 14C will have been converted back to 14N. The time it takes for half of the original concentration of an isotope to decay back to its more stable form is called its half-life. Because the half-life of 14C is long, it is used to date formerly living objects such as old bones or wood. Comparing the ratio of the 14C concentration found in an object to the amount of 14C detected in the atmosphere, the amount of the isotope that has not yet decayed can be determined. On the basis of this amount, the age of the material, such as the pygmy mammoth shown in Figure 2.4, can be calculated with accuracy if it is not much older than about 50,000 years. Other elements have isotopes with different half lives. For example, 40K (potassium-40) has a half-life of 1.25 billion years, and 235U (Uranium 235) has a half-life of about 700 million years. Through the use of radiometric dating, scientists can study the age of fossils or other remains of extinct organisms to understand how organisms have evolved from earlier species. Figure 2.4 The age of carbon-containing remains less than about 50,000 years old, such as this pygmy mammoth, can be determined using carbon dating. (credit: Bill Faulkner, NPS) Based on carbon dating, scientists estimate this pygmy mammoth died 11,000 years ago. How would the ratio of 14 C to 12 C in a living elephant compare to the 14 C to 12 C ratio found in the mammoth? a. The ratio would be the same in the elephant and the mammoth. b. The ratio would be lower in the elephant than the mammoth. c. The ratio would be higher in the elephant than the mammoth. d. The ratio would depend on the diet of each animal. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 47 To learn more about atoms, isotopes, and how to tell one isotope from another, visit this site (http://openstaxcollege.org/ l/atoms_isotopes) and run the simulation. K-41 is one of the naturally occurring isotopes of potassium. Use the periodic table to explain how the structure of K-41 differs from the normal K atom. a. K-41 has a total of 24 neutrons and normal K atom has 22 neutrons b. K-41 has a total of 22 neutrons and normal K atom has 20 neutrons c. K-41 has one more neutron than the normal K atom d. K-41 has one less neutron than normal K atom The Periodic Table The different elements are organized and displayed in the periodic table. Devised by Russian chemist Dmitri Mendeleev (1834–1907) in 1869, the table groups elements that, although unique, share certain chemical properties with other elements. The properties of elements are responsible for their physical state at room temperature: they may be gases, solids, or liquids. Elements also have specific chemical reactivity, the ability to combine and to chemically bond with each other. In the periodic table, shown in Figure 2.5, the elements are organized and displayed according to their atomic number and are arranged in a series of rows and columns based on shared chemical and physical properties. In addition to providing the atomic number for each element, the periodic table also displays the element’s atomic mass. Looking at carbon, for example, its symbol (C) and name appear, as well as its atomic number of six (in the upper left-hand corner) and its atomic mass of 12.11. 48 Chapter 2 \| The Chemical Foundation of Life Figure 2.5 The periodic table shows the atomic mass and atomic number of each element. The atomic number appears above the symbol for the element and the approximate atomic mass appears below it. The periodic table groups elements according to chemical properties. The differences in chemical reactivity between the elements are based on the number and spatial distribution of an atom’s electrons. Atoms that chemically react and bond to each other form molecules. Molecules are simply two or more atoms chemically bonded together. Logically, when two atoms chemically bond to form a molecule, their electrons, which form the outermost region of each atom, come together first as the atoms form a chemical bond. Electron Shells and the Bohr Model It should be stressed that there is a connection between the number of protons in an element, the atomic number that distinguishes one element from another, and the number of electrons it has. In all electrically neutral atoms, the number of electrons is the same as the number of protons. Thus, each element, at least when electrically neutral, has a characteristic number of electrons equal to its atomic number. An early model of the atom was developed in 1913 by Danish scientist Niels Bohr (1885–1962). The Bohr model shows the atom as a central nucleus containing protons and neutrons, with the electrons in circular orbitals at specific distances from the nucleus, as illustrated in Figure 2.6. These orbits form electron shells or energy levels, which are a way of visualizing the number of electrons in the outermost shells. These energy levels are designated by a number and the symbol “n.” For example, 1n represents the first energy level located closest to the nucleus. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 49 Figure 2.6 The Bohr model was developed by Niels Bohr in 1913. In this model, electrons exist within principal shells. An electron normally exists in the lowest energy shell available, which is the one closest to the nucleus. Energy from a photon of light can bump it up to a higher energy shell, but this situation is unstable, and the electron quickly decays back to the ground state. In the process, a photon of light is released. Electrons fill orbitals in a consistent order: they first fill the orbitals closest to the nucleus, then they continue to fill orbitals of increasing energy further from the nucleus. If there are multiple orbitals of equal energy, they will be filled with one electron in each energy level before a second electron is added. The electrons of the outermost energy level determine the energetic stability of the atom and its tendency to form chemical bonds with other atoms to form molecules. Under standard conditions, atoms fill the inner shells first, often resulting in a variable number of electrons in the outermost shell. The innermost shell has a maximum of two electrons but the next two electron shells can each have a maximum of eight electrons. This is known as the octet rule, which states, with the exception of the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence shell, the outermost electron shell. Examples of some neutral atoms and their electron configurations are shown in this figure. Notice that in this Figure 2.7, helium has a complete outer electron shell, with two electrons filling its first and only shell. Similarly, neon has a complete outer 2n shell containing eight electrons. In contrast, chlorine and sodium have seven and one in their outer shells, respectively, but theoretically they would be more energetically stable if they followed the octet rule and had eight. 50 Chapter 2 \| The Chemical Foundation of Life Figure 2.7 Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially filled valence shells and gain or lose electrons to achieve a stable electron configuration. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 51 An atom may give, take, or share electrons with another atom to achieve a full valence shell, the most stable electron configuration. Looking at this figure, how many electrons do elements in group 1 need to lose in order to achieve a stable electron configuration? How many electrons do elements in groups 14 and 17 need to gain to achieve a stable configuration? a. Elements of group 1 need to lose one electron, elements of group 14 need to gain 4 electrons, and elements of group 17 need to gain 1 electron b. Elements of group 1 need to lose 4 electrons while elements of group 14 and 17 need to gain 1 electron each. c. Elements of group 1 need to lose 2 electrons, elements of group 14 need to gain 4 electrons and elements of group 17 need to gain 1 electron. d. Elements of group 1 need to gain 1 electron while, elements of group 14 need to lose 4 electrons and elements of group 17 need to lose 1 electron. Understanding that the organization of the periodic table is based on the total number of protons (and electrons) helps us know how electrons are distributed among the shells. The periodic table is arranged in columns and rows based on the number of electrons and where these electrons are located. Take a closer look at the some of the elements in the table’s far right column in the periodic table. The group 18 atoms helium (He), neon (Ne), and argon (Ar) all have filled outer electron shells, making it unnecessary for them to share electrons with other atoms to attain stability; they are highly stable as single atoms. Their non-reactivity has resulted in their being named the inert gases (or noble gases). Compare this to the group 1 elements in the left-hand column. These elements, including hydrogen (H), lithium (Li), and sodium (Na), all have one electron in their outermost she
lls. That means that they can achieve a stable configuration and a filled outer shell by donating or sharing one electron with another atom or a molecule such as water. Hydrogen will donate or share its electron to achieve this configuration, while lithium and sodium will donate their electron to become stable. As a result of losing a negatively charged electron, they become positively charged ions. Group 17 elements, including fluorine and chlorine, have seven electrons in their outmost shells, so they tend to fill this shell with an electron from other atoms or molecules, making them negatively charged ions. Group 14 elements, of which carbon is the most important to living systems, have four electrons in their outer shell allowing them to make several covalent bonds (discussed below) with other atoms. Thus, the columns of the periodic table represent the potential shared state of these elements’ outer electron shells that is responsible for their similar chemical characteristics. Electron Orbitals Although useful to explain the reactivity and chemical bonding of certain elements, the Bohr model of the atom does not accurately reflect how electrons are spatially distributed surrounding the nucleus. They do not circle the nucleus like the earth orbits the sun, but are found in electron orbitals. These relatively complex shapes result from the fact that electrons behave not just like particles, but also like waves. Mathematical equations from quantum mechanics known as wave functions can predict within a certain level of probability where an electron might be at any given time. The area where an electron is most likely to be found is called its orbital. Recall that the Bohr model depicts an atom’s electron shell configuration. Within each electron shell are subshells, and each subshell has a specified number of orbitals containing electrons. While it is impossible to calculate exactly where an electron is located, scientists know that it is most probably located within its orbital path. Subshells are designated by the letter s, p, d, and f. The s subshell is spherical in shape and has one orbital. Principal shell 1n has only a single s orbital, which can hold two electrons. Principal shell 2n has one s and one p subshell, and can hold a total of eight electrons. The p subshell has three dumbbell-shaped orbitals, as illustrated in Figure 2.8. Subshells d and f have more complex shapes and contain five and seven orbitals, respectively. These are not shown in the illustration. Principal shell 3n has s, p, and d subshells and can hold 18 electrons. Principal shell 4n has s, p, d and f orbitals and can hold 32 electrons. Moving away from the nucleus, the number of electrons and orbitals found in the energy levels increases. Progressing from one atom to the next in the periodic table, the electron structure can be worked out by fitting an extra electron into the next available orbital. 52 Chapter 2 \| The Chemical Foundation of Life Figure 2.8 The s subshells are shaped like spheres. Both the 1n and 2n principal shells have an s orbital, but the size of the sphere is larger in the 2n orbital. Each sphere is a single orbital. p subshells are made up of three dumbbellshaped orbitals. Principal shell 2n has a p subshell, but shell 1 does not. The closest orbital to the nucleus, called the 1s orbital, can hold up to two electrons. This orbital is equivalent to the innermost electron shell of the Bohr model of the atom. It is called the 1s orbital because it is spherical around the nucleus. The 1s orbital is the closest orbital to the nucleus, and it is always filled first, before any other orbital can be filled. Hydrogen has one electron; therefore, it has only one spot within the 1s orbital occupied. This is designated as 1s1, where the superscripted 1 refers to the one electron within the 1s orbital. Helium has two electrons; therefore, it can completely fill the 1s orbital with its two electrons. This is designated as 1s2, referring to the two electrons of helium in the 1s orbital. On the periodic table Figure 2.5, hydrogen and helium are the only two elements in the first row (period); this is because they only have electrons in their first shell, the 1s orbital. Hydrogen and helium are the only two elements that have the 1s and no other electron orbitals in the electrically neutral state. The second electron shell may contain eight electrons. This shell contains another spherical s orbital and three “dumbbell” shaped p orbitals, each of which can hold two electrons, as shown in Figure 2.8. After the 1s orbital is filled, the second electron shell is filled, first filling its 2s orbital and then its three p orbitals. When filling the p orbitals, each takes a single electron; once each p orbital has an electron, a second may be added. Lithium (Li) contains three electrons that occupy the first and second shells. Two electrons fill the 1s orbital, and the third electron then fills the 2s orbital. Its electron configuration is 1s22s1. Neon (Ne), on the other hand, has a total of ten electrons: two are in its innermost 1s orbital and eight fill its second shell (two each in the 2s and three p orbitals); thus, it is an inert gas and energetically stable as a single atom that will rarely form a chemical bond with other atoms. Larger elements have additional orbitals, making up the third electron shell. While the concepts of electron shells and orbitals are closely related, orbitals provide a more accurate depiction of the electron configuration of an atom because the orbital model specifies the different shapes and special orientations of all the places that electrons may occupy. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 53 Watch this visual animation (http://openstaxcollege.org/l/orbitals) to see the spatial arrangement of the p and s orbitals. Use the periodic table to describe what a Bohr model of Fluorine (F) would look like and explain why the Bohr model is not an accurate representation of the electron orbitals in fluorine. a. A Bohr model would have 2 electron rings, and a Bohr model would not give information about atomic mass b. A Bohr model would have one electron ring, and a Bohr model would not show the sub-shells of first ring c. A Bohr model would have 2 electron rings, and a Bohr model would not show the sub-shell of second ring d. A Bohr model would have one electron ring, and a Bohr model would not give information about number of electron in each ring Activity Create diagrams to show the placement of protons, neutrons, and electrons in an atom of carbon-12 and carbon-14, respectably. Based on their subatomic difference(s), determine which element is an organism more likely to use to synthesize glucose (C6H12O6) and give a reason for your choice. Chemical Reactions and Molecules All elements are most stable when their outermost shell is filled with electrons according to the octet rule. This is because it is energetically favorable for atoms to be in that configuration and it makes them stable. However, since not all elements have enough electrons to fill their outermost shells, atoms form chemical bonds with other atoms thereby obtaining the electrons they need to attain a stable electron configuration. When two or more atoms chemically bond with each other, the resultant chemical structure is a molecule. The familiar water molecule, H2O, consists of two hydrogen atoms and one oxygen atom; these bond together to form water, as illustrated in Figure 2.9. Atoms can form molecules by donating, accepting, or sharing electrons to fill their outer shells. 54 Chapter 2 \| The Chemical Foundation of Life Figure 2.9 Two or more atoms may bond with each other to form a molecule. When two hydrogens and an oxygen share electrons via covalent bonds, a water molecule is formed. Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances used in the beginning of a chemical reaction are called the reactants (usually found on the left side of a chemical equation), and the substances found at the end of the reaction are known as the products (usually found on the right side of a chemical equation). An arrow is typically drawn between the reactants and products to indicate the direction of the chemical reaction; this direction is not always a “one-way street.” For the creation of the water molecule shown above, the chemical equation would be: 2H + O → H2 O An example of a simple chemical reaction is the breaking down of hydrogen peroxide molecules, each of which consists of two hydrogen atoms bonded to two oxygen atoms (H2O2). The reactant hydrogen peroxide is broken down into water, containing one oxygen atom bound to two hydrogen atoms (H2O), and oxygen, which consists of two bonded oxygen atoms (O2). In the equation below, the reaction includes two hydrogen peroxide molecules and two water molecules. This is an example of a balanced chemical equation, wherein the number of atoms of each element is the same on each side of the equation. According to the law of conservation of matter, the number of atoms before and after a chemical reaction should be equal, such that no atoms are, under normal circumstances, created or destroyed. 2H2 O2 (hydrogen peroxide) → 2H2 O (water) + O 2 (oxygen) Even though all of the reactants and products of this reaction are molecules (each atom remains bonded to at least one other atom), in this reaction only hydrogen peroxide and water are representatives of compounds: they contain atoms of more than one type of element. Molecular oxygen, on the other hand, as shown in Figure 2.10, consists of two doubly bonded oxygen atoms and is not classified as a compound but as a homonuclear molecule. Figure 2.10 The oxygen atoms in an O2 molecule are joined by a double bond. Some chemical reactions, such as the one shown above, can proceed in one dire
ction until the reactants are all used up. The equations that describe these reactions contain a unidirectional arrow and are irreversible. Reversible reactions are those that can go in either direction. In reversible reactions, reactants are turned into products, but when the concentration of product goes beyond a certain threshold (characteristic of the particular reaction), some of these products will be converted back into reactants; at this point, the designations of products and reactants are reversed. This back and forth continues until a certain relative balance between reactants and products occurs—a state called equilibrium. These situations of reversible reactions are often denoted by a chemical equation with a double headed arrow pointing towards both the reactants and products. For example, in human blood, excess hydrogen ions (H+) bind to bicarbonate ions (HCO3 -) forming an equilibrium state with carbonic acid (H2CO3). If carbonic acid were added to this system, some of it would be converted to bicarbonate and This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 55 hydrogen ions. HCO− + H+ — H2 CO3 In biological reactions, however, equilibrium is rarely obtained because the concentrations of the reactants or products or both are constantly changing, often with a product of one reaction being a reactant for another. To return to the example of excess hydrogen ions in the blood, the formation of carbonic acid will be the major direction of the reaction. However, the carbonic acid can also leave the body as carbon dioxide gas (via exhalation) instead of being converted back to bicarbonate ion, thus driving the reaction to the right by the chemical law known as law of mass action. These reactions are important for maintaining the homeostasis of our blood. Ions and Ionic Bonds HCO− + H+ — H2 CO3 ↔ CO2 + H2 O Some atoms are more stable when they gain or lose an electron (or possibly two) and form ions. This fills their outermost electron shell and makes them energetically more stable. Because the number of electrons does not equal the number of protons, each ion has a net charge. Cations are positive ions that are formed by losing electrons. Negative ions are formed by gaining electrons and are called anions. Anions are designated by their elemental name being altered to end in “-ide”: the anion of chlorine is called chloride, and the anion of sulfur is called sulfide, for example. This movement of electrons from one element to another is referred to as electron transfer. As Figure 2.11 illustrates, sodium (Na) only has one electron in its outer electron shell. It takes less energy for sodium to donate that one electron than it does to accept seven more electrons to fill the outer shell. If sodium loses an electron, it now has 11 protons, 11 neutrons, and only 10 electrons, leaving it with an overall charge of +1. It is now referred to as a sodium ion. Chlorine (Cl) in its lowest energy state (called the ground state) has seven electrons in its outer shell. Again, it is more energy-efficient for chlorine to gain one electron than to lose seven. Therefore, it tends to gain an electron to create an ion with 17 protons, 17 neutrons, and 18 electrons, giving it a net negative (–1) charge. It is now referred to as a chloride ion. In this example, sodium will donate its one electron to empty its shell, and chlorine will accept that electron to fill its shell. Both ions now satisfy the octet rule and have complete outermost shells. Because the number of electrons is no longer equal to the number of protons, each is now an ion and has a +1 (sodium cation) or –1 (chloride anion) charge. Note that these transactions can normally only take place simultaneously: in order for a sodium atom to lose an electron, it must be in the presence of a suitable recipient like a chlorine atom. Figure 2.11 In the formation of an ionic compound, metals lose electrons and nonmetals gain electrons to achieve an octet. Ionic bonds are formed between ions with opposite charges. For instance, positively charged sodium ions and negatively charged chloride ions bond together to make crystals of sodium chloride, or table salt, creating a crystalline molecule with zero net charge. Certain salts are referred to in physiology as electrolytes (including sodium, potassium, and calcium), ions necessary for nerve impulse conduction, muscle contractions and water balance. Many sports drinks and dietary supplements provide these ions to replace those lost from the body via sweating during exercise. Covalent Bonds and Other Bonds and Interactions Another way the octet rule can be satisfied is by the sharing of electrons between atoms to form covalent bonds. These bonds are stronger and much more common than ionic bonds in the molecules of living organisms. Covalent bonds are commonly found in carbon-based organic molecules, such as our DNA and proteins. Covalent bonds are also found in inorganic molecules like H2O, CO2, and O2. One, two, or three pairs of electrons may be shared, making single, double, and triple bonds, respectively. The more covalent bonds between two atoms, the stronger their connection. Thus, triple bonds are the strongest. The strength of different levels of covalent bonding is one of the main reasons living organisms have a difficult time in acquiring nitrogen for use in constructing their molecules, even though molecular nitrogen, N2, is the most abundant gas in the atmosphere. Molecular nitrogen consists of two nitrogen atoms triple bonded to each other and, as with all molecules, 56 Chapter 2 \| The Chemical Foundation of Life the sharing of these three pairs of electrons between the two nitrogen atoms allows for the filling of their outer electron shells, making the molecule more stable than the individual nitrogen atoms. This strong triple bond makes it difficult for living systems to break apart this nitrogen in order to use it as constituents of proteins and DNA. The formation of water molecules provides an example of covalent bonding. The hydrogen and oxygen atoms that combine to form water molecules are bound together by covalent bonds, as shown in Figure 2.9. The electron from the hydrogen splits its time between the incomplete outer shell of the hydrogen atoms and the incomplete outer shell of the oxygen atoms. To completely fill the outer shell of oxygen, which has six electrons in its outer shell but which would be more stable with eight, two electrons (one from each hydrogen atom) are needed: hence the well-known formula H2O. The electrons are shared between the two elements to fill the outer shell of each, making both elements more stable. View this short video (http://openstaxcollege.org/l/ionic_covalent) to see an animation of ionic and covalent bonding. What makes ionic bonds different from covalent bonds? a. Ionic bond involves the transfer of electrons whereas covalent bond involves the sharing of electrons. b. Ionic bond involves the van der Waals force of interaction whereas covalent bond involves the sharing of electrons. c. Ionic bond involves the sharing of electrons whereas a covalent bond involves the transfer of electrons. d. An ionic bond involves the transfer of electrons whereas a covalent bond involves the van der Waals force of interaction. Polar Covalent Bonds There are two types of covalent bonds: polar and nonpolar. In a polar covalent bond, shown in this figure, the electrons are unequally shared by the atoms and are attracted more to one nucleus than the other. Because of the unequal distribution of electrons between the atoms of different elements, a slightly positive (δ+) or slightly negative (δ–) charge develops. This partial charge is an important property of water and accounts for many of its characteristics. Water is a polar molecule, with the hydrogen atoms acquiring a partial positive charge and the oxygen a partial negative charge. This occurs because the nucleus of the oxygen atom is more attractive to the electrons of the hydrogen atoms than the hydrogen nucleus is to the oxygen’s electrons. Thus oxygen has a higher electronegativity than hydrogen and the shared electrons spend more time in the vicinity of the oxygen nucleus than they do near the nucleus of the hydrogen atoms, giving the atoms of oxygen and hydrogen slightly negative and positive charges, respectively. Another way of stating this is that the probability of finding a shared electron near an oxygen nucleus is more likely than finding it near a hydrogen nucleus. Either way, the atom’s relative electronegativity contributes to the development of partial charges whenever one element is significantly more electronegative than the other, and the charges generated by these polar bonds may then be used for the formation of hydrogen bonds based on the attraction of opposite partial charges. (Hydrogen bonds, which are discussed in detail below, are weak bonds between slightly positively charged hydrogen atoms to slightly negatively charged atoms in other molecules.) Since macromolecules often have atoms within them that differ in electronegativity, polar bonds are often present in organic molecules. Nonpolar Covalent Bonds Nonpolar covalent bonds form between two atoms of the same element or between different elements that share electrons equally. For example, molecular oxygen (O2) is nonpolar because the electrons will be equally distributed between the two oxygen atoms. Another example of a nonpolar covalent bond is methane (CH4), also shown in this figure. Carbon has four electrons in its outermost shell and needs four more to fill it. It gets these four from four hydrogen atoms, each atom providing one, making a stable outer shell of eight electrons. Carbon and hydrogen do not have the same electronegativity but are similar; thus, This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation o
f Life 57 nonpolar bonds form. The hydrogen atoms each need one electron for their outermost shell, which is filled when it contains two electrons. These elements share the electrons equally among the carbons and the hydrogen atoms, creating a nonpolar covalent molecule. Figure 2.12 Whether a molecule is polar or nonpolar depends both on bond type and molecular shape. Both water and carbon dioxide have polar covalent bonds, but carbon dioxide is linear, so the partial charges on the molecule cancel each other out. Hydrogen Bonds and Van Der Waals Interactions Ionic and covalent bonds between elements require energy to break. Ionic bonds are not as strong as covalent, which determines their behavior in biological systems. However, not all bonds are ionic or covalent bonds. Weaker bonds can also form between molecules. Two weak bonds that occur frequently are hydrogen bonds and van der Waals interactions. Without these two types of bonds, life as we know it would not exist. Hydrogen bonds provide many of the critical, lifesustaining properties of water and also stabilize the structures of proteins and DNA, the building block of cells. When polar covalent bonds containing hydrogen form, the hydrogen in that bond has a slightly positive charge because hydrogen’s electron is pulled more strongly toward the other element and away from the hydrogen. Because the hydrogen is slightly positive, it will be attracted to neighboring negative charges. When this happens, a weak interaction occurs between the δ+of the hydrogen from one molecule and the δ– charge on the more electronegative atoms of another molecule, usually oxygen or nitrogen, or within the same molecule. This interaction is called a hydrogen bond. This type of bond is common and occurs regularly between water molecules. Individual hydrogen bonds are weak and easily broken; however, they occur in very large numbers in water and in organic polymers, creating a major force in combination. Hydrogen bonds are also responsible for zipping together the DNA double helix. Like hydrogen bonds, van der Waals interactions are weak attractions or interactions between molecules. Van der Waals attractions can occur between any two or more molecules and are dependent on slight fluctuations of the electron densities, which are not always symmetrical around an atom. For these attractions to happen, the molecules need to be very close to one another. These bonds—along with ionic, covalent, and hydrogen bonds—contribute to the three-dimensional structure of the proteins in our cells that is necessary for their proper function. 58 Chapter 2 \| The Chemical Foundation of Life Pharmaceutical chemists are responsible for the development of new drugs and trying to determine the mode of action of both old and new drugs. They are involved in every step of the drug development process. Drugs can be found in the natural environment or can be synthesized in the laboratory. In many cases, potential drugs found in nature are changed chemically in the laboratory to make them safer and more effective, and sometimes synthetic versions of drugs substitute for the version found in nature. After the initial discovery or synthesis of a drug, the chemist then develops the drug, perhaps chemically altering it, testing it to see if the drug is toxic, and then designing methods for efficient large-scale production. Then, the process of getting the drug approved for human use begins. In the United States, drug approval is handled by the Food and Drug Administration (FDA) and involves a series of large-scale experiments using human subjects to make sure the drug is not harmful and effectively treats the condition it aims to treat. This process often takes several years and requires the participation of physicians and scientists, in addition to chemists, to complete testing and gain approval. An example of a drug that was originally discovered in a living organism is Paclitaxel, an anti-cancer drug. This drug was discovered in the bark of the pacific yew tree. Another example is aspirin, originally isolated from willow tree bark. Finding drugs often means testing hundreds of samples of plants, fungi, and other forms of life to see if any biologically active compounds are found within them. Sometimes, traditional medicine can give modern medicine clues to where an active compound can be found. For example, the use of willow bark to make medicine has been known for thousands of years, dating back to ancient Egypt. It was not until the late 1800s, however, that the aspirin molecule, known as acetylsalicylic acid, was purified and marketed for human use. Occasionally, drugs developed for one use are found to have unforeseen effects that allow these drugs to be used in other, unrelated ways. For example, the drug minoxidil was originally developed to treat high blood pressure. When tested on humans, it was noticed that individuals taking the drug would grow new hair. Eventually the drug was marketed to men and women with baldness to restore lost hair. The career of development, all with the goal of making human beings healthier. the pharmaceutical chemist may involve detective work, experimentation, and drug This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 59 Bonds Can Be Flexible Proteins are mostly made up of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. The proteins that make up hair contain sulfur bonded to another sulfur, which is called a disulfide bond. These covalent bonds give hair its shape and texture. Heat from a hair straightener breaks the disulfide bonds, which causes the hair to lose its curl. Why do you think this method of hair straightening isn’t permanent? Figure 2.13 The shape of hair proteins is maintained by a combination of hydrogen bonds and covalent, disulfide bonds. Heat is sufficient to break the hydrogen bonds, but harsh chemicals are required to break the disulfide bonds. Why is it harder to break the disulfide bonds than the hydrogen bonds? a. Covalent bonds are stronger than hydrogen bonds. b. There are many more disulfide bonds than hydrogen bonds. c. Covalent bonds are stronger than disulfide bonds. d. Covalent bonds are less elastic than hydrogen bonds. 2.2 \| Water In this section, you will investigate the following questions: • How does the molecular structure of water result in unique properties of water that are critical to maintaining life? • What are the role of acids, bases, and buffers in dynamic homeostasis? Connection for AP® Courses Covalent bonds form between atoms when they share electrons to fill their valence electron shells. When the sharing of electrons between atoms is equal, such as O2 (oxygen) or CH4 (methane), the covalent bond is said to be nonpolar. However, when electrons are shared, but not equally due to differences in electronegativity (the tendency to attract 60 Chapter 2 \| The Chemical Foundation of Life electrons), the covalent bond is said to be polar. H2O (water) is an example of a polar molecule. Because oxygen is more electronegative than hydrogen, the electrons are drawn toward oxygen and away from the hydrogen atoms; consequently, the oxygen atom acquires a slight negative charge and each hydrogen atoms acquires a slightly positive charge. It is important to remember that the electrons are still shared, just not equally. Water’s polarity allows for the formation of hydrogen bonds between adjacent water molecules, resulting in many unique properties that are critical to maintaining life. For example, water is an excellent solvent because hydrogen bonds allow ions and other polar molecules to dissolve in water. Water’s hydrogen bonds also contribute to its high heat capacity and high heat of vaporization, resulting in greater temperature stability. Hydrogen bond formation makes ice less dense as a solid than as a liquid, insulating aquatic environments. Water’s cohesive and adhesive properties are seen as it rises inside capillary tubes or travels up a large tree from roots to leaves. The pH or hydrogen ion concentration of a solution is highly regulated to help organisms maintain homeostasis; for example, as will be explored in later chapters, the enzymes that catalyze most chemical reactions in cells are pH specific. Thus, the properties of water are connected to the biochemical and physical processes performed by living organisms. Life on Earth would be very different if these properties were altered—if life could exist at all. The information presented and the examples highlighted in this section support concepts and Learning Objectives outlined in Big Idea 2 of the AP® Biology Curriculum. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 2 Enduring Understanding 2.A Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question. 2.8 The student is able to justify the selection of data regarding the types of molecules that an animal, plant, or bacterium will take up as necessary building blocks and excrete as waste products. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.8] [APLO 2.23] Why do scientists spend time looking for water
on other planets? Why is water so important? It is because water is essential to life as we know it. Water is one of the more abundant molecules and the one most critical to life on Earth. Approximately 60–70 percent of the human body is made up of water. Without it, life as we know it simply would not exist. The polarity of the water molecule and its resulting hydrogen bonding make water a unique substance with special properties that are intimately tied to the processes of life. Life originally evolved in a watery environment, and most of an organism’s cellular chemistry and metabolism occur inside the watery contents of the cell’s cytoplasm. Special properties of water are its high heat capacity and heat of vaporization, its ability to dissolve polar molecules, its cohesive and adhesive properties, and its dissociation into ions that leads to the generation of pH. Understanding these characteristics of water helps to elucidate its importance in maintaining life. Water’s Polarity One of water’s important properties is that it is composed of polar molecules: the hydrogen and oxygen within water molecules (H2O) form polar covalent bonds. While there is no net charge to a water molecule, the polarity of water creates a slightly positive charge on hydrogen and a slightly negative charge on oxygen, contributing to water’s properties of attraction. Water’s charges are generated because oxygen is more electronegative than hydrogen, making it more likely that a shared electron would be found near the oxygen nucleus than the hydrogen nucleus, thus generating the partial negative charge near the oxygen. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 61 As a result of water’s polarity, each water molecule attracts other water molecules because of the opposite charges between water molecules, forming hydrogen bonds. Water also attracts or is attracted to other polar molecules and ions. A polar substance that interacts readily with or dissolves in water is referred to as hydrophilic (hydro- = “water”; -philic = “loving”). In contrast, non-polar molecules such as oils and fats do not interact well with water, as shown in Figure 2.14 and separate from it rather than dissolve in it, as we see in salad dressings containing oil and vinegar (an acidic water solution). These nonpolar compounds are called hydrophobic (hydro- = “water”; -phobic = “fearing”). Figure 2.14 Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound. (credit: Gautam Dogra). Water’s States: Gas, Liquid, and Solid The formation of hydrogen bonds is an important quality of the liquid water that is crucial to life as we know it. As water molecules make hydrogen bonds with each other, water takes on some unique chemical characteristics compared to other liquids and, since living things have a high water content, understanding these chemical features is key to understanding life. In liquid water, hydrogen bonds are constantly formed and broken as the water molecules slide past each other. The breaking of these bonds is caused by the motion (kinetic energy) of the water molecules due to the heat contained in the system. When the heat is raised as water is boiled, the higher kinetic energy of the water molecules causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam or water vapor). On the other hand, when the temperature of water is reduced and water freezes, the water molecules form a crystalline structure maintained by hydrogen bonding (there is not enough energy to break the hydrogen bonds) that makes ice less dense than liquid water, a phenomenon not seen in the solidification of other liquids. Water’s lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes: the water molecules are pushed farther apart compared to liquid water. With most other liquids, solidification when the temperature drops includes the lowering of kinetic energy between molecules, allowing them to pack even more tightly than in liquid form and giving the solid a greater density than the liquid. The lower density of ice, illustrated and pictured in Figure 2.15, an anomaly, causes it to float at the surface of liquid water, such as in an iceberg or in the ice cubes in a glass of ice water. In lakes and ponds, ice will form on the surface of the water creating an insulating barrier that protects the animals and plant life in the pond from freezing. Without this layer of insulating ice, plants and animals living in the pond would freeze in the solid block of ice and could not survive. The detrimental effect of freezing on living organisms is caused by the expansion of ice relative to liquid water. The ice crystals that form upon freezing rupture the delicate membranes essential for the function of living cells, irreversibly damaging them. Cells can only survive freezing if the water in them is temporarily replaced by another liquid like glycerol. 62 Chapter 2 \| The Chemical Foundation of Life Figure 2.15 Hydrogen bonding makes ice less dense than liquid water. The (a) lattice structure of ice makes it less dense than the freely flowing molecules of liquid water, enabling it to (b) float on water. (credit a: modification of work by Jane Whitney, image created using Visual Molecular Dynamics (VMD) software ; credit b: modification of work by Carlos Ponte) [1] Click here (http://openstaxcollege.org/l/ice_lattice2) to see a 3-D animation of the structure of an ice lattice. (Image credit: Jane Whitney. Image created using Visual Molecular Dynamics VMD software. [2] ) Identify the red and white balls in the model and explain how arrangement of the molecules supports the fact that ice floats on water. a. Red and white balls represent oxygen and hydrogen, respectively, loose arrangement of molecules results in low density of ice b. Red and white balls represent oxygen and hydrogen respectively, tightly packed arrangement of molecules results in a low density of ice c. Red and white balls represent hydrogen and oxygen, respectively, loose arrangement of molecules results in low density of ice d. Red and white balls represent oxygen and hydrogen, respectively, tightly packed arrangement of molecules results in high density of ice Water’s High Heat Capacity Water’s high heat capacity is a property caused by hydrogen bonding among water molecules. Water has the highest specific 1. W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-38. 2. W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-38. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 63 heat capacity of any liquids. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie. It therefore takes water a long time to heat and long time to cool. In fact, the specific heat capacity of water is about five times more than that of sand. This explains why the land cools faster than the sea. Due to its high heat capacity, water is used by warm blooded animals to more evenly disperse heat in their bodies: it acts in a similar manner to a car’s cooling system, transporting heat from warm places to cool places, causing the body to maintain a more even temperature. Water’s Heat of Vaporization Water also has a high heat of vaporization, the amount of energy required to change one gram of a liquid substance to a gas. A considerable amount of heat energy (586 cal) is required to accomplish this change in water. This process occurs on the surface of water. As liquid water heats up, hydrogen bonding makes it difficult to separate the liquid water molecules from each other, which is required for it to enter its gaseous phase (steam). As a result, water acts as a heat sink or heat reservoir and requires much more heat to boil than does a liquid such as ethanol, whose hydrogen bonding with other ethanol molecules is weaker than water’s hydrogen bonding. Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit), the heat is able to break the hydrogen bonds between the water molecules, and the kinetic energy (motion) between the water molecules allows them to escape from the liquid as a gas. Even when below its boiling point, water’s individual molecules acquire enough energy from other water molecules such that some surface water molecules can escape and vaporize: this process is known as evaporation. The fact that hydrogen bonds need to be broken for water to evaporate means that a substantial amount of energy is used in the process. As the water evaporates, energy is taken up by the process, cooling the environment where the evaporation is taking place. In many living organisms, including in humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of body temperature can be maintained. Water’s Solvent Properties Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can readily dissolve in it. Therefore, water is referred to as a solvent, a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules will form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, as illustrated in Figure 2.16 and serves to keep the particles separated or dispersed in the water. When ionic compounds are added to water, the individual ions react with the polar regions of the water molecules and their ionic
