Complete solutions to all 21 in-text questions and all 13 end-of-chapter exercise questions from NCERT Science, Chapter 5 “Life Processes”, in the English edition’s own wording. Try working out the answer yourself first, then check it against this one.
A In-text Questions
The question boxes printed inside the chapter, after the sections they test, answered first in book order. The end-of-chapter exercises follow below.
Questions after Section 5.1
Q. Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?
Answer: In a multi-cellular organism like a human, most cells are not in direct contact with the surrounding air; they lie deep inside a large body. Diffusion is a slow process, and it can move oxygen only over very short distances. It would take far too long for oxygen to diffuse from the body surface to every one of these cells, so it cannot meet their needs. That is why multi-cellular organisms have special organs to take in oxygen (lungs) and a transport system (blood) to carry it to every cell.
Q. What criteria do we use to decide whether something is alive?
Answer: Visible movement (such as walking, breathing or growing) is one sign of life, but it is not a reliable test, because a sleeping animal or a plant that is not visibly growing is still alive. The real criterion is molecular movement: a living thing keeps its body structures in order by constantly repairing and maintaining them, and this needs molecules to be moving all the time. If such molecular movement (and the life processes that support it, such as nutrition and respiration) is going on, the thing is alive.
Q. What are outside raw materials used for by an organism?
Answer: An organism takes in food, water and oxygen (and, in the case of plants, carbon dioxide and minerals) from outside. Food is the source of energy for all its activities and also supplies the materials needed to grow, develop, repair the body and make proteins and other substances. Oxygen is used to break down food to release energy, and water is needed for the chemical reactions of the body and for transporting substances.
Q. What processes would you consider essential for maintaining life?
Answer: Nutrition (taking in food as a source of energy and materials), respiration (breaking down food to release energy), transportation (carrying food, oxygen and other substances to every cell) and excretion (removing harmful waste products). These life processes together perform the maintenance job of the body.
Questions after Section 5.2
Q. What are the differences between autotrophic nutrition and heterotrophic nutrition?
Answer: Autotrophic nutrition: the organism makes its own food from simple inorganic substances (carbon dioxide and water) using an external energy source, sunlight, in the presence of chlorophyll; for example green plants and some bacteria. Heterotrophic nutrition: the organism cannot make its own food and takes in complex food material already made by other organisms, breaking it down with enzymes; for example animals and fungi. Heterotrophs depend, directly or indirectly, on autotrophs for their survival.
Q. Where do plants get each of the raw materials required for photosynthesis?
Answer: Carbon dioxide: from the atmosphere, taken in through the stomata of the leaves. Water: from the soil, absorbed by the roots and carried to the leaves through the xylem. Sunlight: from the Sun, absorbed by the chlorophyll in the chloroplasts. Minerals such as nitrogen, phosphorus, iron and magnesium are also taken up from the soil.
Q. What is the role of the acid in our stomach?
Answer: The gastric glands release hydrochloric acid in the stomach. It creates an acidic medium, which is needed for the enzyme pepsin to act and digest proteins. The acid also kills many germs (bacteria) that enter the stomach along with food.
Q. What is the function of digestive enzymes?
Answer: Digestive enzymes break down large, complex food molecules into small, simple molecules that can be absorbed into the blood. For example, salivary amylase breaks starch into simple sugar, pepsin and trypsin digest proteins, lipase breaks down fats, and the enzymes of the intestinal juice finally convert proteins to amino acids, complex carbohydrates to glucose, and fats to fatty acids and glycerol.
Q. How is the small intestine designed to absorb digested food?
Answer: The small intestine is the longest part of the alimentary canal, packed into a small space by extensive coiling, which gives digested food a long path and time for absorption. Its inner lining has numerous finger-like projections called villi, which greatly increase the surface area for absorption. The villi are richly supplied with blood vessels, which take the absorbed food to every cell of the body.
Questions after Section 5.3
Q. What advantage over an aquatic organism does a terrestrial organism have with regard to obtaining oxygen for respiration?
Answer: A terrestrial organism breathes the oxygen of the atmosphere, while an aquatic organism has to use the oxygen dissolved in water. The amount of dissolved oxygen is fairly low compared to the amount of oxygen in air. So an aquatic organism has to breathe much faster to get enough oxygen, whereas a terrestrial organism gets the oxygen it needs with a much slower rate of breathing.
