Chapter 6 · Life Processes

NCERT Exemplar Class 10 Science Chapter 6 Short Answer Questions

All 37 questions of NCERT Exemplar Class 10 Science Chapter 6 Short Answer Questions (Life Processes), in the book's order, each with its answer and a step-by-step solution.

1. Q36
   Name the following

   1. (a)The process in plants that links light energy ​with chemical energy
   2. (b)Organisms that can prepare their own ​food
   3. (c)The cell organelle where ​photosynthesis occurs
   4. (d)Cells that surround a stomatal pore
   5. (e)Organisms that cannot prepare their ​own ​food
   6. (f)An enzyme secreted ​from gastric glands in stomach that ​acts on proteins.
   Show answer

   The answers are photosynthesis, autotrophs, chloroplast, guard cells, heterotrophs and pepsin.

   (a) Photosynthesis; (b) Autotrophs; (c) Chloroplast; (d) Guard cells; (e) Heterotrophs; (f) Pepsin.

   Step-by-step solution →
2. Q37
   “All plants give out oxygen during ​day ​and carbon ​dioxide ​during night”. Do ​you agree with ​this statement? Give ​reason.
   Show answer

   No; respiration goes on day and night, so the statement holds only for the net exchange of a green plant in good light.

   No, not as it stands. Respiration goes on in a plant day and night, using oxygen and releasing carbon dioxide. During the day photosynthesis also takes place, and in good light it is much faster than respiration, so all the carbon dioxide of respiration is used up inside the leaf and the surplus oxygen diffuses out. At night photosynthesis stops while respiration continues, so carbon dioxide is released. The statement therefore describes only the net exchange of a green plant in good light, not the processes themselves, and it fails altogether for non-green plants such as Cuscuta and for green plants in very dim light, where photosynthesis is no faster than respiration.

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3. Q38
   How ​do the ​guard cells regulate ​opening and ​closing ​of ​stomatal pores?
   Show answer

   Guard cells take in water and become turgid, bowing apart to open the pore; they lose water and become flaccid, closing it.

   Each stomatal pore is bounded by a pair of bean-shaped guard cells whose inner wall, facing the pore, is thick and inelastic while the outer wall is thin and elastic. When the pore is to open, the guard cells take up solutes such as potassium ions, so water enters them from the neighbouring cells by osmosis and they become turgid. Turgor stretches the thin outer wall much more than the thick inner one, so each guard cell bows outward and the pore between the two widens. When the solutes leave, water follows them out, the guard cells become flaccid, the walls straighten, the two cells come together and the pore closes. In this way the guard cells open the pore for the intake of carbon dioxide and close it to check the loss of water vapour.

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4. Q39
   Two green ​plants are ​kept separately in ​oxygen free containers, one in ​the dark ​and the other ​in continuous light. ​Which one will live ​longer? Give reasons.
   Show answer

   The plant in continuous light lives longer because photosynthesis supplies food and oxygen; the dark plant must respire anaerobically and dies sooner.

   The plant kept in continuous light will live longer. Being in the light it can photosynthesise, so it makes its own food and, more importantly here, releases oxygen inside the sealed container; that oxygen is then used for its own aerobic respiration, and the carbon dioxide respiration returns is used again in photosynthesis. The plant kept in the dark cannot photosynthesise, so it gets neither food nor oxygen. It is forced to respire anaerobically, which yields very little energy per glucose molecule and leaves toxic ethanol behind, so its reserves are exhausted quickly and it dies much sooner.

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5. Q40
   If ​a plant ​is releasing ​carbon dioxide and taking in oxygen ​during the day, does it mean ​that there is no photosynthesis occurring? ​Justify your ​answer.
   Show answer

   No; both processes run together, and the net exchange of oxygen in and carbon dioxide out only shows that respiration is faster than photosynthesis.