bonds are disrupted in the process of dissociation. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na+ and Cl– ions, and spheres of hydration form around the ions, illustrated in Figure 2.16. The positively charged sodium ion is surrounded by the partially negative charge of the water molecule’s oxygen. The negatively charged chloride ion is surrounded by the partially positive charge of the hydrogen on the water molecule. Figure 2.16 When table salt (NaCl) is mixed in water, spheres of hydration are formed around the ions. Water’s Cohesive and Adhesive Properties Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass. Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (heavier) than the water. Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top. It’s even possible to “float” a needle on top of a glass of water if 64 Chapter 2 \| The Chemical Foundation of Life it is placed gently without breaking the surface tension, as shown in Figure 2.17. Figure 2.17 The weight of the needle is pulling the surface downward; at the same time, the surface tension is pulling it up, suspending it on the surface of the water and keeping it from sinking. Notice the indentation in the water around the needle. (credit: Cory Zanker) These cohesive forces are related to water’s property of adhesion, or the attraction between water molecules and other molecules. This attraction is sometimes stronger than water’s cohesive forces, especially when the water is exposed to charged surfaces such as those found on the inside of thin glass tubes known as capillary tubes. Adhesion is observed when water “climbs” up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle. This is because the water molecules are attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it. This type of adhesion is called capillary action, and is illustrated in Figure 2.18. Figure 2.18 Capillary action in a glass tube is caused by the adhesive forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves. (credit: modification of work by Pearson-Scott Foresman, donated to the Wikimedia Foundation) Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants. These forces create a “pull” on the water column. This pull results from the tendency of water molecules being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water from their roots to their leaves. Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider, shown in Figure 2.19, use the surface tension of water to stay afloat on the surface layer of water and even mate there. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 65 Figure 2.19 Water’s cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat. (credit: Tim Vickers) Activity During a process called transpiration, water evaporates through a plant’s leaves. Water in the ground travels up from the roots to the leaves. Based on water’s molecular properties, create a visual representation (e.g., diagrams or models) with annotations to explain how water travels up a 300-ft. California redwood tree. What other unique properties of water are attributed to its molecular structure, and how are these properties important to life? pH, Buffers, Acids, and Bases The pH of a solution indicates its acidity or alkalinity. H2 O(l) ↔ H+(aq) + OH-(aq) litmus or pH paper, filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator, to test how much acid (acidity) or base (alkalinity) exists in a solution. You might have even used some to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures the concentration of hydrogen ions in a given solution. Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H30+). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water. The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water. Moles (mol) are a way to express the amount of a substance (which can be atoms, molecules, ions, etc), with one mole being equal to 6.02 × 1023 particles of the substance. Therefore, 1 mole of water is equal to 6.02 × 1023 water molecules. The pH is calculated as the negative of the base 10 logarithm of this concentration. The log10 of 1 × 10-7 is -7.0, and the negative of this number (indicated by the “p” of “pH”) yields a pH of 7.0, which is also known as neutral pH. The pH inside of human cells and blood are examples of two areas of the body where near-neutral pH is maintained. Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH number, whereas low levels of hydrogen ions result in a high pH. An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, usually by having one of its hydrogen atoms dissociate. A base provides either hydroxide ions (OH–) or other negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water molecules. The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids. Conversely, strong bases are those substances that readily donate OH– or take up hydrogen 66 Chapter 2 \| The Chemical Foundation of Life ions. Sodium hydroxide (NaOH) and many household cleaners are highly alkaline and give up OH– rapidly when placed in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are able to thrive in it. The pH scale is, as previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure 2.20). Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH of 1 to 2. So how do the cells of the stomach survive in such an acidic environment? How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of the human stomach is completely replaced every seven to ten days. Figure 2.20 The pH scale measures the concentration of hydrogen ions (H+) in a solution. (credit: modification of work by Edward Stevens) Watch this video (http://openstaxcollege.org/l/pH_scale) for a straightforward explanation of pH and its logarithmic scale. One of the risks for people with diabetes is diabetic ketoacidosis, a build-up of acid in the blood stream. Explain why this is dangerous to humans. a. Diabetic ketoacidosis decreases the normal pH (8.35-8.45) to a lower value. b. Diabetic ketoacidosis increases normal pH level of blood disrupting biological processes. c. Diabetic ketoacidosis keeps pH level of blood constant which disrupts biological processes. d. Diabetic ketoacidosis decreases normal pH (7.35-7.45) to a lower value. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 67 So how can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers readily absorb excess H+ or OH–, keeping the pH of the body carefully maintained in the narrow range
required for survival. Maintaining a constant blood pH is critical to a person’s –), and well-being. The buffer maintaining the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3 carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, as shown in Figure 2.21, excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs. This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood’s pH. Likewise, if too much OH– is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy. Figure 2.21 This diagram shows the body’s buffering of blood pH levels. The blue arrows show the process of raising pH as more CO2 is made. The purple arrows indicate the reverse process: the lowering of pH as more bicarbonate is created. Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer “heartburn” after eating. The unique properties of water that contribute to this capacity to balance pH—as well as water’s other characteristics—are essential to sustaining life on Earth. To learn more about water. Visit the U.S. Geological Survey Water Science for Schools (http://openstaxcollege.org/l/ all_about_water) All About Water! website. Water takes up 333 million cubic miles on Earth, yet access to drinking water is a critical issue for many communities around the world. Explain why this is so. a. Drinking water is only obtained by rain water harvesting. b. Only 4 percent of the total water on earth is freshwater which is found only in glaciers. c. Only 4 percent of the total water on earth is freshwater, out of which 68 percent is found in glaciers. d. Drinking water is only obtained by desalination treatments of salt water found on earth. 68 Chapter 2 \| The Chemical Foundation of Life Acid Rain Figure 2.22 When rain water is too acidic, it can greatly damage living organisms, such as this forest in the Czech Republic. Limestone is a naturally occurring mineral rich in calcium carbonate ( CaCO3 to form carbonate (CO3 rain, an environment rich in limestone or an environment poor in limestone? 2 − ) , a weak base that acts as a buffer. Which would you expect to be more affected by acid ). In water, calcium carbonate dissolves a. The presence of limestone would not make a difference. b. An environment rich in limestone would be more affected by acid rain. c. An environment poor in limestone would be more affected by acid rain. d. The impact would depend on the type of vegetation present. 2.3 \| Carbon In this section, you will investigate the following questions: • Why is carbon important for life? • How do functional groups determine the properties of biological molecules? Connection for AP® Courses The unique properties of carbon make it a central part of biological molecules. With four valence electrons, carbon can covalently bond to oxygen, hydrogen, and nitrogen to form the many molecules important for cellular function. Carbon This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 69 and hydrogen can form either hydrocarbon chains or rings. Functional groups, such as –CH3 (methyl) and –COOH (carboxyl), are groups of atoms that give specific properties to hydrocarbon chains or rings that define their overall chemical characteristics and function. For example, the attachment of a carboxyl group (-COOH) makes a molecule more acidic, whereas the presence of an amine group (NH2) makes a molecule more basic. (As we will explore in the next chapter, amino acids have both a carboxyl group and an amine group.) Isomers are molecules with the same molecular formula (i.e., same kinds and numbers of atoms), but different molecular structures resulting in different properties or functions. (Don’t confuse “isomer” with “isotope”!) The information presented and examples highlighted in this section support concepts and Learning Objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 2 Enduring Understanding 2.A Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question. 2.8 The student is able to justify the selection of data regarding the types of molecules that an animal, plant, or bacterium will take up as necessary building blocks and excrete as waste products. Cells are made of many complex molecules called macromolecules, such as proteins, nucleic acids (RNA and DNA), carbohydrates, and lipids. The macromolecules are a subset of organic molecules (any carbon-containing liquid, solid, or gas) that are especially important for life. The fundamental component for all of these macromolecules is carbon. The carbon atom has unique properties that allow it to form covalent bonds to as many as four different atoms, making this versatile element ideal to serve as the basic structural component, or “backbone,” of the macromolecules. Individual carbon atoms have an incomplete outermost electron shell. With an atomic number of 6 (six electrons and six protons), the first two electrons fill the inner shell, leaving four in the second shell. Therefore, carbon atoms can form up to four covalent bonds with other atoms to satisfy the octet rule. The methane molecule provides an example: it has the chemical formula CH4. Each of its four hydrogen atoms forms a single covalent bond with the carbon atom by sharing a pair of electrons. This results in a filled outermost shell. Hydrocarbons Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen, such as methane (CH4) described above. We often use hydrocarbons in our daily lives as fuels—like the propane in a gas grill or the butane in a lighter. The many covalent bonds between the atoms in hydrocarbons store a great amount of energy, which is released when these molecules are burned (oxidized). Methane, an excellent fuel, is the simplest hydrocarbon molecule, with a central carbon atom bonded to four different hydrogen atoms, as illustrated in Figure 2.23. The geometry of the methane molecule, where the atoms reside in three dimensions, is determined by the shape of its electron orbitals. The carbons and the four hydrogen atoms form a shape known as a tetrahedron, with four triangular faces; for this reason, methane is described as having tetrahedral geometry. 70 Chapter 2 \| The Chemical Foundation of Life Figure 2.23 Methane has a tetrahedral geometry, with each of the four hydrogen atoms spaced 109.5° apart. As the backbone of the large molecules of living things, hydrocarbons may exist as linear carbon chains, carbon rings, or combinations of both. Furthermore, individual carbon-to-carbon bonds may be single, double, or triple covalent bonds, and each type of bond affects the geometry of the molecule in a specific way. This three-dimensional shape or conformation of the large molecules of life (macromolecules) is critical to how they function. Hydrocarbon Chains Hydrocarbon chains are formed by successive bonds between carbon atoms and may be branched or unbranched. Furthermore, the overall geometry of the molecule is altered by the different geometries of single, double, and triple covalent bonds, illustrated in Figure 2.24. The hydrocarbons ethane, ethene, and ethyne serve as examples of how different carbon-to-carbon bonds affect the geometry of the molecule. The names of all three molecules start with the prefix “eth-,” which is the prefix for two carbon hydrocarbons. The suffixes “-ane,” “-ene,” and “-yne” refer to the presence of single, double, or triple carbon-carbon bonds, respectively. Thus, propane, propene, and propyne follow the same pattern with three carbon molecules, butane, butene, and butyne for four carbon molecules, and so on. Double and triple bonds change the geometry of the molecule: single bonds allow rotation along the axis of the bond, whereas double bonds lead to a planar configuration and triple bonds to a linear one. These geometries have a significant impact on the shape a particular molecule can assume. Figure 2.24 When carbon forms single bonds with other atoms, the shape is tetrahedral. When two carbon atoms form a double bond, the shape is planar, or flat. Single bonds, like those found in ethane, are able to rotate. Double bonds, like those found in ethene cannot rotate, so the atoms on either side are locked in place. Hydrocarbon Rings So far, the hydrocarbons we have discussed have been aliphatic hydrocarbons, which consist of linear chains of carbon atoms. Another type of hydrocarbon, aromatic hydrocarbons, consists of closed rings of carbon atoms. Ring structures are found in hydrocarbons, sometimes with the presence of double bonds, which can be seen by comparing the structure of cyclohexane to benzene in Figure 2.25. Examples of biological molecules that incorporate the benzene ring include some
amino acids and cholesterol and its derivatives, including the hormones estrogen and testosterone. The benzene ring is also found in the herbicide 2,4-D. Benzene is a natural component of crude oil and has been classified as a carcinogen. Some hydrocarbons have both aliphatic and aromatic portions; beta-carotene is an example of such a hydrocarbon. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 71 Figure 2.25 Carbon can form five-and six membered rings. Single or double bonds may connect the carbons in the ring, and nitrogen may be substituted for carbon. Isomers The three-dimensional placement of atoms and chemical bonds within organic molecules is central to understanding their chemistry. Molecules that share the same chemical formula but differ in the placement (structure) of their atoms and/ or chemical bonds are known as isomers. Structural isomers (like butane and isobutene shown in figurea) differ in the placement of their covalent bonds: both molecules have four carbons and ten hydrogens (C4H10), but the different arrangement of the atoms within the molecules leads to differences in their chemical properties. For example, due to their different chemical properties, butane is suited for use as a fuel for torches, whereas isobutene is suited for use as a refrigerant and a propellant in spray cans. Geometric isomers, on the other hand, have similar placements of their covalent bonds but differ in how these bonds are made to the surrounding atoms, especially in carbon-to-carbon double bonds. In the simple molecule butene (C4H8), the two methyl groups (CH3) can be on either side of the double covalent bond central to the molecule, as illustrated in figureb. When the carbons are bound on the same side of the double bond, this is the cis configuration; if they are on opposite sides of the double bond, it is a trans configuration. In the trans configuration, the carbons form a more or less linear structure, whereas the carbons in the cis configuration make a bend (change in direction) of the carbon backbone. 72 Chapter 2 \| The Chemical Foundation of Life Figure 2.26 Molecules that have the same number and type of atoms arranged differently are called isomers. (a) Structural isomers have a different covalent arrangement of atoms. (b) Geometric isomers have a different arrangement of atoms around a double bond. (c) Enantiomers are mirror images of each other. Which of the following statements is false? a. Molecules with the formulas CH3 CH2 COOH and C3 H6 O2 could be structural isomers. b. Molecules must have a double bond to be cis-trans isomers. c. To be enantiomers, a molecule must have at least three different atoms or groups connected to a central carbon. d. To be enantiomers, a molecule must have at least four different atoms or groups connected to a central carbon. In triglycerides (fats and oils), long carbon chains known as fatty acids may contain double bonds, which can be in either the cis or trans configuration, illustrated in Figure 2.27. Fats with at least one double bond between carbon atoms are unsaturated fats. When some of these bonds are in the cis configuration, the resulting bend in the carbon backbone of the chain means that triglyceride molecules cannot pack tightly, so they remain liquid (oil) at room temperature. On the other This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 73 hand, triglycerides with trans double bonds (popularly called trans fats), have relatively linear fatty acids that are able to pack tightly together at room temperature and form solid fats. In the human diet, trans fats are linked to an increased risk of cardiovascular disease, so many food manufacturers have reduced or eliminated their use in recent years. In contrast to unsaturated fats, triglycerides without double bonds between carbon atoms are called saturated fats, meaning that they contain all the hydrogen atoms available. Saturated fats are a solid at room temperature and usually of animal origin. Figure 2.27 These space-filling models show a cis (oleic acid) and a trans (eliadic acid) fatty acid. Notice the bend in the molecule caused by the cis configuration. Enantiomers Enantiomers are molecules that share the same chemical structure and chemical bonds but differ in the three-dimensional placement of atoms so that they are mirror images. As shown in Figure 2.28, an amino acid alanine example, the two structures are non-superimposable. In nature, only the L-forms of amino acids are used to make proteins. Some D forms of amino acids are seen in the cell walls of bacteria, but never in their proteins. Similarly, the D-form of glucose is the main product of photosynthesis and the L-form of the molecule is rarely seen in nature. Figure 2.28 D-alanine and L-alanine are examples of enantiomers or mirror images. Only the L-forms of amino acids are used to make proteins. Functional Groups Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to those molecules. They are found along the “carbon backbone” of macromolecules. This carbon backbone is formed by chains and/or rings of carbon atoms with the occasional substitution of an element such as nitrogen or oxygen. Molecules with other elements in their carbon backbone are substituted hydrocarbons. The functional groups in a macromolecule are usually attached to the carbon backbone at one or more different places along its chain and/or ring structure. Each of the four types of macromolecules—proteins, lipids, carbohydrates, and nucleic acids—has its own characteristic set of functional groups that contributes greatly to its differing chemical properties and its function in living organisms. 74 Chapter 2 \| The Chemical Foundation of Life A functional group can participate in specific chemical reactions. Some of the important functional groups in biological molecules are shown in Figure 2.29; they include: hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl. These groups play an important role in the formation of molecules like DNA, proteins, carbohydrates, and lipids. Functional groups are usually classified as hydrophobic or hydrophilic depending on their charge or polarity characteristics. An example of a hydrophobic group is the non-polar methyl molecule. Among the hydrophilic functional groups is the carboxyl group found in amino acids, some amino acid side chains, and the fatty acids that form triglycerides and phospholipids. This carboxyl group ionizes to release hydrogen ions (H+) from the COOH group resulting in the negatively charged COOgroup; this contributes to the hydrophilic nature of whatever molecule it is found on. Other functional groups, such as the carbonyl group, have a partially negatively charged oxygen atom that may form hydrogen bonds with water molecules, again making the molecule more hydrophilic. Figure 2.29 The functional groups shown here are found in many different biological molecules. Hydrogen bonds between functional groups (within the same molecule or between different molecules) are important to the function of many macromolecules and help them to fold properly into and maintain the appropriate shape for functioning. Hydrogen bonds are also involved in various recognition processes, such as DNA complementary base pairing and the binding of an enzyme to its substrate, as illustrated in Figure 2.30. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 75 Figure 2.30 Hydrogen bonds connect two strands of DNA together to create the double-helix structure. Activity Carbon forms the backbone of important biological molecules. Create a mini-poster of a simple food chain that shows how carbon enters and exits each organism on the chain. Based on the food chain you created, make a prediction regarding the impact of human activity on the supply of carbon in the food chain. 76 Chapter 2 \| The Chemical Foundation of Life KEY TERMS acid molecule that donates hydrogen ions and increases the concentration of hydrogen ions in a solution adhesion attraction between water molecules and other molecules aliphatic hydrocarbon hydrocarbon consisting of a linear chain of carbon atoms anion negative ion that is formed by an atom gaining one or more electrons aromatic hydrocarbon hydrocarbon consisting of closed rings of carbon atoms atom the smallest unit of matter that retains all of the chemical properties of an element atomic mass calculated mean of the mass number for an element’s isotopes atomic number total number of protons in an atom balanced chemical equation statement of a chemical reaction with the number of each type of atom equalized for both the products and reactants base molecule that donates hydroxide ions or otherwise binds excess hydrogen ions and decreases the concentration of hydrogen ions in a solution buffer substance that prevents a change in pH by absorbing or releasing hydrogen or hydroxide ions calorie amount of heat required to change the temperature of one gram of water by one degree Celsius capillary action occurs because water molecules are attracted to charges on the inner surfaces of narrow tubular structures such as glass tubes, drawing the water molecules to the sides of the tubes cation positive ion that is formed by an atom losing one or more electrons chemical bond interaction between two or more of the same or different atoms that results in the formation of molecules chemical reaction process leading to the rearrangement of atoms in molecules chemical reactivity the ability to combine and to chemically bond with each other cohesion intermolecular forces between water molecules caused by the polar nature of water; responsible for surface tension compound substance composed of molecules consisting of atoms of at least two diffe
rent elements covalent bond type of strong bond formed between two atoms of the same or different elements; forms when electrons are shared between atoms dissociation release of an ion from a molecule such that the original molecule now consists of an ion and the charged remains of the original, such as when water dissociates into H+ and OH- electrolyte ion necessary for nerve impulse conduction, muscle contractions and water balance electron negatively charged subatomic particle that resides outside of the nucleus in the electron orbital; lacks functional mass and has a negative charge of –1 unit electron configuration arrangement of electrons in an atom’s electron shell (for example, 1s22s22p6) electron orbital to be found how electrons are spatially distributed surrounding the nucleus; the area where an electron is most likely electron transfer movement of electrons from one element to another; important in creation of ionic bonds electronegativity ability of some elements to attract electrons (often of hydrogen atoms), acquiring partial negative This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 77 charges in molecules and creating partial positive charges on the hydrogen atoms element one of 118 unique substances that cannot be broken down into smaller substances; each element has unique properties and a specified number of protons enantiomers molecules that share overall structure and bonding patterns, but differ in how the atoms are three dimensionally placed such that they are mirror images of each other equilibrium steady state of relative reactant and product concentration in reversible chemical reactions in a closed system evaporation organism separation of individual molecules from the surface of a body of water, leaves of a plant, or the skin of an functional group group of atoms that provides or imparts a specific function to a carbon skeleton geometric isomer isomer with similar bonding patterns differing in the placement of atoms alongside a double covalent bond heat of vaporization of water high amount of energy required for liquid water to turn into water vapor hydrocarbon molecule that consists only of carbon and hydrogen hydrogen bond weak bond between slightly positively charged hydrogen atoms and slightly negatively charged atoms in other molecules hydrophilic describes ions or polar molecules that interact well with other polar molecules such as water hydrophobic describes uncharged non-polar molecules that do not interact well with polar molecules such as water inert gas (also, noble gas) element with filled outer electron shell that is unreactive with other atoms ion atom or chemical group that does not contain equal numbers of protons and electrons ionic bond chemical bond that forms between ions with opposite charges (cations and anions) irreversible chemical reaction chemical reaction where reactants proceed uni-directionally to form products isomers molecules that differ from one another even though they share the same chemical formula isotope one or more forms of an element that have different numbers of neutrons law of mass action substances chemical law stating that the rate of a reaction is proportional to the concentration of the reacting litmus paper (also, pH paper) filter paper that has been treated with a natural water-soluble dye that changes its color as the pH of the environment changes so it can be used as a pH indicator mass number total number of protons and neutrons in an atom matter anything that has mass and occupies space molecule two or more atoms chemically bonded together neutron uncharged particle that resides in the nucleus of an atom; has a mass of one amu noble gas see inert gas nonpolar covalent bond type of covalent bond that forms between atoms when electrons are shared equally between them nucleus core of an atom; contains protons and neutrons octet rule rule that atoms are most stable when they hold eight electrons in their outermost shells 78 Chapter 2 \| The Chemical Foundation of Life orbital region surrounding the nucleus; contains electrons organic molecule any molecule containing carbon (except carbon dioxide) periodic table organizational chart of elements indicating the atomic number and atomic mass of each element; provides key information about the properties of the elements pH paper see litmus paper pH scale scale ranging from zero to 14 that is inversely proportional to the concentration of hydrogen ions in a solution polar covalent bond type of covalent bond that forms as a result of unequal sharing of electrons, resulting in the creation of slightly positive and slightly negative charged regions of the molecule product molecule found on the right side of a chemical equation proton positively charged particle that resides in the nucleus of an atom; has a mass of one amu and a charge of +1 radioisotope isotope that emits radiation composed of subatomic particles to form more stable elements reactant molecule found on the left side of a chemical equation reversible chemical reaction chemical reaction that functions bi-directionally, where products may turn into reactants if their concentration is great enough solvent substance capable of dissolving another substance specific heat capacity degree Celsius the amount of heat one gram of a substance must absorb or lose to change its temperature by one sphere of hydration in solution when polar water molecules surround charged or polar molecules thus keeping them dissolved and structural isomers molecules that share a chemical formula but differ in the placement of their chemical bonds substituted hydrocarbon backbone carbons hydrocarbon chain or ring containing an atom of another element in place of one of the surface tension tension at the surface of a body of liquid that prevents the molecules from separating; created by the attractive cohesive forces between the molecules of the liquid valence shell outermost shell of an atom van der Waals interaction very close together very weak interaction between molecules due to temporary charges attracting atoms that are CHAPTER SUMMARY 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks Matter is anything that occupies space and has mass. It is made up of elements. All of the 98 elements that occur naturally have unique qualities that allow them to combine in various ways to create molecules, which in turn combine to form cells, tissues, organ systems, and organisms. Atoms, which consist of protons, neutrons, and electrons, are the smallest units of an element that retain all of the properties of that element. Electrons can be transferred, shared, or cause charge disparities between atoms to create bonds, including ionic, covalent, and hydrogen bonds, as well as van der Waals interactions. 2.2 Water Water has many properties that are critical to maintaining life. It is a polar molecule, allowing for the formation of hydrogen bonds. Hydrogen bonds allow ions and other polar molecules to dissolve in water. Therefore, water is an excellent solvent. The hydrogen bonds between water molecules cause the water to have a high heat capacity, meaning it This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 79 takes a lot of added heat to raise its temperature. As the temperature rises, the hydrogen bonds between water continually break and form anew. This allows for the overall temperature to remain stable, although energy is added to the system. Water also exhibits a high heat of vaporization, which is key to how organisms cool themselves by the evaporation of sweat. Water’s cohesive forces allow for the property of surface tension, whereas its adhesive properties are seen as water rises inside capillary tubes. The pH value is a measure of hydrogen ion concentration in a solution and is one of many chemical characteristics that is highly regulated in living organisms through homeostasis. Acids and bases can change pH values, but buffers tend to moderate the changes they cause. These properties of water are intimately connected to the biochemical and physical processes performed by living organisms, and life would be very different if these properties were altered, if it could exist at all. 2.3 Carbon The unique properties of carbon make it a central part of biological molecules. Carbon binds to oxygen, hydrogen, and nitrogen covalently to form the many molecules important for cellular function. Carbon has four electrons in its outermost shell and can form four bonds. Carbon and hydrogen can form hydrocarbon chains or rings. Functional groups are groups of atoms that confer specific properties to hydrocarbon (or substituted hydrocarbon) chains or rings that define their overall chemical characteristics and function. REVIEW QUESTIONS 1. What are atoms that vary in the number of neutrons found in their nuclei called? 5. If xenon has an atomic number of 54 and a mass number of 108 , how many neutrons does it have? a. b. c. Ions Isotopes Isobars d. Neutral atoms 2. Potassium has an atomic number of 19. What is its electron configuration? a. Shells 1 and 2 are full, and shell 3 has nine electrons. b. Shells 1, 2, and 3 are full, and shell 4 has three electrons. c. Shells 1, 2, and 3 are full, and shell 4 has one electron. 7. d. Shells 1, 2, and 3 are full, and no other electrons are present. 3. Which type of bond exemplifies a weak chemical bond? a. b. c. d. 27 54 100 108 6. What forms ionic bonds? a. atoms that share electrons equally b. atoms that share electrons unequally c. d. ions with similar charges ions with opposite charges a. Covalent bond b. Hydrogen bond c. Ionic bond d. Nonpolar covalent bond 4. Which of the following statements is false? a. Electrons are unequally shared in polar covalent bonds. b. Electrons are equally shared in nonpolar covalent bonds. c. Hydrogen bonds are weak bonds based on electr
ostatic forces. d. Ionic bonds are generally stronger than covalent bonds. Based on the information provided, which of the following statements is false? 80 Chapter 2 \| The Chemical Foundation of Life a. b. c. d. , the nitrogen atom acquires a partial In NH2 positive charge and the hydrogen atoms acquire a partial negative charge. In H2 O , the hydrogen atoms acquire a partial negative charge, and the oxygen atom acquires a partial positive charge. In HCl , the hydrogen atom acquires a partial positive charge, and the chlorine atom acquires a partial negative charge. In LiF , the lithium atom acquires a negative charge, and the fluorine atom acquires a positive charge. 8. Which of the following statements is not true? a. Water is polar. b. Water can stabilize the temperature of nearby air. c. Water is essential for life. d. Water is the most abundant molecule in the Earth’s atmosphere. 9. Why do hydrogen and oxygen form polar covalent bonds within water molecules? a. a force that allows surface water molecules to escape and vaporize b. c. the attraction between water molecules and other molecules the intermolecular force between water molecules d. the force that keeps particles dispersed in water 13. In a solution, what kind of molecule binds up excess hydrogen ions? a. acid b. base c. donator d. isotope 14. What is the maximum number of atoms or molecules a single carbon atom can bond with? a. 4 b. 1 c. 6 d. 2 a. Hydrogen is more electronegative than oxygen, generating a partial negative charge near the hydrogen atom. b. Hydrogen is more electronegative than oxygen, generating a partial positive charge near the hydrogen atom. c. Oxygen is more electronegative than hydrogen, generating a partial negative charge near the oxygen atoms. d. Oxygen is more electronegative than hydrogen, generating a partial positive charge near the oxygen atoms. 15. Which of the following statements is true? a. Molecules with the formulas CH3 CH2 OH and C3 H6 O2 could be structural isomers. b. Molecules must have a single bond to be cis- trans isomers. c. To be enantiomers, a molecule must have at least three different atoms or groups connected to a central carbon d. To be enantiomers, a molecule must have at least four different atoms or groups connected to a central carbon 10. What happens to the pH of a solution when acids are added? 16. Which of the following is not a functional group that can bond with carbon? a. The pH of the solution decreases. b. The pH of the solution increases. c. The pH of the solution increases and then decreases. a. carbonyl b. hydroxyl c. phosphate d. sodium d. The pH of the solution stays the same. 17. Which of the following functional groups is not polar? 11. Which of the following statements is true? a. Acids and bases cannot mix together. b. Acids and bases can neutralize each other. c. Acids, not bases, can change the pH of a solution. d. Acids donate hydroxide ions ( OH− ); bases donate hydrogen ions ( H+ ). 12. Define water’s property of adhesion. a. carbonyl b. hydroxyl c. methyl d. sulfhydryl 18. What are enantiomers? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 81 a. Hydrocarbon consisting of closed rings of carbon atoms b. Isomers with similar bonding patterns differing in the placement of atoms along a double covalent bond. c. Molecules that share the same chemical bonds but are mirror images of each other. d. Molecules with the same chemical formula but differ in the placement of their chemical bonds CRITICAL THINKING QUESTIONS 19. List the mass number and atomic number of carbon-12 and carbon-13, respectively. a. The mass number and atomic number of carbon-13 is 13 and 6 , while that of carbon-12 is 12 and 6 , respectively. b. The mass number and atomic number of carbon-13 is 13 and 12 , while that of carbon-12 is 12 and 6 , respectively. c. The mass number and atomic number of carbon-13 is 13 and 13 , while that of carbon-12 is 12 and 6 , respectively. d. The mass number and atomic number of carbon-13 is 13 and 12 , while that of carbon-12 is 12 and 12 , respectively. 20. Why are hydrogen bonds and van der Waals interactions necessary for cells? a. Hydrogen bonds and van der Waals interactions form weak associations between molecules, providing the necessary shape and structure of DNA and proteins to function in the body. b. Hydrogen bonds and van der Waals interactions form strong associations between molecules, providing the necessary shape and structure of DNA and proteins to function in the body. c. Hydrogen bonds and van der Waals interactions form weak associations between different molecules, providing the necessary shape and structure for acids to function in the body. d. Hydrogen bonds and van der Waals interactions form strong associations between same molecules, providing the necessary shape and structure for acids to function in the body. 21. Looking at Figure 2.7, can you infer which two groups together will form a strong ionic bond? a. Group 1 and Group 17 b. Group 1 and Group 14 c. Group 14 and Group 18 d. Group 1 and Group 18 22. Why can some insects walk on water? a. b. c. d. Insects can walk on water because of its high surface tension. Insects can walk on water because it is a polar solvent. Insects can walk on water because they are less dense than water. Insects can walk on water because they are denser than water. 23. Discuss how buffers help prevent drastic swings in pH. a. Buffers absorb excess hydrogen and hydroxide ions to prevent increases or decrease in pH. An example is the bicarbonate system in human body. b. Buffers absorb extra hydrogen ions to prevent increases or decreases in pH. An example is the bicarbonate system in the human body. c. Buffers absorb excess hydroxide ions to prevent increases or decreases in pH. An example of that is the bicarbonate system in the human body. d. Buffers absorb excess hydrogen and hydroxide ions to prevent increases or decreases in pH. An example of that is carbonate system in human body. 24. What are three examples of how the characteristics of water are important in maintaining life? 82 Chapter 2 \| The Chemical Foundation of Life a. First, the lower density of water as a solid versus a liquid allows ice to float, forming an insulating surface layer for aquatic life. Second, the high specific heat capacity of water insulates aquatic life or bodily fluids from temperature changes. Third, the high heat of vaporization of water allows animals to cool themselves by sweating. b. First, the higher density of water as a solid versus a liquid allows ice to float, forming an insulating surface layer for aquatic life. Second, the high specific heat capacity of water insulates aquatic life or bodily fluids from temperature changes. Third, the low heat of vaporization of water allows animals to cool themselves by sweating. c. First, the lower density of water as a solid versus a liquid allows ice to float, forming an insulating surface layer for aquatic life. Second, the low specific heat capacity of water insulates aquatic life or bodily fluids from temperature changes. Third, the high heat of vaporization of water allows animals to cool themselves by sweating. d. First, the lower density of water as a solid versus a liquid allows ice to float, forming an insulating surface layer for aquatic life. Second, the low specific heat capacity of water insulates aquatic life or bodily fluids from temperature changes. Third, the low heat of vaporization of water allows animals to cool themselves by sweating. 25. Describe the pH scale and how it relates to living systems. Give an example of how drastic pH changes are prevented in living systems. a. The pH scale ranges from 0 to 14, where anything below 7 is acidic and above 7 is alkaline. The bicarbonate system in the human body buffers the blood. b. The pH scale ranges from 0 to 14, where anything below 7 is alkaline and above 7 is acidic. The bicarbonate system in human body buffers the blood. c. The pH scale ranges from 0 to 7, where anything below 7 is acidic and above 7 is alkaline. Water in the human body buffers the blood. d. pH scale ranges from 0 to 7, where anything below 4 is acidic and above 4 is alkaline. Water in the human body buffers the blood. 26. What property of carbon makes it essential for organic TEST PREP FOR AP® COURSES 29. Why can water be a good insulator within the body of endothermic (warm-blooded) animals? life? a. Carbon can form up to four covalent bonds, allowing it to form long chains. b. Carbon can form more than four covalent bonds, allowing it to form long chains. c. Carbon can form more than four covalent bonds, but can only form short chains. d. Carbon can form up to four covalent bonds, but can only form short chains. 27. What property of carboxyl makes carboxyl containing molecules hydrophilic? Which macromolecules contain carboxyl? a. Carboxyl groups release H+ , making its parent molecule hydrophilic. It is found amino acids and fatty acids. b. Carboxyl groups absorb H+ ion, making its parent molecule hydrophilic. It is found in phospholipids and triglycerides. c. Carboxyl groups release OH− , making its parent molecule hydrophilic. It is found in phospholipids phosphates and triglycerides d. Carboxyl groups release OH− , making its parent molecule hydrophilic. It is found in phospholipids and DNA. 28. Compare and contrast saturated and unsaturated triglycerides. a. Saturated triglycerides contain single bonds and are solids at room temperature, while unsaturated triglycerides contain double bonds and are liquids at room temperature. b. Saturated triglycerides contain double bonds and are solids at room temperature, while unsaturated triglycerides contain single bonds and are liquids at room temperature. c. Saturated triglycerides contain single bonds and are liquids at room temperature, while unsaturated triglycerides contain double bonds and are solids at room tempera
