NCERT Solutions for class 10th Science Chapter 15 Our Environment


Question 1. Why are some substances biodegradable and some non-biodegradable?

Sol. The micro-organisms like bacteria and other decomposer organisms (called saprophytes) present in our environment are ‘specific’ in their action. They breakdown the natural materials or products made from natural materials (say, paper) but do not break down man-made materials such as plastics. So, it is due to the property of decomposer organisms of being specific in their action that some waste materials are biodegradable whereas others are non­ biodegradable.

Question 2. Give any two ways in which biodegradable substances would affect the environment.

Sol. The two ways in which biodegradable substances would affect the
environment are:
(i) Biodegradable wastes usually do not pollute the environment. It pollutes the environment only when their amount is large and cannot be degraded (or decomposed) into harmless substances in nature at the right time.
(ii) The biodegradable substance mainly contains carbon. These substances after decomposition release that carbon back to the atmosphere.

Question 3. Give any two ways in which non-biodegradable substances would affect the environment.

Sol. The two ways in which non-biodegradable substances affect the environment are:

  1. Non-biodegradable wastes are the major pollutants of the environment. For example, the discarded plastic articles, glass articles and metal objects are the non-biodegradable waste materials which cause a lot of pollution in our surroundings.
  2. Use of non-biodegradable substances such as polythene results in blocking of drainage system and also harms animals when they are eaten by animals.

Question 4. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Sol. The various steps in a food chain at which the transfer of food (or energy) takes place are called trophic levels. The producers form the first trophic level as they manufacture food. The primary consumers form the second trophic level, the secondary consumers form the third, and the tertiary consumers form the fourth trophic level.

In the above figure, grass is the producer (first trophic level) which is eaten by the grasshopper known as primary consumer (second trophic level). Then the grasshopper is eaten by shrew, secondary consumer (third trophic level) and lastly shrew eaten up by owl, tertiary consumer (fourth trophic level).

Question 5. What is the role of decomposers in the ecosystem?

Sol. The role of decomposers in the ecosystem are as follows:
(i) The decomposers help in decomposing the dead bodies of plants and animals, and hence act as cleansing agents of environment.
(ii) Decomposer organisms help in recycling the materials in the ecosystem so that the process of life may go on and on like an unending chain.
(iii) They breakdown complex organic matter into its basic components (carbon, nitrogen etc.) and release them into the environment.
(iv) They feed on dead and decaying organisms and thus prevent the foul odour that spreads from the decaying organic matter.

Note: Decomposers include bacteria and fungi. They makes dead things into chemicals and then the chemicals go into the air, ground or water and is used again.

Question 6. What is ozone and how does it affect any ecosystem?

Sol. Ozone is a molecule formed by three atoms of oxygen. At the higher levels of the atmosphere, it shields the surface of the earth from ultraviolet (UV) radiation from the sun. It may affect any ecosystem in the following ways:

  1. At the surface of the earth, it is a deadly poison for all lower forms of life.
  2. Certain chemicals like chlorofluorocarbons (CFCs) which are widely used in refrigeration, fire extinguishers etc. reach the upper atmosphere and react with ozone gas present in the ozone layer and destroy it gradually.
  3. Ozone layer present in the upper atmosphere, is becoming thinner allowing more harmful UV rays to pass through it and reach to the Earth. These UV radiations can cause skin cancer, cataract and damage to the human immune system. It also harms plants and animals.

Note: Ozone concentration in the atmosphere naturally fluctuates depending on seasons and latitudes, but it generally was stable when global measurements began in 1957.

Question 7. How can you help in reducing the problem of waste disposal? Give any two methods.

Sol. We can help in reducing the problem of waste disposal by following two methods:

  1. By minimising the use of disposal items and promoting the use of recycled articles.
  2. By using separate bins (blue and green) for biodegradable and non­ biodegradable waste before dumping them.
  3. Recycling the non-biodegradable waste material.

Note: A disposable (also called disposable product) is a product designed for a single use after which it is recycled or is disposed as solid waste.


Question 1. Which of the following groups contain 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

Sol. The term biodegradable is used to describe materials that decompose through the actions of bacteria, fungi, and other living organisms. Temperature and sunlight may also play roles in the decomposition of biodegradable plastics and other substances.
Example: human and animal excreta, plant products like rubber, paper, wood, leaves, cotton, and wool, dead remains of living organisms, kitchen waste, agricultural waste.

Question 2. Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat

Sol. (b) Food chain is a linear sequence of organisms which starts from producer organism and ends with decomposer species. In the given group, grass is a producer, goat is a herbivore and human is top carnivore.

