Chapter 15 · Our Environment

NCERT Exemplar Class 10 Science Chapter 15 Short Answer Questions

All 9 questions of NCERT Exemplar Class 10 Science Chapter 15 Short Answer Questions (Our Environment), in the book's order, each with its answer and a step-by-step solution.

1. Q22
   Why ​is ​improper disposal ​of waste a ​curse ​to environment?
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   Improper disposal spoils land, pollutes water and air and spreads disease, while useful compost and recyclable material is wasted.

   Waste thrown away carelessly is never really removed, so it goes on damaging the surroundings. (i) Land: the non-biodegradable part — plastic, glass, metal, e-waste — stays for decades, dumps occupy and spoil land, and cattle die after swallowing polythene. (ii) Water: rain soaking through a dump carries nitrates and toxic metals down into the groundwater, and nitrate and phosphate washed into ponds cause eutrophication, whose decaying algal bloom uses up the dissolved oxygen and kills the fish. (iii) Air: open burning of waste gives smoke, carbon monoxide and toxic gases, and rotting heaps give off methane and a foul smell. (iv) Health: flies, mosquitoes and rats breed on the heaps and spread cholera, dysentery, dengue and malaria. In addition, material that could have become compost or a recycled article is lost. Because a single badly-managed heap harms land, water, air and people all at once, improper disposal is a curse to the environment; the same waste, segregated at source, composted and recycled, is a resource.

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2. Q23
   Write the common ​food chain ​of ​a pond ​ecosystem.
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   Phytoplankton are eaten by zooplankton, then small fish, then large fish; decomposers return nutrients to the water.

   The common food chain of a pond ecosystem is: Phytoplankton (algae, diatoms) $\to$ Zooplankton $\to$ Small fish $\to$ Large fish. Here the phytoplankton are the producers ($T_1$), the zooplankton the primary consumers ($T_2$), the small fish the secondary consumers ($T_3$) and the large fish the tertiary consumers ($T_4$). If a fish-eating bird such as a kingfisher or crane, or a human being, feeds on the large fish, it forms one more level. Each arrow points from the organism eaten to the organism that eats it. Decomposers — bacteria and fungi — act on the dead remains of every level and return simple nutrients to the pond water, where the phytoplankton take them up again.

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3. Q24
   What are the advantages ​of ​cloth bags ​over ​plastic ​bags ​during ​shopping?
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   Cloth bags are renewable, strong, reusable and biodegradable, whereas plastic bags are non-biodegradable and choke drains and harm animals.

   Cloth bags are better than plastic carry bags because: (i) They are made of cotton or jute, natural fibres that are grown again each season, whereas polythene is made from petroleum, a non-renewable fossil fuel; (ii) Cloth is strong and carries a heavy load without tearing; (iii) A cloth bag can be washed and reused hundreds of times, so one of them replaces a great many plastic bags and far less waste is made in the first place; (iv) A worn-out cloth bag is biodegradable — decomposers break it down and the material returns to the soil — while a plastic bag is non-biodegradable and remains for hundreds of years; (v) Plastic bags choke drains and cause waterlogging, are swallowed by cattle and by marine animals and kill them, and give off toxic gases when burnt, and cloth bags do none of these. Carrying a cloth bag is therefore the simplest eco-friendly habit a shopper can adopt.

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4. Q25
   Why are ​crop ​fields ​known ​as ​artificial ecosystems?
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   A crop field is artificial because humans create and maintain it, choosing the crop, supplying inputs and removing the harvest.

   A crop field has both biotic and abiotic components and so is an ecosystem, but it is created and maintained entirely by human beings, and that is what makes it artificial. The land is ploughed, the natural vegetation is removed and one selected crop is sown in rows; irrigation replaces natural rainfall; manure and chemical fertilisers replace the nutrients that natural litter would have returned; weeds, insects, rodents and birds are killed, so the number of species and the food web are deliberately kept small and simple; and the crop is harvested and taken away, so the energy and minerals it holds leave the field instead of being recycled in it. If the farmer stops ploughing, sowing, irrigating and weeding, the crop disappears and wild vegetation takes over within a season or two. Because its composition and its very survival depend on continuous human effort, a crop field is called an artificial (man-made) ecosystem, like a garden, a park or an aquarium.

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5. Q26
   Differentiate between biodegradable and ​non-biodegradable ​substances. Cite ​examples.
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   Biodegradable substances are broken down by decomposers and recycled; non-biodegradable ones, such as plastics, persist and accumulate.