   No, it does not. Respiration and photosynthesis go on together in a green plant during the day, and the gases exchanged with the air are only the difference between the two. Taking in oxygen and giving out carbon dioxide therefore means that the rate of respiration is greater than the rate of photosynthesis at that moment, not that photosynthesis has stopped. This happens when photosynthesis is slowed, for example in very dim light at dawn, dusk or on a heavily overcast day, when the stomata are closed because the plant is short of water so that carbon dioxide cannot enter, or when the leaves are old, dusty or damaged. Photosynthesis would be truly absent only in the dark or in a plant with no chlorophyll, such as Cuscuta.

   Step-by-step solution →
6. Q41
   Why ​do fishes ​die ​when taken ​out ​of water?
   Show answer

   Out of water the gill filaments collapse and dry, so the fish cannot absorb oxygen and dies of asphyxiation.

   A fish breathes by gills, which absorb the oxygen dissolved in water. The gills are made of very fine filaments that are held apart and kept moist by the water flowing over them. When the fish is taken out of water, the filaments are no longer supported, so they collapse and stick together under their own weight and their surface dries. The large, thin, moist surface that gaseous exchange needs is therefore lost, and the fish cannot absorb oxygen even though air is much richer in oxygen than water is. Respiration in its tissues stops for want of oxygen and the fish dies of asphyxiation.

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7. Q42
   Differentiate ​between ​an autotroph ​and ​a heterotroph.
   Show answer

   An autotroph makes its own food from carbon dioxide and water; a heterotroph depends on other organisms for ready-made food.

   An autotroph prepares its own food from simple inorganic substances, carbon dioxide and water, using an external source of energy, sunlight trapped by the chlorophyll it contains; the food it makes is already simple, so it needs no digestion, and it occupies the first trophic level of a food chain as a producer. Examples are green plants and blue-green algae. A heterotroph cannot prepare its own food and depends on other organisms for ready-made organic food; it has no chlorophyll and gets its energy from the chemical bonds of that food, which must first be digested into simple absorbable molecules, and it occupies a later trophic level as a consumer or a decomposer. Examples are human beings, a deer, fungi and the parasitic plant Cuscuta.

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8. Q43
   Is ​‘nutrition’ ​a necessity ​for ​an organism? Discuss.
   Show answer

   Yes, nutrition is necessary because food is the only source of energy and of materials for growth and repair.

   Yes, nutrition is a necessity for every organism. Nutrition is the taking in of food and its use, and food is the only source of two things an organism cannot do without. First, it is the source of energy: respiration releases energy from food, and that energy pays for every other life process, transport, excretion, movement, growth and reproduction included, so without food there is nothing to respire. Second, it is the source of materials: growth, the replacement of worn-out cells and the repair of injuries all need proteins, carbohydrates and fats as raw material. In addition, vitamins and minerals, which supply almost no energy, are needed in small amounts for enzymes and hormones to work and for resistance to disease, which is why their absence causes deficiency diseases. An organism deprived of nutrition survives only as long as its stored food lasts and then dies.

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9. Q44
   What ​would ​happen ​if green plants ​disappear ​from earth?
   Show answer

   Life would end: food chains would collapse, oxygen would fall, carbon dioxide would rise and the earth would become hotter.

   Life on earth would come to an end. Green plants are the producers, the point at which energy from the sun and oxygen enter the living world. With them gone, every food chain would collapse from its base: herbivores would starve first, then the carnivores that feed on them, and finally every heterotroph including human beings, since the stored food already on earth would not be replaced. At the same time respiration, decay and the burning of fuels would go on removing oxygen from the air and adding carbon dioxide, with nothing to reverse them, so the oxygen of the atmosphere would fall steadily and the carbon dioxide would rise. The rising carbon dioxide would strengthen the greenhouse effect and the earth would become hotter, the carbon and other nutrient cycles would break down, transpiration and hence rainfall would fall, and the soil, no longer held by roots, would be eroded.

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10. Q45
    Leaves of a ​healthy potted ​plant ​were coated ​with ​vaseline. Will ​this plant remain healthy for long? ​Give reasons for your ​answer.
    Show answer

    No; vaseline blocks the stomata, stopping photosynthesis, gaseous exchange and transpiration, so the plant will wilt and die.