ture. d. Saturated triglycerides contain double bonds and are liquids at room temperature, while unsaturated triglycerides contain single bond and are solids at room temperature. a. adhesive properties b. surface tension c. heat of vaporization d. specific heat capacity This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 83 30. The unique properties of water are important in biological processes. For the following three properties of water, define the property and give one example of how the property affects living organisms: 1. cohesion 2. adhesion 3. high heat of vaporization a. Cohesion is the attraction between the water molecules, which helps create surface tension. Insects can walk on water because of cohesion. Adhesion is the attraction between water molecules and other molecules. Water moving up from the roots of plants to the leaves as a result of capillary action is because of adhesion. Heat of vaporization is the amount of energy required to convert liquid into gas. This property helps humans maintain homeostasis of body temperature by evaporation. b. Cohesion is the attraction between water and other molecules, which help create surface tension. Insects can walk on water because of cohesion. Adhesion is the attraction between water molecules. Water moving up from the roots of plants to the leaves as a result of capillary action is because of adhesion. Heat of vaporization is the amount of energy required to convert liquid into gas. This property helps humans maintain homeostasis of body temperature by evaporation. c. Cohesion is the attraction between the water molecules, which helps create surface tension. Insects can walk on water because of cohesion. Adhesion is the attraction between water molecules and other molecules. Water moving up from the roots of plants to the leaves as a result of capillary action is because of adhesion. Heat of vaporization is the amount of energy required to convert solid into gas. This property helps humans maintain homeostasis of body temperature by evaporation. d. Cohesion is the attraction between the water molecules, which helps create surface tension. Water moving up from the roots of plants to the leaves as a result of capillary action is because of cohesion. Adhesion is the attraction between water molecules and other molecules. Some insects can walk on water because of adhesion. Heat of vaporization is the amount of energy required to convert solid into gas. This property helps humans maintain homeostasis of body temperature by evaporation. SCIENCE PRACTICE CHALLENGE QUESTIONS 31. At a time when the theory of evolution was controversial (the year following the Scopes Monkey Trial), Macallum (Physiological Reviews, 2, 1926) made an observation that is still contested by some who do not see the pattern in the data below showing percentages (g solute /100 g solution) of major biologically important inorganic elements in a variety of sources. Na+ K+ Ca+2 Mg+2 Cl– Ocean water 0.306 0.011 0.012 0.0037 0.55 Lobster 0.903 0.0337 0.0438 0.0156 1.547 0.5918 0.02739 0.01609 0.0146 0.9819 Dog fish Table 2.3 Na+ K+ Ca+2 Mg+2 Cl– ratio implied about the oceans of early Earth: Chapter 2 \| The Chemical Foundation of Life “At once it is suggested that as the cell is older than its media is [presently] the relative proportions of the inorganic elements in it are of more ancient origin than the relative proportions of the same amount of elements which prevail in the media, blood plasma and lymph or in the ocean and river water of today.” D. In your own words, summarize the argument that Macallum is using to justify this claim. 32. Approximately half the energy that flows through the Earth’s biosphere is captured by phytoplankton, photosynthetic microscopic organisms in the surface waters of the oceans. Scientists think the growth of phytoplankton in the Atlantic Ocean is limited by the availability of nitrogen, whereas growth in the Pacific Ocean is limited by the availability of iron. The concentration of oxygen (O2) in the atmosphere of early Earth was low and, therefore, so was the concentration of dissolved oxygen in the early ocean. Because insoluble iron oxides (rust) do not form in the absence of oxygen, soluble iron ions (Fe2+) were more available in the early ocean than at present since the concentration of oxygen is high. Nitrogen (N2), while always abundant in the atmosphere, was not available until the evolution of molybdenum-based nitrogen-fixing proteins. Figure 2.31 84 Sand shark 0.6173 0.0355 0.0184 0.0172 1.042 Cod 0.416 0.0395 0.0163 0.00589 0.6221 Pollock 0.4145 0.017497 0.01286 0.00608 0.5613 Frog 0.195 0.0233 0.00627 0.00155 0.2679 0.3033 0.0201 0.0085 0.0023 0.4231 Dog lymph Human Blood 0.302 0.0204 0.0094 0.0021 0.389 Lung 0.2956 0.02095 0.00839 0.0021 0.3425 Testes 0.3023 0.01497 0.00842 0.001914 0.3737 0.2935 0.0164 0.0091 0.00184 0.3888 Abdominal cavity Table 2.3 A. Using a spreadsheet, or by sharing calculations with your classmates, construct a quantitative model of these data from these percentages as ratios of mass fractions relative to that of sodium, %X/%Na. Of course, you will not be asked to use a spreadsheet on the AP Biology Exam. However, the ability to develop a quantitative model through the transformation of numerical data can be assessed. The question that led Macallum to investigate the elemental composition of different species and compare these with the composition of seawater follows from the central organizing principle of biology: the theory of evolution. B. The elements in the table above all occur in aqueous solution as ions. The net charges on the inside and outside of a cell are both zero. A very large difference in the concentrations of ions, though, results in stresses that the cell must expend energy to relieve. Based on this constraint on the total number of ions, connect this refined model based on ratios of ion concentration rather than absolute ionic concentrations to the modern concept of shared ancestry. Frequently, a follow-up question regarding scientific data on the AP Biology Exam will ask you to pose questions that are raised by the data. Credit will be awarded for scientific questions. These questions usually look for a cause-and-effect relationship, and are testable. C. Examine relative concentrations of potassium and magnesium ions in terrestrial and marine organisms. Pose a question that could be investigated to connect concentrations of these ions to adaptations to a change in the environment. Macallum noted the high potassium to sodium ratio relative to seawater, and made this claim about what the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 2 \| The Chemical Foundation of Life 85 double line. Modern concentrations of dissolved iron and molybdenum (relative to dissolved carbon) are shown. A. The principle chemical processes of life today have been conserved through evolution from early Earth conditions. Using this fact, justify the selection of these data shown in the graphs in the construction of a model of ocean photosynthetic productivity. Iron and molybdenum are two of 30 elements that are required by the chemical processes supporting life on Earth. Concentrations of these two and 15 others are shown in the graph at the right. Of these elements, the three most abundant in cells are also found in seawater in approximately the same concentrations. By increasing the mass of phytoplankton in the ocean, we may be able to compensate for the increasing concentration of carbon produced by the combustion of gas, oil, and coal. B. Select, with justification, the element or elements that, if added in large amounts to the ocean, could boost the growth of phytoplankton. C. Before implementing a large-scale geo-engineering effort to avert the effects of climate change due to carbon pollution, we must test the legitimacy of this solution. Describe a plan for collecting data that could be used to evaluate the effect of enrichment on phytoplankton productivity. Figure 2.32 The graphs (Anbar and Knoll, Science, 297, 2002) show models of concentrations of two trace elements, iron (Fe) and molybdenum (Mo), in ocean waters. The model describes the change over time of these elements from early Earth (>1.85 billion years ago, Gya) to a modern era (<1.25 Gya) and a period of transition between these. Surface waters of the oceans lie to the left of the vertical 86 Chapter 2 \| The Chemical Foundation of Life This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 87 3 \| BIOLOGICAL MACROMOLECULES Figure 3.1 Foods such as bread, fruit, and cheese are rich sources of biological macromolecules. (credit: modification of work by Bengt Nyman) Chapter Outline 3.1: Synthesis of Biological Macromolecules 3.2: Carbohydrates 3.3: Lipids 3.4: Proteins 3.5: Nucleic Acids Introduction Food provides the body with the nutrients it needs to survive. Many of these critical nutrients are biological macromolecules, or large molecules, necessary for and built by living things. For example, the amino acids found in protein are needed to build healthy bone and muscle. The body uses fat molecules to build new cells, store energy, and for proper digestion. Carbohydrates are the primary source of the body’s energy. Nucleic acids contain genetic information. While all living things, including humans, need macromolecules in their daily diet, an imbalance of any one of them can lead to health problems. For example, eating too much fat can lead to cardiovascular problems, and too much protein can lead to problems with the kidneys. Some people think that removing whole grains, such as wheat, from one’s diet can be beneficial. However, scientists have found that to not be true for the majority of people. In fact, just the opposite may be true, because whole wheat contains more dietary fib
er than other types of grains. The full research review can be found here (http://openstaxcollege.org/l/32wholegrain) . 88 Chapter 3 \| Biological Macromolecules 3.1 \| Synthesis of Biological Macromolecules In this section, you will explore the following questions: • How are complex macromolecule polymers synthesized from monomers? • What is the difference between dehydration (or condensation) and hydrolysis reactions? Connection for AP® Courses Living organisms need food to survive as it contains critical nutrients in the form of biological macromolecules. These large molecules are composed mainly of six elements—sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen (SPONCH)—in different quantities and arrangements. Complex polymers are built from combinations of smaller monomers by dehydration synthesis, a chemical reaction in which a molecule of water is removed between two linking monomers. (Think of a train: each boxcar, including the caboose, represents a monomer, and the entire train is a polymer.) During digestion, polymers can be broken down by hydrolysis, or the addition of water. Both dehydration and hydrolysis reactions in cells are catalyzed by specific enzymes. Dehydration reactions typically require an investment of energy for new bond formation, whereas hydrolysis reactions typically release energy that can be used to power cellular processes. The four categories of macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Evidence supports scientists’ claim that the organic precursors of these biological molecules were present on primitive Earth. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A learning objective merges required content with one or more of the seven Science Practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.D The origin of living systems is explained by natural processes. Essential Knowledge 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. Science Practice 1.2 The student can make claims and predictions about natural phenomena based on scientific theories and models. Learning Objective 1.27 The student is able to describe a scientific hypothesis about the origin of life on Earth. Essential Knowledge 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. Science Practice 3.3 The student can evaluate scientific questions. Learning Objective 1.28 The student is able to evaluate scientific questions based on hypotheses about the origin of life on Earth. Dehydration Synthesis As you’ve learned, biological macromolecules are large molecules, necessary for life, that are built from smaller organic molecules. There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids); each is an important cell component and performs a wide array of functions. Combined, these molecules make up the majority of a cell’s dry mass (recall that water makes up the majority of its complete mass). Biological macromolecules are organic, meaning they contain carbon. In addition, they may contain hydrogen, oxygen, nitrogen, and additional minor elements. Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers combine with each other using covalent bonds to form larger molecules known as polymers. In doing so, monomers release water This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 89 molecules as byproducts. This type of reaction is known as dehydration synthesis, which means “to put together while losing water.” Figure 3.2 In the dehydration synthesis reaction depicted above, two molecules of glucose are linked together to form the disaccharide maltose. In the process, a water molecule is formed. In a dehydration synthesis reaction (Figure 3.2), the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water. At the same time, the monomers share electrons and form covalent bonds. As additional monomers join, this chain of repeating monomers forms a polymer. Different types of monomers can combine in many configurations, giving rise to a diverse group of macromolecules. Even one kind of monomer can combine in a variety of ways to form several different polymers: for example, glucose monomers are the constituents of starch, glycogen, and cellulose. Hydrolysis Polymers are broken down into monomers in a process known as hydrolysis, which means “to split with water.” Hydrolysis is a reaction in which a water molecule is used during the breakdown of another compound (Figure 3.3). During these reactions, the polymer is broken into two components: one part gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–) from a split water molecule. Figure 3.3 In the hydrolysis reaction shown here, the disaccharide maltose is broken down to form two glucose monomers with the addition of a water molecule. Note that this reaction is the reverse of the synthesis reaction shown in Figure 3.2. Dehydration and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes; dehydration reactions involve the formation of new bonds, requiring energy, while hydrolysis reactions break bonds and release energy. These reactions are similar for most macromolecules, but each monomer and polymer reaction is specific for its class. For example, in our bodies, food is hydrolyzed, or broken down, into smaller molecules by catalytic enzymes in the digestive system. This allows for easy absorption of nutrients by cells in the intestine. Each macromolecule is broken down by a specific enzyme. For instance, carbohydrates are broken down by amylase, sucrase, lactase, or maltase. Proteins are broken down by the enzymes pepsin and peptidase, and by hydrochloric acid. Lipids are broken down by lipases. Breakdown of these macromolecules provides energy for cellular activities. 90 Chapter 3 \| Biological Macromolecules Visit this site (http://openstaxcollege.org/l/hydrolysis) to see visual representations of dehydration synthesis and hydrolysis. What role do electrons play in dehydration synthesis and hydrolysis? a. Sharing of electrons between monomers occurs in both dehydration synthesis and hydrolysis. b. The sharing of electrons between monomers occurs in hydrolysis only. c. H+ and OH− ions share electrons with the respective monomers in dehydration synthesis. d. H+ and OH− ions share electrons with the respective monomers in hydrolysis. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 91 Recreating Primordial Earth Many people wonder how life formed on Earth. In 1953, Stanley Miller and Harold Urey developed an apparatus like the one shown in Figure 3.4 to model early conditions on earth. They wanted to test if organic molecules could form from inorganic precursors believed to exist very early in Earth’s history. They used boiling water to mimic early Earth’s oceans. Steam from the “ocean” combined with methane, ammonia, and hydrogen gases from the early Earth’s atmosphere and was exposed to electrical sparks to act as lightning. As the gas mixture cooled and condensed, it was found to contain organic compounds, such as amino acids and nucleotides. According to the abiogenesis theory, these organic molecules came together to form the earliest form of life about 3.5 billion years ago. (credit: Yassine Mrabet) Figure 3.4 Think About It How does Stanley Miller’s and Harold Urey’s model support the claim that organic precursors present on early Earth could have assembled into large, complex molecules necessary for life? What chemical “ingredients” were present on early Earth? 92 Chapter 3 \| Biological Macromolecules 3.2 \| Carbohydrates By the end of this section, you will be able to: • What is the role of carbohydrates in cells and in the extracellular materials of animals and plants? • What are the different classifications of carbohydrates? • How are monosaccharide building blocks assembled into disaccharides and complex polysaccharides? Connection for AP® Courses Carbohydrates provide energy for the cell and structural support to plants, fungi, and arthropods such as insects, spiders, and crustaceans. Consisting of carbon, hydrogen, and oxygen in the ratio CH2O or carbon hydrated with water, carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the macromolecule. Monosaccharides are linked by glycosidic bonds that form as a result of dehydration synthesis. Glucose, galactose, and fructose are common isomeric monosaccharides, whereas sucrose or table sugar is a disaccharide. Examples of polysaccharides include cellulose and starch in plants and glycogen in animals. Although storing glucose in the form of polymers like starch or glycogen makes it less accessible for metabolism, this prevents it from leaking out of cells or creating a high osmotic pressure that could cause excessive water uptake by the cell. Insects have a hard outer skeleton made of chitin, a unique nitrogen-containing polysaccharide. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 4 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities,
and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 4 Enduring Understanding 4.A Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Learning Objective 7.1 The student can connect phenomena and models across spatial and temporal scales. 4.1 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Learning Objective 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain. 4.2 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice 6.1 The student can justify claims with evidence. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 93 Science Practice Learning Objective 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecules. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 4.15] [APLO 2.5] Molecular Structures Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants. Carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides. Monosaccharides Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is known as an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R'), it is known as a ketose. Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons). See Figure 3.5 for an illustration of the monosaccharides. 94 Chapter 3 \| Biological Macromolecules Figure 3.5 Monosaccharides are classified based on the position of their carbonyl group and the number of carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three-, five-, and six-carbon backbones, respectively. The chemical formula for glucose is C6H12O6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants. Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon (Figure 3.6). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 95 Figure 3.6 Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the same chemical formula (C6H12O6) but a different arrangement of atoms. Identify each sugar as an aldose or ketose. 1. fructose 2. galactose 3. glucose a. Glucose and galactose are aldoses. Fructose is a ketose b. Glucose and fructose are aldoses. Galactose is a ketose. c. Galactose and fructose are ketoses. Glucose is an aldose. d. Glucose and fructose are ketoses. Galactose is an aldose. Glucose, galactose, and fructose are isomeric monosaccharides (hexoses), meaning they have the same chemical formula but have slightly different structures. Glucose and galactose are aldoses, and fructose is a ketose. Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms (Figure 3.7). Glucose in a ring form can have two different arrangements of the hydroxyl group (OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation). If the hydroxyl group is below carbon number 1 in the sugar, it is said to be in the alpha (α) position, and if it is above the plane, it is said to be in the beta (β) position. 96 Chapter 3 \| Biological Macromolecules Figure 3.7 Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. When the ring forms, the side chain it closes on is locked into an α or β position. Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose. Disaccharides Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond. A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond (Figure 3.8). Glycosidic bonds (also called glycosidic linkages) can be of the alpha or the beta type. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 97 Figure 3.8 Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose. Common disaccharides include lactose, maltose, and sucrose (Figure 3.9). Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose. 98 Chapter 3 \| Biological Macromolecules Figure 3.9 Common disaccharides include maltose (grain sugar), lactose (milk sugar), and sucrose (table sugar). Polysaccharides A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of monomers joined. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides. Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose. The cells can then absorb the glucose. Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. As illustrated in Figure 3.10, amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a br
anched polysaccharide (α 1-6 linkages at the branch points). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 99 Figure 3.10 Amylose and amylopectin are two different forms of starch. Amylose is composed of unbranched chains of glucose monomers connected by α 1,4 glycosidic linkages. Amylopectin is composed of branched chains of glucose monomers connected by α 1,4 and α 1,6 glycosidic linkages. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen (not shown) is similar in structure to amylopectin but more highly branched. Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis. Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by β 1-4 glycosidic bonds (Figure 3.11). 100 Chapter 3 \| Biological Macromolecules Figure 3.11 In cellulose, glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure. As shown in Figure 3.11, every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. While the β 1-4 linkage cannot be broken down by human digestive enzymes, herbivores such as cows, koalas, and buffalos are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix of grazing animals also contains bacteria that digest cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal. Termites are also able to break down cellulose because of the presence of other organisms in their bodies that secrete cellulases. Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have an outer skeleton, called the exoskeleton, which protects their internal body parts (as seen in the bee in Figure 3.12). This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing nitrogen. It is made of repeating units of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a major component of fungal cell walls; fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya. Figure 3.12 Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide. (credit: Louise Docker) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 101 Registered dietitians help plan nutrition programs for individuals in various settings. They often work with patients in health care facilities, designing nutrition plans to treat and prevent diseases. For example, dietitians may teach a patient with diabetes how to manage blood sugar levels by eating the correct types and amounts of carbohydrates. Dietitians may also work in nursing homes, schools, and private practices. To become a registered dietitian, one needs to earn at least a bachelor’s degree in dietetics, nutrition, food technology, or a related field. In addition, registered dietitians must complete a supervised internship program and pass a national exam. Those who pursue careers in dietetics take courses in nutrition, chemistry, biochemistry, biology, microbiology, and human physiology. Dietitians must become experts in the chemistry and physiology (biological functions) of food (proteins, carbohydrates, and fats). Benefits of Carbohydrates Are carbohydrates good for you? People who wish to lose weight are often told that carbohydrates are bad for them and should be avoided. Some diets completely forbid carbohydrate consumption, claiming that a low-carbohydrate diet helps people to lose weight faster. However, carbohydrates have been an important part of the human diet for thousands of years; artifacts from ancient civilizations show the presence of wheat, rice, and corn in our ancestors’ storage areas. Carbohydrates should be supplemented with proteins, vitamins, and fats to be parts of a well-balanced diet. Calorie-wise, a gram of carbohydrate provides 4.3 Kcal. For comparison, fats provide 9 Kcal/g, a less desirable ratio. Carbohydrates contain soluble and insoluble elements; the insoluble part is known as fiber, which is mostly cellulose. Fiber has many uses; it promotes regular bowel movement by adding bulk, and it regulates the rate of consumption of blood glucose. Fiber also helps to remove excess cholesterol from the body: fiber binds to the cholesterol in the small intestine, then attaches to the cholesterol and prevents the cholesterol particles from entering the bloodstream, and then cholesterol exits the body via the feces. In addition, a meal containing whole grains and vegetables gives a feeling of fullness. As an immediate source of energy, glucose is broken down during the process of cellular respiration, which produces ATP, the energy currency of the cell. Without the consumption of carbohydrates, the availability of “instant energy” would be reduced. Eliminating carbohydrates from the diet is not the best way to lose weight. A low-calorie diet that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercise and plenty of water, is the more sensible way to lose weight. For an additional perspective on carbohydrates, explore “Biomolecules: the Carbohydrates” through this interactive animation (http://openstaxcollege.org/l/carbohydrates) . Fiber is not really a nutrient, because it passes through our body undigested. Why can't fiber be digested and why is it important to our diet? a. The enzymes required to digest cellulose are not produced in human body; undigested fiber adds bulk to the food easing bowel movements. b. The enzymes that digests cellulose cannot bind to the cellulose due to altered active sites; undigested fiber adds bulk to the food easing bowel movements. c. The enzymes required to digest cellulose are not produced in human body; fiber produces energy for the metabolism. d. Competitive inhibitors are not the reason that fiber is indigestible. 102 Chapter 3 \| Biological Macromolecules Activity Use a molecular model kit to construct a polysaccharide from several different monosaccharide monomers. Explain how the structure of the polysaccharide determines its primary function as an energy storage molecule. Then use your model to describe how changes in structure result in changes in function. Think About It • Explain why athletes often “carb-load” before a big game or tournament. • Explain why it is difficult for some animals, including humans, to digest cellulose. Describe a structural difference between cellulose and starch, which is easily digested by humans. How are cows and other ruminants able to digest cellulose? 3.3 \| Lipids In this section, you will explore the following questions: • What are the four major types of lipids? • What are functions of fats in living organisms? • What is the difference between saturated and unsaturated fatty acids? • What is the molecular structure of phospholipids, and what is the role of phospholipids in cells? • What is the basic structure of a steroid, and what are examples of their functions? • How does cholesterol help maintain the fluid nature of the plasma membrane of cells? Connection for AP® Courses Lipids also are sources of energy that power cellular processes. Like carbohydrates, lipids are composed of carbon, hydrogen, and oxygen, but these atoms are arranged differently. Most lipids are nonpolar and hydrophobic. Major types include fats and oils, waxes, phospholipids, and steroids. A typical fat consists of three fatty acids bonded to one molecule of glycerol, forming triglycerides or triacylglycerols. The fatty acids may be saturated or unsaturated, depending on the presence or absence of double bonds in the hydrocarbon chain; a saturated fatty acid has the maximum number of hydrogen atoms bonded to carbon and, thus, only single bonds. In general, fats that are liquid at room temperature (e.g., canola oil) tend to be more unsaturated than fats that are solid at room temperature. In the food industry, oils are artificially hydrogenated to make them chemically more appropriate for use in processed foods. During this hydrogenation process, double bonds in the cis- conformation in the hydrocarbon chain may be converted to double bonds in the transconformation; unfortunately, trans fats have been shown to contribute to heart disease. Phospholipids are a special type of lipid associated with cell membranes and typically have a glycerol (or sphingosine) backbone to which two fatty acid chains and a phosphate-containing group are attached. As a result, phospholipids are considered amphipathic because they have both hydrophobic and hydrophilic components. (In Chapters 4 and 5 we will explore in more detail how the amphipathic nature of phospholipids in plasma cell membranes helps regulate the passage of substances into and out of the cell.) Although the molecular structures of steroids differ from that of trig
lycerides and phospholipids, steroids are classified as lipids based on their hydrophobic properties. Cholesterol is a type of steroid in animal cells’ plasma membrane. Cholesterol is also the precursor of steroid hormones such as testosterone. Information presented and the examples highlighted in the section, support concepts outlined in Big Idea 4 of the AP® Biology Curriculum Framework. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 103 Big Idea 4 Enduring Understanding 4.A Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Learning Objective 7.1 The student can connect phenomena and models across spatial and temporal scales. 4.1 The student is able to explain the connection between the sequence and the subcomponents of a biological polymer and its properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Learning Objective 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain. 4.2 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Science Practice Learning Objective 6.1 The student can justify claims with evidence. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecules. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.9] [APLO 2.10] [APLO 2.12] [APLO 2.13][APLO 2.14][APLO 4.14] Fats and Oils Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals (Figure 3.13). For example, their water-repellant hydrophobic nature can help keep aquatic birds and mammals dry by forming a protective layer over fur or feathers. Lipids are also the building blocks of many hormones and an important constituent of all cellular membranes. Lipids include fats, waxes, phospholipids, and steroids. 104 Chapter 3 \| Biological Macromolecules Figure 3.13 Hydrophobic lipids in the fur of aquatic mammals, such as this river otter, protect them from the elements. (credit: Ken Bosma) A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36; most common are those containing 12–18 carbons. In a fat molecule, the fatty acids are attached to each of the three carbons of the glycerol molecule with an ester bond through an oxygen atom (Figure 3.14). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 105 Figure 3.14 Triacylglycerol is formed by the joining of three fatty acids to a glycerol backbone in a dehydration reaction. Three molecules of water are released in the process. During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. Fats are also called triacylglycerols or triglycerides because of their chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid, is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts. Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is said to be saturated. Saturated fatty acids are saturated with hydrogen; in other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid (Figure 3.15) Figure 3.15 Stearic acid is a common saturated fatty acid. 106 Chapter 3 \| Biological Macromolecules When the hydrocarbon chain contains a double bond, the fatty acid is said to be unsaturated. Oleic acid is an example of an unsaturated fatty acid (Figure 3.16). Figure 3.16 Oleic acid is a common unsaturated fatty acid. Most unsaturated fats are liquid at room temperature and are called oils. If there is one double bond in the molecule, then it is known as a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is known as a polyunsaturated fat (e.g., canola oil). When a fatty acid has no double bonds, it is known as a saturated fatty acid because no more hydrogen may be added to the carbon atoms of the chain. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds tend to get packed tightly and are solid at room temperature. Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialized cells called adipocytes, where globules of fat occupy most of the cell’s volume. In plants, fat or oil is stored in many seeds and is used as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the same plane, it is referred to as a cis fat; if the hydrogen atoms are on two different planes, it is referred to as a trans fat. The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature (Figure 3.17). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels whereas saturated fats contribute to plaque formation in the arteries. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 107 Figure 3.17 Saturated fatty acids have hydrocarbon chains connected by single bonds only. Unsaturated fatty acids have one or more double bonds. Each double bond may be in a cis or trans configuration. In the cis configuration, both hydrogens are on the same side of the hydrocarbon chain. In the trans configuration, the hydrogens are on opposite sides. A cis double bond causes a kink in the chain. Trans Fats In the food industry, oils are artificially hydrogenated to make them semi-solid and of a consistency desirable for many processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis- conformation in the hydrocarbon chain may be converted to double bonds in the transconformation. Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats. Recent studies have shown that an increase in trans fats in the human diet may lead to an increase in levels of low-density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned the use of trans fats, and food labels are required to display the trans fat content. Omega Fatty Acids Essential fatty acids are fatty acids required but not synthesized by the human body. Consequently, they have to be supplemented through ingestion via the diet. Omega-3 fatty acids (like that shown in Figure 3.18) fall into this category and are one of only two known for humans (the other being omega-6 fatty acid). These are polyunsaturated fatty acids and are called omega-3 because the third carbon from the end of the hydrocarbon chain is connected to its neighboring carbon by a double bond. 108 Chapter 3 \| Biological Macromolecules Figure 3.18 Alpha-linolenic acid is an example of an omega-3 fatty acid. It has three cis double bonds and, as a result, a curved shape. For clarity, the carbons are not shown. Each singly bonded carbon has two hydrogens associated with it, also not shown. The farthest carbon away from the carboxyl group is numbered as the omega (ω) carbon, and if the double bond is between the third and fourth carbon from that end, it is known as an omega-3 fatty acid. Nutritionally important because the body does not make them, omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmo
n, trout, and tuna are good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden death from heart attacks, reduce triglycerides in the blood, lower blood pressure, and prevent thrombosis by inhibiting blood clotting. They also reduce inflammation, and may help reduce the risk of some cancers in animals. Like carbohydrates, fats have received a lot of bad publicity. It is true that eating an excess of fried foods and other “fatty” foods leads to weight gain. However, fats do have important functions. Many vitamins are fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide insulation for the body. Therefore, “healthy” fats in moderate amounts should be consumed on a regular basis. Think About It Explain why trans fats have been banned from some restaurants. How are trans fats made, and what effect does a simple chemical change have on the properties of the lipid? Waxes Wax covers the feathers of some aquatic birds and the leaf surfaces of some plants. Because of the hydrophobic nature of waxes, they prevent water from sticking on the surface (Figure 3.19). Waxes are made up of long fatty acid chains esterified to long-chain alcohols. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 109 Figure 3.19 Waxy coverings on some leaves are made of lipids. (credit: Roger Griffith) Phospholipids Phospholipids are major constituents of the plasma membrane, the outermost layer of all living cells. Like fats, they are composed of fatty acid chains attached to a glycerol or sphingosine backbone. Instead of three fatty acids attached as in triglycerides, however, there are two fatty acids forming diacylglycerol, and the third carbon of the glycerol backbone is occupied by a modified phosphate group (Figure 3.20). A phosphate group alone attached to a diaglycerol does not qualify as a phospholipid; it is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. The phosphate group is modified by an alcohol. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are found in plasma membranes. Figure 3.20 A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. The phosphate may be modified by the addition of charged or polar chemical groups. A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water, whereas the phosphate-containing group is hydrophilic and interacts with water (Figure 3.21). 110 Chapter 3 \| Biological Macromolecules Figure 3.21 The phospholipid bilayer is the major component of all cellular membranes. The hydrophilic head groups of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer. The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the matrix of the structure, the fatty acid tails of phospholipids face inside, away from water, whereas the phosphate group faces the outside, aqueous side (Figure 3.21). Phospholipids are responsible for the dynamic nature of the plasma membrane. If a drop of phospholipids is placed in water, it spontaneously forms a structure known as a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure. Fats are amphiphilic molecules. In other words, the long hydrocarbon tail is hydrophobic, and the glycerol part of the molecule is hydrophilic. When in water, fats will arrange themselves into a ball called a micelle so that the hydrophilic “heads” are on the outer surface, and the hydrophobic “tails” are on the inside where they are protected from the surrounding water. Figure 3.22 Steroids Unlike the phospholipids and fats discussed earlier, steroids have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail (Figure 3.23). Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 111 Figure 3.23 Steroids such as cholesterol and cortisol are composed of four fused hydrocarbon rings. Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver and is the precursor to many steroid hormones such as testosterone and estradiol, which are secreted by the gonads and endocrine glands. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help in the emulsification of fats and their subsequent absorption by cells. Although cholesterol is often spoken of in negative terms by lay people, it is necessary for proper functioning of the body. It is a component of the plasma membrane of animal cells and is found within the phospholipid bilayer. Being the outermost structure in animal cells, the plasma membrane is responsible for the transport of materials and cellular recognition and it is involved in cell-to-cell communication. For an additional perspective on lipids, explore the interactive animation “Biomolecules: The Lipids” (http://openstaxcollege.org/l/lipids) . What is cholesterol specifically classified as? a. a lipid b. a phospholipid c. a steroid d. a wax 112 Chapter 3 \| Biological Macromolecules 3.4 \| Proteins In this section, you will investigate the following questions: • What are functions of proteins in cells and tissues? • What is the relationship between amino acids and proteins? • What are the four levels of protein organization? • What is the relationship between protein shape and function? Connection for AP® Courses Proteins are long chains of different sequences of the 20 amino acids that each contain an amino group (-NH2), a carboxyl group (-COOH), and a variable group. (Think of how many protein “words” can be made with 20 amino acid “letters”). Each amino acid is linked to its neighbor by a peptide bond formed by a dehydration reaction. A long chain of amino acids is known as a polypeptide. Proteins serve many functions in cells. They act as enzymes that catalyze chemical reactions, provide structural support, regulate the passage of substances across the cell membrane, protect against disease, and coordinate cell signaling pathways. Protein structure is organized at four levels: primary, secondary, tertiary, and quaternary. The primary structure is the unique sequence of amino acids. A change in just one amino acid can change protein structure and function. For example, sickle cell anemia results from just one amino acid substitution in a hemoglobin molecule consisting of 574 amino acids. The secondary structure consists of the local folding of the polypeptide by hydrogen bond formation; leading to the α helix and β pleated sheet conformations. In the tertiary structure, various interactions, e.g., hydrogen bonds, ionic bonds, disulfide linkages, and hydrophobic interactions between R groups, contribute to the folding of the polypeptide into different three-dimensional configurations. Most enzymes are of tertiary configuration. If a protein is denatured, loses its three-dimensional shape, it may no longer be functional. Environmental conditions such as temperature and pH can denature proteins. Some proteins, such as hemoglobin, are formed from several polypeptides, and the interactions of these subunits form the quaternary structure of proteins. Information presented and the examples highlighted in the section, support concepts and Learning Objectives outlined in Big Idea 4 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven science practices. Big Idea 4 Enduring Understanding 4.A Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Learning Objective 7.1 The student can connect phenomena and models across spatial and temporal scales. 4.1 The student is able to explain the connection between the sequence and the subcomponents of a biological polymer and its properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 113 Learning Objective 4.2 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Science Practice Learning Objective 6.1 The student can justify claims with evidence. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecules. The Science Practice
Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1.14] [APLO 2.12] [APLO 4.1] [APLO 4.3][APLO 4.15][APLO 4.22] Types and Functions of Proteins Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective; they may serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly. They are all, however, polymers of amino acids, arranged in a linear sequence. Enzymes, which are produced by living cells, are catalysts in biochemical reactions (like digestion) and are usually complex or conjugated proteins. Each enzyme is specific for the substrate (a reactant that binds to an enzyme) it acts on. The enzyme may help in breakdown, rearrangement, or synthesis reactions. Enzymes that break down their substrates are called catabolic enzymes, enzymes that build more complex molecules from their substrates are called anabolic enzymes, and enzymes that affect the rate of reaction are called catalytic enzymes. It should be noted that all enzymes increase the rate of reaction and, therefore, are considered to be organic catalysts. An example of an enzyme is salivary amylase, which hydrolyzes its substrate amylose, a component of starch. Hormones are chemical-signaling molecules, usually small proteins or steroids, secreted by endocrine cells that act to control or regulate specific physiological processes, including growth, development, metabolism, and reproduction. For example, insulin is a protein hormone that helps to regulate the blood glucose level. The primary types and functions of proteins are listed in Table 3.1. Protein Types and Functions Examples Functions Type Digestive Enzymes Amylase, lipase, pepsin, trypsin Transport Hemoglobin, albumin Help in digestion of food by catabolizing nutrients into monomeric units Carry substances in the blood or lymph throughout the body Structural Actin, tubulin, keratin Construct different structures, like the cytoskeleton Hormones Insulin, thyroxine Coordinate the activity of different body systems Defense Immunoglobulins Protect the body from foreign pathogens Contractile Actin, myosin Effect muscle contraction Legume storage proteins, egg white (albumin) Provide nourishment in early development of the embryo and the seedling Storage Table 3.1 114 Chapter 3 \| Biological Macromolecules Proteins have different shapes and molecular weights; some proteins are globular in shape whereas others are fibrous in nature. For example, hemoglobin is a globular protein, but collagen, found in our skin, is a fibrous protein. Protein shape is critical to its function, and this shape is maintained by many different types of chemical bonds. Changes in temperature, pH, and exposure to chemicals may lead to permanent changes in the shape of the protein, leading to loss of function, known as denaturation. All proteins are made up of different arrangements of the most common 20 types of amino acids. Amino Acids Amino acids are the monomers that make up proteins. Each amino acid has the same fundamental structure, which consists of a central carbon atom, also known as the alpha (α) carbon, bonded to an amino group (NH2), a carboxyl group (COOH), and to a hydrogen atom. Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group (Figure 3.24). Figure 3.24 Amino acids have a central asymmetric carbon to which an amino group, a carboxyl group, a hydrogen atom, and a side chain (R group) are attached. The name "amino acid" is derived from the fact that they contain both amino group and carboxyl-acid-group in their basic structure. As mentioned, there are 20 common amino acids present in proteins. Nine of these are considered essential amino acids in humans because the human body cannot produce them and they are obtained from the diet. For each amino acid, the R group (or side chain) is different (Figure 3.25). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 115 Figure 3.25 There are typically 20 common amino acids commonly found in proteins, each with a different R group (variant group) that determines its chemical nature. Which categories of amino acid would you expect to find on the surface of a soluble protein, and which would you expect to find in the interior? a. Polar and charged amino acids will be found on the surface. Non-polar amino acids will be found in the interior. b. Polar and charged amino acids will be found in the interior. Non-polar amino acids will be found on the surface. c. Non-polar and uncharged proteins will be found on the surface as well as in the interior. The chemical nature of the side chain determines the nature of the amino acid (that is, whether it is acidic, basic, polar, or nonpolar). For example, the amino acid glycine has a hydrogen atom as the R group. Amino acids such as valine, methionine, and alanine are nonpolar or hydrophobic in nature, while amino acids such as serine, threonine, and cysteine are polar and have hydrophilic side chains. The side chains of lysine and arginine are positively charged, and therefore these amino acids are also known as basic amino acids. Proline has an R group that is linked to the amino group, forming a ringlike structure. Proline is an exception to the standard structure of an animo acid since its amino group is not separate from the side chain (Figure 3.25). Amino acids are represented by a single upper case letter or a three-letter abbreviation. For example, valine is known by the letter V or the three-letter symbol val. Just as some fatty acids are essential to a diet, some amino acids are necessary as well. They are known as essential amino acids, and in humans they include isoleucine, leucine, and cysteine. Essential 116 Chapter 3 \| Biological Macromolecules amino acids refer to those necessary for construction of proteins in the body, although not produced by the body; which amino acids are essential varies from organism to organism. The sequence and the number of amino acids ultimately determine the protein's shape, size, and function. Each amino acid is attached to another amino acid by a covalent bond, known as a peptide bond, which is formed by a dehydration reaction. The carboxyl group of one amino acid and the amino group of the incoming amino acid combine, releasing a molecule of water. The resulting bond is the peptide bond (Figure 3.26). Figure 3.26 Peptide bond formation is a dehydration synthesis reaction. The carboxyl group of one amino acid is linked to the amino group of the incoming amino acid. In the process, a molecule of water is released. The products formed by such linkages are called peptides. As more amino acids join to this growing chain, the resulting chain is known as a polypeptide. Each polypeptide has a free amino group at one end. This end is called the N terminal, or the amino terminal, and the other end has a free carboxyl group, also known as the C or carboxyl terminal. While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids, whereas the term protein is used for a polypeptide or polypeptides that have combined together, often have bound nonpeptide prosthetic groups, have a distinct shape, and have a unique function. After protein synthesis (translation), most proteins are modified. These are known as post-translational modifications. They may undergo cleavage or phosphorylation, or may require the addition of other chemical groups. Only after these modifications is the protein completely functional. Click through the steps of protein synthesis in this interactive tutorial (http://openstaxcollege.org/l/protein_synth) . Why is the process of protein synthesis critical to life? a. Protein is the body’s preferred source for energy for rapid energy production. b. Protein is stored in the liver and muscles to supply energy for future use. c. Protein is required for tissue formation and constitutes hormones and enzymes. d. Proteins are required for the absorption of all fat soluble vitamins. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 117 Cytochrome c is an important component of the electron transport chain, a part of cellular respiration, and it is normally found in the cellular organelle, the mitochondrion. This protein has a heme prosthetic group, and the central ion of the heme gets alternately reduced and oxidized during electron transfer. Because this essential protein’s role in producing cellular energy is crucial, it has changed very little over millions of years. Protein sequencing has shown that there is a considerable amount of cytochrome c amino acid sequence homology among different species; in other words, evolutionary kinship can be assessed by measuring the similarities or differences among various species’ DNA or protein sequences. Scientists have determined that human cytochrome c contains 104 amino acids. For each cytochrome c molecule from different organisms that has been sequenced to date, 37 of these amino acids appear in the same position in all samples of cytochrome c. This indicates that there may have been a common ancestor. On comparing the human and chimpanzee protein sequences, no sequence difference was found. When human and rhesus monkey sequences were compared, the single difference found was in one amino acid. In another comparison, human to yeast sequencing shows a difference in the 44th position. The protein sequence of cytochrome c from chimpanzees and humans is identical. The protein
sequence of cytochrome c from rhesus monkeys differs from the human sequence by one amino acid. What do these comparisons suggest? a. Rhesus monkeys are more closely related to humans than chimpanzees. b. Chimpanzees are more closely related to rhesus monkeys than to humans. c. Humans are related to chimpanzees, but are not related to rhesus monkeys. d. Chimpanzees are more closely related to humans than rhesus monkeys. Protein Structure As discussed earlier, the shape of a protein is critical to its function. For example, an enzyme can bind to a specific substrate at a site known as the active site. If this active site is altered because of local changes or changes in overall protein structure, the enzyme may be unable to bind to the substrate. To understand how the protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary. Primary Structure The unique sequence of amino acids in a polypeptide chain is its primary structure. For example, the pancreatic hormone insulin has two polypeptide chains, A and B, and they are linked together by disulfide bonds. The N terminal amino acid of the A chain is glycine, whereas the C terminal amino acid is asparagine (Figure 3.27). The sequences of amino acids in the A and B chains are unique to insulin. 118 Chapter 3 \| Biological Macromolecules Figure 3.27 Bovine serum insulin is a protein hormone made of two peptide chains, A (21 amino acids long) and B (30 amino acids long). In each chain, primary structure is indicated by three-letter abbreviations that represent the names of the amino acids in the order they are present. The amino acid cysteine (cys) has a sulfhydryl (SH) group as a side chain. Two sulfhydryl groups can react in the presence of oxygen to form a disulfide (S-S) bond. Two disulfide bonds connect the A and B chains together, and a third helps the A chain fold into the correct shape. Note that all disulfide bonds are the same length, but are drawn different sizes for clarity. The unique sequence for every protein is ultimately determined by the gene encoding the protein. A change in nucleotide sequence of the gene’s coding region may lead to a different amino acid being added to the growing polypeptide chain, causing a change in protein structure and function. In sickle cell anemia, the hemoglobin β chain (a small portion of which is shown in Figure 3.28) has a single amino acid substitution, causing a change in protein structure and function. Specifically, the amino acid glutamic acid is substituted by valine in the β chain. What is most remarkable to consider is that a hemoglobin molecule is made up of two alpha chains and two beta chains that each consist of about 150 amino acids. The molecule, therefore, has about 600 amino acids. The structural difference between a normal hemoglobin molecule and a sickle cell molecule—which dramatically decreases life expectancy—is a single amino acid of the 600. What is even more remarkable is that those 600 amino acids are encoded by three nucleotides each, and the mutation is caused by a single base change (point mutation), 1 in 1800 bases. Figure 3.28 The beta chain of hemoglobin is 147 residues in length, yet a single amino acid substitution leads to sickle cell anemia. In normal hemoglobin, the amino acid at position seven is glutamate. In sickle cell hemoglobin, this glutamate is replaced by a valine. Because of this change of one amino acid in the chain, hemoglobin molecules form long fibers that distort the biconcave, or disc-shaped, red blood cells and cause them to assume a crescent or “sickle” shape, which clogs blood vessels (Figure 3.29). This can lead to myriad serious health problems such as breathlessness, dizziness, headaches, and abdominal pain for those affected by this disease. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 119 Figure 3.29 In this blood smear, visualized at 535x magnification using bright field microscopy, sickle cells are crescent shaped, while normal cells are disc-shaped. (credit: modification of work by Ed Uthman; scale-bar data from Matt Russell) Secondary Structure The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The most common are the α-helix and β-pleated sheet structures (Figure 3.30). Both structures are held together by hydrogen bonds. In the α-helix structure, the hydrogen bonds form between the oxygen atom in the carbonyl group in one amino acid and another amino acid that is four amino acids farther along the chain. Figure 3.30 The α-helix and β-pleated sheet are secondary structures of proteins that form because of hydrogen bonding between carbonyl and amino groups in the peptide backbone. Certain amino acids have a propensity to form an α-helix, while others have a propensity to form a β-pleated sheet. Every helical turn in an alpha helix has 3.6 amino acid residues. The R groups (the variant groups) of the polypeptide protrude out from the α-helix chain. In the β-pleated sheet, the “pleats” are formed by hydrogen bonding between atoms on the backbone of the polypeptide chain. The R groups are attached to the carbons and extend above and below the folds of the pleat. The pleated segments align parallel or antiparallel to each other, and hydrogen bonds form between the partially positive nitrogen atom in the amino group and the partially negative oxygen atom in the carbonyl group of the peptide backbone. The α-helix and β-pleated sheet structures are found in most globular and fibrous proteins and they play an important structural role. Tertiary Structure The unique three-dimensional structure of a polypeptide is its tertiary structure (Figure 3.31). This structure is in part due to chemical interactions at work on the polypeptide chain. Primarily, the interactions among R groups creates the 120 Chapter 3 \| Biological Macromolecules complex three-dimensional tertiary structure of a protein. The nature of the R groups found in the amino acids involved can counteract the formation of the hydrogen bonds described for standard secondary structures. For example, R groups with like charges are repelled by each other and those with unlike charges are attracted to each other (ionic bonds). When protein folding takes place, the hydrophobic R groups of nonpolar amino acids lie in the interior of the protein, whereas the hydrophilic R groups lie on the outside. The former types of interactions are also known as hydrophobic interactions. Interaction between cysteine side chains forms disulfide linkages in the presence of oxygen, the only covalent bond forming during protein folding. Figure 3.31 The tertiary structure of proteins is determined by a variety of chemical hydrophobic interactions, ionic bonding, hydrogen bonding and disulfide linkages. interactions. These include All of these interactions, weak and strong, determine the final three-dimensional shape of the protein. When a protein loses its three-dimensional shape, it may no longer be functional. Quaternary Structure In nature, some proteins are formed from several polypeptides, also known as subunits, and the interaction of these subunits forms the quaternary structure. Weak interactions between the subunits help to stabilize the overall structure. For example, insulin (a globular protein) has a combination of hydrogen bonds and disulfide bonds that cause it to be mostly clumped into a ball shape. Insulin starts out as a single polypeptide and loses some internal sequences in the presence of post-translational modification after the formation of the disulfide linkages that hold the remaining chains together. Silk (a fibrous protein), however, has a β-pleated sheet structure that is the result of hydrogen bonding between different chains. The four levels of protein structure (primary, secondary, tertiary, and quaternary) are illustrated in Figure 3.32. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 121 Figure 3.32 The four levels of protein structure can be observed in these illustrations. (credit: modification of work by National Human Genome Research Institute) Denaturation and Protein Folding Each protein has its own unique sequence and shape that are held together by chemical interactions. If the protein is subject to changes in temperature, changes in pH, or exposure to chemicals, the protein structure may change, losing its shape without losing its primary sequence in what is known as denaturation. Denaturation is often reversible because the primary structure of the polypeptide is conserved in the process if the denaturing agent is removed, allowing the protein to resume its function. Sometimes denaturation is irreversible, leading to loss of function. One example of irreversible protein denaturation is when an egg is fried. The albumin protein in the liquid egg white is denatured when placed in a hot pan. Not all proteins are denatured at high temperatures; for instance, bacteria that survive in hot springs have proteins that function at temperatures close to boiling. The stomach is also very acidic, has a low pH, and denatures proteins as part of the digestion process; however, the digestive enzymes of the stomach retain their activity under these conditions. Protein folding is critical to its function. It was originally thought that the proteins themselves were responsible for the folding process. Only recently was it found that often they receive assistance in the folding process from protein helpers known as chaperones (or chaperonins) that associate with the target protein during the folding process. They act by preventing aggregation of polypeptides that make up the complete protein structure, and they disassociate from the protein once the target protein is folded. 122 Chapter 3 \| Biological Macromolecules For an
additional perspective on proteins, view this animation (http://openstaxcollege.org/l/proteins) called “Biomolecules: The Proteins.” Vegans are people who do not consume any animal products in their diet. Why do vegans need to pay special attention to the protein they eat? a. Plant proteins contain all of the essential as well as non-essential amino acids. b. It is more difficult to obtain all essential amino acids from single plant sources. c. Plant proteins contain only non-essential amino acids. d. Plants proteins do not have all of the non-essential amino acids, but do contain the essential amino acids. Think About It • Predict what happens if even one amino acid is substituted for another in a polypeptide and provide a specific example. • What categories of amino acids would you expect to find on the surface of a soluble protein, and which would you expect to find in the interior? What distribution of amino acids would you expect to find in a protein embedded in a lipid bilayer of a plasma cell membrane? Activity Folding is an important property of proteins, especially enzymes. Proteins have a narrow range of conditions in which they fold properly; outside that range, proteins can unfold (denature) and often cannot refold and become functional again. Investigate one disease that results from improper folding of a protein. Describe causes of the unfolding and consequences to the molecular structure of the polypeptide that result in the disease. 3.5 \| Nucleic Acids In this section, you will investigate the following questions: • What are the two types of nucleic acid? • What is the structure and role of DNA? • What is the structure and roles of RNA? Connection for AP® Courses Nucleic acids (DNA and RNA) comprise the fourth group of biological macromolecules and contain phosphorus (P) in This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 123 addition to carbon, hydrogen, oxygen, and nitrogen. Conserved through evolution in all organisms, nucleic acids store and transmit hereditary information. As will be explored in more detail in Chapters 14-17, DNA contains the instructions for the synthesis of proteins by dictating the sequences of amino acids in polypeptides through processes known as transcription and translation. Nucleic acids are made up of nucleotides; in turn, each nucleotide consists of a pentose sugar (deoxyribose in DNA and ribose in RNA), a nitrogenous base (adenine, cytosine, guanine, and thymine or uracil), and a phosphate group. DNA carries the genetic blueprint of the cell that is passed from parent to offspring via cell division. DNA has a double-helical structure with the two strands running in opposite directions (antiparallel), connected by hydrogen bonds and complementary to each other. In DNA, purines pair with pyrimidines: adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G). In RNA, uracil replaces thymine to pair with adenine (U-A). RNA also differs from DNA in that it is single-stranded and has many forms, such as messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) that all participate in the synthesis of proteins. MicroRNAs (miRNAs) regulate the use of mRNA. The flow of genetic information is usually DNA → RNA → protein, also known as the Central Dogma of Life. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 and Big Idea 4 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 3 Enduring Understanding 3.A Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A Living systems store, retrieve, transmit and respond to information essential to life processes. Heritable information provides for continuity of life. 3.A.1 DNA, and in some cases RNA, is the primary source of heritable information. 6.5 The student can evaluate alternative scientific explanations. 3.1 The student is able to construct scientific explanations that use the structures and mechanisms of DNA and RNA to support the claim that DNA and, in some cases, that RNA are the primary sources of heritable information. 3.A.1 DNA, and in some cases RNA, is the primary source of heritable information. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 3.6 The student can predict how a change in a specific DNA or RNA sequence can result in changes in gene expression. Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice Learning Objective 7.1 The student can connect phenomena and models across spatial and temporal scales. 4.1 The student is able to explain the connection between the sequence and the subcomponents of a biological polymer and its properties. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. 124 Science Practice Learning Objective Chapter 3 \| Biological Macromolecules 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain. 4.2 The student is able to refine representations and models to explain how the subcomponents of a biological polymer and their sequence determine the properties of that polymer. Essential Knowledge 4.A.1 The subcomponents of biological molecules and their sequence determine the properties of that molecule. Science Practice 6.1 The student can justify claims with evidence. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. Learning Objective 4.3 The student is able to use models to predict and justify that changes in the subcomponents of a biological polymer affect the functionality of the molecules. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.1] [APLO 4.17] DNA and RNA Nucleic acids are the most important macromolecules for the continuity of life. They carry the genetic blueprint of a cell and carry instructions for the functioning of the cell. The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is found in the nucleus of eukaryotes and in the organelles, chloroplasts, and mitochondria. In prokaryotes, the DNA is not enclosed in a membranous envelope. The entire genetic content of a cell is known as its genome, and the study of genomes is genomics. In eukaryotic cells but not in prokaryotes, DNA forms a complex with histone proteins to form chromatin, the substance of eukaryotic chromosomes. A chromosome may contain tens of thousands of genes. Many genes contain the information to make protein products; other genes code for RNA products. DNA controls all of the cellular activities by turning the genes “on” or “off.” The other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave the nucleus but instead use an intermediary to communicate with the rest of the cell. This intermediary is the messenger RNA (mRNA). Other types of RNA—like rRNA, tRNA, and microRNA—are involved in protein synthesis and its regulation. DNA and RNA are made up of monomers known as nucleotides. The nucleotides combine with each other to form a polynucleotide, DNA or RNA. Each nucleotide is made up of three components: a nitrogenous base, a pentose (fivecarbon) sugar, and a phosphate group (Figure 3.33). Each nitrogenous base in a nucleotide is attached to a sugar molecule, which is attached to one or more phosphate groups. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 125 Figure 3.33 A nucleotide is made up of three components: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Carbon residues in the pentose are numbered 1′ through 5′ (the prime distinguishes these residues from those in the base, which are numbered without using a prime notation). The base is attached to the 1′ position of the ribose, and the phosphate is attached to the 5′ position. When a polynucleotide is formed, the 5′ phosphate of the incoming nucleotide attaches to the 3′ hydroxyl group at the end of the growing chain. Two types of pentose are found in nucleotides, deoxyribose (found in DNA) and ribose (found in RNA). Deoxyribose is similar in structure to ribose, but it has an H instead of an OH at the 2′ position. Bases can be divided into two categories: purines and pyrimidines. Purines have a double ring structure, and pyrimidines have a single ring. The nitrogenous bases, important components of nucleotides, are organic molecules and are so named because they contain carbon and nitrogen. They are bases because they contain an amino group that has the potential of binding an extra hydrogen, thus decreasing the hydrogen ion concentration in its environment, making it more basic. Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G) cytosine (C), and thymine (T). Adenine and guanine are classified as purines. The primary structure of a purine is two carbon-nitrogen rings. Cytosine, th
ymine, and uracil are classified as pyrimidines which have a single carbon-nitrogen ring as their primary structure (Figure 3.33). Each of these basic carbon-nitrogen rings has different functional groups attached to it. In molecular biology shorthand, the nitrogenous bases are simply known by their symbols A, T, G, C, and U. DNA contains A, T, G, and C whereas RNA contains A, U, G, and C. The pentose sugar in DNA is deoxyribose, and in RNA, the sugar is ribose (Figure 3.33). The difference between the sugars is the presence of the hydroxyl group on the second carbon of the ribose and hydrogen on the second carbon of the deoxyribose. The carbon atoms of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one prime”). The phosphate residue is attached to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, which forms a 5′–3′ phosphodiester linkage. The phosphodiester linkage is not formed by simple dehydration reaction like the other linkages connecting monomers in macromolecules: its formation involves the 126 Chapter 3 \| Biological Macromolecules removal of two phosphate groups. A polynucleotide may have thousands of such phosphodiester linkages. DNA Double-Helix Structure DNA has a double-helix structure (Figure 3.34). The sugar and phosphate lie on the outside of the helix, forming the backbone of the DNA. The nitrogenous bases are stacked in the interior, like the steps of a staircase, in pairs; the pairs are bound to each other by hydrogen bonds. Every base pair in the double helix is separated from the next base pair by 0.34 nm. The two strands of the helix run in opposite directions, meaning that the 5′ carbon end of one strand will face the 3′ carbon end of its matching strand. (This is referred to as antiparallel orientation and is important to DNA replication and in many nucleic acid interactions.) Figure 3.34 Native DNA is an antiparallel double helix. The phosphate backbone (indicated by the curvy lines) is on the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base from the opposing strand. (credit: Jerome Walker/Dennis Myts) Only certain types of base pairing are allowed. For example, a certain purine can only pair with a certain pyrimidine. This means A can pair with T, and G can pair with C, as shown in Figure 3.35. This is known as the base complementary rule. In other words, the DNA strands are complementary to each other. If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG. During DNA replication, each strand is copied, resulting in a daughter DNA double helix containing one parental DNA strand and a newly synthesized strand. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 127 Figure 3.35 In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5′ to 3′ and the other 3′ to 5′. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine. A mutation occurs, and cytosine is replaced with adenine. What impact do you think this will have on the DNA structure? a. The DNA will normally pair its adenine with thymine, causing no change in the DNA structure. b. The DNA will bulge in the places where cytosine is replaced by adenine. c. The adenine substituted in the place of cytosine will get methylated and will not be transcribed further. d. The DNA will cause another mutation to replace this incorrect DNA base. RNA Ribonucleic acid, or RNA, is mainly involved in the process of protein synthesis under the direction of DNA. RNA is usually single-stranded and is made of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G, and C), and the phosphate group. There are four major types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and microRNA (miRNA). The first, mRNA, carries the message from DNA, which controls all of the cellular activities in a cell. If a cell requires a certain protein to be synthesized, the gene for this product is turned “on” and the messenger RNA is synthesized in the nucleus. The RNA base sequence is complementary to the coding sequence of the DNA from which it has been copied. However, in RNA, the base T is absent and U is present instead. If the DNA strand has a sequence AATTGCGC, the sequence of the complementary RNA is UUAACGCG. In the cytoplasm, the mRNA interacts with ribosomes and other cellular machinery (Figure 3.36). 128 Chapter 3 \| Biological Macromolecules Figure 3.36 A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two subunits. A tRNA molecule recognizes a codon on the mRNA, binds to it by complementary base pairing, and adds the correct amino acid to the growing peptide chain. The mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid. In this way, the mRNA is read and the protein product is made. Ribosomal RNA (rRNA) is a major constituent of ribosomes on which the mRNA binds. The rRNA ensures the proper alignment of the mRNA and the ribosomes; the rRNA of the ribosome also has an enzymatic activity (peptidyl transferase) and catalyzes the formation of the peptide bonds between two aligned amino acids. Transfer RNA (tRNA) is one of the smallest of the four types of RNA, usually 70–90 nucleotides long. It carries the correct amino acid to the site of protein synthesis. It is the base pairing between the tRNA and mRNA that allows for the correct amino acid to be inserted in the polypeptide chain. microRNAs are the smallest RNA molecules and their role involves the regulation of gene expression by interfering with the expression of certain mRNA messages. Table 3.2 summarizes features of DNA and RNA. Features of DNA and RNA DNA RNA Function Carries genetic information Involved in protein synthesis Location Remains in the nucleus Leaves the nucleus Structure Double helix Usually single-stranded Sugar Deoxyribose Ribose Pyrimidines Cytosine, thymine Cytosine, uracil Purines Adenine, guanine Adenine, guanine Table 3.2 Even though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function. As you have learned, information flow in an organism takes place from DNA to RNA to protein. DNA dictates the structure of mRNA in a process known as transcription, and RNA dictates the structure of protein in a process known as translation. This is known as the Central Dogma of Life, which holds true for all organisms; however, exceptions to the rule occur in connection with viral infections. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 129 To learn more about DNA, explore the Howard Hughes Medical (http://openstaxcollege.org/l/DNA) on the topic of DNA. Institute BioInteractive animations Why is DNA replication like an assembly line? a. b. c. d. It consists of many biochemical machines that functions specifically in order to drive a specific action like an assembly line. It consists of many biochemical machines that have the same function in order to drive a specific action like an assembly line. It consists of many biochemical machines where each functions randomly in order to drive a specific action like an assembly line. It consists of many biochemical machines that functions in order to drive a non-specific action like an assembly line. Activity Using construction paper, markers, and scissors, construct a model of DNA with at least 8 nucleotides. Then, use the model to distinguish between DNA and RNA and hypothesize how the DNA molecule is replicated during cell division. (Keep your molecule to model the processes of transcription and translation that you will explore in Chapter 15.) Think About It A mutation occurs, and cytosine is replaced with adenine. Explain how this affects how the changed strand will base pair with its complimentary strand of DNA. 130 Chapter 3 \| Biological Macromolecules KEY TERMS alpha-helix structure (α-helix) type of secondary structure of proteins formed by folding of the polypeptide into a helix shape with hydrogen bonds stabilizing the structure amino acid monomer of a protein; has a central carbon or alpha carbon to which an amino group, a carboxyl group, a hydrogen, and an R group or side chain is attached; the R group is different for the most common 20 amino acids beta-pleated sheet (β-pleated) secondary structure found in proteins in which “pleats” are formed by hydrogen bonding between atoms on the backbone of the polypeptide chain biological macromolecule large molecule necessary for life that is built from smaller organic molecules carbohydrate biological macromolecule in which the ratio of carbon to hydrogen and to oxygen is 1:2:1; carbohydrates serve as energy sources and structural support in cells and form the a cellular exoskeleton of arthropods cellulose polysaccharide that makes up the cell wall of plants; provides structural support to the cell chaperone (also, chaperonin) protein that helps nascent protein in the folding process chitin type of carbohydrate that forms the outer skeleton of all arthropods that include crustaceans and insects; it also forms the cell walls of fungi dehydration synthesis (also, condensation) reaction that links monomer molecules together, releasing a molecule of water for each bond formed denaturation loss of shape in a protein as a result of changes in temperature, pH, or exposure to chemicals deoxyribonucleic acid (DNA) double-helical molec
ule that carries the hereditary information of the cell disaccharide two sugar monomers that are linked together by a glycosidic bond enzyme catalyst in a biochemical reaction that is usually a complex or conjugated protein glycogen storage carbohydrate in animals glycosidic bond molecule bond formed by a dehydration reaction between two monosaccharides with the elimination of a water hormone chemical signaling molecule, usually protein or steroid, secreted by endocrine cells that act to control or regulate specific physiological processes hydrolysis reaction that causes breakdown of larger molecules into smaller molecules with the utilization of water lipid macromolecule that is nonpolar and insoluble in water messenger RNA (mRNA) RNA that carries information from DNA to ribosomes during protein synthesis monomer smallest unit of larger molecules called polymers monosaccharide single unit or monomer of carbohydrates nucleic acid biological macromolecule that carries the genetic blueprint of a cell and carries instructions for the functioning of the cell nucleotide monomer of nucleic acids; contains a pentose sugar, one or more phosphate groups, and a nitrogenous base omega fat type of polyunsaturated fat that is required by the body; the numbering of the carbon omega starts from the methyl end or the end that is farthest from the carboxylic end peptide bond bond formed between two amino acids by a dehydration reaction phosphodiester linkage covalent chemical bond that holds together the polynucleotide chains, with a phosphate group This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 131 linking two pentose sugars of neighboring nucleotides phospholipid major constituent of the membranes; composed of two fatty acids and a phosphate-containing group attached to a glycerol backbone polymer chain of monomer residues that is linked by covalent bonds; polymerization is the process of polymer formation from monomers by condensation polynucleotide long chain of nucleotides polypeptide long chain of amino acids linked by peptide bonds polysaccharide long chain of monosaccharides; may be branched or unbranched primary structure linear sequence of amino acids in a protein protein biological macromolecule composed of one or more chains of amino acids purine type of nitrogenous base in DNA and RNA; adenine and guanine are purines pyrimidine type of nitrogenous base in DNA and RNA; cytosine, thymine, and uracil are pyrimidines quaternary structure association of discrete polypeptide subunits in a protein ribonucleic acid (RNA) single-stranded, often internally base paired, molecule that is involved in protein synthesis ribosomal RNA (rRNA) RNA that ensures the proper alignment of the mRNA and the ribosomes during protein synthesis and catalyzes the formation of the peptide linkage saturated fatty acid long-chain of hydrocarbon with single covalent bonds in the carbon chain; the number of hydrogen atoms attached to the carbon skeleton is maximized secondary structure regular structure formed by proteins by intramolecular hydrogen bonding between the oxygen atom of one amino acid residue and the hydrogen attached to the nitrogen atom of another amino acid residue starch storage carbohydrate in plants steroid type of lipid composed of four fused hydrocarbon rings forming a planar structure tertiary structure three-dimensional conformation of a protein, including interactions between secondary structural elements; formed from interactions between amino acid side chains trans fat fat formed artificially by hydrogenating oils, leading to a different arrangement of double bond(s) than those found in naturally occurring lipids transcription process through which messenger RNA forms on a template of DNA transfer RNA (tRNA) RNA that carries activated amino acids to the site of protein synthesis on the ribosome translation process through which RNA directs the formation of protein triacylglycerol (also, triglyceride) fat molecule; consists of three fatty acids linked to a glycerol molecule unsaturated fatty acid long-chain hydrocarbon that has one or more double bonds in the hydrocarbon chain wax lipid made of a long-chain fatty acid that is esterified to a long-chain alcohol; serves as a protective coating on some feathers, aquatic mammal fur, and leaves CHAPTER SUMMARY 3.1 Synthesis of Biological Macromolecules Proteins, carbohydrates, nucleic acids, and lipids are the four major classes of biological macromolecules—large 132 Chapter 3 \| Biological Macromolecules molecules necessary for life that are built from smaller organic molecules. Macromolecules are made up of single units known as monomers that are joined by covalent bonds to form larger polymers. The polymer is more than the sum of its parts: it acquires new characteristics, and leads to an osmotic pressure that is much lower than that formed by its ingredients; this is an important advantage in the maintenance of cellular osmotic conditions. A monomer joins with another monomer with the release of a water molecule, leading to the formation of a covalent bond. These types of reactions are known as dehydration or condensation reactions. When polymers are broken down into smaller units (monomers), a molecule of water is used for each bond broken by these reactions; such reactions are known as hydrolysis reactions. Dehydration and hydrolysis reactions are similar for all macromolecules, but each monomer and polymer reaction is specific to its class. Dehydration reactions typically require an investment of energy for new bond formation, while hydrolysis reactions typically release energy by breaking bonds. 3.2 Carbohydrates Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides depending on the number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that are formed as a result of dehydration reactions, forming disaccharides and polysaccharides with the elimination of a water molecule for each bond formed. Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched. Cellulose is an example of an unbranched polysaccharide, whereas amylopectin, a constituent of starch, is a highly branched molecule. Storage of glucose, in the form of polymers like starch of glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell. 3.3 Lipids Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids, and steroids. Fats are a stored form of energy and are also known as triacylglycerols or triglycerides. Fats are made up of fatty acids and either glycerol or sphingosine. Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain. If only single bonds are present, they are known as saturated fatty acids. Unsaturated fatty acids may have one or more double bonds in the hydrocarbon chain. Phospholipids make up the matrix of membranes. They have a glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the fluid nature of the membrane. It is also the precursor of steroid hormones such as testosterone. 3.4 Proteins Proteins are a class of macromolecules that perform a diverse range of functions for the cell. They help in metabolism by providing structural support and by acting as enzymes, carriers, or hormones. The building blocks of proteins (monomers) are amino acids. Each amino acid has a central carbon that is linked to an amino group, a carboxyl group, a hydrogen atom, and an R group or side chain. There are 20 commonly occurring amino acids, each of which differs in the R group. Each amino acid is linked to its neighbors by a peptide bond. A long chain of amino acids is known as a polypeptide. Proteins are organized at four levels: primary, secondary, tertiary, and (optional) quaternary. The primary structure is the unique sequence of amino acids. The local folding of the polypeptide to form structures such as the α helix and β-pleated sheet constitutes the secondary structure. The overall three-dimensional structure is the tertiary structure. When two or more polypeptides combine to form the complete protein structure, the configuration is known as the quaternary structure of a protein. Protein shape and function are intricately linked; any change in shape caused by changes in temperature or pH may lead to protein denaturation and a loss in function. 3.5 Nucleic Acids Nucleic acids are molecules made up of nucleotides that direct cellular activities such as cell division and protein synthesis. Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group. There are two types of nucleic acids: DNA and RNA. DNA carries the genetic blueprint of the cell and is passed on from parents to offspring (in the form of chromosomes). It has a double-helical structure with the two strands running in opposite directions, connected by hydrogen bonds, and complementary to each other. RNA is single-stranded and is made of a pentose sugar (ribose), a nitrogeno
