Chapter 15 Our Environment

Two components of the ecosystem are biotic and abiotic; components whose interaction maintain ecological balance so life sustain life on earth. In this chapter we will study food chain the unidirectional flow of energy when one organism is eaten by other. Internetwork of food chain form food web; Trophic level always ten percent flow of energy occurs in each step of trophic level the remaining energy is lost in form of heat and returned back to the environment. We will also cover ozone depletion and management of garbage.

Download pdf of NCERT Solutions for Class Science Chapter 15 Our Environment

Download pdf of NCERT Examplar with Solutions for Class Science Chapter 15 Our Environment

Exercise 1

  • Q1 What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

     The sequential inter- linking of organisms involves the transfer of food in the form of energy from producers by the series of organisms with repeated eating and being eaten is referred to as the food chain. And, these sequential steps in the food chain are referred as different tropic levels.

    Example:- Grassland ecosystem food chain and their different tropic levels

         Grasses      ←        Insects       ←        Frogs      ←         Snake       ←      Predatory birds 
      Producers              Herbivores                Primary             Secondary          Tertiary carnivores
                                                                   carnivores            carnivores

    Trophic levels are the nutritional levels in which food energy is transferred from one level to another level. In this above example, producers are the green plant (grasses) which is first trophic level; herbivores (insects) are the secondary trophic levels which are the plant eaters also called primary consumers belongs to secondary trophic level; then came primary carnivores (frogs) also called secondary consumers which feeds on primary consumers. And, then secondary carnivores (snake) which are the flesh eaters as like primary carnivores also called tertiary consumers; And then quaternary consumer (predatory birds) level came which are the tertiary carnivores. The maximum step in the food chain is 4 or 5.

    Q2 What is the role of decomposers in the ecosystem?

    Decomposition is the process of breaking down the substances into the smaller particles or the parts. Decomposition of the decay organic matter like plants, wastage of animals etc. , also known as putrefaction. In the process of decomposition mainly two decomposers are responsible, i.e, bacteria and the fungi. These two organisms breaks the dead organic matter into smaller organic molecules (utilized as nutrients by themselves) or into inorganic compounds which are released into the environment.

    Decomposition involves some enzymes for the degradation of organic matter. Mainly 3 steps are involved in this process fragmentation (breakdown of the organic matter into smaller fragments), leaching (fragmented detritus particles like sugar and several nutrients get dissolved in water percolating through the soil and removed due to the leaching process) and the catabolism (release the extracellular enzymes to decompose decay particles into the simpler compounds which are carried out by the decomposers like bacteria and fungi) process occurs.

    The decomposition process plays vital role in any ecosystem because it involves the release of the nutrients from the dead matter. If there is no decomposers, all the nutrients would remain locked in the dead remains of plants and the animals, so these will not be available for the indefinite recreation of the living matter. 


Exercise 2

  • Q1 Why are some substances biodegradable and some non-biodegradable?

     Some substances are biodegradable and some are non- biodegradable because some microbial actions, radiations and some pollutants may get biologically magnified as they passed through the food chain and some bacteria and the decomposers have a specific role to play. 

    Like, biodegradable substances (like paper, wood) are those which are degraded naturally. These can disposed by the action of microbes and the radiations. And, non- biodegradable substances are those which cannot be degraded or if degradation occurs, it will be very slow in the process. Non- biodegradable substances are man- made products like D.D.T (Dichloro- diphenyl- trichloroethane), glass, heavy metals, tins, radioactive materials and plastics.


    Q2 Give any two ways in which biodegradable substances would affect the environment.

     The biodegradable substances would affect the environment and they can easily degraded by the natural actions. Two ways are following in which biodegradable substances affect the environment:-

    1. Biodegradable substances decomposed easily, they are less harmful than the non- biodegradable substances. When the decomposition process occurs through the microbial action or the radiations they produce foul smell. Biodegradable wastes like green waste, food waste, paper etc. pollutes the environment only when they are present in the excess amount and produce some harmful gases like ammonia, methane gas, carbon dioxide that may cause global warming.

    2. When the decomposition process occurs by some microbes of biodegradable wastes, which contains houseflies, mosquitoes which carry the parasites which are the main causes of the human diseases.

    Q3 Give any two ways in which non-biodegradable substances would affect the environment.