    No, the plant will not remain healthy for long. Vaseline is an oily film that seals the stomata, the pores through which a leaf exchanges gases and loses water vapour. Carbon dioxide can no longer enter, so photosynthesis slows and stops and the plant makes no food; oxygen for respiration cannot enter and the carbon dioxide produced cannot leave, so gaseous exchange in the leaf is hampered as well. Transpiration also stops, and with it the transpiration pull that draws water and dissolved minerals up the xylem from the roots, so the ascent of sap and the mineral supply fail; the evaporative cooling that transpiration gave the leaf is lost too, so the leaves overheat in the sun. Deprived of food, of gaseous exchange and of its water and mineral supply, the plant will yellow, wilt and die. The effect is greatest when the lower surface is coated, since most stomata are there.

    Step-by-step solution →
11. Q46
    How does ​aerobic respiration ​differ ​from anaerobic ​respiration?
    Show answer

    Aerobic respiration uses oxygen and fully breaks down glucose in the mitochondria, releasing much energy; anaerobic respiration needs no oxygen and releases little.

    Aerobic respiration takes place in the presence of oxygen and is completed in the mitochondria, where glucose is completely oxidised to carbon dioxide and water, releasing a large amount of energy, about thirty molecules of ATP per molecule of glucose; it is the usual mode of respiration in most organisms. Anaerobic respiration takes place in the absence of oxygen and is completed in the cytoplasm, where glucose is only partly broken down, to ethanol and carbon dioxide in yeast or to lactic acid in our muscle cells, and it releases only a small amount of energy, two molecules of ATP per molecule of glucose; it occurs in yeast and many bacteria and temporarily in our muscles during vigorous exercise. Both begin alike, with glucose broken down to pyruvate in the cytoplasm, and they differ only in what becomes of that pyruvate.

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12. Q47
    Match the ​words of ​Column (A) ​with that of ​Column ​(B)
    Show answer

    (a)-(ii), (b)-(i), (c)-(iv), (d)-(iii). That is, phloem carries out the translocation of food, the nephron carries out excretion, veins carry deoxygenated blood, and platelets bring about the clotting of blood.

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13. Q48
    Differentiate ​between an artery and ​a ​vein.
    Show answer

    Arteries carry blood away from the heart under high pressure and have thick walls; veins bring it back, with thin walls and valves.

    An artery carries blood away from the heart to the organs, while a vein collects blood from the organs and carries it back to the heart; this is the difference that defines them. An artery usually carries oxygenated blood and a vein deoxygenated blood, but the pulmonary artery and the pulmonary vein are exceptions. Because blood enters an artery straight from the contracting ventricle at high pressure, an artery has a thick, muscular and elastic wall and a narrow lumen, and the blood in it flows rapidly and in spurts so that a pulse can be felt; a vein has a thin, much less muscular wall and a wide lumen, and its blood flows slowly and smoothly at low pressure with no pulse. Arteries have no valves along their length, only the semilunar valves where they leave the heart, whereas veins have valves at intervals that prevent the blood from flowing backwards. Arteries are mostly deep-seated in the body, while many veins lie close to the surface.

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14. Q49
    What are ​the adaptations ​of ​leaf for ​photosynthesis?
    Show answer

    A broad, thin lamina, chloroplast-rich palisade cells, stomata with air spaces and a network of veins suit the leaf for photosynthesis.

    A leaf is adapted for photosynthesis in several ways. Its lamina is broad and flat and the leaves are arranged so as not to shade one another, giving the largest possible area for absorbing light and exchanging gases. The lamina is very thin, so that light reaches every layer and carbon dioxide has only a short distance to diffuse. The cuticle and epidermis are thin and transparent, so light passes through them to the palisade mesophyll just beneath, whose closely packed cells carry the greatest number of chloroplasts and so lie where the light is strongest. The lower epidermis has numerous stomata with guard cells that regulate them, through which carbon dioxide enters and oxygen leaves, and behind them the spongy mesophyll is loosely packed with large intercellular air spaces and moist cell walls, so that carbon dioxide reaches and dissolves at every cell. A dense network of veins brings water and minerals to the mesophyll through the xylem and carries the food made away through the phloem, and also keeps the lamina spread out.