us base, and a phosphate group. RNA is involved in protein synthesis and its regulation. Messenger This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 133 RNA (mRNA) is copied from the DNA, is exported from the nucleus to the cytoplasm, and contains information for the construction of proteins. Ribosomal RNA (rRNA) is a part of the ribosomes at the site of protein synthesis, whereas transfer RNA (tRNA) carries the amino acid to the site of protein synthesis. MicroRNA regulates the use of mRNA for protein synthesis. REVIEW QUESTIONS 1. Dehydration synthesis leads to the formation of what? a. carbohydrate a. monomers b. polymers c. carbohydrates only d. water only b. protein c. RNA d. triglyceride 8. What is an example of a monosaccharide? 2. What is removed during the formation of nucleic acid polymers? a. carbon b. hydroxyl groups c. phosphates d. amino acids 3. During the breakdown of polymers, which of the following reactions takes place? a. condensation b. covalent bond c. dehydration d. hydrolysis 4. Energy is released as a result of which of the following chemical reactions? a. condensation b. dehydration synthesis c. hydrolysis d. dissolution a. cellulose b. c. d. fructose lactose sucrose 9. Cellulose and starch are examples of ________. a. disaccharides b. lipids c. monosaccharides d. polysaccharides 10. What type of bond joins the molecules in the disaccharide lactose? What molecule is joined with glucose to form lactose? a. a glycosidic bond between glucose and lactose b. a glycosidic bond between glucose and galactose c. a hydrogen bond between glucose and sucrose d. a hydrogen bond between glucose and fructose 11. What is structurally different about cellulose when compared to starch? 5. In the metabolism of cell, why is hydrolysis used? a. an extra hydrogen atom is left on the monomer a. Hydrolysis breaks down polymers. b. Hydrolysis is used to form linkages in DNA. c. Hydrolysis is used to produce proteins. b. β -1,4 glycosidic linkages are used c. α -1,6 glycosidic linkages are used d. an extra hydroxyl group is removed during d. Hydrolysis synthesizes new macromolecules. synthesis 6. Plant cell walls contain which of the following in abundance? a. cellulose b. glycogen c. d. lactose starch 7. What makes up the outer layer of some insects? 12. Which of the following are classified as lipids? a. disaccharides and cellulose b. essential amino acids c. mRNA and DNA d. oils and waxes 13. What is cholesterol specifically classified as? 134 Chapter 3 \| Biological Macromolecules a. a lipid b. a phospholipid c. a steroid d. a wax 14. Which fat serves as an animal’s major form of energy storage? a. cholesterol b. glycerol c. phospholipid d. triglycerides 15. Which hormones are made from cholesterol? a. estradiol and testosterone b. insulin and growth hormone c. progesterone and glucagon d. prolactin and thyroid hormone a. mRNA transport b. production of bile salts c. water reabsorption in the kidney d. wax production 22. Where is cholesterol found in cell membranes? a. attached to the inner side of the membrane b. attached to the outer side of the membrane c. floating in the phospholipid tail layer d. penetrating both lipid layers 23. Which type of body cell would have a higher amount of cholesterol in its membrane? a. a cartilage cell b. a liver cell c. a red blood cell d. a spleen cell 16. Which of the following characteristics is not true for saturated fats? 24. Which of the following is a function of proteins in cells? a. They are solid at room temperature. a. energy storage b. They have single bonds within the carbon chain. b. gene storage and access c. They tend to dissolve in water easily. c. membrane fluidity 17. Which fat has the least number of hydrogen atoms? d. structure a. b. trans fat saturated fat c. unsaturated fat d. wax 18. Of what are phospholipids important components? a. b. c. d. the double bond in hydrocarbon chains the plasma membrane of animal cells the ring structure of steroids the waxy covering on leaves 19. What is a diacylglycerol 3-phosphate? a. phospholipid b. phosphatidylcholine c. phosphatidylserine d. phosphatidate 20. What is the basic structure of a steroid? a. four fused hydrocarbon rings b. glycerol with three fatty acid chains c. d. two fatty acid chains and a phosphate group two six carbon rings 21. Besides its use in hormone production, for what does the body use cholesterol? 25. What type of protein facilitates or accelerates chemical reactions? a. an enzyme b. a hormone c. a membrane transport protein d. a tRNA molecule 26. What type of amino acids would you expect to find on the surface of proteins that must interact closely with water? 27. What are the monomers that make up proteins called? a. amino acids b. chaperones c. disaccharides d. nucleotides 28. Where is the linkage made that combines two amino acids? a. between the R group of one amino acid and the R group of the second b. between the carboxyl group of one amino acid and the amino group of the other c. between the 6 carbon of both amino acids d. between the nitrogen atoms of the amino groups in the amino acids This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 135 29. The α -helix and the β -pleated sheet are part of which protein structure? 34. What may a nucleotide of DNA contain? a. ribose, uracil, and a phosphate group a. b. c. d. the primary structure the secondary structure the tertiary structure the quaternary structure b. deoxyribose, uracil, and a phosphate group c. deoxyribose, thymine, and a phosphate group d. ribose, thymine, and a phosphate group 35. What is DNA’s structure described as? 30. Which structural level of proteins is most often associated with their biological function? a. a step ladder b. a double helix a. b. c. d. the primary structure the secondary structure the tertiary structure the quaternary structure 31. Which of the following may cause a protein to denature? a. changes in pH b. high temperatures c. the addition of some chemicals d. all of the above 32. What is a protein’s chaperone? a. a chemical that assists the protein in its enzymatic functions c. a tertiary protein-like structure d. barber pole 36. What is found in RNA that is not in DNA? a. deoxyribose and adenine b. fructose and thymine c. glucose and quinine d. ribose and uracil 37. What is the smallest type of RNA? a. mRNA b. microRNA c. d. rRNA tRNA b. a second protein that completes the quaternary structure 38. Where is the largest amount of DNA found in a eukaryotic cell? c. a chemical that helps the protein fold properly a. attached to the inner layer of the cell membrane d. a chemical that functions as a cofactor for the protein 33. What are the building blocks of nucleic acids? b. c. in the nucleus in the cytoplasm d. on ribosomes a. nitrogenous bases b. nucleotides c. peptides d. sugars CRITICAL THINKING QUESTIONS 39. The word hydrolysis is defined as the lysis of water. How does this apply to polymers? 40. What role do electrons play in dehydration synthesis and hydrolysis? a. Polymers break by separating water into hydrogen and hydroxyl group that are added to the monomers. b. Polymers are synthesized by using the energy released by the breaking of water molecules into hydrogen and hydroxyl group. c. Polymers are separated into monomers producing energy and water molecules. d. Polymers are hydrolyzed into monomers using water in the process and are called as dehydration synthesis. 136 Chapter 3 \| Biological Macromolecules a. Electrons are added to OH and H ion in the dehydration synthesis. They are removed from OH and H in hydrolysis. b. Electrons are transferred from OH and H ions to the monomers in dehydration synthesis. They are taken up by the H and OH ions from the monomers in hydrolysis. c. Electrons are removed from OH and H in the dehydration synthesis. They are added to OH and H in hydrolysis. d. Electrons are transferred from monomers to H and OH ions in hydrolysis and from OH and H to monomers in dehydration synthesis. 41. Which of the following bodily process would most likely be hindered by a lack of water in the body? a. Glucose and fructose combine to form sucrose. Glucose and galactose combine to form lactose. Two glucose monomers combine to form maltose. b. Glucose and fructose combine to form sucrose. Glucose and galactose combine to form maltose. Two glucose combine to form lactose. c. Two glucose combine to form lactose. Glucose and galactose combine to form sucrose. Glucose and fructose combine to form maltose. d. Two galactose combine to form sucrose. Fructose and glucose combine to form lactose. Two glucose combine to form maltose. 45. What are the four classes of lipids and what is an example of each? a. 1. lipids like margarine a. digestion b. protein synthesis c. copying DNA d. breathing 42. Why is it impossible for humans to digest food that contains cellulose? a. There is no energy available in fiber. b. An inactive form of cellulase in human digestive tract renders it undigested and removes it as waste. c. The acidic environment in the human stomach makes it impossible to break the bonds in cellulose. d. Human digestive enzymes cannot break down the β -1,4 glycosidic linkage in cellulose, which requires a special enzyme that is absent in humans. 43. Which of these describe some of the similarities and differences between glycogen and starch? a. Glycogen is less branched than starch and is found in animals. b. Glycogen is more highly branched than starch and is found in plants. c. Starch is less branched than glycogen and is found in plants. d. Starch is more branched than glycogen and is found in animals. 44. Which of these best describes the production of sucrose, maltose, and lactose? 2. wax like the coating on feathers 3. phospholipids like cell membrane constituents steroid like cholesterol lipids like phosphatidylserine 4. 1. 2. wax lik
e phosphatidic acid 3. phospholipids like oleic acid 4. 1. steroid like epinephrine lipids like phosphatidic acid 2. waxes like margarine 3. phospholipids like phosphatidylcholine 4. 1. steroids like testosterone lipids like cholesterol 2. waxes like the coating on feathers 3. phospholipids like phosphatidylserine 4. steroids like margarine b. c. d. 46. What are three functions that lipids serve in plants and/ or animals? a. Lipids serve in the storage of energy, as a structural component of hormones, and also as signaling molecules. b. Lipids serve in the storage of energy, as carriers for the transport of proteins across the membrane, and as signaling molecules. c. Lipids serve in the breakdown of stored energy molecules, as signaling molecules, and as structural components of hormones. d. Lipids serve in the breakdown of stored energy molecules, as signaling molecules, and as channels for protein transport. 47. Why have trans fats been banned from some restaurants? How are they created? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 137 a. Trans fat is produced by the hydrogenation of oil that makes it more saturated and isomerized. It increases LDL amounts. b. The dehydrogenation of oil forms the trans fat, which contains single bonds in its structure. This increases HDL in the body and has been banned. c. Trans fat is produced by dehydrogenation of oils, which makes it unsaturated. It increases LDL in body. d. The hydrogenation of oil makes the trans fat, which contains double bonds in its structure. It decreases HDL in the body. a. Different amino acids produce different proteins based on the bonds formed between them. b. Differences in amino acids lead to the recycling of proteins, which produces other functional proteins. c. Different amino acids cause rearrangements of amino acids to produce a functional protein. d. Differences in the amino acids cause post- translational modification of the protein, which reassembles to produce a functional protein. 52. What causes the changes in protein structure through the three or four levels of structure? 48. How do phospholipids contribute to cell membrane structure? a. Phospholipids orient their heads towards the polar molecules and tails in the interior of the membrane, thus forming a bilayer. b. Phospholipids orient their tails towards the polar molecules of water solutions, and heads in the interior of the membrane, thus forming a bilayer. c. Phospholipids orient their heads towards the non-polar molecules and tails in the interior of the membrane, forming a bilayer. a. The primary chain forms secondary α-helix and β-pleated sheets which fold onto each other forming the tertiary structure. b. The primary structure undergoes alternative splicing to form secondary structures, which fold on other protein chains to form tertiary structures. c. The primary structure forms secondary α-helix and β-pleated sheets. This further undergoes phosphorylation and acetylation to form the tertiary structure. d. Phospholipids orient their tails towards the polar molecules and heads in the non-polar side of the membrane, forming a bilayer. d. The primary structure undergoes alternative splicing to form a secondary structure, and then disulfide bonds give way to tertiary structures. 49. What type of compound functions in hormone production, contributes to membrane flexibility, and is the starting molecule for bile salts? a. All steroid molecules help in the mentioned functions. b. Cholesterol, which is a lipid and also a steroid, functions here. c. Glycogen, which is a multi-branched polysaccharide of glucose, is the compound. d. Phosphatidylcholine that is a phospholipid with a choline head group, which serves the functions. 50. What part of cell membranes gives flexibility to the structure? a. carbohydrates b. cytoskeleton filaments c. lipids d. proteins 51. How do the differences in amino acid sequences lead to different protein functions? 53. What structural level of proteins is functional? Why? a. The secondary structure is functional as it attains its 2-dimensional shape which has the necessary bonds. b. The tertiary structure is functional as it possesses the geometric shape showing the necessary loops and bends. c. The tertiary structure is functional as it has the non-covalent and covalent bonds along with the subunits attached at the right places, which help it function properly. d. Quaternary structure is functional as it has the essential set of subunits. 54. How does a chaperone work with proteins? a. Chaperones assist proteins in folding. b. Chaperones cause the aggregation of polypeptides. c. Chaperones associate with proteins once the target protein is folded. d. Chaperones escort proteins during translation. 55. What are some differences between DNA and RNA? 138 Chapter 3 \| Biological Macromolecules a. DNA is made from nucleotides; RNA is not. b. DNA contains deoxyribose and thymine, while RNA contains ribose and uracil. c. DNA contains adenine, while RNA contains guanine. d. DNA is double stranded, while RNA may be double stranded in animals. 56. Which molecule carries information in a form that is inherited from one generation to another? a. Hereditary information is stored in DNA. b. Hereditary information is stored in mRNA. c. Hereditary information is stored in proteins. d. Hereditary information is stored in tRNA. 57. What are the four types and functions of RNA? a. mRNA is a single stranded transcript of DNA. rRNA is found in ribosomes. tRNA transfers specific amino acids to a developing protein strand. miRNA regulates the expression of mRNA strands. b. mRNA is a single stranded transcript of rRNA. rRNA is translated in ribosomes to make proteins. tRNA transfers specific amino acids to a developing protein strand. microRNA (miRNA) regulates the expression of the mRNA strand. c. mRNA regulates the expression of the miRNA strand. rRNA are found in ribosomes. tRNA transfers specific amino acids to a developing protein strand. miRNA is a single stranded transcript of DNA. d. mRNA is a single stranded transcript of DNA. rRNA transfers specific amino acids to a developing protein strand. tRNA is found in ribosomes. miRNA regulates the expression of the mRNA strand. TEST PREP FOR AP® COURSES 58. Urey and Miller constructed an experiment to illustrate the early atmosphere of the Earth and possible development of organic molecules in the absence of living cells. Which assumption did Urey and Miller make regarding conditions on Earth? a. electric sparks occurred to catalyze the reaction b. c. d. the composition of the gases in the atmosphere there was sufficient oxygen for creating life it produced water-soluble organic molecules 59. Urey and Miller proposed that a series of reactions occurred, which ultimately resulted in amino acid formation. Which of the following is true based upon their theory? a. The simple molecules assembled to form amino acids and nucleic acids. b. The organic molecules assembled to form the large complexes such as water and methane. c. The inorganic molecules assembled to form the amino acids and nucleic acids. d. The inorganic molecules assembled to form the large complexes such as water and methane. 61. Which statement most accurately describes the importance of the condensation stage during Urey and Miller’s experiment? a. Condensed water enabled the formation of monomers. a. Hydrogen and nitrogen combined to create b. Condensation and evaporation simulated amino acids. lightning storms. b. Hydrogen and oxygen combined to create c. Condensation and evaporation simulated the macromolecules. water cycle. c. Nitrogenous bases combined to form monomers d. Condensed water enabled the formation of then RNA. polymers. d. Periodic elements combined to create molecules then DNA. 60. How does Stanley Miller and Harold Urey’s model support the claim that simple precursors present on early Earth could have assembled into complex molecules necessary for life? 62. According to the findings of the Urey and Miller experiment, the primitive atmosphere consisted of water in the form of steam, methane, ammonia, and hydrogen gases. If there was so much hydrogen gas in the early atmosphere, why is there so little now? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 139 a. Hydrogen gas is so light with a molecular weight of 1 that the excess diffused into space over time and is now absent from the atmosphere. b. Hydrogen combined with ammonia to make ammonium. c. It was all used up in the production of organic molecules. d. The excess hydrogen gas was dissolved in the early oceans. 63. Could the primitive atmosphere illustrated by the Urey and Miller experiment be reproduced on today’s Earth? Why or why not? a. The primitive atmosphere cannot be created due to the oxidizing atmosphere and lack of hydrogen. b. The primitive atmosphere can be created as the atmosphere is reducing and the Earth has sufficient hydrogen to reproduce the conditions. c. The primitive atmosphere cannot be created due to the presence of abundant water and hydrogen in the atmosphere. d. The primitive atmosphere can be created as the atmosphere is oxidizing and has less of hydrogen. 64. What is structurally different between starch and cellulose that gives them different physical properties? Complex polymers are built from combinations of smaller monomers. What type of reaction is shown, and what is a product of the following reaction? Assume water is also produced. 65. a. Cellulose is formed by β -1,4 glycosidic a. a synthesis reaction producing glucose and water linkages and crosslinks, making it rigid. Starch has α -1.4 and α -1.6 glycosidic linkages without the tight crosslinks of cellulose. b. Cellulose has rigid α -1,4 glycosidic linkages while starch has less rigid β -1,4 glycosidic linkages c. Cellulose has amylose and amylopectin, mak
ing it more rigid than starch. d. Starch has amylose and amylopectin that make it more rigid than cellulose. b. a hydrolysis reaction producing fructose and water c. a condensation reaction producing lactose and water d. a dehydration reaction producing sucrose and water 66. The fatty acids of triglycerides are classified as saturated, unsaturated, or trans fats. What is it about the structure of these compounds that gives them their physical characteristics? a. Saturated fats and trans fats contain the greatest possible number of hydrogen atoms, while unsaturated fats do not. b. Saturated and unsaturated fats have stable configurations, while trans fats are transient. c. Unsaturated fats and trans fats have some double bonded carbon atoms, while saturated fats do not. d. Unsaturated and trans fats are the same; the fatty acids are just found on opposite sides of a trans fat. 67. Carbohydrates serve various functions in different animals. Arthropods like insects, crustaceans, and others, have an outer layer, called the exoskeleton, which protects 140 Chapter 3 \| Biological Macromolecules their internal body parts. This exoskeleton is made mostly of chitin. Chitin is also a major component of the cell walls of fungi, the kingdom that includes molds and mushrooms. Chitin is a polysaccharide. What is the major difference between chitin and other types of polysaccharides? a. Chitin is a nitrogen-containing polysaccharide, with repeating units of N-acetyl- β -Dglucosamine, a modified sugar. b. Chitin is similar to amylase, but with sulfur linkages between the monomers. c. Chitin is similar to inulin, a polysaccharide with fructose, but with additional glucose monomers. d. Chitin contains phosphate groups that give it a stiffness not found in other polysaccharides. 68. What categories of amino acids would you expect to find on the surface of a soluble protein and which would you expect to find in the interior? Which of these are some examples for each part of the answer? a. Non-polar and charged amino acids will be present on the surface and polar in the interior of the membrane whereas non-polar will be found in the membrane embedded proteins. b. Non-polar and uncharged proteins will be found on the surface with non-polar in the interior, while only non-polar will be found in the embedded proteins. c. Polar and charged amino acids will be found on the surface whereas non-polar in the interior. d. Polar and charged amino acids will be found on the surface of a membrane protein whereas nonpolar in the interior. The membrane protein will be polar and hydrophobic. 69. You have been identifying the sequence of a segment of a protein. The sequence to date is: leucine-methioninetyrosine-alanine-glutamine-lysine-glutamate. You insert arginine between the leucine and methionine. What effect would this have on the segment? a. Arginine is a negatively charged amino acid and could attach to the glutamate at the end of the segment b. Inserting arginine places a positively charged amino acid in a portion that is non-polar, creating the possibility of a hydrogen bond in this area. c. There would be no effect other than an additional amino acid. a. The change will definitely not be sufficient to have any effect on the function and structure of the protein. b. The amino acid may not show any significant effect the protein structure and function or it may have a significant effect, as in the case of hemoglobin in individuals with sickle cell trait. c. These changes would increase the possibility of having extra bends and loops in the proteins as in Leber congenital disease. d. These changes would modify the structures of proteins making them nonfunctional. 71. HIV is an RNA virus that affects CD4 cells, also known as T cells, in the human body. Which mechanism is most likely responsible for the fast rate at which HIV can spread? a. recombination b. mutation c. d. reassortment formation errors 72. For many years, scientist believed that proteins were the source of heritable information. There are many thousands of different proteins in a cell, and they mediate the cell’s metabolism, producing the traits and characteristics of a species. Researchers working with DNA viruses proved that it is DNA that stores and passes on genes. They worked with viruses with an outer coat of protein and a DNA strand inside. How did they prove that it was DNA, not protein, which is the primary source of heritable information? a. The DNA and protein of the virus were tagged with different isotopes and exposed to host cell where only the DNA was transferred to the host. b. The DNA was tagged with an isotope, which was retained in the virus, proving it to be the genetic material. c. The viral protein was tagged with an isotope, and the host cell was infected by it. This protein was transferred to the host. d. The viral DNA, when sequenced, was found to be present in the host cell proving it to be the hereditary material instead of protein. 73. The genetic code is based on each amino acid being coded for by a distinctive series of three nucleic acid bases called a codon. The following is a short segment of DNA using the slash symbol ( / ) to separate the codons for easy viewing: ATC/GTT/GAA/CTG/TAG/GAT/AAA d. The arginine could attach to the lysine and bend the protein chain at this point. A change has occurred in the segment resulting in the following: 70. What would happen if even one amino acid is substituted for another in a polypeptide? What would be an example? ATC/GTT/GTA/CTG/TAG/GAT/AAA What kind of change has occurred? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 141 a. A substitution of T for A , changing the coding for the third codon b. An addition of C for G , lengthening the strand and changing every codon past the addition c. A deletion of an A , resulting in a shortening and changing every codon past the deletion d. No change has occurred; the same one base was replaced with the same one 74. A change in DNA on a chromosome affects all proteins made from that gene for the life of the cell. A change in the RNA involved in protein production is short lived. What is the difference between the effects of the changes in the two types of nucleic acids? a. DNA is the genetic material that is passed from parent cells to daughter cells and to future generations. b. DNA would not affect the individuals as the proteins made are finally altered and modified. RNA would cause harm to the person as the RNA is encoded by the DNA and is not altered. c. DNA is the genetic material and is transferred from one generation to another making use of repair mechanisms for every mutation. The RNA does not use a repair mechanism. d. DNA, when mutated, makes use of the repair mechanisms and can be repaired whereas RNA is not repaired and is transferred in generations. SCIENCE PRACTICE CHALLENGE QUESTIONS 75. The capture of radiant energy through the conversion of carbon dioxide and water into carbohydrates is the engine that drives life on Earth. Ribose, C5H10O5, and hexose, C6H12O6, form stable five- and six-carbon rings. A. Create visual representations to show how when bonds in the glucose molecules are broken between carbon number 1 and the oxygen atom and between carbons 3 and 4, two molecules of pyruvic acid are produced. Several enzymes in the cell are involved in converting glucose to pyruvic acid. These enzymes are proteins whose amino acid sequences provide these functions. This protein structure is information that was inherited from the cell’s parent, and is stored in deoxyribonucleic acid (DNA). The “deoxyribo” component of that name is a shorthand for 2-deoxyribose. B. Create a visual representation of 2-deoxyribose, 5-phosphate by replacing the OH at carbon 2 with a hydrogen atom and replacing the OH at carbon 5 with a hydrogen phosphate ion, HPO3 shown in problem AP3.2. Use your representation to show that both phosphorylation (the addition of a phosphate ion) at carbon 5 and removal of the hydroxide at carbon 2 produce water molecules in an aqueous solution where hydrogen ions are abundant. -2, whose structure is DNA is a polymer formed from a chain with repeated 2-deoxyribose, 5-phosphate molecules. C. Create a visual representation of three 2-deoxyribose, 5-phosphate molecules forming a chain in which an oxygen atom in the phosphate that is attached to the 5-carbon replaces the OH on the 3-carbon of the next ribose sugar. 76. Cells are bounded by membranes composed of phospholipids. A phospholipid consists of a pair of fatty acids that may or may not have carbon-carbon double bonds, fused at the carboxylic acid with a three-carbon glycerol that is terminated by a phosphate, as shown in the figure below. Most cell membranes comprise two phospholipid layers with the hydrophilic phosphate ends of each molecule in the outer and inner surfaces. The hydrophobic chains of carbon atoms extend into the space between these two surfaces. Figure 3.37 The numbering of the carbons on these rings is important in organizing our description of the role these molecules play in biological energy transfer and information storage and retrieval. Glycolysis is a sequence of chemical reactions that convert glucose to two three-carbon compounds called pyruvic acid. Figure 3.38 142 Chapter 3 \| Biological Macromolecules molecules, O2, through three types of membranes were made (Widomska et al., Biochimica et Biophysica Acta, 1,768, 2007) and compared with the speed of movement of O2 through water. These measurements were carried out at four different temperatures. One type of membrane was obtained from the cells in the eyeball of a calf (lens lipid). Synthetic membranes composed of palmitic acid with cholesterol (POPC/CHOL) and without cholesterol (POPC) were also used. The results from these experiments are shown in the table below. Temperature (°C) 15 25 35 45 Material Speed (cm/s) Lens lipids 15 POPC/C
HOL 15 POPC Water Table 3.3 55 45 30 30 65 60 110 95 100 155 280 55 65 75 B. Represent these data graphically. The axes should be labeled, and different symbols should be used to plot data for each material. C. Analyze the data by comparing transport of oxygen through the biological membrane, water, and the synthetic membranes. Consider both membrane composition and temperature in your analysis. The plasma membrane separates the interior and the exterior of the cell. A potential to do work is established by defining regions inside and outside the cell with different concentrations of key molecules and net charge. In addition to the membrane defining the cell boundary, eukaryotic cells have internal membranes. D. Explain how internal membranes significantly increase the functional capacity of the cells of eukaryotes relative to those of prokaryotes. 77. Proteins are polymers whose sub-components are amino acids connected by peptide bonds. The carboxylic acid carbon, O = C – OH, of one amino acid can form a bond with the amine, NH2, of another amino acid. In the formation of this peptide bond, the amine replaces the OH to form O = C – NH2. The other product of this reaction is water, H2O. Figure 3.39 The exchange of matter between the interior of the cell and the environment is mediated by this membrane with selective permeability. A. Pose questions that identify • • • the important characteristics of this lipid bilayer structure the molecules that must be acquired from the environment and eliminated from the cell relationships between the structures of these molecules and the structure of the bilayer Because the plasma cell membrane has both hydrophilic and hydrophobic properties, few types of molecules possess structures that allow them to pass between the interior of the cell and the environment through passive diffusion. The fluidity of the membrane affects passive transport, and the incorporation of other molecules in the membrane, in particular cholesterols, has a strong effect on its fluidity. Fluidity is also affected by temperature. Measurements of the speed of movement of oxygen This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 143 are distributed unequally. Polarity in a molecule also is caused by charge asymmetry. Life on Earth has evolved within a framework of water, H2O, one of the most polar molecules. The polarities of the amino acids that compose a protein determine the properties of the polymer. The electric polarity of an amino acid in an aqueous solution depends on the pH of the solution. Here are three forms of the general structure of an amino acid. Figure 3.41 C. Qualitatively predict the relationship between solution pH and the form of the amino acid for three solutions of pH: pH < 7, pH = 7, and pH > 7. Figure 3.40 Amino acids can be synthesized in the laboratory from simpler molecules of ammonia (NH3), water (H2O), methane (CH4), and hydrogen (H2) if energy is provided by processes that simulate lightning strikes or volcanic eruptions (Miller, Science, 117, 1953; Johnson et al., Science, 322, 2008). A. The synthesis of amino acids in solutions under laboratory conditions consistent with early Earth was a step toward an explanation of how life began. Pose a question that should have been asked but was not until 2014 (Parker et al., Angewandte Chemie, 53, 2014), when these solutions that had been stored in a refrigerator were analyzed. The diversity and complexity of life begins in the variety of sequences of the 20 common amino acids. B. Apply mathematical reasoning to explain the source of biocomplexity by calculating the possible variations in a polymer composed of just three amino acids. Polarity in a bond between atoms occurs when electrons Figure 3.42 The properties of proteins are determined by interactions among the amino acids in the peptide-bonded chain. The protein subcomponents, especially amino R (variable) groups, can interact with very strong charge-charge forces, with attractive forces between groups of atoms with opposite polarities and with repulsive forces between groups of atoms with the same or no polarity. Attractive polar forces often arise between molecules through interactions between oxygen and hydrogen atoms or between nitrogen and hydrogen atoms. D. Consider particular orientations of pairs of three different amino acids. Predict the relative strength of attractive interaction of all pairs; rank them and provide your reasoning. In an amino acid, the atoms attached to the α carbon are 144 called the R group. Chapter 3 \| Biological Macromolecules Figure 3.43 Interactions between R groups of a polypeptide give threedimensional structure to the one-dimensional, linear sequence of amino acids in a polypeptide. E. Construct an explanation for the effect of R-group interactions on the properties of a polymer with drawings showing molecular orientations with stronger and weaker polar forces between R groups on asparagine and threonine and between asparagine and alanine. Figure 3.44 78. The nucleobase part of deoxyribonucleic acid encodes information in each component in the sequence making up the polymer. There are five nucleobases that are commonly represented by only a single letter: A (adenine), C (cytosine), G (guanine), T (thymine), and U (uracil). These molecules form a bond with the 1-carbon of deoxyribose. In this problem, we need to look at the molecules in slightly more detail so that you can development the ability to explain why DNA, and sometimes RNA, is the primary source of heritable information. Edwin Chargaff and his team isolated nucleobases from salmon sperm and determined the fraction of each (Chargaff et al., Journal of Biological Chemistry, 192, 1951). Experiments in which the fraction of all four nucleobases was determined are shown. Also shown are averages as two standard deviations and the sum of total fractions for each experiment. Precision is calculated with each average. Shown below are the chemical structures of these four nucleobases. In these structures, the nitrogen that attaches to the 2-deoxyribose, 5-phosphate polymer is indicated as N*. The partial charges of particular atoms are indicated This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 3 \| Biological Macromolecules 145 with δ+ and δ-. Figure 3.45 A. Analyze Chargaff’s data in terms of the partial charges on these molecules to show how molecular interactions affect the function of these molecules in the storage and retrieval of biological information. Figure 3.46 The interactions between nucleobase molecules are strong enough to produce the association of pairs observed in Chargaff’s data. However, these pairs are bonded by much weaker hydrogen bonds, chemical bonds within the molecules. Demonstrating an understanding of the replication of DNA requires the ability to explain how the two polymer strands of the double helix interact and grow. To retrieve information from DNA, the strands must be separated. The proteins that perform that task interact with the polymer without forming new chemical bonds. In their paper (Watson and Crick, Nature, 3, 1953) announcing the structure of the polymer that we consider in this problem, Watson and Crick stated, “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Eschenmoser and Lowenthal (Chemical Society Reviews, 21, 1992) asked why the 5-carbon sugar ribose is used in DNA when the 6-carbon sugar glucose is so common in biological systems. To answer the question, they synthesized polymeric chains with this alternative form of sugar. They discovered that the strength of the interaction between pairs of nucleobases increased in the new material. Paired strands of hexose-based polymers were more stable. The AP Biology Curriculum Framework (College Board, 2012) states, “The double-stranded structure of DNA provides a simple and elegant solution for the transmission of heritable information to the next generation; by using each strand as a template, existing information can be preserved and duplicated with high fidelity within the replication process. However, the process of replication is imperfect….” B. Explain why the weaker interaction observed by Eschenmoser and Lowenthal, and the acknowledgement in the Framework that “replication is imperfect,” support the claim implied by Watson and Crick that DNA is the source of heritable information. 146 Chapter 3 \| Biological Macromolecules This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 147 4 \| CELL STRUCTURE Figure 4.1 (a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope), and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different organisms, yet all share certain characteristics of basic cell structure. (credit a: modification of work by Ed Uthman, MD; credit b: modification of work by Umberto Salvagnin; credit c: modification of work by Anthony D'Onofrio, William H. Fowle, Eric J. Stewart, and Kim Lewis of the Lewis Lab at Northeastern University; scale-bar data from Matt Russell) Chapter Outline 4.1: Studying Cells 4.2: Prokaryotic Cells 4.3: Eukaryotic Cells 4.4: The Endomembrane System and Proteins 4.5: Cytoskeleton 4.6: Connections between Cells and Cellular Activities Introduction Close your eyes and picture a brick wall. What is the basic building block of that wall? A single brick, of course. Like a brick wall, your body is composed of basic building blocks called “cells.” Your body has many kinds of cells, each specialized for a specific purpose. Just as a home is made from a variety of building materials, the human body is constructed from many cell types. For example, epithelial cells protect the surface of the body and cover
the organs and body cavities within. Bone cells help to support and protect the body. Immune system cells fight invading pathogens. Additionally, blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide and other waste. Each of these cell types plays a vital role during the growth, development, and ongoing maintenance of the body. In spite of their enormous variety, however, cells from all organisms—even organisms as diverse as bacteria, onion, and human—share certain fundamental characteristics. In humans, before a cell develops into its specialized type, it is called a stem cell. A stem cell is a cell that has not undergone the changes involved in specialization. In this state, it may differentiate to become one of many different specialized cells, and it may divide to produce more stem cells. Under normal circumstances, once a cell becomes specialized, it remains that way. However, scientists have been working on coaxing stem cells in the laboratory to become a particular specialization. For example, scientists at the Cincinnati Children’s Hospital Medical Center have learned how to use stem cells to grow stomach tissue in plastic cell and tissue culture dishes. This accomplishment will enable researchers to study gastric human diseases, such as stomach cancer. You can read more about it here (http://openstaxcollege.org/l/32cellsize) . 148 Chapter 4 \| Cell Structure 4.1 \| Studying Cells In this section, you will explore the following questions: • What is the role of cells in organisms? • What is the difference between light microscopy and electron microscopy? • What is the cell theory? Connection for AP® Courses A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms. Several cells of one kind that interconnect with each other and perform a shared function form a tissue; several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells. There are many types of cells, all grouped into one of two broad categories: prokaryotic and eukaryotic. For example, both animal and plant cells are classified as eukaryotic cells, whereas bacterial cells are classified as prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or eukaryotic, let’s first examine how biologists study cells. Microscopy Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists use microscopes (micro- = “small”; -scope = “to look at”) to study them. A microscope is an instrument that magnifies an object. Most photographs of cells are taken with a microscope, and these images can also be called micrographs. The optics of a compound microscope’s lenses change the orientation of the image that the user sees. A specimen that is right-side up and facing right on the microscope slide will appear upside-down and facing left when viewed through a microscope, and vice versa. Similarly, if the slide is moved left while looking through the microscope, it will appear to move right, and if moved down, it will seem to move up. This occurs because microscopes use two sets of lenses to magnify the image. Because of the manner by which light travels through the lenses, this system of two lenses produces an inverted image (binocular, or dissecting microscopes, work in a similar manner, but include an additional magnification system that makes the final image appear to be upright). Light Microscopes To give you a sense of cell size, a typical human red blood cell is about eight millionths of a meter or eight micrometers (abbreviated as eight μm) in diameter; the head of a pin of is about two thousandths of a meter (two mm) in diameter. That means about 250 red blood cells could fit on the head of a pin. Most student microscopes are classified as light microscopes (Figure 4.2a). Visible light passes and is bent through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells. Light microscopes commonly used in the undergraduate college laboratory magnify up to approximately 400 times. Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the ability of a microscope to distinguish two adjacent structures as separate: the higher the resolution, the better the clarity and detail of the image. When oil immersion lenses are used for the study of small objects, magnification is usually increased to 1,000 times. In order to gain a better understanding of cellular structure and function, scientists typically use electron microscopes. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 149 Figure 4.2 (a) Most light microscopes used in a college biology lab can magnify cells up to approximately 400 times and have a resolution of about 200 nanometers. (b) Electron microscopes provide a much higher magnification, 100,000x, and a have a resolution of 50 picometers. (credit a: modification of work by "GcG"/Wikimedia Commons; credit b: modification of work by Evan Bench) Electron Microscopes In contrast to light microscopes, electron microscopes (Figure 4.2b) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail (Figure 4.3), it also provides higher resolving power. The method used to prepare the specimen for viewing with an electron microscope kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in a vacuum, so living cells cannot be viewed with an electron microscope. In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes. (a) (b) Figure 4.3 (a) These Salmonella bacteria appear as tiny purple dots when viewed with a light microscope. (b) This scanning electron microscope micrograph shows Salmonella bacteria (in red) invading human cells (yellow). Even though subfigure (b) shows a different Salmonella specimen than subfigure (a), you can still observe the comparative increase in magnification and detail. (credit a: modification of work by CDC/Armed Forces Institute of Pathology, Charles N. Farmer, Rocky Mountain Laboratories; credit b: modification of work by NIAID, NIH; scale-bar data from Matt Russell) 150 Chapter 4 \| Cell Structure For another perspective on cell size, try the HowBig interactive at this site (http://openstaxcollege.org/l/cell_sizes) . Why are electron microscopes crucial for the study of cell biology? a. Only electron microscopes can be used to view internal structures. b. Some electron microscopes allow visualization of three dimensional external shapes at very high magnification in a way that is not possible with standard light microscopes. c. Scanning electron microscopes can show internal structures clearly at very high magnifications. d. Electron microscopes are easier to use and less expensive than light microscopes. Cell Theory The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of single-celled organisms, which he collectively termed “animalcules.” In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells. By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 151 Have you ever heard of a medical test called a Pap smear (Figure 4.4)? In this test, a doctor takes a small sample of cells from the uterine cervix of a patient and sends it to a medical lab where a cytotechnologist stains the cells and examines them for any changes that could indicate abnormal cell growth or a microbial infection. Cytotechnologists (cyto- = “cell”) are professionals who study cells via microscopic examinations and other laboratory tests. They are trained to determine which cellular changes are within normal limits and which are abnormal. Their focus is not limited to cervical cells; they study cellular specimens that come from all organs. When they notice abnormalities, they consult a pathologist, who is a medical doctor who can make a clinical diagnosis. Cytotechnologists play a vital role in saving people’s lives. When abnormalities are discovered early, a patient’s treatment can begi
