Chapter 13 · Our Environment

NCERT Class 10 Science Chapter 13 Intext Questions (Page 214)

All 3 questions of NCERT Class 10 Science Chapter 13 Intext Questions (Page 214) — Our Environment — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Why are some ​substances ​biodegradable ​and ​some ​non-biodegradable?
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   Decomposer enzymes are specific: natural substances have matching enzymes and are biodegradable, whereas man-made materials such as plastics have none and are non-biodegradable.

   The difference lies in the decomposers and in the specificity of their enzymes. Bacteria, fungi and other saprophytes break substances down using enzymes, and enzymes are specific in their action: a particular enzyme can act only on a particular substance, which is why the same enzyme does not break down everything we eat and why we get no energy if we try to eat coal. Substances of plant and animal origin, such as leaves, wood, paper, cotton, leather, food waste and cow-dung, have been part of nature for as long as decomposers have existed, so those decomposers already possess enzymes that fit them; these substances are broken down by biological processes and are therefore biodegradable. Human-made materials such as plastics, polythene, synthetic fibres and pesticides like DDT are new to nature, so no organism has ever needed to digest them and no enzyme that can attack them exists. They are not broken down by bacteria or other saprophytes; they will be acted upon by physical processes like heat and pressure, but under the ambient conditions found in our environment they persist for a long time, and this makes them non-biodegradable. The test is therefore biological, not physical: what decides the group is whether a matching enzyme exists, and not how hard, heavy or durable the material is.

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2. Q2
   Give any ​two ​ways ​in ​which ​biodegradable ​substances would ​affect the environment.
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   Rotting waste on land smells and spreads disease, and in water it uses up dissolved oxygen so aquatic animals die.

   Biodegradable substances are broken down by decomposers in the end, but they still affect the environment while that is happening, because we generate them much faster than the decomposers can work. Two ways are these. (i) On land, the accumulated waste rots and gives off foul-smelling gases, so the surroundings become dirty and unhygienic; the heaps breed flies, mosquitoes, cockroaches and rats and let disease-causing microorganisms multiply, so diseases such as cholera, dysentery, typhoid, malaria and dengue spread from them, and the heaps occupy land as well. (ii) In water, when such waste is thrown into a pond, river or lake it becomes food for the decomposers there; their numbers rise, they respire using the oxygen dissolved in the water, and the dissolved oxygen falls so low that fish and other aquatic animals suffocate and die although nothing poisonous was ever added; the nutrients released also make algae grow thickly and choke the water body. Two further effects may be added: waste buried without air gives off methane, a greenhouse gas, and, on the useful side, biodegradable waste broken down completely enriches the soil, which is why it can be composted into good manure.

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3. Q3
   Give any two ways ​in ​which non-biodegradable substances would ​affect the ​environment.
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   They accumulate because no decomposer breaks them down, and they are magnified along food chains, reaching the highest concentration in humans.

   No decomposer has an enzyme that can act on non-biodegradable substances such as plastics, polythene, glass, metals and pesticides, so nothing removes them and everything thrown away stays in the environment, in some cases for hundreds of years. Two ways in which they affect the environment are these. (i) They accumulate: heaps of plastic and glass occupy land and make the surroundings dirty, polythene in the soil makes it impermeable so that rainwater cannot seep down to recharge the groundwater and the soil grows less fertile, discarded bags choke drains and sewers where the stagnant water that collects breeds mosquitoes, and animals are killed by the litter, stray cattle swallowing polythene bags that lodge undigested in the stomach and sea turtles and fish being killed by floating plastic. (ii) They enter food chains and are biologically magnified: pesticides washed into the soil and water are taken up by plants and by plankton at the first trophic level, and because they can neither be broken down nor excreted their concentration rises at each successive trophic level, so that the maximum concentration collects in human beings at the top of the chain, which is why our food grains, vegetables, fruits and even meat carry pesticide residues that washing cannot always remove. In addition they kill useful insects, earthworms and soil microorganisms, and burning them releases poisonous gases into the air.

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