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15. Q50
    Why is ​small ​intestine in ​herbivores ​longer than ​in carnivores?
    Show answer

    Plant cellulose is hard to digest and needs time and surface, but flesh digests quickly, so herbivores need a longer small intestine.

    Because of the food each animal eats. A herbivore lives on grass and leaves, whose cells are enclosed in walls of cellulose, and cellulose is very difficult to digest; no mammal makes cellulase of its own, so herbivores depend on microorganisms in the gut that ferment it slowly. A slow digestion needs the food to be held for a long time and needs a large inner surface to absorb the sugars released, and a long small intestine provides both. A carnivore eats flesh, which is protein and fat and is digested rapidly and almost completely by the animal's own pepsin, trypsin and lipase, so a long intestine would gain it nothing; it would also mean flesh standing and putrefying in a warm gut and extra weight to carry while hunting. The small intestine of a carnivore is therefore much shorter than that of a herbivore of similar size.

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16. Q51
    What will happen if mucus is ​not secreted ​by ​the gastric ​glands?
    Show answer

    The acid and pepsin would attack the stomach lining, causing gastritis and then peptic ulcers.

    The stomach would begin to digest its own wall. The gastric glands secrete hydrochloric acid and the protein-digesting enzyme pepsin along with the mucus, and the lining of the stomach is itself living tissue made largely of protein. Normally the mucus, with the bicarbonate secreted into it, spreads as a slimy layer over the lining and keeps the acid and the enzyme away from the living cells, besides lubricating the food. If it were not secreted, the acid would corrode the lining and the pepsin would digest its protein. The wall would become inflamed, that is gastritis, and with continued exposure the lining would be eroded into open sores, that is peptic ulcers, which are painful and may bleed and in severe cases perforate the wall.

    Step-by-step solution →
17. Q52
    What is ​the significance ​of ​emulsification ​of ​fats?
    Show answer

    Bile salts break fat globules into tiny droplets, greatly increasing the surface exposed to lipase and so speeding up fat digestion.

    Fats are insoluble in water and reach the small intestine as large globules, while lipase, the enzyme that digests them, is water soluble and can act only at the surface where water meets fat. Bile salts from the liver break these large globules into a great many tiny droplets and keep them dispersed in the watery contents; this is emulsification, and it is a purely physical change, since bile contains no enzyme. Its significance is that it enormously increases the surface of fat exposed to the enzyme, so that lipase can act on far more fat at once and the rate of fat digestion rises greatly. Dividing one globule into a thousand equal droplets, for instance, makes the surface ten times as large without changing the amount of fat. The emulsion also keeps the fat from running together again, and the bile, being alkaline, neutralises the acid food from the stomach and provides the alkaline medium the pancreatic enzymes need. Lipase then converts the fats into fatty acids and glycerol, which the villi can absorb.

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18. Q53
    What ​causes ​movement of ​food ​inside ​the alimentary ​canal?
    Show answer

    Peristalsis, a wave of contraction in the involuntary smooth muscle of the canal wall, pushes food along.

    The movement of food inside the alimentary canal is caused by peristalsis. The wall of the canal contains two layers of involuntary smooth muscle, an outer one whose fibres run lengthwise and an inner one whose fibres run round the tube. Behind the mass of food the circular muscle contracts and narrows the tube, while ahead of it the circular muscle relaxes and the longitudinal muscle contracts so that the tube widens; this point of constriction travels steadily along the canal as a wave and pushes the food ahead of it, helped by the mucus that lubricates the lining. Peristalsis begins in the oesophagus the moment food is swallowed and continues all along the canal to the anus, and being involuntary it goes on without our control.

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19. Q54
    Why ​does ​absorption of digested ​food occur ​mainly in ​the small ​intestine?
    Show answer

    The small intestine completes digestion, and its villi, thin walls, blood supply and length ensure thorough absorption.