n sooner, which usually increases the chances of a successful outcome. Figure 4.4 These uterine cervix cells, viewed through a light microscope, were obtained from a Pap smear. Normal cells are on the left. The cells on the right are infected with human papillomavirus (HPV). Notice that the infected cells are larger; also, two of these cells each have two nuclei instead of one, the normal number. (credit: modification of work by Ed Uthman, MD; scale-bar data from Matt Russell) Section Summary A cell is the smallest unit of life. Most cells are so tiny that they cannot be seen with the naked eye. Therefore, scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than light microscopes. The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. 4.2 \| Prokaryotic Cells In this section, you will explore the following questions: • What are the major structures of prokaryotic cells? • What limits the size of a cell? Connection for AP® Courses According to the cell theory, all living organisms, from bacteria to humans, are composed of cells, the smallest units of living matter. Often too small to be seen without a microscope, cells come in all sizes and shapes, and their small size allows for a large surface area-to-volume ratio that enables a more efficient exchange of nutrients and wastes with the environment. There are three basic types of cells: archaea, bacteria, and eukaryotes. Both archaea and bacteria are classified as 152 Chapter 4 \| Cell Structure prokaryotes, whereas cells of animals, plants, fungi, and protists are eukaryotes. Archaea are a unique group of organisms and likely evolved in the harsh conditions of early Earth and are still prevalent today in extreme environments, such as hot springs and polar regions. All cells share features that reflect their evolution from a common ancestor; these features are 1) a plasma membrane that separates the cell from its environment; 2) cytoplasm comprising the jelly-like cytosol inside the cell; 3) ribosomes that are important for the synthesis of proteins, and 4) DNA to store and transmit hereditary information. Prokaryotes may also have a cell wall that acts as an extra layer of protection against the external environment. The term “prokaryote” means “before nucleus,” and prokaryotes do not have nuclei. Rather, their DNA exists as a single circular chromosome in the central part of the cell called the nucleoid. Some bacterial cells also have circular DNA plasmids that often carry genes for resistance to antibiotics (Chapter 17). Other common prokaryotic cell features include flagella and pili. The content presented in this section supports the learning objectives outlined in Big Idea 1 and Big Idea 2 of the AP® Biology Curriculum Framework. The AP® Learning Objectives merge essential knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP® Biology course, along with inquirybased laboratory experiences, instructional activities, and AP® exam questions. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.D The origin of living systems is explained by natural processes. Essential Knowledge 1.D.2 Scientific evidence from many different disciplines supports models of the origin of life. Science Practice Learning Objective 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question. 1.32 The student is able to justify the selection of geological, physical, chemical, and biological data that reveal early Earth conditions. Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 2.6 The student is able to use calculated surface area-to-volume ratios to predict which cell(s) might eliminate wastes or procure nutrients faster by diffusion. Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.7 The student will be able to explain how cell sizes and shapes affect the overall rate of nutrient intake and the rate of waste elimination. Cells fall into one of two broad categories: prokaryotic and eukaryotic. Only the predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro- = “before”; -kary- = “nucleus”). Cells of animals, plants, fungi, and protists are all eukaryotes (eu- = “true”) and have a nucleus. Components of Prokaryotic Cells All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; 2) cytoplasm, consisting of a jelly-like cytosol within the cell in which other cellular components are found; 3) DNA, the genetic material of the cell; and 4) ribosomes, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways. A prokaryote is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid (Figure 4.5). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 153 Figure 4.5 This figure shows the generalized structure of a prokaryotic cell. All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some, but not all, bacteria. While the Earth is approximately 4.6 billion years old, the earliest fossil evidence for life are of microbial mats that date back to 3.5 billion years. What type of evidence for life was most likely found in a 3.5 billion year old rock? a. Scientists found bones buried in the rock that resemble bones of living animals. b. Dead cells buried in the rock superficially resemble living prokaryotic cells. c. The fossil superficially resembles living microbial mats that exist today. d. Scientists found fossilized prokaryotic cells in the rock that are able to grow and divide. Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule (Figure 4.5). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Fimbriae are used by bacteria to attach to a host cell. 154 Chapter 4 \| Cell Structure Microbiologist The most effective action anyone can take to prevent the spread of contagious illnesses is to wash his or her hands. Why? Because microbes (organisms so tiny that they can only be seen with microscopes) are ubiquitous. They live on doorknobs, money, your hands, and many other surfaces. If someone sneezes into his hand and touches a doorknob, and afterwards you touch that same doorknob, the microbes from the sneezer’s mucus are now on your hands. If you touch your hands to your mouth, nose, or eyes, those microbes can enter your body and could make you sick. However, not all microbes (also called microorganisms) cause disease; most are actually beneficial. You have microbes in your gut that make vitamin K. Microbiologists are scientists who study microbes. Microbiologists can pursue a number of careers. Not only do they work in the food industry, they are also employed in the veterinary and medical fields. They can work in the pharmaceutical sector, serving key roles in research and development by identifying new sources of antibiotics that could be used to treat bacterial infections. Environmental microbiologists may look for new ways to use specially selected or genetically engineered microbes for the removal of pollutants from soil or groundwater, as well as hazardous elements from contaminated sites. These uses of microbes are called bioremediation technologies. Microbiologists can also work in the field of bioinformatics, providing specialized knowledge and insight for the design, development, and specificity of computer models of, for example, bacterial epidemics. Cell Size At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm (Figure 4.6). The small size of prokaryotes allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport. Figure 4.6 This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured). Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 155 sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr2, while the formula for its volume is 4
πr3/3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube (see this figure). If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide; another way is to develop organelles that perform specific tasks. These adaptations lead to the development of more sophisticated cells called eukaryotic cells. Besides the volume of the cell, the size of the cell is also important for survival. As mentioned before, most cells are approximately spherical in shape. This is because a sphere is the shape with the largest surface area-to-volume ratio. As nutrients diffuse into the cell, a sphere is the shape where nutrients would have to travel the least distance to reach the center. This is important because nutrients and wastes are always exchanged at the periphery of the cell. The shorter the distance these nutrients and wastes have to travel, the faster the exchange of these molecules are. Figure 4.7 Notice that as a cell increases in size, its surface area-to-volume ratio decreases. When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die. The cell on the left has a volume of 1 mm3 and a surface area of 6 mm2, with a surface area-to-volume ratio of 6 to 1, whereas the cell on the right has a volume of 8 mm3 and a surface area of 24 mm2, with a surface area-to-volume ratio of 3 to 1. On average, prokaryotic cells are smaller than eukaryotic cells. What are some advantages to small cell size? What are some advantages to large cell size? a. Small, prokaryotic cells do not expend energy in intracellular transport of substances. Larger eukaryotic cells have organelles, which enable them to produce complex substances. b. Small, prokaryotic cells easily escape the spontaneous changes in environmental conditions. Large, eukaryotic cells have complex mechanisms to cope with such changes. c. Small, prokaryotic cells divide at a higher rate. Large, eukaryotic cells show division with genetic variations. d. Small, prokaryotic cells have fewer phospholipids in their membrane. Large, eukaryotic cells have more transport proteins in their phospholipid bilayer, supporting efficient transport of molecules. 156 Chapter 4 \| Cell Structure Activity Create an annotated diagram to explain how approximately 300 million alveoli in a human lung increases surface area for gas exchange to the size of a tennis court. Use the diagram to explain how the cellular structures of alveoli, capillaries, and red blood cells allow for rapid diffusion of O2 and CO2 among them. Think About It Which of the following cells would likely exchange nutrients and wastes with its environment more efficiently: a spherical cell with a diameter of 5 μm or a cubed-shaped cell with a side length of 7μm? Provide a quantitative justification for your answer based on surface area-to-volume ratios. 4.3 \| Eukaryotic Cells In this section, you will explore the following questions: • How does the structure of the eukaryotic cell resemble as well as differ from the structure of the prokaryotic cell? • What are structural differences between animal and plant cells? • What are the functions of the major cell structures? Connection for AP® Courses Eukaryotic cells possess many features that prokaryotic cells lack, including a nucleus with a double membrane that encloses DNA. In addition, eukaryotic cells tend to be larger and have a variety of membrane-bound organelles that perform specific, compartmentalized functions. Evidence supports the hypothesis that eukaryotic cells likely evolved from prokaryotic ancestors; for example, mitochondria and chloroplasts feature characteristics of independently-living prokaryotes. Eukaryotic cells come in all shapes, sizes, and types (e.g. animal cells, plant cells, and different types of cells in the body). (Hint: This a rare instance where you should create a list of organelles and their respective functions because later you will focus on how various organelles work together, similar to how your body’s organs work together to keep you healthy.) Like prokaryotes, all eukaryotic cells have a plasma membrane, cytoplasm, ribosomes, and DNA. Many organelles are bound by membranes composed of phospholipid bilayers embedded with proteins to compartmentalize functions such as the storage of hydrolytic enzymes and the synthesis of proteins. The nucleus houses DNA, and the nucleolus within the nucleus is the site of ribosome assembly. Functional ribosomes are found either free in the cytoplasm or attached to the rough endoplasmic reticulum where they perform protein synthesis. The Golgi apparatus receives, modifies, and packages small molecules like lipids and proteins for distribution. Mitochondria and chloroplasts participate in free energy capture and transfer through the processes of cellular respiration and photosynthesis, respectively. Peroxisomes oxidize fatty acids and amino acids, and they are equipped to break down hydrogen peroxide formed from these reactions without letting it into the cytoplasm where it can cause damage. Vesicles and vacuoles store substances, and in plant cells, the central vacuole stores pigments, salts, minerals, nutrients, proteins, and degradation enzymes and helps maintain rigidity. In contrast, animal cells have centrosomes and lysosomes but lack cell walls. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 1, Big Idea 2, and Big Idea 4 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 1 The process of evolution drives the diversity and unity of life. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 157 Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice 7.2 The student can connect concepts in and across domains to generalize or extrapolate in and/or across enduring understandings Learning Objective Big Idea 2 Enduring Understanding 2.B 1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis. Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. Essential Knowledge 2.B.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions. Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.13 The student is able to explain how internal membranes and organelles contribute to cell functions. Essential Knowledge 2.B.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions. Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 2.14 The student is able to use representations and models to describe differences in prokaryotic and eukaryotic cells. Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.2 The structure and function of subcellular components, and their interactions, provide essential cellular processes. Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 1.15] [APLO 2.5][APLO 2.25][APLO 1.16] Have you ever heard the phrase “form follows function?” It’s a philosophy practiced in many industries. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should be built with several elevator banks; a hospital should be built so that its emergency room is easily accessible. Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear 158 Chapter 4 \| Cell Structure as we explore eukaryotic cells (Figure 4.8). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus; 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and 3) several, rod-shaped chromosomes. Because a eukaryotic cell’s nucleus is surro
unded by a membrane, it is often said to have a “true nucleus.” The word “organelle” means “little organ,” and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions. At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 159 (a) (b) Figure 4.8 These figures show the major organelles and other cell components of (a) a typical animal cell and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central vacuole—structures not found in animal cells. Most plant cells do not have lysosomes or centrosomes. If the nucleolus were not able to carry out its function, what other cellular organelles would be affected? a. The structure of endoplasmic reticulum would not form. b. The function of lysosomes would be hindered, as hydrolases are formed by nucleolus. c. The free ribosomes and the rough endoplasmic reticulum, which contains ribosomes, would not form. 160 Chapter 4 \| Cell Structure d. The Golgi apparatus will not be able to sort proteins properly. The Plasma Membrane Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 4.9), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane. Figure 4.9 The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it. The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus); (Figure 4.10). Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet. Figure 4.10 Microvilli, shown here as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only found on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit "micrograph": modification of work by Louisa Howard) The Cytoplasm The cytoplasm is the entire region of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals (Figure 4.8). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 161 simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there, too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm. The Nucleus Typically, the nucleus is the most prominent organelle in a cell (Figure 4.8). The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail (Figure 4.11). Figure 4.11 The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed region of chromatin where ribosome synthesis occurs. The boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus. The Nuclear Envelope The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 4.11). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus. Chromatin and Chromosomes To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins are attached to chromosomes, and they resemble an unwound, jumbled bunch of threads. These unwound protein-chromosome complexes are called chromatin (Figure 4.12); chromatin describes the material that makes up the chromosomes both when condensed and decondensed. 162 Chapter 4 \| Cell Structure Figure 4.12 (a) This image shows various levels of the organization of chromatin (DNA and protein). (b) This image shows paired chromosomes. (credit b: modification of work by NIH; scale-bar data from Matt Russell) (a) (b) The Nucleolus We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm. Ribosomes Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope (Figure 4.8). Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small (Figure 4.13). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins. Figure 4.13 Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins. Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure Mitochondria 163 Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct. In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid. Mitochondria are oval-shaped, double membrane organelles (Figure 4.14) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration. Figure 4.14 This electron microgr
aph shows a mitochondrion as viewed with a transmission electron microscope. This organelle has an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its surface area. The space between the two membranes is called the intermembrane space, and the space inside the inner membrane is called the mitochondrial matrix. ATP synthesis takes place on the inner membrane. (credit: modification of work by Matthew Britton; scale-bar data from Matt Russell) Peroxisomes Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H2O2, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H2O2 into oxygen and water.) Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Vesicles and Vacuoles Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them: The membranes of vesicles can fuse with either the plasma membrane or other membrane systems within the cell. Additionally, some agents such as enzymes within plant vacuoles break down macromolecules. The membrane of a vacuole does not fuse with the membranes of other cellular components. Animal Cells versus Plant Cells At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes, mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells also have centrioles associated with the MTOC: a complex called the centrosome. Animal cells each have a centrosome and lysosomes, whereas most plant cells do not. Plant cells have a cell wall, chloroplasts and other specialized plastids, and a large central vacuole, whereas animal cells do not. The Centrosome The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of centrioles, two structures that lie perpendicular to each other (Figure 4.15). Each centriole is a cylinder of nine triplets of microtubules. 164 Chapter 4 \| Cell Structure Figure 4.15 The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder made up of nine triplets of microtubules. Nontubulin proteins (indicated by the green lines) hold the microtubule triplets together. The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division isn’t clear, because cells that have had the centrosome removed can still divide, and plant cells, which lack centrosomes, are capable of cell division. Lysosomes Animal cells have another set of organelles not found in most plant cells: lysosomes. The lysosomes are the cell’s “garbage disposal.” In plant cells, the digestive processes take place in vacuoles. Enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. These enzymes are active at a much lower pH than that of the cytoplasm. Therefore, the pH within lysosomes is more acidic than the pH of the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, so again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent. The Cell Wall If you examine Figure 4.8b, the diagram of a plant cell, you will see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protistan cells also have cell walls. While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose (Figure 4.16), a polysaccharide made up of glucose units. Have you ever noticed that when you bite into a raw vegetable, like celery, it crunches? That’s because you are tearing the rigid cell walls of the celery cells with your teeth. Figure 4.16 Cellulose is a long chain of β-glucose molecules connected by a 1-4 linkage. The dashed lines at each end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an entire cellulose molecule. Chloroplasts Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major difference between plants and animals; plants (autotrophs) are able to make their own food, like sugars, while animals (heterotrophs) must ingest their food. Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs called thylakoids (Figure 4.17). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane that surrounds the grana is called the stroma. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 165 Figure 4.17 The chloroplast has an outer membrane, an inner membrane, and membrane structures called thylakoids that are stacked into grana. The space inside the thylakoid membranes is called the thylakoid space. The light harvesting reactions take place in the thylakoid membranes, and the synthesis of sugar takes place in the fluid inside the inner membrane, which is called the stroma. Chloroplasts also have their own genome, which is contained on a single circular chromosome. The chloroplasts contain a green pigment called chlorophyll, which captures the light energy that drives the reactions of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform photosynthesis, but their chlorophyll is not relegated to an organelle. Endosymbiosis We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation. Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine. Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just as mitochondria and chloroplasts do. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts. Based on what you know about plant and animals cells, which of the following events are most likely to have occurred? a. A host cell that ingested aerobic bacteria gave rise to modern animals, while ancestor of that cell that also ingested photoautotrophic bacteria that gave rise to modern plants. b. A host cell that gave rise to modern plants ingested photoautotrophic bacteria only, while a host cell that gave rise to modern animals ingested aerobic bacteria only. c. A host cell that gave rise to modern plants ingested both aerobic and photoautotrophic bacteria, while a host cell that gave rise to modern animals ingested photoautotrophic bacteria only. d. A host cell that gave rise to modern plants and animals ingested both aerobic and photoautotrophic bacteria. 166 Chapter 4 \| Cell Structure The Central Vacuole Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 4.8b, you will see that plant cells each have a large central vacuole that occupies most of the area of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of plant cells results in the wilted appearance of the plant. The central vacuole also supports the expansion of the cell. When the central vacuole holds more water, the cell gets larger without having to invest a lot of energy in synthesizing new cytoplasm. Activity • Construct a concept map or Venn diagram to describe the relationships that exist among the three domains of life (Archaea, Bacteria, and Eukarya) based on cellular features. Share your diagram with other stu
dents in the class for review and revision. • Mystery Cell ID. Using a microscope, identify several types of cells, e.g., prokaryote/eukaryote, plant/ animal, based on general features and justify your identification. • Ten-Minute Debate. Working in small teams, create a visual representation to support the claim that eukaryotes evolved from symbiotic relationships among groups of prokaryotes. Think About It • If the nucleolus were not able to carry out its function, what other cellular organelles would be affected? Would a human liver cell that lacked endoplasmic reticulum be able to metabolize toxins? • Antibiotics are medicines that are used to fight bacterial infections. These medicines kill prokaryotic cells without harming human cells. What part(s) of the bacterial cell do antibiotics target and provide reasoning for your answer. Section Summary Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus’s nucleolus is the site of ribosome assembly. Ribosomes are either found in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration; they are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules. Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells. Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm. 4.4 \| The Endomembrane System and Proteins In this section, you will explore the following questions: • What is the relationship between the structure and function of the components of the endomembrane system, especially with regard to the synthesis of proteins? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 167 Connection for AP® Courses In addition to the presence of nuclei, eukaryotic cells are distinguished by an endomembrane system that includes the plasma membrane, nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus. These subcellular components work together to modify, tag, package, and transport proteins and lipids. The rough endoplasmic reticulum (RER) with its attached ribosomes is the site of protein synthesis and modification. The smooth endoplasmic reticulum (SER) synthesizes carbohydrates, lipids including phospholipids and cholesterol, and steroid hormones; engages in the detoxification of medications and poisons; and stores calcium ions. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens. Just like your body uses different organs that work together, cells use these organelles interact to perform specific functions. For example, proteins that are synthesized in the RER then travel to the Golgi apparatus for modification and packaging for either storage or transport. If these proteins are hydrolytic enzymes, they can be stored in lysosomes. Mitochondria produce the energy needed for these processes. This functional flow through several organelles, a process which is dependent on energy produced by yet another organelle, serves as a hallmark illustration of the cell’s complex, interconnected dependence on its organelles. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 2 and Big Idea 4 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 2 Enduring Understanding 2.B Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. Essential Knowledge 2.B.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions. Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.13 The student is able to explain how internal membranes and organelles contribute to cell functions. Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.2 The structure and function of subcellular components, and their interactions, provide essential cellular processes. Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 4.6] The Endoplasmic Reticulum The endomembrane system (endo = “within”) is a group of membranes and organelles (Figure 4.18) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which we’ve already mentioned, and the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. 168 Chapter 4 \| Cell Structure Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include the membranes of either mitochondria or chloroplasts. Figure 4.18 Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum (RER). The RER also sometimes modifies proteins. In this illustration, a (green) integral membrane protein in the ER is modified by attachment of a (purple) carbohydrate. Vesicles with the integral protein bud from the ER and fuse with the cis face of the Golgi apparatus. As the protein passes along the Golgi’s cisternae, it is further modified by the addition of more carbohydrates. After its synthesis is complete, it exits as an integral membrane protein of the vesicles that bud from the Golgi’s trans face. When the vesicle fuses with the cell membrane, the protein becomes an integral portion of that cell membrane. (credit: modification of work by Magnus Manske) If a peripheral membrane protein were synthesized inside the lumen of the ER, would it end up on the inside or outside of the plasma membrane? a. The vesicle travels from the endoplasmic reticulum to get embedded in plasma membrane. b. The vesicle travels from the Golgi to the plasma membrane to release the protein outside. c. The vesicle travels from the endoplasmic reticulum to the plasma membrane, and returns to the Golgi apparatus to get modified. d. The vesicle moves from the endoplasmic reticulum into the cytoplasmic area, remaining there. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 169 The endoplasmic reticulum (ER) (Figure 4.18) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER, respectively. The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope. Rough ER The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope (Figure 4.19). Figure 4.19 This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell. (credit: modification of work by Louisa Howard) Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane (Figure 4.18). Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with cells of the liver
, for example. Smooth ER The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (Figure 4.18). Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions. In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells. 170 Chapter 4 \| Cell Structure You can watch an excellent animation of the endomembrane system here (http://openstaxcollege.org/l/endomembrane) . How do the nucleus and the endomembrane system work together for protein synthesis? a. The endomembrane system processes and ships proteins specified by the nucleus. In the nucleus, DNA is used to make RNA which exits the nucleus and enters the cytoplasm of the cell. The ribosomes on the rough ER use the RNA to create the different types of protein needed by the body. b. The endomembrane system processes and ships proteins specified by the nucleus. From the nucleus, RNA exits and enters the cytoplasm of the cell. The ribosomes on the rough ER use the RNA to create the different types of protein needed by the body. c. The endomembrane system processes and ships proteins specified by the nucleus. In the nucleus, DNA is used to make RNA which exits the nucleus and enters the cytoplasm of the cell. The smooth ER uses the RNA to create the different types of protein needed by the body. d. The endomembrane system processes and ships proteins specified by the nucleus. In the nucleus, DNA is used to make RNA which exits the nucleus and enters the cytoplasm of the cell. The ribosomes on the smooth ER use the RNA to create the different types of protein needed by the body. Cardiologist Heart disease is the leading cause of death in the United States. This is primarily due to our sedentary lifestyle and our high trans-fat diets. Heart failure is just one of many disabling heart conditions. Heart failure does not mean that the heart has stopped working. Rather, it means that the heart can’t pump with sufficient force to transport oxygenated blood to all the vital organs. Left untreated, heart failure can lead to kidney failure and failure of other organs. The wall of the heart is composed of cardiac muscle tissue. Heart failure occurs when the endoplasmic reticula of cardiac muscle cells do not function properly. As a result, an insufficient number of calcium ions are available to trigger a sufficient contractile force. including heart Cardiologists (cardi- = “heart”; -ologist = “one who studies”) are doctors who specialize in treating heart failure via physical diseases, examination, results from an electrocardiogram (ECG, a test that measures the electrical activity of the heart), a chest X-ray to see whether the heart is enlarged, and other tests. If heart failure is diagnosed, the cardiologist will typically prescribe appropriate medications and recommend a reduction in table salt intake and a supervised exercise program. failure. Cardiologists can make a diagnosis of heart The Golgi Apparatus We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to be sorted, packaged, and tagged so that they wind up in the right place. Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes (Figure 4.20). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 171 Figure 4.20 The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower portion of the image. Several vesicles can be seen near the Golgi apparatus. (credit: modification of work by Louisa Howard) The receiving side of the Golgi apparatus is called the cis face. The opposite side is called the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is the addition of short chains of sugar molecules. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations. Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell. In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi. In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell. Geneticist Many diseases arise from genetic mutations that prevent the synthesis of critical proteins. One such disease is Lowe disease (also called oculocerebrorenal syndrome, because it affects the eyes, brain, and kidneys). In Lowe disease, there is a deficiency in an enzyme localized to the Golgi apparatus. Children with Lowe disease are born with cataracts, typically develop kidney disease after the first year of life, and may have impaired mental abilities. Lowe disease is a genetic disease caused by a mutation on the X chromosome. The X chromosome is one of the two human sex chromosomes, as these chromosomes determine a person's sex. Females possess two X chromosomes while males possess one X and one Y chromosome. In females, the genes on only one of the two X chromosomes are expressed. Females who carry the Lowe disease gene on one of their X chromosomes are carriers and do not show symptoms of the disease. However, males only have one X chromosome and the genes on this chromosome are always expressed. Therefore, males will always have Lowe disease if their X chromosome carries the Lowe disease gene. The location of the mutated gene, as well as the locations of many other mutations that cause genetic diseases, has now been identified. Through prenatal testing, a woman can find out if the fetus she is carrying may be afflicted with one of several genetic diseases. Geneticists analyze the results of prenatal genetic tests and may counsel pregnant women on available options. They may also conduct genetic research that leads to new drugs or foods, or perform DNA analyses that are used in forensic investigations. 172 Lysosomes Chapter 4 \| Cell Structure In addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes are considered to be parts of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis or endocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen (Figure 4.21). Figure 4.21 A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium which then fuses with a lysosome within the cell to destroy the pathogen. Other organelles are present in the cell but for simplicity are not shown. Activity Homemade Cell Project. Using inexpensive and common household items, create a model of a specific eukaryotic cell (e.g., neuron, white blood cell, plant root cell, or Paramecium) that demonstrates how at least three organelles work together to perform a specific function. Think About It A certain cell type functions primarily to synthesize proteins for export. What is the most likely route the newly made protein takes through the cell? Justify your prediction. Section Summary The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the ER, and Golgi apparatus, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport proteins and lipids that form the membranes. The RER modifies proteins and synthesizes phospholipids used in cell membranes. The SER synthesizes carbohydrates, lipids, and steroid hormones; engages in the detoxification of medications and poisons; and stores calcium ions. Sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus. Lysosomes are created by the budding of the membranes of the RER and Golgi. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 173 4.5 \| Cytoskeleton In this section, you will explore the following questions: • How do the various components of the cytoskeleton perform their functions? Connection for AP® Courses All cells, from simple bacteria to complex eukaryotes, possess a cytoskeleton composed of different types of protein elements, including microfilaments, intermediate filaments, and microtubules. The cytoskeleton serves a variety of purposes: provides rigidity and shape to the cell, facilitates cell
ular movement, anchors the nucleus and other organelles in place, moves vesicles through the cell, and pulls replicated chromosomes to the poles of a dividing cell. These protein elements are also integral to the movement of centrioles, flagella, and cilia. The information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 1 of the AP Biology Curriculum Framework, as shown in the table below. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry Essential Knowledge 1.B.1 Organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. Learning Objective 1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains or within one domain of life and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. Microfilaments If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that help maintain the shape of the cell, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move. Collectively, this network of protein fibers is known as the cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, intermediate filaments, and microtubules (Figure 4.22). Here, we will examine each. 174 Chapter 4 \| Cell Structure Figure 4.22 Microfilaments thicken the cortex around the inner edge of a cell; like rubber bands, they resist tension. Microtubules are found in the interior of the cell where they maintain cell shape by resisting compressive forces. Intermediate filaments are found throughout the cell and hold organelles in place. Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in cellular movement, have a diameter of about 7 nm, and are made of two intertwined strands of a globular protein called actin (Figure 4.23). For this reason, microfilaments are also known as actin filaments. Figure 4.23 Microfilaments are made of two intertwined strands of actin. Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 175 called myosin. This enables actin to engage in cellular events requiring motion, such as cell division in eukaryotic cells and cytoplasmic streaming, which is the circular movement of the cell cytoplasm in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract. Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection-fighting cells) make good use of this ability. They can move to the site of an infection and phagocytize the pathogen. To see an example of a white blood cell in action, click here (http://openstaxcollege.org/l/chasing_bcteria) and watch a short time-lapse video of the cell capturing two bacteria. It engulfs one and then moves on to the other. The Human Immunodeficiency Virus (HIV) infects and kills white blood cells. Over time, what affect does this have on the body’s immune system? a. The body’s immune system would not be affected by this. b. The body’s immune system would not be able to fight off pathogens like bacteria with fewer white blood cells. This can increase the risk of illness in HIV patients. c. The body’s immune system, in order to recoup this loss, will produce more WBC’s. d. The body’s immune system will fight the pathogens more vigorously in order to compensate for the fewer white blood cells. Intermediate Filaments Intermediate filaments are made of several strands of fibrous proteins that are wound together (Figure 4.24). These elements of the cytoskeleton get their name from the fact that their diameter, 8 to 10 nm, is between those of microfilaments and microtubules. Figure 4.24 Intermediate filaments consist of several intertwined strands of fibrous proteins. Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension, thus maintaining the shape of the cell, and anchor the nucleus and other organelles in place. Figure 4.22 shows how intermediate filaments create a supportive scaffolding inside the cell. The intermediate filaments are the most diverse group of cytoskeletal elements. Several types of fibrous proteins are found in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens your hair, nails, and the epidermis of the skin. Microtubules As their name implies, microtubules are small hollow tubes. The walls of the microtubule are made of polymerized dimers of α-tubulin and β-tubulin, two globular proteins (Figure 4.25). With a diameter of about 25 nm, microtubules are the widest components of the cytoskeleton. They help the cell resist compression, provide a track along which vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like microfilaments, microtubules can disassemble and reform quickly. 176 Chapter 4 \| Cell Structure Figure 4.25 Microtubules are hollow. Their walls consist of 13 polymerized dimers of α-tubulin and β-tubulin (right image). The left image shows the molecular structure of the tube. Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the two perpendicular bodies of the centrosome). In fact, in animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells, flagella and cilia are quite different structurally from their counterparts in prokaryotes, as discussed below. Flagella and Cilia To refresh your memory, flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, many of them extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell (for example, the cilia of cells lining the Fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter and move it toward your nostrils.) Despite their differences in length and number, flagella and cilia share a common structural arrangement of microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center (Figure 4.26). Figure 4.26 This transmission electron micrograph of two flagella shows the 9 + 2 array of microtubules: nine microtubule doublets surround a single microtubule doublet. (credit: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College; scale-bar data from Matt Russell) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 177 Think About It The ribosomes in bacterial cells and in human cells are made up of proteins and ribosomal RNA, suggesting that both kinds of cells share a common ancestor cell type. What are examples of other features of cells that provide evidence for common ancestry? You have now completed a broad survey of the components of prokaryotic and eukaryotic cells. For a summary of cellular components in prokaryotic and eukaryotic cells, see Table 4.1. Components of Prokaryotic and Eukaryotic Cells Cell Component Function Present in Prokaryotes? Plasma membrane Cytoplasm Nucleolus Nucleus Separates cell from external environment; controls passage of organic molecules, ions, water, oxygen, and wastes into and out of cell Provides turgor pressure to plant cells as fluid inside the central vacuole; site of many metabolic reactions; medium in which organelles are found Darkened area within the nucleus where ribosomal subunits are synthesized. Cell organelle that houses DNA and directs synthesis of ribosomes and proteins Ribosomes Protein synthesis Mitochondria ATP production/cellular respiration Peroxisomes Oxidizes and thus breaks down fatty acids and amino acids, and detoxifies poisons Vesicles and vacuoles Storage and transport; digestive function in plant cells Centrosome Lysosomes Unspecified role in cell division in animal cells; source of microtubules in animal cells Digestion of macromolecules; recycling of worn-out organelles Yes Yes No No Yes No No No No No Cell wall Protection, structural support and maintenance of cell shape Yes, primarily peptidoglycan Chloroplasts Photosynthesis Endoplasmic reticulum Table 4.1 Modifies proteins and synthesizes lipids No No Present in Animal Cells? Present in Plant Cells? Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes Yes No No Yes Yes Yes Yes Yes Yes Yes No No Yes, primarily cellulose Yes Yes 178 Chapter 4 \| Cell Structure Components of Prokaryotic and Eukaryotic Cells Cell Component Function Present in Prokaryotes? Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within cell, and enables unicellular organisms to move independently No Yes Present in Animal Cells? Present in Plant Cells? Yes Yes Yes Yes Flagella Cellular locomotion Some Some No, except for some plant sperm cells. Cellular locomotion, movement of particles along extracellular surface of plasma membrane, and filtration Some Some No Cilia Table 4.1 Section Summary The cytoskeleton has three different types of protein elements. From narrowest to widest, they are the microfilaments (actin filaments), intermediate filaments, and microtubules. Microfilaments are often associated with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia. 4.6 \| Connections between Cells and Cellular Activities In this section, you will explore the following questions: • What are the components of the extracellular matrix? • What are the roles of tight junctions, gap junctions, and plasmodesmata in allowing cells to exchange materials with the environment and communicate with other cells? Connection for AP® Courses With the exception of gap junctions between animal cells and plasmodesmata between plant cells that facilitate the exchange of substances, the information presented in Section 4.6\| Connections between Cells and Cellular Activities is not required for AP®. Concepts about cell communication and signaling processes that are required for AP®, including the features of cells that make communication possible, are covered in Chapter 9. You already know that a group of similar cells working together is called a tissue. As you might expect that, if cells are to work together, they must communicate with one another, just as you need to communicate with others when you work on a group project. Let’s take a look at how cells communicate with one another. You already know that a group of similar cells working together is called a tissue. As you might expect, if cells are to work together, they must communicate with each other, just as you need to communicate with others if you work on a group project. Let’s take a look at how cells communicate with each other. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 179 the AP exam. These questions address the following standards: [APLO 4.5][APLO 3.32][APLO 1.16][APLO 3.33][APLO 1.14][APLO 2.7][APLO 4.4] Extracellular Matrix of Animal Cells Most animal cells release materials into the extracellular space. The primary components of these materials are proteins, and the most abundant protein is collagen. Collagen fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Collectively, these materials are called the extracellular matrix (Figure 4.27). Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other. How can this happen? Figure 4.27 The extracellular matrix consists of a network of proteins and carbohydrates. Cells have protein receptors on the extracellular surfaces of their plasma membranes. When a molecule within the matrix binds to the receptor, it changes the molecular structure of the receptor. The receptor, in turn, changes the conformation of the microfilaments positioned just inside the plasma membrane. These conformational changes induce chemical signals inside the cell that reach the nucleus and turn “on” or “off” the transcription of specific sections of DNA, which affects the production of associated proteins, thus changing the activities within the cell. Blood clotting provides an example of the role of the extracellular matrix in cell communication. When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When tissue factor binds with another factor in the extracellular matrix, it causes platelets to adhere to the wall of the damaged blood vessel, stimulates the adjacent smooth muscle cells in the blood vessel to contract (thus constricting the blood vessel), and initiates a series of steps that stimulate the platelets to produce clotting factors. Intercellular Junctions Cells can also communicate with each other via direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata are junctions between plant cells, whereas animal cell contacts include tight junctions, gap junctions, and desmosomes. Plasmodesmata In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell wall that surrounds each cell (Figure 4.8b). How then, can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular tissues (xylem and phloem) primarily. There also exist structural modifications called plasmodesmata (singular = plasmodesma), numerous channels that pass between cell walls of adjacent plant cells, connect their cytoplasm, and enable materials to be transported from cell to cell, and thus throughout the plant (Figure 4.28). 180 Chapter 4 \| Cell Structure Figure 4.28 A plasmodesma is a channel between the cell walls of two adjacent plant cells. Plasmodesmata allow materials to pass from the cytoplasm of one plant cell to the cytoplasm of an adjacent cell. Tight Junctions A tight junction is a watertight seal between two adjacent animal cells (Figure 4.29). The cells are held tightly against each other by proteins (predominantly two proteins called claudins and occludins). Figure 4.29 Tight junctions form watertight connections between adjacent animal cells. Proteins create tight junction adherence. (credit: modification of work by Mariana Ruiz Villareal) This tight adherence prevents materials from leaking between the cells; tight junctions are typically found in epithelial tissues that line internal organs and cavities, and comprise most of the skin. For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space. Desmosomes Also found only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells (Figure 4.30). Short proteins called cadherins in the plasma membrane connect to intermediate filaments to create desmosomes. The cadherins join two adjacent cells together and maintain the cells in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 181 Figure 4.30 A desmosome forms a very strong spot weld between cells. It is created by the linkage of cadherins and intermediate filaments. (credit: modification of work by Mariana Ruiz Villareal) Gap Junctions Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 4.31). Structurally, however, gap junctions and plasmodesmata differ. Figure 4.31 A gap junction is a protein-lined pore that allows water and small molecules to pass between adjacent animal cells. (credit: modification of work by Mariana Ruiz Villareal) Gap junctions develop when a set of six proteins (called connexins) in the plasma membrane arrange themselves in an elongated donut-like configuration called a connexon. When the pores (“doughnut holes”) of connexons in adjacent animal cells align, a channel between the two cells forms. Gap junctions are particularly important in cardiac muscle: The electrical signal for the muscle to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in tandem. 182 Chapter 4 \| Cell Structure To conduct a virtual microscopy lab and review the parts of a cell, work through the steps of this interactive assignment (http://openstaxcollege.org/l/microscopy_lab) . What are two similarities and two differences between plant and animal cells that can be seen under a microscope? a. Plant cells have cell walls which provide structure to the plant and also chloroplasts which allow for photosynthesis. Animal cells do not have either of these structures. Both cells have nuclei, the command center of the cell, and cytoplasm, the gel-like solution that fills the cell. b. Plant cells and animal cells have cell walls as well as nuclei. Plant cells have chloroplasts as well as plasmodesmata which are lacking in animal cells. c. Plant cells have cell walls which provide structure to the plant and also chloroplasts which allow for photosynthesis. Animal cells do not have either of these structures. Animal cells and plant cells both have glyoxysomes as well cytoplasm. d. Plant cells and animal cells both have a rigid plasma membrane as well as cytoplasm which is the gel-like solution that fills the cell. Plant cells have cell walls which provide structure to the plant and also chloroplasts which allow for photosynthesis. Animal