Question 3. Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping.
(b) Switching off unnecessary lights and fans.
(c) Walking to school instead of getting your mother to drop you on her scooter.
(d) All of the above

Sol. (d) Eco-friendly products promote green living that helps to conserve energy and also prevent air, water and noise pollution. They prove to be a boon for the environment and also prevent human health from deterioration.

Question 4. What will happen if we kill all the organisms in one trophic level?

Sol. If we kill all the organisms in one trophic level, then it will disturb the entire ecosystem. The transfer of food energy to the next trophic level will stop. This will cause a break in the food chain. As a result, the organisms of the higher trophic level will also die, while the individuals of the lower trophic level will exhibit enormous growth in their population.

Question 5. 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?

Sol. Yes, removing all the organisms in a tropic level is different for different tropic levels. If the organisms of any trophic level be removed it will certainly damage the ecosystem. For example, if all the producers are killed, it will cause death or migration of primary consumers in an ecosystem. But if primary consumers are removed, it will lead to the cause of death of higher trophic level, while those are of lower level (producers) will show an exponential growth.
The survival of organism of one trophic level depends upon the existence of the member of second trophic level. All the categories of organisms are linked with food chain, so removal of the organisms in a trophic level will disturb the whole ecosystem.

Question 6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

Sol. The increase in concentration of harmful chemical substances like pesticides in the body of living organisms at each trophic level of a food chain is called biological magnification.
Yes, magnification will be different at different trophic levels. The level of these harmful substances will go on increasing from one trophic level to the next because when certain harmful substances enter the food chain at the level of primary producers, they get concentrated many times at each subsequent trophic level.

Note: In biological magnification, secondary and tertiary consumers located on top of the food chain get poison into their body and accumulate in large concentration.

Question 7. What are the problems caused by the non-biodegradable wastes that we generate?

Sol. The problems caused by the non-biodegradable wastes that we generate are as follows:
(i) Non-biodegradable wastes persist in the environment for a long time and cause greater harm to the various members of the ecosystem by causing biological magnification.
(ii) Some of the non-biodegradable waste (like metals, DDT, plastics) enter the food chain of the human and damage their health for long run.
(iii) Non-biodegradable waste such as fertilisers, pesticides, weedicides, etc., changes the soil chemistry which affects the fertility of soil and subsequently reduces the crop yield.
(iv) Burning non-biodegradable wastes can lead to produce harmful gases in the environment causing air pollution.
(v) It leads to the destruction of balance of ecological environment.

Note: Non-biodegradable wastes like plastic bags, glass bottles etc., cannot be broken down by decomposers, their disposal can be managed by practicing the concept of 3Rs- Reduce, Reuse and Recycle.

Question 8. If all the waste we generate is biodegradable, will this have no impact on the environment?

Sol. If all the waste we generate is biodegradable then it will have an adverse effect on the environment. It affects the environment in the following ways:
(i) Too much of biodegradable waste will not be broken down into simpler substances by decomposers at the right time, which will go on accumulating in the environment and act as a pollutant.
(ii) Slow decomposition of biodegradable waste leads to the release of foul smell and harmful gases. When these gases inhaled by human beings, they may cause nausea, irritation, etc.
(iii) Increase in the population of micro-organism in an aquatic medium, leads to the deficiency of oxygen in water bodies.
(iv) Blockage of drains due to the accumulation of large amount of bio­ degradable waste also causes unhygienic conditions and multiplication of mosquitoes, which spread diseases like malaria and dengue.

Question 9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

Sol. Ozone (03) is a molecule formed by three atoms of oxygen. Ozone, is a
deadly poison. However. at the higher levels of the atmosphere, ozone performs an essential function. It shields the surface of the earth from ultraviolet (UV) radiation from the Sun. This radiation is highly damaging to organisms.

Ozone at the higher levels of the atmosphere is a product of UV radiation acting on oxygen (02) molecule. The higher energy UV radiations split apart some moleculer oxygen (02) into free oxygen(0) atoms.

The steps taken to limit the ozone layer depletion are:

  1. Banning on those companies which are producing ozone gas at large scale.
  2. Less use of refrigerators and air conditioner.
  3. Reduce the use of ozone depleting substances like CFCs.
  4. Use ozone friendly sprayers in perfumes, insect repellents etc.
  5. Do not release untreated CFCs present in ACs, fire extinguishers and refrigerators directly into the atmosphere when repairing them.
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