Q. What are the different ways in which glucose is oxidised to provide energy in various organisms?
Answer: In all organisms, glucose (a six-carbon molecule) is first broken down in the cytoplasm into pyruvate (a three-carbon molecule), releasing some energy. After this: (i) in yeast, in the absence of oxygen, pyruvate is converted into ethanol and carbon dioxide (fermentation, anaerobic respiration); (ii) in our muscle cells, when there is a lack of oxygen, pyruvate is converted into lactic acid; (iii) in the presence of oxygen, pyruvate is broken down in the mitochondria into carbon dioxide and water (aerobic respiration), which releases much more energy than the anaerobic pathways.
Q. How is oxygen and carbon dioxide transported in human beings?
Answer: Oxygen is carried by the respiratory pigment haemoglobin, present in the red blood corpuscles; haemoglobin has a very high affinity for oxygen, takes it up in the lungs and releases it in the tissues. Carbon dioxide is more soluble in water than oxygen is, so it is mostly transported from the tissues to the lungs in the dissolved form in the blood (plasma).
Q. How are the lungs designed in human beings to maximise the area for exchange of gases?
Answer: Inside the lungs, the air passage divides into smaller and smaller tubes, which finally end in balloon-like structures called alveoli. The very large number of alveoli provides a huge surface for gas exchange: if the alveolar surface were spread out, it would cover about 80 m². The walls of the alveoli are very thin and contain an extensive network of blood vessels, so oxygen and carbon dioxide are exchanged quickly between the air in the alveoli and the blood.
Questions after Section 5.4
Q. What are the components of the transport system in human beings? What are the functions of these components?
Answer: (i) Heart: a muscular pump that pumps blood around the body. (ii) Blood vessels: arteries carry blood away from the heart to the organs, veins bring blood back to the heart, and thin-walled capillaries allow exchange of materials between blood and the body cells. (iii) Blood: plasma carries food, carbon dioxide and nitrogenous wastes in dissolved form; red blood corpuscles carry oxygen; platelets help blood to clot and plug leaks at an injury. (iv) Lymph: carries digested and absorbed fat from the intestine and drains excess fluid from the tissue spaces back into the blood.
Q. Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?
Answer: Mammals and birds are warm-blooded: they constantly use energy to maintain their body temperature, so they need a very high supply of energy and therefore of oxygen. Keeping oxygenated and deoxygenated blood separate (in a four-chambered heart with double circulation) ensures that fully oxygen-rich blood reaches the body, giving a highly efficient supply of oxygen.
Q. What are the components of the transport system in highly organised plants?
Answer: Two conducting tissues: (i) xylem, which moves water and minerals obtained from the soil (through vessels and tracheids); (ii) phloem, which transports the products of photosynthesis from the leaves, where they are made, to other parts of the plant (through sieve tubes, with the help of companion cells).
Q. How are water and minerals transported in plants?
Answer: Through the xylem. The xylem vessels and tracheids of the roots, stems and leaves are interconnected into a continuous system of water-conducting channels. At the roots, cells in contact with the soil actively take up ions; this creates a difference in concentration, so water moves into the root from the soil, and a column of water is steadily pushed upwards (root pressure). The major force, especially during the day, is transpiration: water evaporating from the leaves through the stomata creates a suction that pulls water, with its dissolved minerals, up the xylem from the roots.
Q. How is food transported in plants?
Answer: Food is transported through the phloem; this is called translocation. Sucrose and other materials are loaded into the sieve tubes of the phloem using energy from ATP. This raises the osmotic pressure of the tissue, so water moves into it, and the resulting pressure moves the material through the phloem to tissues with less pressure. In this way food can be moved upwards or downwards according to the plant’s needs, for example from the leaves to storage organs, or from roots and stems to growing buds in spring.