cells do not have either of these structures. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 183 KEY TERMS cell theory see unified cell theory cell wall rigid cell covering made of various molecules that protects the cell, provides structural support, and gives shape to the cell central vacuole large plant cell organelle that regulates the cell’s storage compartment, holds water, and plays a significant role in cell growth as the site of macromolecule degradation centrosome region in animal cells made of two centrioles chlorophyll green pigment that captures the light energy that drives the light reactions of photosynthesis chloroplast plant cell organelle that carries out photosynthesis chromatin protein-DNA complex that serves as the building material of chromosomes chromosome structure within the nucleus that is made up of chromatin that contains DNA, the hereditary material cilium (plural = cilia) short, hair-like structure that extends from the plasma membrane in large numbers and is used to move an entire cell or move substances along the outer surface of the cell cytoplasm entire region between the plasma membrane and the nuclear envelope, consisting of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals cytoskeleton network of protein fibers that collectively maintain the shape of the cell, secure some organelles in specific positions, allow cytoplasm and vesicles to move within the cell, and enable unicellular organisms to move independently cytosol gel-like material of the cytoplasm in which cell structures are suspended desmosome linkage between adjacent epithelial cells that forms when cadherins in the plasma membrane attach to intermediate filaments electron microscope an instrument that magnifies an object using a beam of electrons passed and bent through a lens system to visualize a specimen endomembrane system group of organelles and membranes in eukaryotic cells that work together modifying, packaging, and transporting lipids and proteins endoplasmic reticulum (ER) series of interconnected membranous structures within eukaryotic cells that collectively modify proteins and synthesize lipids eukaryotic cell cell that has a membrane-bound nucleus and several other membrane-bound compartments or sacs extracellular matrix material (primarily collagen, glycoproteins, and proteoglycans) secreted from animal cells that provides mechanical protection and anchoring for the cells in the tissue flagellum (plural = flagella) long, hair-like structure that extends from the plasma membrane and is used to move the cell gap junction channel between two adjacent animal cells that allows ions, nutrients, and low molecular weight substances to pass between cells, enabling the cells to communicate Golgi apparatus eukaryotic organelle made up of a series of stacked membranes that sorts, tags, and packages lipids and proteins for distribution intermediate filament cytoskeletal component, composed of several intertwined strands of fibrous protein, that bears tension, supports cell-cell junctions, and anchors cells to extracellular structures light microscope an instrument that magnifies an object using a beam visible light passed and bent through a lens system to visualize a specimen 184 Chapter 4 \| Cell Structure lysosome organelle in an animal cell that functions as the cell’s digestive component; it breaks down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles microfilament movements narrowest element of the cytoskeleton system; it provides rigidity and shape to the cell and enables cellular microscope an instrument that magnifies an object microtubule widest element of the cytoskeleton system; it helps the cell resist compression, provides a track along which vesicles move through the cell, pulls replicated chromosomes to opposite ends of a dividing cell, and is the structural element of centrioles, flagella, and cilia mitochondria (singular = mitochondrion) cellular organelles responsible for carrying out cellular respiration, resulting in the production of ATP, the cell’s main energy-carrying molecule nuclear envelope double-membrane structure that constitutes the outermost portion of the nucleus nucleoid central part of a prokaryotic cell in which the chromosome is found nucleolus darkly staining body within the nucleus that is responsible for assembling the subunits of the ribosomes nucleoplasm semi-solid fluid inside the nucleus that contains the chromatin and nucleolus nucleus cell organelle that houses the cell’s DNA and directs the synthesis of ribosomes and proteins organelle compartment or sac within a cell peroxisome small, round organelle that contains hydrogen peroxide, oxidizes fatty acids and amino acids, and detoxifies many poisons plasma membrane phospholipid bilayer with embedded (integral) or attached (peripheral) proteins, that separates the internal content of the cell from its surrounding environment plasmodesma (plural = plasmodesmata) channel that passes between the cell walls of adjacent plant cells, connects their cytoplasm, and allows materials to be transported from cell to cell prokaryote unicellular organism that lacks a nucleus or any other membrane-bound organelle ribosome cellular structure that carries out protein synthesis rough endoplasmic reticulum (RER) region of the endoplasmic reticulum that is studded with ribosomes and engages in protein modification and phospholipid synthesis smooth endoplasmic reticulum (SER) region of the endoplasmic reticulum that has few or no ribosomes on its cytoplasmic surface and synthesizes carbohydrates, lipids, and steroid hormones; detoxifies certain chemicals (like pesticides, preservatives, medications, and environmental pollutants), and stores calcium ions tight junction firm seal between two adjacent animal cells created by protein adherence unified cell theory a biological concept that states that all organisms are composed of one or more cells; the cell is the basic unit of life; and new cells arise from existing cells vacuole membrane-bound sac, somewhat larger than a vesicle, which functions in cellular storage and transport vesicle small, membrane-bound sac that functions in cellular storage and transport; its membrane is capable of fusing with the plasma membrane and the membranes of the endoplasmic reticulum and Golgi apparatus CHAPTER SUMMARY 4.1 Studying Cells A cell is the smallest unit of life. Most cells are so tiny that they cannot be seen with the naked eye. Therefore, scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 185 light microscopes. The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. 4.2 Prokaryotic Cells Prokaryotes are single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 μm. As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. 4.3 Eukaryotic Cells Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus (meaning its DNA is surrounded by a membrane), and has other membrane-bound organelles that allow for compartmentalization of functions. The plasma membrane is a phospholipid bilayer embedded with proteins. The nucleus’s nucleolus is the site of ribosome assembly. Ribosomes are either found in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration; they are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids, and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also help break down macromolecules. Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of animal cells. Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall, whose primary component is cellulose, protects the cell, provides structural support, and gives shape to the cell. Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more cytoplasm. 4.4 The Endomembrane System and Proteins The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the ER, and Golgi apparatus, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport proteins and lipids that form the membranes. The RER modifies proteins and synthesizes phospholipids used in cell membranes. The SER synthesizes carbohydrates, lipids, and steroid hormones; engages in the detoxification of medications and poisons; and stores calcium ions. Sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus. Lysosomes are created by the budding of the membranes of the RER and Golgi. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens. 4.5 Cytoskeleton The cytoskeleton has three different types of protein elements. From narrowest to widest, they are the m
icrofilaments (actin filaments), intermediate filaments, and microtubules. Microfilaments are often associated with myosin. They provide rigidity and shape to the cell and facilitate cellular movements. Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles, flagella, and cilia. 4.6 Connections between Cells and Cellular Activities Animal cells communicate via their extracellular matrices and are connected to each other via tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other via plasmodesmata. When protein receptors on the surface of the plasma membrane of an animal cell bind to a substance in the extracellular matrix, a chain of reactions begins that changes activities taking place within the cell. Plasmodesmata are channels between adjacent plant cells, while gap junctions are channels between adjacent animal cells. However, their structures are quite different. A tight junction is a watertight seal between two adjacent cells, while a desmosome acts like a spot weld. 186 Chapter 4 \| Cell Structure REVIEW QUESTIONS 1. When viewing a specimen through a light microscope, what is a method that scientists use to make it easier to see individual components of cells? a. a beam of electrons b. high temperatures c. d. radioactive isotopes special stains 2. What is the basic unit of life? a. cell b. organism c. organ d. tissue a. cell division b. diffusion c. d. flagellar motion ribosomes 8. When bacteria lack fimbriae, what are they less likely to do? a. Adhere to cell surfaces b. c. d. retain the ability to divide swim through bodily fluids synthesize proteins 9. What is a difference between prokaryotic and eukaryotic cells? 3. Which of the following statements is part of the cell theory? a. Both cells have a nucleus but prokaryotic cells lack cytoplasm. a. All living organisms are made of cells. b. Both cells have cytoplasm but prokaryotic cells b. All cells contain DNA that they pass on to daughter cells. c. All cells depend on their surroundings to provide energy. d. All cells have a nucleus. 4. Which of the following could most effectively be visualized with a scanning electron microscope? a. cells swimming in a drop of pond water. b. details of structures inside cells c. a three-dimensional view of the surface of a membrane d. the movement of molecules inside the cell 5. Who was the first to clearly identify and name individual cells? a. Anton van Leeuwenhoek. b. Matthias Schleiden c. Robert Hooke d. Theodore Schwann 6. Which of the following observations contributed to the cell theory? a. Animal and plant cells have nuclei and organelles. b. Non-living material cannot give rise to living organisms. c. Prokaryotic and eukaryotic cells are surrounded by a plasma membrane. d. Viruses replicate. 7. In order to obtain some materials and remove waste, what process is used by prokaryotes? lack a nucleus. c. Both cells have DNA but prokaryotic cells lack a cell membrane. d. Both cells have a cell membrane but prokaryotic cells lack DNA. 10. Eukaryotic cells contain complex organelles that carry out their chemical reactions. Prokaryotes lack many of these complex organelles, although they have a variety of unique structures of their own. However, most prokaryotic cells can exchange nutrients with the outside environment faster than most eukaryotic cells. Why is this so? a. Most prokaryotic cells are smaller, and have a higher surface-to-volume ratio, than eukaryotic cells. b. Most prokaryotic cells are larger, and have a higher surface-to-volume ratio than eukaryotic cells. c. Most prokaryotic cells are smaller, and have a lower surface-to-volume ratio than eukaryotic cells. d. Prokaryotic cells are larger and have a lower surface-to-volume ratio than eukaryotic cells. 11. Which of the following is surrounded by two phospholipid bilayers? a. b. lysosomes ribosomes c. nucleolus d. nucleus 12. Peroxisomes got their name because hydrogen peroxide is ______. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 187 a. a cofactor for the organelles’ enzymes b. incorporated into their membranes c. produced during their oxidation reactions d. used in their detoxification reactions 13. In plant cells, the function of the lysosomes is carried out by what? a. nuclei b. peroxisomes c. ribosomes d. vacuole 14. Which of the following is found both in eukaryotic and prokaryotic cells? a. mitochondrion b. nucleus c. ribosomes d. centrosomes 15. Which of the following structures is not found in prokaryotic cells? a. plasma membrane b. chloroplast c. nucleoid d. ribosome 16. Where would you find DNA, the genetic material, in an animal cell? a. in the centriole b. only in the mitochondria c. in the mitochondria and the nucleus 17. Which of the following is most likely to have the greatest concentration of smooth endoplasmic reticulum (SER)? a. a cell that secretes enzymes b. a cell that destroys pathogens c. a cell that makes steroid hormones d. a cell that engages in photosynthesis 18. Which of the following sequences correctly lists in order the steps involved in the incorporation of a protein within a cell membrane? a. b. c. d. synthesis of the protein on the ribosome; modification in the Golgi apparatus; packaging in the endoplasmic reticulum; modification in the vesicle synthesis of the protein on the lysosome; modification in the Golgi; packaging in the vesicle; distribution in the endoplasmic reticulum synthesis of the protein on the ribosome; modification in the endoplasmic reticulum; tagging in the Golgi; distribution via the vesicle synthesis of the protein on the lysosome; packaging in the vesicle; distribution via the Golgi; modification in the endoplasmic reticulum 19. Which of the following is not a component of the endomembrane system? a. endoplasmic reticulum b. Golgi apparatus c. lysosome d. mitochondrion 20. Which of the following have the ability to disassemble and reform quickly? a. intermediate filaments and microtubules b. microfilaments and intermediate filaments c. microfilaments and microtubules d. only intermediate filaments 21. Which of the following do not play a role in intracellular movement? a. intermediate filaments and microtubules b. microfilaments and intermediate filaments c. microfilaments and microtubules d. only intermediate filaments 22. Which components of the cytoskeleton are responsible for the contraction of muscles? a. intermediate filaments b. microfilaments c. microtubules 23. What type of junctions prevent the movement of chemicals between two adjacent animal cells? a. desmosomes b. gap junctions c. plasmodesmata d. tight junctions 24. Gap junctions are formed by ________. 188 Chapter 4 \| Cell Structure a. gaps in the cell wall of plants b. protein complexes that form channels between cells c. tight, rivet-like regions in the membranes of adjacent cells d. a tight knitting of membranes 25. Some animal cells produce extensive extracellular matrix. You would expect their ribosomes to synthesize large amounts of which of the following proteins? CRITICAL THINKING QUESTIONS 27. Which element of the cell theory has practical applications in health care because it promotes the use of sterilization and disinfection? a. All cells come from pre-existing cells. b. All living organisms are composed of one or more cells. c. A cell is the basic unit of life. d. A nucleus and organelles are found in prokaryotic cells. 28. What are the advantages and disadvantages of light microscopes? What are the advantages and disadvantages of electron microscopes? a. actin b. collagen c. myosin d. tubulin 26. Which of the following molecules are typically found in the extracellular matrix? a. nucleic acids such as DNA b. peptidoglycans c. cellulose d. proteoglycans a. Advantage: In light microscopes, the light beam does not kill the cell. Electron microscopes are helpful in viewing intricate details of a specimen and have high resolution. Disadvantage: Light microscopes have low resolving power. Electron microscopes are costly and require killing the specimen. b. Advantage: Light microscopes have high resolution. Electron microscopes are helpful in viewing surface details of a specimen. Disadvantage: Light microscopes kill the cell. Electron microscopes are costly and low resolution. c. Advantage: Light microscopes have high resolution. Electron microscopes are helpful in viewing surface details of a specimen. Disadvantage: Light microscopes can be used only in the presence of light and are costly. Electron microscopes uses short wavelength of electrons and hence have lower magnification. d. Advantage: Light microscopes have high magnification. Electron microscopes are helpful in viewing surface details of a specimen. Disadvantage: Light microscopes can be used only in the presence of light and have lower resolution. Electron microscopes can be used only for viewing ultra-thin specimens. 29. Mitochondria are observed in plant cells that contain chloroplasts. Why do you find mitochondria in photosynthetic tissue? a. Mitochondria are not needed but are an evolutionary relic. b. Mitochondria and chloroplasts work together to use light energy to make sugars. c. Mitochondria participate in the Calvin cycle/ light independent reactions of photosynthesis. d. Mitochondria are required to break down sugars and other materials for energy. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 189 30. In what situation, or situations, would the use of a light microscope be ideal? Why? a. A light microscope is used to view the details of the surface of a cell as it cannot be viewed in detail by the trans
mission microscope. b. A light microscope allows visualization of small living cells, which have been stained and cannot be viewed by scanning electron microscope. c. A standard light microscope is used to view living organisms with little contrast to distinguish them from the background, which would be harder to see with the electron microscope. d. A light microscope reveals the internal structures of a cell, which cannot be viewed by transmission electron microscopy. 31. The major role of the cell wall in bacteria is protecting the cell against changes in osmotic pressure, pressure caused by different solute concentrations in the environment. Bacterial cells swell, but do not burst, in low solute concentrations. What happens to bacterial cells if a compound that interferes with the synthesis of the cell wall is added to an environment with low solute concentrations? a. Bacterial cells will shrink due to the lack of cell wall material. b. Bacterial cells will shrink in size. c. Bacterial cells may burst due to the influx of water. d. Bacterial cells remain normal; they have alternative pathways to synthesize cell walls. 32. We have discussed the upper limits of cell size; yet, there is a lower limit to cell size. What determines how small a cell can be? a. The cell should be large enough to escape detection. b. The cell should be able to accommodate all the structures and metabolic activities necessary to survival. c. The size of the cell should be large enough to reproduce itself. d. The cell should be large enough to adapt to the changing environmental conditions. 33. Which of these is a possible explanation for the presence of a rigid cell wall in plants? a. Plants remain exposed to changes in temperature and thus require rigid cell walls to protect themselves. b. Plants are subjected to osmotic pressure and a cell wall helps them against bursting or shrinking. c. Plant cells have a rigid cell wall to protect themselves from grazing animals. d. Plant cells have a rigid cell wall to prevent the influx of waste material. 34. Bacteria do not have organelles; yet, the same reactions that take place on the mitochondria inner membrane, the phosphorylation of ADP to ATP, and chloroplasts, photosynthesis, take place in bacteria. Where do these reactions take place? a. These reactions take place in the nucleoid of the bacteria. b. These reactions occur in the cytoplasm present in the bacteria. c. These reactions occur on the plasma membrane of bacteria. d. These reactions take place in the mesosomes. 35. What are the structural and functional similarities and differences between mitochondria and chloroplasts? a. Similarities: double membrane, inter-membrane space, ATP production, contain DNA. Differences: mitochondria have inner folds called cristae, chloroplast contains accessory pigments in thylakoids, which form grana and a stroma. b. Similarities: DNA, inter-membrane space, ATP production, and chlorophyll. Differences: mitochondria have a matrix and inner folds called cristae; chloroplast contains accessory pigments in thylakoids, which form grana and a stroma. c. Similarities: double membrane and ATP production. Differences: mitochondria have inter-membrane space and inner folds called cristae; chloroplast contains accessory pigments in thylakoids, which form grana and a stroma. d. Similarities: double membrane and ATP production. Differences: mitochondria have inter-membrane space, inner folds called cristae, ATP synthase for ATP synthesis, and DNA; chloroplast contains accessory pigments in thylakoids, which, form grana and a stroma. 36. Is the nuclear membrane part of the endomembrane system? Why or why not? 190 Chapter 4 \| Cell Structure a. The nuclear membrane is not a part of the endomembrane system as the endoplasmic reticulum is a separate organelle of the cell. b. The nuclear membrane is considered a part of the endomembrane system as it is continuous with the Golgi body. c. The nuclear membrane is part of the endomembrane system as it is continuous with the rough endoplasmic reticulum. d. The nuclear membrane is not considered a part of the endomembrane system as the nucleus is a separate organelle. 37. What happens to the proteins that are synthesized on free ribosomes in the cytoplasm? Do they go through the Golgi apparatus? a. These proteins move through the Golgi apparatus and enter in the nucleus. b. These proteins go through the Golgi apparatus and remain in the cytosol. c. The proteins do not go through the Golgi apparatus and move into the nucleus for processing. d. The proteins do not go through the Golgi apparatus and remain free in the cytosol. 38. What are the similarities and differences between the structures of centrioles and flagella? a. Centrioles and flagella are made of microtubules but show different arrangements. b. Centrioles are made of microtubules but flagella are made of microfilaments and both show the same arrangement. c. Centrioles and flagella are made of microfilaments. Centrioles have a 9 + 2 arrangement. d. Centrioles are made of microtubules and flagella are made of microfilaments and both have different structures. 39. Inhibitors of microtubule assembly, vinblastine for example, are used for cancer chemotherapy. How does an inhibitor of microtubule assembly affect cancerous cells? a. The inhibitors restrict the separation of chromosomes, thereby stopping cell division. b. The inhibition of microtubules interferes with the synthesis of proteins. c. The inhibitors bind the microtubule to the nuclear membrane, stopping cell division. d. The inhibitor interferes with energy production. 40. How do cilia and flagella differ? a. Cilia are made of microfilaments and flagella of microtubules. b. Cilia are helpful in the process of engulfing food. Flagella are involved in the movement of the organism. c. Cilia are short and found in large numbers on the cell surface whereas flagella are long and fewer in number. d. Cilia are found in prokaryotic cells and flagella in eukaryotic cells. 41. In which human tissues would you find desmosomes? Think of tissues that undergo strong mechanical stress and must be held together with some flexibility. a. bone cells and cartilage cells b. muscle cells and skin cells c. nerve cells and muscle cells d. secretory cells and muscle cells 42. If there is a mutation in the gene for collagen, such as the one involved in Ehlers-Danlos syndrome, and the individual produces defective collagen, how would it affect coagulation? a. The syndrome affects the clotting factors and platelet aggregation. b. The disease leads to hyper-coagulation of blood. c. Coagulation is not affected because collagen is not required for coagulation. d. The disease occurs due to the breakdown of platelets. 43. How does the structure of a plasmodesma differ from that of a gap junction? a. Gap junctions are essential for transportation in animal cells and plasmodesmata are essential for the movement of substances in plant cells. b. Gap junctions are found to provide attachment in animal cells and plasmodesmata are essential for attachment of plant cells. c. Plasmodesmata are essential for communication between animal cells and gap junctions are necessary for attachment of cells in plant cells. d. Plasmodesmata help in transportation and gap junctions help in attachment, in plant cells. TEST PREP FOR AP® COURSES 44. Which of the following organisms appear first in the fossil record? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 191 a. archaea b. fish c. protists d. plants 45. Why is it challenging to study bacterial fossils and determine if the fossils are members of the domain archaea, rather than bacteria? 48. a. Bacteria lack rigid structures, thus do not form fossils. sphere and the cube? a. b. c. d. Bacteria have rigid structures, but their fossil impression is scarce. c. Fossils of bacteria are rarely found because bacteria were not abundant in the past. d. A fossil of bacteria changes overtime due to the presence of new bacteria living on them. 46. Pictured are two cells along with their radius. What does cell B likely have when compared to cell A? a. b. c. d. smaller surface area and larger volume larger surface area and smaller volume smaller surface area-to-volume ratio larger surface area-to-volume ratio 47. Consider the shapes. The diameter of the sphere is equal to 1 mm and the side of the cube is also equal to 1 mm . What is the ratio of the surface to volume ratios for the Which of the following is true regarding the surface-tovolume ratios of the cube and the sphere? a. The sphere will have a higher surface area than the cube. b. The sphere will have a higher volume than the cube. c. The sphere will have a higher surface area-to- volume ratio than the cube. d. Their surface area-to-volume ratios will be equal. e. The sphere will have a lower surface area-to- volume ratio than the cube. 49. What is the major consideration in setting the lower limit of cell size? a. The cell must be large enough to fight the pathogens b. The cell must be large enough to attach to a substrate. c. The lower limit should be small enough, for the cell to move in the fluid efficiently. d. The cell size must be small as to fit all the processes and structures to support life. 50. Which of the following structures has the same general structure in Archaea, Bacteria, and Eukarya, pointing to a common origin? a. centriole b. cytoplasmic membrane c. Golgi apparatus d. nucleus 51. Why does the structure of the cytoplasmic membrane point to a common ancestor? 192 Chapter 4 \| Cell Structure a. The presence of a cytoplasmic membrane in every organism does not point to a common ancestry. b. The similar arrangement of phospholipids and proteins points to common ancestry. c. The lipid nature of the membrane makes it the most primitive trait. d. The similar effect of temperature on the membrane makes it the ancestral trait. 52. Which or
ganelles would be present in high numbers in the leg muscles of a marathon runner? a. centrioles b. chloroplasts c. mitochondria d. peroxisome 53. Macrophages ingest and digest many pathogens. Which organelle plays a major role in the activity of macrophages? a. chloroplast b. lysosome c. nucleus d. peroxisome 54. You are looking at a sample under a light microscope and observe a new type of cell. You come to the conclusion that it is a bacterium and not a eukaryotic cell. What would you observe to come to this conclusion? a. b. c. the cell has a cell wall the cell has a flagellum the cell does not have a nucleus 55. Thiomargarita namibiensis is a large single cell organism, which can reach lengths of 700 µm . The cell is classified as a bacterium. What is the main argument to justify the classification? a. This organism shows simple diffusion for the uptake of nutrients and is thus classified as a bacterium. b. This organism does not show presence of any cell organelles, and thus is classified as a bacterium. c. the existence of these organisms in long chains and pearl appearance d. The organism demonstrates characteristics of gram-negative bacteria, and thus is classified as a bacterium. 56. Radioactive amino acids are fed to a cell in culture for a short amount of time. This is called a pulse. You follow the appearance of radioactive proteins in the cell compartments. In which organelles and in what order does radioactivity appear? a. endoplasmic reticulum - lysosomes - Golgi body - vesicle - extracellular region b. endoplasmic reticulum - vesicles - Golgi body - vesicles - extracellular region c. Golgi Body - vesicles - endoplasmic reticulum - vesicles - extracellular region d. nucleus - endoplasmic reticulum - Golgi body - vesicle - extracellular region 57. With which cellular structure does the extracellular matrix interact? a. cytoskeleton b. nucleus c. smooth endoplasmic reticulum 58. Which structure or structures allow bacteria to move about? a. b. c. fimbriae only flagella only flagella and fimbriae d. plasmid and capsule 59. Cells lining the intestine absorb a lot of nutrients. How did those cells adapt to their function? a. Cells use cilia to move nutrients to their surface. b. Cells grow much larger than adjacent cells to increase intake c. Cells are flat and thin to absorb more nutrients. d. Membrane folds called microvilli increase the surface area. SCIENCE PRACTICE CHALLENGE QUESTIONS 60. Describe structural and functional similarities between mitochondria and chloroplasts that provide evidence of common ancestry. 61. Explain how the structural and functional differences between mitochondria and chloroplasts provide evidence of adaptations among common ancestral organisms. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 4 \| Cell Structure 193 62. Examine the differences and similarities in the structural features of animal and plant cells. Justify the claim that both animals and plants have common ancestors based on your observations. 63. What conserved core processes are common to both animals and plants? Construct an explanation of the differences based on the selective advantages provided in different environments. 64. Louis Sullivan described architectural design as “form follows function.” For example, a window is designed to add light to a space without heat transport. A door is designed to allow access to a space. Windows and doors have different functions and so take different forms. Biological systems are not designed, but selected from random trials by interaction with the environment. Apply Sullivan’s principle to explain the relationship of function and form for each pair of cellular structures below. a. Plasma membrane and endoplasmic reticulum b. Mitochondrion and chloroplast c. Rough endoplasmic reticulum and smooth endoplasmic reticulum d. Flagella and cilia e. Muscle cells and secretory cells 65. Complex multicellular organisms share nutrients and resources, and their cells communicate with each other. A society may encourage cooperation among individuals while discouraging selfish behavior to increase the overall success of the social system, sometimes at the expense of the individual. Scientific questions are testable and often attempt to reveal a mechanism responsible for a phenomenon. Pose three questions that can be used to examine the ways in which a social system regulates itself. Be prepared to share these in small group discussions with your classmates about the similarities between these regulatory strategies and the analogous roles of plasmodesmata and gap junctions in cell communication. 66. Plasmodesmata in vascular plants and gap junctions in animals are examples of specialized features of cells. Mechanisms by which transport occurs between cells evolved independently within several eukaryotic clades. Explain, in terms of cellular cooperation, the selective advantages provided by such structures. 67. Mammalian red blood cells have no nuclei, must originate in other tissue systems, are relatively long-lived, are small with shapes that actively respond to their environment, and are metabolic anaerobes. Other vertebrates have red blood cells that are usually nucleated and are often relatively large, aerobic, self-replicating, and short-lived. To connect these facts to biology, questions need to be asked. The questions that you pose will depend on the path your class is taking through the curriculum. Begin by summarizing what you know: • What are the functions of a eukaryotic cell nucleus? • What is the approximate average size of a human red blood cell? • What is the range of blood vessel diameters in adult humans? • What is the range of red blood cell size in vertebrates? • What is the average lifetime of a human red blood cell? • How can you show how cell production is stimulated using examples from particular systems? • How is cell death controlled? • What biochemical cycles are associated with anaerobic and aerobic respiration, and what are the important differences between these? • What process is involved in the transport of oxygen and carbon dioxide into and out of red blood cells? • What behaviors and dynamic homeostatic processes might be associated with the properties of red blood cells in mammalian and nonmammalian organisms? • What do you know about the evolutionary divergences among vertebrates? Your summary has revealed some similarities and differences among vertebrate erythrocyte and circulatory system structures. Scientific questions are testable. They can be addressed by making observations and measurements and analyzing the resulting data. A. Pose three scientific questions that arise from your summaries of what you know about erythrocytes and capillary size. B. For each question you pose, predict what you believe would be the answer and provide reasoning for your prediction. C. Describe an approach you think can be used to obtain data to test your prediction. D. In the production of mammalian red blood cells, erythrocytes that have not yet matured and are still synthesizing heme proteins are surrounded by a macrophage. Predict the role of the macrophage in the maturation of a red blood cell. 68. Mitochondria have DNA that encode proteins related to the structures and functions of the organelles. The replication appears to occur continuously, however, many questions about control of replication rate and segregation during mitosis are yet unanswered. Many diseases are caused by mitochondrial dysfunction. Mitophagy, as the name suggests, leads to the destruction of mitochondria. Predict whether or not cellular control mechanisms involving the regulation of mitochondrial DNA by the nucleus exist. Make use of what you know about selection and homeostasis as they apply to both the organism and to the organelle. 194 Chapter 4 \| Cell Structure This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 195 5 \| STRUCTURE AND FUNCTION OF PLASMA MEMBRANES Figure 5.1 Despite its seeming hustle and bustle, Grand Central Station functions with a high level of organization: People and objects move from one location to another, they cross or are contained within certain boundaries, and they provide a constant flow as part of larger activity. Analogously, a plasma membrane’s functions involve movement within the cell and across boundaries in the process of intracellular and intercellular activities. (credit: modification of work by Randy Le’Moine) Chapter Outline 5.1: Components and Structure 5.2: Passive Transport 5.3: Active Transport 5.4: Bulk Transport Introduction The plasma membrane, which is also called the cell membrane, has many functions; but, the most basic one is to define the borders and act as gatekeeper for the cell. The plasma membrane is selectively permeable, meaning some molecules can freely enter or leave the cell. Others require help from specialized structures, other molecules, or require energy in order to cross. One example of a molecule that assists other molecules across the plasma membrane is a protein called NPC1. This protein is involved in moving cholesterol and other types of fats across the plasma membrane. Some people have a genetic condition resulting in improperly functioning NPC1. As a result, excessive cholesterol accumulates within cells causing a condition called NPC Disease. Scientists from the Albert Einstein College of Medicine, Harvard Medical School, and the Whitehead Institute for Biomedical Research discovered that the Ebola virus also uses NPC1 to hitch a ride into cells and replicate. The scientists used mice that lacked the NPC1 protein to test this hypothesis. When the scientists tried to infect these mice with Ebola, none of the mice got sick. Then they tried to infect mice with partially functioning NPC1 and found that they got sick, but did not die. In other words, without properly functioning NPC1