    Unlike the biodegradable substances, the non- biodegradable substances would affect the environment in the negative ways and they cannot be degraded easily. They can be accumulated and biologically magnified when they get pass through the food chain. The two ways are following in which non- biodegradable substances affect the environment:-

    1. As they do not degrade or degrade very slowly in nature. These non- biodegradable substances get accumulated in the ecosystem and get biologically magnified as they passed through the food chain, they remains in the environment and does not get decompose. This can cause pollution and kills many animals by eating these non- biodegradable substances.

    2. Plastics, tins, glass containers, radioactive materials, polythene, biomagnifications of Dichloro- diphenyl- trichloroethane in aquatic food chain, detergents- these non- biodegradable substances affect the environment when they get accumulated it causes soil pollution, water pollution, electronic waste pollution, thermal pollution etc.

Exercise 3

  • Q1 What is ozone and how does it affect any ecosystem?

    Ozone is a clear, blue gas layer which is also known as the layer of protection of the environment. Ozone is the form of oxygen and the molecules of ozone contains three oxygen atoms (O3) It is present in the stratosphere which protects the earth from the ultraviolet (UV) radiations that came from the sun. Stratosphere is present at the altitude of 15-50 km above the earth’s surface and at the lower altitude it has thick sheet of ozone known as ozonosphere. The amount of atmospheric ozone is measured by the Dobson Spectrometer. Reaction:-                       O2  +  O    →   O3 (Ozone)

    Ozone affects the environment in following ways:-

    • Ozone is very poisonous at the ground level but at the higher altitude it is a layer of protection present in the stratosphere which protects the earth’s surface from the ultraviolet radiations, specially UV- B rays (290- 320 nm). Increase in the amount of UV rays reaches the earth surface when ozone depletion occurs, it can affect the ecosystem like it can cause skin cancer, dimming of the eye sight, herpes and diminishing the functioning of immune system in human beings.

    • It can cause death (mortality) of the young ones of the human beings and the animals.

    • It can cause mutation and blindness of the land animals.

    • All food chains can get disturbed.

    The main cause of the ozone depletion is chlorofluorocarbon (CFCs).

    Q2 How can you help in reducing the problem of waste disposal? Give any two methods.

    Waste disposal problems can be reduced. Two methods are following:-

    1. When we buy plastic bottle of water or plastic glasses, single use plastic or any disposable items, they pollute the environment because they are non- biodegradable. So, by minimizing their use, we should promote the use of recycling items which will be beneficial for the ecosystem.

    2.  We should use jute bags instead of polythene when we go for shopping or for purchasing anything. It can help in reducing the waste disposal, and will reduce the pollution ultimately.

Exercise 4

  • Q1 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

    All options contains biodegradable substances which can decompose easily except the grass, wood and plastic because plastic is a non- biodegradable substance which cannot be degraded and get accumulated in the environment which is the main cause of the pollution.

    Q2 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

    Grass, goat and human

    Food chain is defined as the sequential steps of interlinking of the organisms which involves the transfer of food energy from one trophic level to another with repeated eating or being eaten process. In this, grass is the producer makes first trophic level and goat is the herbivore or the primary consumers which feeds on the grass makes secondary trophic levels. And, then flesh eaters, carnivores come, also known as secondary consumers, i.e, human beings which eats goat.


    Q3 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

    All of the above.

    If we carry cloth- bags to put purchases while shopping, we can deny the use of polythene bags that is provided by the shopkeepers because polythene is a non- biodegradable substance. We should switch off the unnecessary lights or fans for the conservation of energy, light and the bill. And, at last if we prefer walking instead of getting our mother to drop on scooter, we can minimise the use of petrol or diesel or can contribute in reducing the air pollution that is released by the vehicles, gases release like CO2. So, all of the above options contribute in environment- friendly practices.  

    Q4 What will happen if we kill all the organisms in one trophic level?

     Tropic levels in the food chain are interlinked with one to another level in the sequential manner. If we kill all the organisms in one trophic level, the flow of energy at each tropic level is reduced 10 percent and the food chain get disturbed that creates an imbalance in the ecosystem.

    For example, food chain is:-                       crops         ←          Rats       ←      Owls

    In this food chain, crops are eaten by rats and then rats are eaten by owl population. If all the owls in the one trophic level get killed, there will be increase in the rats population and will lead to the overeating of the crops will happen. And, due to the overeating can lead to barren land which will affect the another trophic levels due to the soil erosion. So, as a result if we kill one trophic level, the lower trophic level will reproduce and population will get increases that disturbs the ecosystem.

    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?