Questions after Section 5.5
Q. Describe the structure and functioning of nephrons.
Answer: Structure: each kidney has a large number of nephrons, its basic filtration units. A nephron begins with a cup-shaped end called Bowman’s capsule, which holds a cluster of very thin-walled blood capillaries called the glomerulus. From Bowman’s capsule a long, coiled tubule runs on and joins a collecting duct. Functioning: blood is filtered in the glomerulus, and the filtrate (containing water, glucose, amino acids, salts and nitrogenous wastes such as urea) collects in Bowman’s capsule. As the filtrate flows along the tubule, useful substances such as glucose, amino acids, salts and a major amount of water are selectively re-absorbed into the surrounding blood capillaries. The remaining fluid, urine, passes into the collecting duct and on to the ureter.
Q. What are the methods used by plants to get rid of excretory products?
Answer: Oxygen (a by-product of photosynthesis) and carbon dioxide are removed through the stomata; excess water is removed by transpiration. Plants can also lose some tissues: many waste products are stored in cellular vacuoles, some are stored in leaves that later fall off, some are stored as resins and gums, especially in old xylem, and some waste substances are excreted into the soil around the plant.
Q. How is the amount of urine produced regulated?
Answer: By controlling how much water is re-absorbed in the tubules of the nephrons. The amount of water re-absorbed depends on how much excess water there is in the body and on how much dissolved waste has to be excreted. When the body has excess water, less water is re-absorbed and more urine is produced; when the body needs to save water, more is re-absorbed and less, more concentrated urine is produced.
B End-of-chapter Exercises
1. The kidneys in human beings are a part of the system for
(a) nutrition (b) respiration (c) excretion (d) transportation
Answer: (c) excretion. The kidneys are the organs that filter nitrogenous waste (urea, uric acid) out of the blood, which is exactly what the excretory system does.
2. The xylem in plants are responsible for
(a) transport of water (b) transport of food (c) transport of amino acids (d) transport of oxygen
Answer: (a) transport of water. Xylem carries water and minerals absorbed by the roots up to the rest of the plant; the transport of food (including amino acids) is phloem’s job instead.
3. The autotrophic mode of nutrition requires
(a) carbon dioxide and water (b) chlorophyll (c) sunlight (d) all of the above
Answer: (d) all of the above. Photosynthesis, the process behind autotrophic nutrition, needs carbon dioxide and water as raw material, chlorophyll to absorb light, and sunlight as the energy source, so all three are required together.
4. The breakdown of pyruvate to give carbon dioxide, water and energy takes place in
(a) cytoplasm (b) mitochondria (c) chloroplast (d) nucleus
Answer: (b) mitochondria. This is the aerobic respiration pathway: pyruvate is broken down further inside the mitochondria, using oxygen, to give carbon dioxide, water and a large amount of energy.
5. How are fats digested in our bodies? Where does this process take place?
Answer: Fats enter the small intestine as large globules, which bile juice from the liver first emulsifies (breaks into smaller globules) using bile salts, greatly increasing the surface area available for enzyme action. Lipase, an enzyme present in the pancreatic juice, then breaks down these emulsified fats into fatty acids and glycerol. This entire process of fat digestion takes place in the small intestine.
6. What is the role of saliva in the digestion of food?
Answer: Saliva, secreted by the salivary glands in the mouth, wets and softens food so it can pass smoothly along the alimentary canal, and it is mixed with the food by the muscular tongue while chewing. Saliva also contains the enzyme salivary amylase, which begins the chemical digestion of food by breaking down starch, a complex molecule, into simple sugar.
7. What are the necessary conditions for autotrophic nutrition and what are its by-products?
Answer: Autotrophic nutrition (photosynthesis) needs carbon dioxide, water, sunlight and chlorophyll. Chlorophyll absorbs the light energy; this energy converts to chemical energy and splits water molecules into hydrogen and oxygen; the carbon dioxide is then reduced to carbohydrates using this energy. The main by-product is oxygen, which is released into the atmosphere; the carbohydrate made is either used immediately for energy or stored as starch.
8. What are the differences between aerobic and anaerobic respiration? Name some organisms that use the anaerobic mode of respiration.