, the Ebola virus cannot infect a mouse. If this pattern also 196 Chapter 5 \| Structure and Function of Plasma Membranes exists in humans, it means that anyone with NPC Disease and its subsequent problem with high cholesterol may also be protected from Ebola. The complete research report can be found here (http://openstaxcollege.org/l/32ebolaentry) . 5.1 \| Components and Structure In this section, you will explore the following questions: • How does the fluid mosaic model describe the structure and components of the plasma cell membrane? • How do the molecular components of the membrane provide fluidity? Connection for AP® Courses Like an art mosaic, the plasma membrane consists of several different components. Phospholipids (which we studied in previously) form a bilayer; the hydrophobic, fatty acid tails are in contact with each other and hydrophilic portions of the phospholipids are oriented toward the aqueous internal and external environments. Several types of proteins with different functions stud the membrane. Integral proteins often span the membrane and can transport materials into or out of the cells; these embedded proteins can be hydrophilic or hydrophobic, depending on their placement within the membrane. Peripheral proteins found on the exterior and interior surfaces of membranes can serve as enzymes, structural attachments for fibers of the cytoskeleton, and part of a cell’s recognition sites. These “cell-specific” proteins play a vital role in immune function; enable cells of a certain type (e.g., liver cells) to identify each other when forming a tissue; and allow hormones and other molecules to recognize target cells. These proteins “float” throughout the membrane, constantly in flux. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The learning objectives provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 2 Enduring Understanding 2.B Essential Knowledge Science Practice Science Practice Learning Objective Essential Knowledge Science Practice Science Practice Science Practice Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. 2.B.1 Cell membranes are selectively permeable due to their structure. 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 3.1 The student can pose scientific questions. 2.10 The student is able to use representations and models to pose scientific questions about the properties of cell membranes and selective permeability based on molecular structure. 2.B.1 Cell membranes are selectively permeable due to their structure. 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 7.1 The student can connect phenomena and models across spatial and temporal scales. 7.2 The student can connect concepts in and across domain(s) to generalize and extrapolate in and/or across enduring understandings and/or big ideas. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 197 Learning Objective 2.11 The student is able to construct models that connect the movement of molecules across membrane with membrane structure and function. A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment (see Figure 5.2 for a summary). Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane must be very flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and which later plays a role in the “self” versus “non-self” distinction of the immune response. Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound. Occasionally, receptors are hijacked by viruses that use them to gain entry into cells, and at times, the genes encoding receptors become mutated, causing the process of signal transduction to malfunction with disastrous consequences. Fluid Mosaic Model The existence of the plasma membrane was identified in the 1890s, and its chemical components were identified in 1915. The principal components identified at that time were lipids and proteins. The first widely accepted model of the plasma membrane’s structure was proposed in 1935 by Hugh Davson and James Danielli; it was based on the “railroad track” appearance of the plasma membrane in early electron micrographs. They theorized that the structure of the plasma membrane resembles a sandwich, with protein being analogous to the bread, and lipids being analogous to the filling. In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the core of the plasma membrane consisted of a double, rather than a single, layer. A new model that better explains both the microscopic observations and the function of that plasma membrane was proposed by S.J. Singer and Garth L. Nicolson in 1972. The explanation proposed by Singer and Nicolson is called the fluid mosaic model. The model has evolved somewhat over time, but it still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane. The membrane does look a bit like a sandwich (Figure 5.2). Figure 5.2 The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates 198 Chapter 5 \| Structure and Function of Plasma Membranes attached to some of the lipids and some of the proteins. A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group. Cholesterol, another lipid composed of four fused carbon rings, is found alongside the phospholipids in the core of the membrane. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type, but for a typical human cell, protein accounts for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent of the composition by mass, with the remaining 10 percent of the composition by mass being carbohydrates. However, the concentration of proteins and lipids varies with different cell membranes. For example, myelin, an outgrowth of the membrane of specialized cells that insulates the axons of the peripheral nerves, contains only 18 percent protein and 76 percent lipid. The mitochondrial inner membrane contains 76 percent protein and only 24 percent lipid. The plasma membrane of human red blood cells is 30 percent lipid. Carbohydrates are present only on the exterior surface of the plasma membrane and are attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids. Phospholipids The main fabric of the membrane is composed of amphiphilic, phospholipid molecules. The hydrophilic or “water-loving” areas of these molecules (which look like a collection of balls in an artist’s rendition of the model) (Figure 5.2) are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic, or water-hating molecules, tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The hydrophilic regions of the phospholipids tend to form hydrogen bonds with water and other polar molecules on both the exterior and interior of the cell. Thus, the membrane surfaces that face the interior and exterior of the cell are hydrophilic. In contrast, the interior of the cell membrane is hydrophobic and will not interact with water. Therefore, phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside of the cell. A phospholipid molecule (Figure 5.3) consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement gives the overall molecule an area described as its head (the phosphate-containing
group), which has a polar character or negative charge, and an area called the tail (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot. A molecule with this arrangement of a positively or negatively charged area and an uncharged, or non-polar, area is referred to as amphiphilic or “dual-loving.” Figure 5.3 This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains. This characteristic is vital to the structure of a plasma membrane because, in water, phospholipids tend to become arranged with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they form a lipid This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 199 bilayer—a barrier composed of a double layer of phospholipids that separates the water and other materials on one side of the barrier from the water and other materials on the other side. In fact, phospholipids heated in an aqueous solution tend to spontaneously form small spheres or droplets (called micelles or liposomes), with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside (Figure 5.4). Figure 5.4 In an aqueous solution, phospholipids tend to arrange themselves with their polar heads facing outward and their hydrophobic tails facing inward. (credit: modification of work by Mariana Ruiz Villareal) Proteins Proteins make up the second major component of plasma membranes. Integral proteins (some specialized types are called integrins) are, as their name suggests, integrated completely into the membrane structure, and their hydrophobic membranespanning regions interact with the hydrophobic region of the the phospholipid bilayer (Figure 5.2). Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20–25 amino acids. Some span only part of the membrane—associating with a single layer—while others stretch from one side of the membrane to the other, and are exposed on either side. Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane (Figure 5.5). This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions. This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid. 200 Chapter 5 \| Structure and Function of Plasma Membranes Figure 5.5 Integral membranes proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3). (credit: “Foobar”/Wikimedia Commons) Peripheral proteins are found on the exterior and interior surfaces of membranes, attached either to integral proteins or to phospholipids. Peripheral proteins, along with integral proteins, may serve as enzymes, as structural attachments for the fibers of the cytoskeleton, or as part of the cell’s recognition sites. These are sometimes referred to as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens. Carbohydrates Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids) (Figure 5.2). These carbohydrate chains may consist of 2–60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other. These sites have unique patterns that allow the cell to be recognized, much the way that the facial features unique to each person allow him or her to be recognized. This recognition function is very important to cells, as it allows the immune system to differentiate between body cells (called “self”) and foreign cells or tissues (called “non-self”). Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them. These carbohydrates on the exterior surface of the cell—the carbohydrate components of both glycoproteins and glycolipids—are collectively referred to as the glycocalyx (meaning “sugar coating”). The glycocalyx is highly hydrophilic and attracts large amounts of water to the surface of the cell. This aids in the interaction of the cell with its watery environment and in the cell’s ability to obtain substances dissolved in the water. As discussed above, the glycocalyx is also important for cell identification, self/non-self determination, and embryonic development, and is used in cell-cell attachments to form tissues. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 201 How Viruses Infect Specific Organs Glycoprotein and glycolipid patterns on the surfaces of cells give many viruses an opportunity for infection. HIV and hepatitis viruses infect only specific organs or cells in the human body. HIV is able to penetrate the plasma membranes of a subtype of lymphocytes called T-helper cells, as well as some monocytes and central nervous system cells. The hepatitis virus attacks liver cells. These viruses are able to invade these cells, because the cells have binding sites on their surfaces that are specific to and compatible with certain viruses (Figure 5.6). Other recognition sites on the virus’s surface interact with the human immune system, prompting the body to produce antibodies. Antibodies are made in response to the antigens or proteins associated with invasive pathogens, or in response to foreign cells, such as might occur with an organ transplant. These same sites serve as places for antibodies to attach and either destroy or inhibit the activity of the virus. Unfortunately, these recognition sites on HIV change at a rapid rate because of mutations, making the production of an effective vaccine against the virus very difficult, as the virus evolves and adapts. A person infected with HIV will quickly develop different populations, or variants, of the virus that are distinguished by differences in these recognition sites. This rapid change of surface markers decreases the effectiveness of the person’s immune system in attacking the virus, because the antibodies will not recognize the new variations of the surface patterns. In the case of HIV, the problem is compounded by the fact that the virus specifically infects and destroys cells involved in the immune response, further incapacitating the host. Figure 5.6 HIV binds to the CD4 receptor, a glycoprotein on the surfaces of T cells. (credit: modification of work by NIH, NIAID) Why does the immune system attack a transplanted organ? a. Glycoproteins and glycolipids on the surface of the organ are similar to those found on pathogens. b. Glycoproteins and glycolipids on the surface of the organ are not recognized by the immune system. c. Glycoproteins and glycolipids on the surface of the organ are toxic to the body. d. Glycoproteins and glycolipids on the surface of the organ are similar to those found on immune cells. 202 Chapter 5 \| Structure and Function of Plasma Membranes Membrane Fluidity The mosaic characteristic of the membrane, described in the fluid mosaic model, helps to illustrate its nature. The integral proteins and lipids exist in the membrane as separate but loosely attached molecules. These resemble the separate, multicolored tiles of a mosaic picture, and they float, moving somewhat with respect to one another. The membrane is not like a balloon, however, that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst, and the membrane will flow and self-seal when the needle is extracted. The mosaic characteristics of the membrane explain some but not all of its fluidity. There are two other factors that help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms. There are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. In contrast, unsaturated fatty acids do not contain a maximal number of hydrogen atoms, but they do contain some double bonds between adjacent carbon atoms; a double bond results in a bend in the string of carbons of approximately 30 degrees (Figure 5.3). Thus, if saturated fatty acids, with their straight tails, are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane. If unsaturated fatty acids are compressed, the “kinks” in their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This “elbow room” helps to maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phospholipids would “freeze” or solidify. The relative fluidity of the membrane is particularly important in a cold environment. A cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing.
Many organisms (fish are one example) are capable of adapting to cold environments by changing the proportion of unsaturated fatty acids in their membranes in response to the lowering of the temperature. Visit this site (http://openstaxcollege.org/l/biological_memb) to see animations of the fluidity and mosaic quality of membranes. Explain why glucose cannot pass directly through the cell membrane. a. The plasma membrane is impermeable to polar molecules, so transport proteins are required. b. The plasma membrane is selectively permeable to polar molecules, and a transport protein is required for larger molecules. c. The plasma membrane is permeable to all polar molecules, but a transport protein is required. d. The plasma membrane is selectively permeable to all polar molecules and a transport protein is never required for them. Animals have an additional membrane constituent that assists in maintaining fluidity. Cholesterol, which lies alongside the phospholipids in the membrane, tends to dampen the effects of temperature on the membrane. Thus, this lipid functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. Thus, cholesterol extends, in both directions, the range of temperature in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 203 The Components and Functions of the Plasma Membrane Component Location Phospholipid Cholesterol Integral proteins (for example, integrins) Peripheral proteins Carbohydrates (components of glycoproteins and glycolipids) Table 5.1 Main fabric of the membrane Attached between phospholipids and between the two phospholipid layers Embedded within the phospholipid layer(s). May or may not penetrate through both layers On the inner or outer surface of the phospholipid bilayer; not embedded within the phospholipids Generally attached to proteins on the outside membrane layer Immunologist The variations in peripheral proteins and carbohydrates that affect a cell’s recognition sites are of prime interest in immunology. These changes are taken into consideration in vaccine development. Many infectious diseases, such as smallpox, polio, diphtheria, and tetanus, were conquered by the use of vaccines. Immunologists are the physicians and scientists who research and develop vaccines, as well as treat and study allergies or other immune problems. Some immunologists study and treat autoimmune problems (diseases in which a person’s immune system attacks his or her own cells or tissues, such as lupus) and immunodeficiencies, whether acquired (by a virus, for example) or hereditary (such as severe combined immunodeficiency, or SCID). Immunologists are called in to help treat organ transplantation patients, who must have their immune systems suppressed so that their bodies will not reject a transplanted organ. Some immunologists work to understand natural immunity and the effects of a person’s environment on it. Others work on questions about how the immune system affects the development of certain chronic diseases. To work as an immunologist, a PhD or MD is required. In addition, immunologists undertake at least 2–3 years of training in an accredited program and must pass an examination given by the American Board of Allergy and Immunology. Immunologists must possess knowledge of the functions of the human body as they relate to issues beyond immunization, and knowledge of pharmacology and medical technology, such as medications, therapies, test materials, and surgical procedures. Activity Using appropriate media, construct a model of the plasma membrane and its molecular components. In the next section, you will use the model to demonstrate the movement of different substances across the membrane. Think About It What research questions can be asked about plasma membranes? State three questions relating to plasma membranes along with possible solutions to the questions. 204 Chapter 5 \| Structure and Function of Plasma Membranes 5.2 \| Passive Transport By the end of this section, you will be able to: • Identify and describe the properties of life. Why and how does passive transport occur across membranes? • What is tonicity, and how is it relevant to passive transport? Connection for AP® Courses Preventing dehydration is important for both plants and animals. Water moves across plasma membranes by a specific type of diffusion called osmosis. The concentration gradient of water across a membrane is inversely proportional to the concentration of solutes; that is, water moves through channel proteins called aquaporins from higher water concentration to lower water concentration. Solute concentration outside and inside the cell influences the rate of osmosis. Tonicity describes how the extracellular concentration of solutes can change the volume of a cell by affecting osmosis, often correlating with the osmolarity of the solution, i.e., the total solute concentration of the solution. In a hypotonic situation, because the extracellular fluid has a lower concentration of solutes (lower osmolarity) than the fluid inside the cell, water enters the cell, causing it to swell and possibly burst. The cell walls of plants prevent them from bursting, but animal cells, such as red blood cells, can lyse. When a cell is placed in a hypertonic solution, water leaves the cell because the cell has a higher water potential than the extracellular solution. When the concentrations of solute are equal on both sides of the membrane (isotonic), no net movement of water into or out of the cell occurs. Living organisms have evolved a variety of ways to maintain osmotic balance; for example, marine fish secrete excess salt through the gills to maintain dynamic homeostasis. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Essential Knowledge 2.B.2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes. Science Practice Science Practice 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 3.1 The student can pose scientific questions. Learning Objective 2.11 The student is able to construct models that connect the movement of molecules across membranes with membrane structure and function. Essential Knowledge 2.B.2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes. Science Practice Science Practice Learning Objective 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 3.1 The student can pose scientific questions. 2.12 The student is able to use representations and models to analyze situation or solve problems qualitatively and quantitatively to investigate whether dynamic homeostasis is maintained by the active movement of molecules across membranes. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.25][APLO 2.27][APLO 4.3][APLO 4.17][APLO1.9] [APLO 2.16][APLO 2.17][APLO 2.18] Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable—they This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 205 allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells; they must have a way of obtaining these materials from extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP), to obtain these materials. Red blood cells use some of their energy doing just that. Most cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the interior and exterior of the cell. The most direct forms of membrane transport are passive. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a range of concentrations of a single substance is said to have a concentration gradient. Selective Permeability Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane. In fact, there is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane. On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton. There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix. Carbohydrates, attached to lipi
ds or proteins, are also found on the exterior surface of the plasma membrane. These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes (Figure 5.7). Figure 5.7 The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane. Recall that plasma membranes are amphiphilic: They have hydrophilic and hydrophobic regions. This characteristic helps the movement of some materials through the membrane and hinders the movement of others. Lipid-soluble material with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain easy entry into cells and are readily transported into the body’s tissues and organs. Similarly, molecules of oxygen and carbon dioxide have no charge and so pass through membranes by simple diffusion. Polar substances present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, while small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes, achieved by various transmembrane proteins (channels). Diffusion Diffusion is a passive process of transport. A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. You are familiar with diffusion of substances through the air. For example, think about someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its highest concentration in the bottle; its lowest concentration is at the edges of the room. The ammonia vapor will diffuse, 206 Chapter 5 \| Structure and Function of Plasma Membranes or spread away, from the bottle, and gradually, more and more people will smell the ammonia as it spreads. Materials move within the cell’s cytosol by diffusion, and certain materials move through the plasma membrane by diffusion (Figure 5.8). Diffusion expends no energy. On the contrary, concentration gradients are a form of potential energy, dissipated as the gradient is eliminated. Figure 5.8 Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm). (credit: modification of work by Mariana Ruiz Villareal) Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient, independent of the concentration gradients of other materials. In addition, each substance will diffuse according to that gradient. Within a system, there will be different rates of diffusion of the different substances in the medium. Factors That Affect Diffusion Molecules move constantly in a random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for the diffusion of molecules through whatever medium in which they are localized. A substance will tend to move into any space available to it until it is evenly distributed throughout it. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. This lack of a concentration gradient in which there is no net movement of a substance is known as dynamic equilibrium. While diffusion will go forward in the presence of a concentration gradient of a substance, several factors affect the rate of diffusion. • Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the rate of diffusion becomes. • Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules. • Temperature: Higher temperatures increase the energy and therefore the movement of the molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the molecules, thus decreasing the rate of diffusion. • Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The molecules slow down because they have a more difficult time getting through the denser medium. If the medium is less dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density will inhibit the movement of the materials. An example of this is a person experiencing dehydration. As the body’s cells lose water, the rate of diffusion decreases in the cytoplasm, and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect. Dehydration frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the cells. • Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion. • Surface area and thickness of the plasma membrane: Increased surface area increases the rate of diffusion, whereas a thicker membrane reduces it. • Distance travelled: The greater the distance that a substance must travel, the slower the rate of diffusion. This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 207 center of the cell, respectively. Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes. A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by pressure, causing the substances to filter more rapidly. This occurs in the kidney, where blood pressure forces large amounts of water and accompanying dissolved substances, or solutes, out of the blood and into the renal tubules. The rate of diffusion in this instance is almost totally dependent on pressure. One of the effects of high blood pressure is the appearance of protein in the urine, which is “squeezed through” by the abnormally high pressure. Facilitated transport In facilitated transport, also called facilitated diffusion, materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are polar molecules that are repelled by the hydrophobic parts of the cell membrane. Facilitated transport proteins shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell. The material being transported is first attached to protein or glycoprotein receptors on the exterior surface of the plasma membrane. This allows the material that is needed by the cell to be removed from the extracellular fluid. The substances are then passed to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta pleated sheets that form a pore or channel through the phospholipid bilayer. Others are carrier proteins which bind with the substance and aid its diffusion through the membrane. Channels The integral proteins involved in facilitated transport are collectively referred to as transport proteins, and they function as either channels for the material or carriers. In both cases, they are transmembrane proteins. Channels are specific for the substance that is being transported. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids; they additionally have a hydrophilic channel through their core that provides a hydrated opening through the membrane layers (Figure 5.9). Passage through the channel allows polar compounds to avoid the nonpolar central layer of the plasma membrane that would otherwise slow or prevent their entry into the cell. Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate. Figure 5.9 Facilitated transport moves substances down their concentration gradients. They may cross the plasma membrane with the aid of channel proteins. (credit: modification of work by Mariana Ruiz Villareal) Channel proteins are either open at all times or they are “gated,” which controls the opening of the channel. The attachment of a particular ion to the channel protein may control the opening, or other mechanisms or substances may be involved. In some tissues, sodium and chloride ions pass freely through open channels, whereas in other tissues a gate must be opened to allow passage. An example of this occurs in the kidney, where both forms of channels are found in different parts of the renal tubules. Cells involved in the transmission of electrical impulses, such as nerve and muscle cells, have 208 Chapter 5 \| Structure and Function of Plasma Membranes gated channels for sodium, potassium, and calcium in their membranes. Opening and closing of these channels changes the relative concentrations on opposing sides of the membrane of these io
ns, resulting in the facilitation of electrical transmission along membranes (in the case of nerve cells) or in muscle contraction (in the case of muscle cells). Carrier Proteins Another type of protein embedded in the plasma membrane is a carrier protein. This aptly named protein binds a substance and, in doing so, triggers a change of its own shape, moving the bound molecule from the outside of the cell to its interior (Figure 5.10); depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. This selectivity adds to the overall selectivity of the plasma membrane. The exact mechanism for the change of shape is poorly understood. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough of the material for the cell to function properly. When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum. Increasing the concentration gradient at this point will not result in an increased rate of transport. Figure 5.10 Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane. (credit: modification of work by Mariana Ruiz Villareal) An example of this process occurs in the kidney. Glucose, water, salts, ions, and amino acids needed by the body are filtered in one part of the kidney. This filtrate, which includes glucose, is then reabsorbed in another part of the kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than the proteins can handle, the excess is not transported and it is excreted from the body in the urine. In a diabetic individual, this is described as “spilling glucose into the urine.” A different group of carrier proteins called glucose transport proteins, or GLUTs, are involved in transporting glucose and other hexose sugars through plasma membranes within the body. Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly than do carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second, whereas carrier proteins work at a rate of a thousand to a million molecules per second. Osmosis Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. While diffusion transports material across membranes and within cells, osmosis transports only water across a membrane and the membrane limits the diffusion of solutes in the water. Not surprisingly, the aquaporins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules. Mechanism Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves (Figure 5.11). On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 209 membrane is the same, but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane. Figure 5.11 In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can. To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it. Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. Osmosis proceeds constantly in living systems. The beaker example here occurs in an open system where the volume of fluid can increase and decrease freely. Cells, on the other hand, are composed of proteins and other substances embedded in the aqueous cytoplasm. These substances could be considered solutes for the purposes of predicting osmosis. The cell membrane keeps most of the proteins and other substances within the cell, causing the cell to have a higher osmolarity than pure water. Suppose you perform an experiment where you placed red blood cells in an environment of pure water. What do you suppose would happen to the cells? Because the concentration of solute is higher in the red blood cell than it is in the beaker, water would rush into the red blood cell. What do you think would happen to the red blood cell, given that its cell membrane is made up of a fixed surface area? It is likely that the red blood cell will undergo hemolysis, where they swell up with water and burst. It should be noted, however, that most cells have mechanisms to prevent them from taking on too much water. However, red blood cells lack these controls, making them ideal for osmolarity studies. This is an important consideration for clinicians delivering drugs intravenously. How would the drug have to be formulated, in terms of osmolarity, to prevent red blood cells from undergoing hemolysis? In order to prevent hemolysis of red blood cells in the blood, drugs are typically formulated in an isotonic solution with the blood to maintain osmolarity. Tonicity Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution's tonicity often directly correlates with the osmolarity of the solution. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles. In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity (and more water) to the side with higher osmolarity (and less water). This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient. An important distinction that concerns living systems is that osmolarity measures the number of particles (which may be molecules) in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear, if the second solution contains more dissolved molecules than there are cells. 210 Chapter 5 \| Structure and Function of Plasma Membranes Hypotonic Solutions Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. (In living systems, the point of reference is always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower concentration of solutes, or a lower osmolarity, than the cell cytoplasm.) It also means that the extracellular fluid has a higher concentration of water in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell. Hypertonic Solutions As for a hypertonic solution, the prefix hyper- refers to the extracellular fluid having a higher osmolarity than the cell’s cytoplasm; therefore, the fluid contains less water than the cell does. Because the cell has a relatively higher concentration of water, water will leave the cell. Isotonic Solutions In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances (Figure 5.12). Figure 5.12 Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions (credit: Mariana Ruiz Villareal) For a video illustrating the process of diffusion in solutions, visit
this site (http://openstaxcollege.org/l/dispersion) . Explain the difference between the two beakers (http://openstaxcollege.org/l/dispersion) . a. The lower temperature of left beaker causes yellow dye to diffuse faster than the right beaker. b. The lower temperature in left beaker causes yellow dye to diffuse slower in it than in the right beaker. c. The higher temperature of left beaker causes faster diffusion of yellow dye in the left beaker. d. The higher temperature of right beaker causes slower diffusion of yellow dye in the right beaker. Tonicity in Living Systems In a hypotonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane. There is no net water movement; therefore, there is no change in the size of the cell. In a hypertonic solution, water leaves a cell and the cell shrinks. If either the hypo- or hyper- condition goes to excess, the cell’s functions become compromised, and the cell may be destroyed. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 211 A red blood cell will burst, or lyse, when it swells beyond the plasma membrane’s capability to expand. Remember, the membrane resembles a mosaic, with discrete spaces between the molecules composing it. If the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart. In contrast, when excessive amounts of water leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell. The cell’s ability to function will be compromised and may also result in the death of the cell. Various living things have ways of controlling the effects of osmosis—a mechanism called osmoregulation. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant (Figure 5.13). In nonwoody plants, turgor pressure supports the plant. Conversly, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell. In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt (Figure 5.14). Figure 5.13 The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. (credit: modification of work by Mariana Ruiz Villareal) Figure 5.14 Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it (right). (credit: Victor M. Vicente Selvas) Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment (Figure 5.15). 212 Chapter 5 \| Structure and Function of Plasma Membranes field light microscopy at 480x Figure 5.15 A paramecium’s contractile vacuole, here visualized using bright magnification, continuously pumps water out of the organism’s body to keep it from bursting in a hypotonic medium. (credit: modification of work by NIH; scale-bar data from Matt Russell) Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis. Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water. Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine. In vertebrates, the kidneys regulate the amount of water in the body. Osmoreceptors are specialized cells in the brain that monitor the concentration of solutes in the blood. If the levels of solutes increase beyond a certain range, a hormone is released that retards water loss through the kidney and dilutes the blood to safer levels. Animals also have high concentrations of albumin, which is produced by the liver, in their blood. This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues. Activity Use the model of the plasma cell membrane you constructed to demonstrate how O2 and CO2, H2O, Na+ and K+, and glucose are transported across the membrane. Think About It Why should farmers consider the salinity of the soil in which they grow crops? Answer: Farmers need to consider the salinity of soil, because the movement of water into and out of plant cells depends on the solute concentration of their environment. In soil high in saline, water will be drawn out of root cells causing the cells to shrivel, and the plant to die. 5.3 \| Active Transport By the end of this section, you will be able to: • How do electrochemical gradients affect the active transport of ions and molecules across membranes? Connection for AP® Courses If a substance must move into the cell against its concentration gradient, the cell must use free energy, often provided by ATP, and carrier proteins acting as pumps to move the substance. Substances that move across membranes by this mechanism, a process called active transport, include ions, such as Na+ and K+. The combined gradients that affect movement of an ion are its concentration gradient and its electrical gradient (the difference in charge across the membrane); together these gradients are called the electrochemical gradient. To move substances against an electrochemical gradient This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 213 requires free energy. The sodium-potassium pump, which maintains electrochemical gradients across the membranes of nerve cells in animals, is an example of primary active transport. The formation of H+ gradients by secondary active transport (co-transport) is important in cellular respiration and photosynthesis and moving glucose into cells. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices (SP). Big Idea 2 Enduring Understanding 2.B Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments. Essential Knowledge 2.B.2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes. Science Practice Learning Objective 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 2.12 The student is able to use representations and models to analyze situations or solve problems qualitatively and quantitatively to investigate whether dynamic homeostasis is maintained by the active movement of molecules across membranes. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.10][APLO 2.17][APLO 1.2][APLO 3.24] Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules. Electrochemical Gradient We have discussed simple concentration gradients—differential concentrations of a substance across a space or a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than does the extracellular fluid. So in a living cell, the concentration gradient of Na+ tends to drive it into the cell, and the electrical gradient of Na+ (a positive ion) also tends to drive it inward to the negatively charged interior. The situation is more complex, however, for other
elements such as potassium. The electrical gradient of K+, a positive ion, also tends to drive it into the cell, but the concentration gradient of K+ tends to drive K+ out of the cell (Figure 5.16). The combined gradient of concentration and electrical charge that affects an ion is called its electrochemical gradient. 214 Chapter 5 \| Structure and Function of Plasma Membranes Figure 5.16 Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. Structures labeled A represent proteins. (credit: “Synaptitude”/Wikimedia Commons) If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell to increase or decrease? a. Transport of amino acids into the cell increases b. Transport of amino acids into the cell stops. c. Transport of amino acids into the cell is not affected by pH. d. Transport of amino acid into the cell decreases. Moving Against a Gradient To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy is harvested from ATP generated through the cell’s metabolism. Active transport mechanisms, collectively called pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements. Much of a cell’s supply of metabolic energy may be spent maintaining these processes. (Most of a red blood cell’s metabolic energy is used to maintain the imbalance between exterior and interior sodium and potassium levels required by the cell.) Because active transport mechanisms depend on a cell’s metabolism for energy, they are sensitive to many metabolic poisons that interfere with the supply of ATP. Two mechanisms exist for the transport of small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport describes the movement of material that is due to the electrochemical gradient established by primary active transport that does not directly require ATP. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 215 Carrier Proteins for Active Transport An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three types of these proteins or transporters (Figure 5.17). A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An antiporter also carries two different ions or molecules, but in different directions. All of these transporters can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also found in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na+-K+ ATPase, which carries sodium and potassium ions, and H+-K+ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps. Figure 5.17 A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the same direction. An antiporter also carries two different molecules or ions, but in different directions. (credit: modification of work by “Lupask”/Wikimedia Commons) 216 Chapter 5 \| Structure and Function of Plasma Membranes The primary active transport that functions with the active transport of sodium and potassium allows secondary active transport to occur. The second transport method is still considered active because it depends on the use of energy as does primary transport (illustrative example). Figure 5.18 Primary active transport moves ions across a membrane, creating an electrochemical gradient (electrogenic transport). (credit: modification of work by Mariana Ruiz Villareal) One of the most important pumps in animal cells is the sodium-potassium pump (Na+-K+ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells. The sodium-potassium pump moves K+ into the cell while moving Na+ out at the same time, at a ratio of three Na+ for every two K+ ions moved in. The Na+-K+ ATPase exists in two forms, depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions. The process consists of the following six steps: 1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein. 2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it. 3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier. 4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier. 5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell. 6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again. Several things have happened as a result of this process. At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. For every three ions of sodium that move out, two ions of potassium move in. This results in the interior being slightly more negative relative to the exterior. This difference in charge is important to creating the conditions necessary for the secondary process. Therefore, the sodium-potassium pump is an electrogenic pump (a pump that creates a charge imbalance) contributing to the membrane potential. What will happen to the opening of the sodium-potassium pump if no ATP is present in a cell? a. b. c. It will remain facing the extracellular space, with sodium ions bound. It will remain facing the extracellular space, with potassium ions bound. It will remain facing the cytoplasm, but no sodium ions would bind. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 5 \| Structure and Function of Plasma Membranes 217 d. It will remain facing the cytoplasm, with sodium ions bound. Visit the site (http://openstaxcollege.org/l/Na_K_ATPase) to see a simulation of active transport in a sodium-potassium ATPase. Sodium and potassium are necessary electrolytes. As a result, the human body uses a great deal of energy keeping these electrolytes in balance. Explain why the body needs to use energy for this process. a. ATP is required to move sodium ions against their concentration gradient outside the cell. b. ATP is required to allow entry of potassium ions inside the cell. c. ATP is required to allow entry of sodium ions inside the cell. d. ATP is required to release potassium ions outside the cell. Activity Create a representation/diagram (or use the model you constructed of the plasma cell membrane) to explain how the sodium-potassium pump contributes to the net negative change of the interior of an animal nerve cell. Think About It If the pH outside the cell decreases, would you expect the amount of amino acids and glucose transported into the cell to increase or decrease? Justify your reasoning. Secondary Active Transport (Co-transport) Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created. If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane (Figure 5.19). Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP. The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP. 218 Chapter 5 \| Structure and Function of Plasma Membranes Figure 5.19 An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport. (credit: modification of work by Mariana Ruiz Villareal) Injection of a potassium solution into a person’s blood is lethal. Potassium is used in capital punishment and euthanasia. Why do you think a potassium solution injection is lethal? a. Excess potassium disrupts the membrane components b. Excess potassium increases action potential generation, leading to uncoordinated organ activity. c. Potassium dissipates the electrochemical gradient in cardiac muscle cells, preventing them from contracting. d. Potassium creates a new concentration gradient across the cell membrane, preventing sodium from leaving the cell. 5.4 \| Bulk Transport By the end of this section, you will be able to: • What are the differences among the different types of endocytosis: (phagocytosis, pinocytosis, and receptor- mediated endocytosis) and exocytosis? Connection for AP® Courses Diffusion, osmosis, an

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