    Because the small intestine is the only part of the canal where all the conditions for absorption are met at once. Digestion is completed there, bile, pancreatic juice and intestinal juice together breaking carbohydrates down to glucose, proteins to amino acids and fats to fatty acids and glycerol, and only molecules as small as these can be absorbed. Its inner lining is thrown into folds and bears millions of finger-like villi, each covered with microvilli, so the absorbing surface is enormous. The wall of a villus is only one cell thick, and inside each villus is a dense network of blood capillaries together with a lacteal, so that glucose and amino acids pass into the blood and fatty acids and glycerol into the lymph and are carried away at once, keeping the concentration gradient steep. Finally, the small intestine is the longest part of the canal and the food is moved through it slowly, so it stays in contact with this surface long enough for absorption to be practically complete.

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20. Q55
    Match Group (A) with Group (B)
    Show answer

    (a)-(iv), (b)-(iii), (c)-(i), (d)-(ii). That is, autotrophic nutrition is shown by the green plant, heterotrophic nutrition by the deer, parasitic nutrition by the leech, and digestion in food vacuoles by Paramecium.

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21. Q56
    Why is the rate ​of breathing ​in aquatic ​organisms ​much faster ​than in terrestrial ​organisms?
    Show answer

    Water holds far less oxygen than air, so aquatic organisms must pass much more water over their gills, breathing faster.

    Because the medium they breathe holds far less oxygen. A litre of air contains about $210$ mL of oxygen, while a litre of water holds only about $5$ to $8$ mL of dissolved oxygen, roughly thirty times less; oxygen also diffuses much more slowly through water than through air. Since the oxygen collected is the volume of medium passed over the respiratory surface multiplied by its oxygen content, an aquatic organism has to sweep a far larger volume of water across its gills to obtain the same amount of oxygen, and it does this by breathing much faster than a terrestrial organism.

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22. Q57
    Why is blood ​circulation ​in ​human ​heart called ​double circulation?
    Show answer

    Blood passes through the heart twice in one complete round of the body: once on the lung circuit, once on the body circuit.

    Because in one complete round of the body the blood passes through the heart twice. On the pulmonary circuit, deoxygenated blood goes right atrium $\to$ right ventricle $\to$ pulmonary artery $\to$ lungs, where it is oxygenated, and returns by the pulmonary veins to the left atrium. On the systemic circuit it goes left atrium $\to$ left ventricle $\to$ aorta $\to$ body tissues, where it gives up its oxygen, and returns by the venae cavae to the right atrium. Two circuits, each beginning and ending at the heart, mean two passages through the heart for every single circuit of the body, and a complete septum keeps the oxygenated and deoxygenated blood from mixing while this happens. Hence the name double circulation.

    Step-by-step solution →
23. Q58
    What ​is ​the ​advantage ​of having ​four chambered ​heart?
    Show answer

    A complete septum keeps oxygenated and deoxygenated blood apart, so tissues get fully oxygenated blood for a high rate of aerobic respiration.

    The four chambers, separated by a complete septum, keep oxygenated and deoxygenated blood from mixing: the right atrium and right ventricle carry only deoxygenated blood to the lungs, and the left atrium and left ventricle carry only oxygenated blood to the body. The advantages that follow are that the body always receives fully oxygenated blood, so oxygen is delivered to the tissues as efficiently as possible and they can respire aerobically at a high rate; that this supports the high, continuous energy demand of warm-blooded animals such as birds and mammals, which must spend energy all the time to keep their body temperature constant; and that the delicate low-pressure lung circuit and the high-pressure body circuit can each be driven at the pressure that suits it. In a three-chambered heart the single ventricle mixes the two bloods, so the blood sent to the body carries less oxygen than it could.

    Step-by-step solution →
24. Q59
    Mention ​the ​major events during ​photosynthesis
    Show answer

    Chlorophyll absorbs light energy, which is converted to chemical energy with splitting of water, and carbon dioxide is then reduced to carbohydrate.

