Chapter 13 · Our Environment

NCERT Class 10 Science Chapter 13 Exercises

All 9 questions of NCERT Class 10 Science Chapter 13 Exercises (Our Environment), in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Which ​of ​the ​following ​groups ​contain ​only ​biodegradable items?

   1. (a)Grass, flowers and leather
   2. (b)Grass, wood and plastic
   3. (c)Fruit-peels, cake and lime-juice
   4. (d)Cake, wood and grass
   Show answer

   (a) Grass, flowers and leather

   (c) Fruit-peels, cake and lime-juice

   (d) Cake, wood and grass

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2. Q2
   Which of the ​following ​constitute ​a ​food-chain?

   1. (a)Grass, wheat and mango
   2. (b)Grass, goat and human
   3. (c)Goat, cow and elephant
   4. (d)Grass, fish and goat
   Show answer

   (b) Grass, goat and human

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3. Q3
   Which ​of the ​following ​are ​environment-friendly ​practices?

   1. (a)Carrying cloth-bags to put purchases in while shopping
   2. (b)Switching off unnecessary lights and fans
   3. (c)Walking to school instead of getting your mother to drop you on her scooter
   4. (d)All of the above
   Show answer

   (d) All of the above

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4. Q4
   What ​will happen if ​we kill all ​the ​organisms in ​one trophic level?
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   The food chain breaks: the level above starves, the level below multiplies unchecked, and the ecosystem loses its balance.

   The food chain is broken and the balance of the ecosystem is destroyed, and the damage travels in both directions from the gap. Above the gap: the organisms of the level immediately above ate nothing but the organisms that have been killed, so they lose their food supply, starve and die; as they die, the level above them loses its food in turn, and the shortage passes on up to the top of the chain, because energy can reach those levels by no other route. Below the gap: the organisms of the level immediately below were being kept in check by being eaten, so with that check removed their numbers grow very rapidly; they overrun the ecosystem and consume the level beneath them faster than it can recover, until they have exhausted their own food and starve as well. The gap cannot be filled, because an organism belongs to the trophic level that its food habits give it and those habits cannot be changed, so the organisms of a trophic level cannot be replaced. If the level removed is that of the producers the effect is worst of all, since energy then has no way of entering the ecosystem at all and every level above dies out.

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5. Q5
   Will the ​impact of removing all ​the organisms in a trophic level ​be ​different for different ​trophic levels? Can the organisms of any trophic level be removed without causing ​any ​damage to the ecosystem?
   Show answer

   Yes, the impact differs, being most severe for producers; no, no trophic level can be removed without damaging the ecosystem.

   Yes to the first question and No to the second. The impact is different at different trophic levels, because each level occupies a different position in the chain. If the producers are removed, energy has no way of entering the ecosystem at all — no food is made, every level above starves in turn, and the whole ecosystem collapses; this is the most severe case. If a middle level such as the herbivores is removed, the damage runs in both directions: the carnivores above lose their only food and starve, while the plants below are no longer grazed and grow unchecked until they exhaust the space, water and minerals available. If the top carnivores are removed, nothing starves for want of them, but the level below is released from the check that being eaten placed on it, multiplies without limit, eats out the level beneath it and then collapses from starvation. The general rule is that the lower the level removed, the more levels are cut off from their supply of energy and the more immediate and complete the damage; this is also why so few organisms occupy the higher levels, since only about ten per cent of the energy passes from one level to the next. But no trophic level can be removed without damaging the ecosystem. Every level does two things — it feeds the level above it and it keeps the numbers of the level below it in check — so removing any level destroys at least one of them, and because an organism belongs to the trophic level that its food habits give it, nothing can move in to fill the gap. What differs from level to level is the severity, the speed and the direction of the damage, never whether damage is caused at all.

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6. Q6
   What ​is biological ​magnification? Will ​the ​levels of this magnification be different ​at different levels of ​the ​ecosystem?
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   Biological magnification is the progressive rise in concentration of harmful chemicals at each trophic level; yes, it differs, being greatest at the top.

   Biological magnification is the progressive increase in the concentration of a harmful chemical in the bodies of organisms at each successive trophic level of a food chain. The chemicals responsible are non-biodegradable ones such as the pesticides used to protect crops. Sprayed on the fields, they are washed into the soil and into water, and from there they are taken up by plants and by aquatic plankton, so they enter the food chain at the first trophic level. Because they cannot be broken down and are not excreted, they are stored in the body, largely in the fatty tissues; every animal therefore keeps all of the chemical it has ever taken in and hands it on complete to whatever eats it. A consumer eats a very large number of organisms of the level below, yet only about a tenth of what it eats becomes its own body, so the chemical collected from a great mass of food ends up in a far smaller mass of tissue and its concentration rises. Yes, the levels of magnification are different at different levels of the ecosystem: the concentration is least in the producers, greater in the herbivores, greater still in the small carnivores and greatest of all at the top of the chain. Since human beings occupy the top of food chains, the maximum concentration of these chemicals accumulates in our own bodies.