    The impact of removing all the organisms in the trophic level will be the same at each independent trophic level. When we remove or kill the organism of any trophic level it will be an damage to the ecosystem.

    For example, in a pond food chain:-

    Phytoplanktons       ←       Zooplanktons      ←       Small fishes       ←       Large fishes       ←         Snakes, birds

    In this food chain phytoplanktons are eaten by zooplanktons and then zooplanktons get eaten by the small fishes; small fishes are eaten by the large fishes in the pond and then at the last trophic level large fishes get caught and eaten by the snake or the birds. So, we can see that all the trophic levels in the ecosystem are dependent on each other. If we remove any one trophic level population like we remove zooplanktons in the food chain then the lower trophic level, i.e, phytoplanktons population will get increase in number which will disturb the entire ecosystem.

    Q6 What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

    Some toxic substances chemicals and large number of pesticides like D.D.T (Dichloro- diphenyl- trichloroethane) are non- biodegradable. When the concentration of these chemicals and pesticides increases at each trophic level of the food chain, this phenomenon is known as biological magnification or biomagnifications. They are non- biodegradable substances, so they do not decompose then they get accumulated in the environment and affect each of the trophic level. And, our body get directly affected at some stage of trophic level or mostly human beings are on the top level because we consume most of the chemicals in our body.

    The level of biological magnification will be different at the different trophic levels. The concentration of the biomagnification got increased with the increase in trophic levels. For example:- biomagnification of pesticide D.D.T in aquatic food chain;

            Water           ←         Zooplankton         ←       Small fishes       ←     Large fishes        ←      Fish eating birds
    (DDT 0.003ppm)           (DDT 0.04 ppm)               (DDT 0.5 ppm)             (DDT 2 ppm)                   (DDT 5 ppm)

    In the above example, when the pesticides are sprayed on the agricultural fields then they are carried out by running off water into the aquatic bodies in the pond or lakes etc. The pesticide DDT get accumulate into the water and then in the zooplanktons, the concentration of DDT goes on increasing with each trophic level in the food chain. The fish eating birds have the higher concentration of biomagnification, which affects the egg shells due to the effect on calcium metabolism. Egg shell becomes thin and breaks at the premature stage. Then mercury discharge into the ponds, rivers or lakes that is changed by the bacteria into the toxic forms which is very harmful, absorbes by the fishes and they may get killed.

    Q7 What are the problems caused by the non-biodegradable wastes that we generate?

    Non- biodegradable resources cannot be degraded or decomposed easily. And when they do not decompose they get accumulated and affect the environment.

    Problems caused by the non- biodegradable wastes that is generated by us:-

    1. When we use plastic bottles and plastic glasses, we throw them on the road, lakes, beaches instead of dustbins and they get accumulate and then pollute the environment. And, many types of pollutants are present which are non- biodegradable like radioactive substances, tins, glasses, electronic wastes etc all pollutes the environment which causes biomagnifications.

    2. These pollutants when get accumulated in the environment and cause very serious diseases to the human beings and the animals.

    Q8 If all the waste we generate is biodegradable, will this have no impact on the environment?

    Biodegradable substances decompose easily but when the amount of degradable substances get increased it will create an negative impact on the environment but positive impact occurs more than the negative. If we burn maximum amount of biodegradable matter it will produce foul smell by the reaction of microbes and this foul smell carried by houseflies and mosquitoes which carries the disease with them and humans get affected by these diseases.

    But biodegradable decomposition possess more the positive impact on the environment because they produces raw materials which are again used by the ecosystem after the decomposition process.


    Q9 Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

     Ozone layer is formed of three oxygen atoms and known as the layer of protection which protects the earth from the UV rays. The photodissociation of ozone and generation are present in the equilibrium state. This ozone layer releases the energy of ultraviolet radiations as heat. So, ozone layer acts as the shield in the stratosphere which protect us from the harmful radiations. Decline in the thickness of the ozone layer over a restricted area is known as ozone hole. And, the chlorofluorocarbons are the main cause of the ozone depletion. CFCs are used as the coolants in refrigerators and air conditioners or as cleaning solvents etc. They get released into the troposphere and then they reach into the stratosphere and produce chlorine in the presence of the UV rays and these chlorine radicals does the ozone depletion by converting them into the oxygen.

    This damage can be overcome by minimizing the release of chlorofluorocarbons. CFCs used as air conditioners or refrigerators should be replaced with environment friendly alternatives. Or CFCs released through the industries, should be controlled.