    The three major events are: (i) absorption of light energy by chlorophyll; (ii) conversion of that light energy into chemical energy, together with the splitting of water molecules into hydrogen and oxygen, the oxygen being released as a by-product; and (iii) reduction of carbon dioxide to carbohydrate, using the hydrogen and the chemical energy obtained in the previous event. These events need not take place one immediately after another, since the energy and hydrogen produced in light can be used later; overall, $6{\text{CO}}_2+6{\text{H}}_2\text{O}\underset{\text{chlorophyll}}{\overset{\text{sunlight}}{\to }}{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$.

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25. Q60
    In ​each of the following ​situations ​what happens ​to the rate ​of photosynthesis?

    1. (a)Cloudy days
    2. (b)​No rainfall in the area
    3. (c)Good manuring in ​the area
    4. (d)Stomata get blocked due to ​dust
    Show answer

    Photosynthesis decreases on cloudy days, with no rainfall and when stomata are blocked, and increases with good manuring.

    (a) The rate decreases, because clouds cut down the light intensity and light is what chlorophyll absorbs to drive the process. (b) The rate decreases: less water reaches the leaf, and to conserve it the plant closes its stomata, which also shuts out the carbon dioxide, so the fall is sharp. (c) The rate increases, because manure supplies minerals such as nitrogen and magnesium that the plant needs to make chlorophyll and enzymes, giving greener, healthier leaves. (d) The rate decreases, because dust plugging the stomata prevents carbon dioxide from diffusing into the leaf, and a thick layer shades it as well.

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26. Q61
    Name ​the energy currency ​in ​the living organisms. ​When and ​where is ​it produced?
    Show answer

    ATP (adenosine triphosphate) is the energy currency; it forms during respiration in the cytoplasm and mitochondria, and in chloroplasts during the light reaction.

    The energy currency of living organisms is ATP, adenosine triphosphate. It is produced during cellular respiration, that is whenever glucose and other food molecules are broken down, and in green plants also during the light reaction of photosynthesis. It is produced in the cytoplasm, where glycolysis yields a small amount of it; chiefly in the mitochondria, where pyruvate is completely oxidised in the presence of oxygen, which is why mitochondria are called the powerhouse of the cell; and, in green plants, in the chloroplasts while light is available. Energy is stored by adding a phosphate group to ADP, $\text{ADP}+{\text{P}}_i+\text{energy}\to \text{ATP}$, and released again when that terminal bond is broken, giving about $30.5$ kJ per mole.

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27. Q62
    What ​is ​common ​for cuscuta, ​ticks ​and ​leeches?
    Show answer

    All three are parasites: each lives on a host and takes ready-made food from it.

    All three are parasites: they show parasitic nutrition. Each lives on the body of another living organism, called the host, and takes ready-made food from it, harming the host without killing it outright. Cuscuta, or amarbel, has no chlorophyll and so cannot make its own food; it twines over a host plant and pushes haustoria into its stem to draw out food and water. Ticks attach to the skin of cattle and other animals and suck their blood, and leeches attach by a sucker and do the same. Since none of the three can make its own food, all three are heterotrophs of the parasitic kind.

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28. Q63
    Explain ​the role ​of ​mouth ​in ​digestion of food.
    Show answer

    The mouth chews food into small pieces, mixes it with saliva into a bolus, and starts starch digestion with salivary amylase.

    The mouth begins both the mechanical and the chemical digestion of food. Food is taken in, and the teeth cut, tear and grind it into small pieces, which greatly increases the surface area available to the digestive enzymes that will act on it later. The salivary glands pour in saliva, and the muscular tongue mixes it thoroughly through the food; the mucus of saliva moistens and lubricates the food so that the tongue can roll it into a soft ball, the bolus, that is easily swallowed, and the tongue also bears the taste buds. Saliva contains the enzyme salivary amylase, or ptyalin, which breaks starch down into the sugar maltose, $\text{starch}\stackrel{\text{salivary amylase}}{\to }\text{maltose}$; this is the only chemical digestion that occurs in the mouth, and it is why chewed bread or rice starts to taste sweet. The bolus is then swallowed into the oesophagus.

