Answers to all in-text and exercise questions of Chapter 13, Our Environment (NCERT Class 10 Science, 2026-27 reprint): trophic levels and food chains, decomposers, the 10% law, biodegradable and non-biodegradable wastes, biological magnification, the ozone layer and CFCs, and ways to reduce waste. All 16 questions are answered, with the key answer highlighted.
Free NCERT solutions by Notes Bazar · www.notesbazar.in/ncert-solutions/class-10-science/chapter-13-our-environment
An ecosystem has biotic components (producers, consumers, decomposers) and abiotic components (air, water, soil, temperature). Each step of a food chain is a trophic level. Only about 10% of the energy at one level is passed on to the next, so food chains usually have only three or four levels. Harmful non-biodegradable chemicals such as pesticides build up along a food chain (biological magnification).
Decomposers, such as bacteria and fungi, break down the dead remains and wastes of plants and animals into simple substances. These substances return to the soil and are used again by plants. So decomposers recycle nutrients and also keep the environment clean by removing dead matter.
They break down dead organisms and wastes into simple substances, returning nutrients to the soil and cleaning the environment.
Why are some substances biodegradable and some non-biodegradable?
Solution
Biological processes such as the action of bacteria and other decomposers can break down substances made by living things (food, paper, wood, cotton), so these are biodegradable. Decomposers have no enzymes to break down many man-made substances, such as plastics, glass and DDT, so these are non-biodegradable and remain in the environment for a long time.
Decomposers have enzymes to break down some substances (biodegradable) but not others, such as plastics (non-biodegradable).
Give any two ways in which biodegradable substances would affect the environment.
Solution
When they pile up, they rot and smell, and become breeding grounds for flies, mosquitoes and germs that spread disease.
When properly decomposed, they add nutrients to the soil (compost) and are recycled; in water bodies, excess waste uses up dissolved oxygen as it decays, harming aquatic life.
Heaps of such waste smell and breed disease carriers; properly decomposed, they enrich the soil.
What is ozone and how does it affect any ecosystem?
Solution
Ozone (O₃) is a molecule made of three oxygen atoms. Near the ground it is a poisonous gas. High in the atmosphere, however, the ozone layer shields the Earth from the Sun's harmful ultraviolet (UV) radiation.
UV radiation causes skin cancer, cataracts and damage to the immune system in humans, and harms plants, plankton and other organisms. So the ozone layer protects every ecosystem; if it thins, these harmful effects increase.
Ozone (O₃) in the upper atmosphere absorbs harmful UV rays from the Sun, protecting living things from skin cancer and other damage.
Which group contains only biodegradable items? (a) Grass, flowers and leather (b) Grass, wood and plastic (c) Fruit-peels, cake and lime-juice (d) Cake, wood and grass
Solution
Plastic in (b) is non-biodegradable. Groups (a), (c) and (d) contain only items made by living things, which decomposers can break down.
(a), (c) and (d) all contain only biodegradable items; (b) does not, because of plastic.
Which are environment-friendly practices? (a) Carrying cloth bags for shopping (b) Switching off unnecessary lights and fans (c) Walking to school instead of being dropped on a scooter (d) All of the above
What will happen if we kill all the organisms in one trophic level?
Solution
The food chain breaks, and the whole ecosystem is disturbed:
The organisms of the next higher level lose their food, so their numbers fall and they may die out.
The organisms of the level below are no longer eaten, so their numbers increase sharply. For example, if all deer are killed, tigers starve while grass and plants are overgrazed by nothing and grow unchecked (and other herbivores multiply).
The level above starves while the level below multiplies, upsetting the balance of the ecosystem.
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 damaging the ecosystem?
Solution
Yes, the impact differs.
Removing producers would be the most damaging: no energy would enter the ecosystem, and all other organisms would die.
Removing herbivores would starve the carnivores and let plants grow unchecked.
Removing carnivores would let herbivores multiply, overgraze and destroy vegetation.
Removing decomposers would stop the recycling of nutrients, and dead matter would pile up.
No, the organisms of no trophic level can be removed without damaging the ecosystem, because all levels are interdependent.
Yes, the effects differ (removing producers is the worst); no level can be removed without harming the ecosystem.
What is biological magnification? Will its levels be different at different levels of the ecosystem?
Solution
Harmful non-biodegradable chemicals, such as pesticides (e.g. DDT), are taken up by plants from soil and water. They are not broken down or excreted, so they accumulate in the bodies of organisms and pass on through the food chain. Their concentration increases at each trophic level. This is biological magnification.
Yes, the levels differ: the concentration is lowest in producers and highest in the top consumers. Humans, who are at the top of many food chains, get the largest amount.
The increase in concentration of harmful chemicals at each higher level of a food chain; it is highest at the top level.
Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?
Solution
The ozone layer protects the Earth from the Sun's harmful UV radiation. If it is damaged, more UV rays reach the Earth, causing skin cancer, cataracts and weaker immunity in humans, and harming plants, crops and marine life. The ozone layer has been thinning since the 1980s, mainly because of synthetic chemicals called CFCs (chlorofluorocarbons), used in refrigerators, air conditioners and fire extinguishers.
Steps taken: in 1987, the United Nations Environment Programme (UNEP) got countries to agree to freeze CFC production at 1986 levels; later agreements (the Montreal Protocol) have phased out CFCs. CFC-free refrigerators and safer alternatives are now used.
Ozone depletion lets harmful UV rays reach the Earth; CFC production was frozen (UNEP, 1987) and is being phased out in favour of CFC-free alternatives.