Answer:
| Aerobic respiration | Anaerobic respiration | |
|---|---|---|
| Oxygen | Required | Not required |
| Where it happens | Mitochondria | Cytoplasm |
| End products | Carbon dioxide and water | Ethanol and carbon dioxide (in yeast), or lactic acid (in muscle cells) |
| Breakdown of glucose | Complete: the three-carbon pyruvate is broken down to give three molecules of carbon dioxide and water | Incomplete: pyruvate is converted into ethanol and carbon dioxide, or into lactic acid (also a three-carbon molecule) |
| Energy released | A large amount | Much less |
| Examples | Most plants and animals, including human beings | Yeast (fermentation); our muscle cells during a lack of oxygen |
Organisms that use the anaerobic mode: yeast, which carries out fermentation and gives ethanol and carbon dioxide. Our own muscle cells also use an anaerobic pathway for a short time when there is a lack of oxygen during sudden activity; the lactic acid that builds up causes muscle cramps.
9. How are the alveoli designed to maximise the exchange of gases?
Answer: The air passage inside the lungs divides into smaller and smaller tubes that finally end in balloon-like structures called alveoli. Alveoli give a very large combined surface area for gas exchange, and their walls contain an extensive network of blood vessels, so oxygen and carbon dioxide can be exchanged with the blood quickly and efficiently. Spread out, the alveolar surface would cover about 80 m², which makes the exchange of gases very efficient. The lungs also always retain a residual volume of air, giving enough time for the oxygen to be absorbed and the carbon dioxide to be released with every breath.
10. What would be the consequences of a deficiency of haemoglobin in our bodies?
Answer: Haemoglobin is the respiratory pigment of human beings. It is present in the red blood corpuscles, has a very high affinity for oxygen, and carries oxygen from the lungs to the tissues that are deficient in oxygen. A deficiency of haemoglobin (a condition called anaemia) would have these consequences:
- The blood would carry less oxygen, so the body’s cells would get less oxygen for aerobic respiration.
- Less energy (ATP) would be released in the cells, so the person would feel weak and tired easily and get breathless on exertion.
- Diffusion cannot make up for it: the book estimates that diffusion alone would take about 3 years to move a molecule of oxygen from the lungs to the toes.
- Muscle cells short of oxygen may switch to the anaerobic pathway, and the lactic acid that builds up can cause cramps.
11. Describe double circulation of blood in human beings. Why is it necessary?
Answer: Oxygen-rich blood from the lungs enters the left atrium, which contracts to pass it into the left ventricle; the left ventricle then contracts and pumps this oxygenated blood out to the whole body. Deoxygenated blood returning from the body enters the right atrium, which contracts to pass it into the right ventricle; the right ventricle then pumps this blood to the lungs to be oxygenated. Since blood passes through the heart twice in one complete circuit of the body, once to the lungs and once to the body, this is called double circulation.
It is necessary because, together with the complete separation of the right and left sides of the heart, it keeps oxygen-rich and carbon dioxide-rich blood separate, giving a much more efficient supply of oxygen to the body than the single circulation seen in fish. This efficiency is especially important for animals like humans, birds and other mammals, which need a large, steady supply of energy to maintain their own constant body temperature.
12. What are the differences between the transport of materials in xylem and phloem?
| Xylem | Phloem | |
|---|---|---|
| Carries | Water and minerals from the soil | Products of photosynthesis and other substances, from the leaves |
| Direction | Upward only | Both upward and downward, as the plant needs |
| Driven by | Mostly physical forces: root pressure and transpiration pull | Uses energy (ATP) to build osmotic pressure and move material |
13. Compare the functioning of alveoli in the lungs and nephrons in the kidneys with respect to their structure and functioning.
Answer: Both alveoli and nephrons are the basic functional units of an organ built around a cluster of very thin-walled blood capillaries, and both are present in very large numbers to maximise the exchange or filtration they carry out.
Alveoli (lungs): Balloon-like structures at the end of the smallest air passages, walled with an extensive network of blood vessels. Their job is gas exchange: oxygen from the inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses out into the alveolar air to be breathed out.
Nephrons (kidneys): Each nephron’s cluster of thin-walled capillaries (the glomerulus) is cupped inside Bowman’s capsule, which collects the fluid filtered out of the blood. As this filtrate then flows along the rest of the nephron’s tubule, useful substances such as glucose, amino acids, salts and most of the water are selectively re-absorbed back into the blood, and what remains becomes urine.
So both structures work by bringing blood into very close contact with a thin, permeable surface to move substances in or out efficiently, but alveoli exchange gases by simple diffusion in both directions, while nephrons filter the blood and then selectively re-absorb only the substances the body wants to keep.