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29. Q64
    What ​are the functions ​of ​gastric glands present ​in the wall ​of the ​stomach?
    Show answer

    They secrete gastric juice: hydrochloric acid for acidity and killing germs, pepsin to digest proteins, and mucus to protect the stomach lining.

    The gastric glands in the wall of the stomach secrete gastric juice, which has three components, each with its own function. (i) Hydrochloric acid makes the contents of the stomach strongly acidic, which provides the medium the enzyme pepsin needs in order to act, and it also kills the bacteria and other germs that enter with the food. (ii) Pepsin, a protein-digesting enzyme, begins the digestion of proteins by breaking them into shorter fragments called peptones, $\text{proteins}\stackrel{\text{pepsin}}{\to }\text{peptones}$. (iii) Mucus coats the inner lining of the stomach and protects it from being damaged by the acid and from being digested by the pepsin. The three work as one system: the acid makes the enzyme effective and the mucus makes the acid safe.

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30. Q65
    Match ​the ​terms in ​Column ​(A) ​with those ​in Column ​(B)
    Show answer

    (a)-(i), (b)-(iv), (c)-(ii), (d)-(iii). That is, trypsin is secreted by the pancreas, amylase by the saliva, bile by the liver and pepsin by the gastric glands. Bile and the liver pair first because bile is the only entry that is not an enzyme and the liver the only gland that secretes none; pepsin pairs with the gastric glands because it works only in the acidic medium they provide; trypsin pairs with the pancreas because no other listed source can supply it; and amylase, which the pancreas also secretes, is then left with saliva, where salivary amylase begins the digestion of starch in the mouth.

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31. Q66
    Name ​the correct ​substrates for the following enzymes

    1. (a)​Trypsin
    2. (b)Amylase
    3. (c)​Pepsin
    4. (d)​Lipase
    Show answer

    Trypsin and pepsin act on proteins, amylase on starch, and lipase on emulsified fats.

    (a) Trypsin acts on proteins, in particular on the partly digested proteins or peptones that reach the small intestine, breaking them down into peptides and amino acids. (b) Amylase acts on starch, breaking it down into the sugar maltose. (c) Pepsin acts on proteins, breaking them down into peptones in the acidic medium of the stomach. (d) Lipase acts on fats, and specifically on emulsified fats, the fine droplets that bile salts have made from large fat globules, breaking them down into fatty acids and glycerol.

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32. Q67
    Why do ​veins ​have thin ​walls ​as compared ​to arteries?
    Show answer

    Blood reaches a vein at low pressure after passing through the capillaries, so a thin wall is enough, unlike the high-pressure artery.

    Because a vein never has to hold high pressure. An artery receives blood directly from a contracting ventricle, so the blood enters it at high pressure and in spurts; the artery must therefore have a thick, strong and elastic wall, strong enough not to burst and elastic enough to stretch with each beat and recoil between beats, and its lumen is narrow. Between the artery and the vein lies the capillary bed, and forcing blood through those very fine vessels uses up almost all of its pressure and smooths the spurts into a steady flow. The blood entering a vein is therefore at low pressure, so a thin wall is sufficient and the vein can have a wide lumen instead. Because that low pressure would let the blood slip backwards, veins carry valves that allow flow only towards the heart. Note that the difference follows from pressure and not from the kind of blood: the pulmonary artery carries deoxygenated blood and is still thick-walled.

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33. Q68
    What ​will ​happen if ​platelets ​were absent ​in ​the blood?
    Show answer

    Blood would not clot, so even a small cut would keep bleeding, with severe blood loss and a risk of infection.

    The blood would lose its power to clot. Platelets are the fragments that gather at a cut, plug the leak and set off the reactions that turn liquid blood into a solid clot, so in their absence nothing would seal a broken vessel. Even a small cut would go on bleeding instead of stopping within a few minutes, and the loss of blood from an ordinary injury could become severe or even fatal. The same failure would occur inside the body, where small vessels are damaged all the time: blood would leak into joints, muscles and other tissues from injuries too slight to notice. In addition, since the clot normally forms a seal over a wound, its absence would leave the broken skin open to bacteria and other germs.