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7. Q7
   What are ​the problems ​caused by ​the ​non-biodegradable ​wastes that we ​generate?
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   They are not decomposed, so they accumulate, harming land, animals and food chains; pesticides build up in humans and burning causes air pollution.

   Non-biodegradable wastes such as plastics, polythene, glass, metals and pesticides are not broken down by decomposers, because no organism has the enzymes to act on them. Everything we throw away therefore stays in the environment, in some cases for hundreds of years, and the quantity present only grows. On land the waste piles up: rubbish heaps and landfill sites take up land and make the surroundings dirty and unhygienic, buried plastic makes the soil impermeable so that rainwater cannot seep down to recharge the groundwater, and litter chokes drains and sewers, where the stagnant water that collects breeds mosquitoes and spreads disease. Animals are killed by it: stray cattle swallow polythene bags that then lodge undigested in the stomach, and turtles and fish in the sea are killed by floating plastic. In food chains the harm is chemical: pesticides washed into the soil and water are taken up by plants and plankton, and because they cannot be broken down or excreted their concentration rises at each successive trophic level, so that the maximum concentration collects in us at the top of the chain; these chemicals also kill useful insects and soil organisms that were never their target. The materials locked into such waste are lost to the cycles of nature, since nothing returns them to the soil. And burning the waste is no remedy, because plastics give off poisonous gases when burnt and land pollution is merely exchanged for air pollution. The only real answers are to generate less of it, and to reuse and recycle what is generated.

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8. Q8
   If all the ​waste we generate ​is biodegradable, ​will ​this ​have no ​impact on ​the environment?
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   No: biodegradable waste still harms the environment, causing disease, oxygen loss in water and methane, because decomposition is slow.

   No. Biodegradable waste will still have an impact on the environment, because 'biodegradable' means only that decomposers are able to break the waste down in the end; it does not mean that the waste is harmless while they are doing so, and it says nothing about how fast they can work. Decomposition is slow and needs a large enough population of decomposers together with warmth, moisture and air, so if waste is generated faster than it can be broken down the excess accumulates. Heaps of rotting waste give off foul-smelling gases, make the surroundings dirty and unhygienic and occupy land; they breed flies, mosquitoes and rats and harbour disease-causing microorganisms, so diseases such as cholera, dysentery, typhoid, malaria and dengue spread. The most serious effect is in water: decomposers respire while they work, and in a pond or river they take the oxygen they need from the water itself, so a large quantity of organic waste supports so many decomposers that the dissolved oxygen is used up and the fish and other aquatic animals suffocate and die, although nothing poisonous was ever added. Decomposition also releases nutrients such as nitrates and phosphates, which make algae and water plants grow so thickly that the water body is choked, and that growth in turn has to be decomposed and takes still more oxygen; and waste buried deep in a dump, where there is no air, decomposes anaerobically and gives off methane, which is inflammable and is a greenhouse gas. The impact is different in kind from that of non-biodegradable waste, which persists indefinitely and is magnified along food chains, but it is a real impact, and it grows with the quantity we generate. The remedy is to generate less waste, to compost what we do generate, and to keep it out of our water bodies.

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9. Q9
   Why is damage to ​the ​ozone ​layer a cause ​for concern? ​What steps are being taken to ​limit this damage?
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   Ozone damage lets harmful ultraviolet radiation reach the earth; CFC production was frozen at 1986 levels in 1987 and CFC-free refrigerators are mandatory.

   Ozone is a molecule of three atoms of oxygen, and high up in the atmosphere it forms a layer. It is made there when ultraviolet radiation from the Sun splits an oxygen molecule into two free atoms, each of which then joins another oxygen molecule. This layer absorbs the ultraviolet radiation coming from the Sun and prevents it from reaching the surface of the earth, and it is the only shield we have against it. Damage to the layer is a cause for concern because ultraviolet radiation is highly damaging to organisms: a thinner layer lets more of it through, and in human beings that means skin cancer, damage to the eyes leading to cataract and a weakening of the immune system, while plants are damaged and crop yields fall, and the plankton of the surface waters of the sea are killed, so the harm is passed up every marine food chain. The amount of ozone began to drop sharply in the 1980s, and the cause was traced to synthetic chemicals, chiefly the chlorofluorocarbons or CFCs, used as refrigerants in refrigerators and air-conditioners and in fire extinguishers; released at the ground they reach the layer and destroy the ozone there, and because one molecule destroys many, a small quantity does damage out of all proportion to its amount. As to the steps being taken: in 1987 the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at its 1986 levels, and it is now mandatory for all manufacturing companies throughout the world to make CFC-free refrigerators. CFCs have been replaced in refrigerators, air-conditioners, aerosol sprays and fire extinguishers by chemicals that do not attack ozone, products are labelled CFC-free, and old appliances are disposed of in ways that recover the gas instead of releasing it.

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