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34. Q69
    Plants have ​low ​energy ​needs as ​compared ​to animals. Explain.
    Show answer

    Plants do not move, do not keep a constant body temperature and are largely dead tissue, so they need little energy.

    Because a plant does very little of the work an animal does. It is fixed in one place and does not move about in search of food, a mate or shelter, so it spends nothing on locomotion; it does not maintain a constant body temperature; and it has no muscular, nervous, digestive or circulatory system to run, water rising through it largely by transpiration pull and osmosis rather than by a pump. In addition a large part of a plant's body is dead: the xylem vessels, the fibres, the bark and the bulk of a woody stem are non-living and need no energy at all, so even a very large tree supports only a comparatively small mass of living cells. The result is a low rate of energy use, and it shows in the plant's design: it needs no system for transporting oxygen at all, gases simply diffusing in through the stomata and lenticels to reach each cell separately, and different parts respiring independently. Note that this is about how much energy is spent, not about where it comes from; being able to make its own food does not by itself make a plant's demand small.

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35. Q70
    Why and ​how ​does water enter ​continuously into the ​root ​xylem?
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    Active ion uptake makes root cells more concentrated, so water enters by osmosis; ion uptake and transpiration pull keep it flowing.

    Water enters because the root itself creates a concentration difference and then keeps renewing it. The cells of the root, especially the root hairs, actively take up mineral ions from the soil using energy from respiration, so the cell sap of the root hair becomes more concentrated in solutes than the very dilute soil water. Water therefore moves across the selectively permeable cell membrane of the root hair from the soil into the cell by osmosis. This dilutes the root hair cell, which now becomes more dilute than the cortex cell beside it, so water passes on by osmosis from cell to cell across the cortex and finally into the xylem. The entry does not stop, because two things keep the gradient alive: ions continue to be taken up actively at the root, keeping the cell sap concentrated, and water is continuously lost by transpiration at the leaves, whose pull draws water away up the xylem and keeps the far end of the chain short of water. The steady entry of water into the root xylem builds up root pressure, which pushes the column upwards.

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36. Q71
    Why ​is ​transpiration ​important for ​plants?
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    Transpiration pulls water and minerals up the xylem, cools the leaves and keeps cells turgid.

    Although transpiration loses the plant most of the water it absorbs, the loss does useful work. Firstly, evaporation from the leaves creates the transpiration pull, a suction on the unbroken column of water in the xylem, and this is the chief force that lifts water from the roots to the leaves, high enough to reach the top of the tallest trees. Secondly, the mineral ions absorbed by the roots are dissolved in that rising stream, so transpiration distributes them to the stem, the leaves and every other part of the shoot. Thirdly, evaporation takes heat with it, so transpiration cools the leaves and protects them from being damaged in bright sunlight. Fourthly, the steady flow keeps the cells supplied with water and turgid, which holds the leaves spread and the young stems upright. Transpiration is unavoidable, because the stomata must stay open to admit carbon dioxide, and the plant turns that unavoidable loss to these uses.

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37. Q72
    How ​do ​leaves of ​plants ​help in ​excretion?
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    Leaves excrete gaseous wastes and excess water vapour through stomata, and store other wastes that leave the plant when old leaves are shed.

    Plants have no specialised excretory organs, so the leaves take on the work in two ways. First, the stomata of the leaves let gaseous wastes diffuse out: oxygen, which is a waste product of photosynthesis whenever more is made than the plant's own respiration can use, and carbon dioxide, the waste product of respiration, which is given out chiefly at night. Excess water is got rid of through the same stomata as water vapour in transpiration, so transpiration acts as an excretory process as well as a transport one. Secondly, wastes that cannot escape as gases are stored in the cells of the leaves, often in the vacuoles, as crystals, tannins, resins and similar harmless deposits; when the old leaves are shed, these stored wastes are carried out of the plant with them, so leaf fall is itself an excretory event.

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