   Biodegradable substances are those that decomposers — bacteria and fungi — can break down into simple substances; non-biodegradable substances are those that no decomposer can break down. The differences are: (i) Agent: biodegradable substances are broken down by the enzymes of microorganisms, non-biodegradable ones are not acted upon by any organism. (ii) Time: biodegradable substances decay in days to a few months, non-biodegradable ones remain for years or centuries. (iii) End products: biodegradation gives simple inorganic substances — carbon dioxide, water, nitrates, phosphates and minerals — which return to the soil, air and water and re-enter the nutrient cycle, whereas non-biodegradable substances accumulate as pollutants and, if toxic, are biomagnified up the food chain. (iv) Management: biodegradable waste can be composted or turned into biogas, while non-biodegradable waste can only be reduced, reused, recycled or landfilled. Examples — biodegradable: vegetable and fruit peels, leftover food, tea leaves, paper, cotton and jute cloth, wood, leaves, cow dung, sewage. Non-biodegradable: plastics and polythene bags, glass, metal cans, aluminium foil, thermocol, synthetic fibres, DDT, batteries and e-waste.

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6. Q27
   Suggest one word for each of the following ​statements/ definitions

   1. (a)The ​physical and biological world where we live in
   2. (b)Each level of ​food chain where transfer of energy takes place
   3. (c)The physical factors like temperature, rainfall, wind ​and soil of an ​ecosystem
   4. (d)Organisms which ​depend on the producers either directly or indirectly for ​food
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   The four words are environment, trophic level, abiotic components and consumers.

   (a) Environment; (b) Trophic level; (c) Abiotic components (abiotic factors); (d) Consumers.

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7. Q28
   Explain the ​role of ​decomposers ​in ​the ​environment?
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   Decomposers break down dead matter, recycle nutrients, clean the environment and keep the soil fertile.

   Decomposers are saprophytic microorganisms — mainly bacteria and fungi — that break the complex organic matter of dead plants, dead animals and their excreta into simple inorganic substances. Their roles are: (i) Nutrient recycling: they release carbon dioxide, water, nitrates, phosphates and minerals into the soil, air and water, where producers absorb them again, so the same atoms are used over and over and the biogeochemical cycles keep running. (ii) Cleansing agents of nature: they clear away dead bodies, fallen leaves, excreta and other biodegradable waste that would otherwise pile up, and so make room for new organisms. (iii) Soil fertility: the humus they leave improves the texture, the water-holding capacity and the nutrient content of the soil — the principle used in composting and vermicomposting. (iv) They form the last link of every food chain and the bridge between the biotic and the abiotic world, since they return to the non-living environment the matter that the living world has taken from it.

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8. Q29
   Select the mis-matched pair ​in the following ​and correct it.

   1. (a)Biomagnification — Accumulation of chemicals at the successive trophic levels of a food chain
   2. (b)​Ecosystem — ​Biotic components ​of ​environment
   3. (c)Aquarium — A man-made ecosystem
   4. (d)Parasites — Organisms which obtain food from other living organisms
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   (b) Ecosystem — Biotic components of environment is mis-matched; an ecosystem includes the abiotic components as well.

   The mis-matched pair is (b) Ecosystem — Biotic components of environment. It is wrong because an ecosystem is not made of living things alone. Corrected: Ecosystem — all the living organisms (biotic components) of an area together with the non-living or abiotic components of the environment such as light, temperature, rainfall, wind, water, soil and air, the two interacting with one another as a self-supporting unit; for example a pond, a forest, a crop field or an aquarium. The other three pairs are correctly matched: (a) Biomagnification really is the accumulation of a non-biodegradable chemical at successive trophic levels of a food chain, (c) An aquarium really is a man-made ecosystem, and (d) Parasites really do obtain their food from other living organisms.

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9. Q30
   We do ​not clean ponds ​or ​lakes, but an aquarium needs to ​be cleaned. Why?
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   A pond cleans itself through decomposers and water flow; an aquarium lacks these, so waste builds up and it must be cleaned by hand.

   A pond or a lake is a natural, complete and self-sustaining ecosystem. It contains producers (algae and water plants), consumers of every level and, above all, plenty of decomposers, and its water is renewed by rain and by streams. Dead plants and animals and the excreta of the fish are broken down by the decomposers into simple nutrients, which the producers absorb and use again, while the currents and rainwater dilute and carry away the rest. The nutrient cycle is therefore closed and nothing accumulates, so no one has to clean a lake. An aquarium is an artificial ecosystem made and maintained by man. It has only a few chosen plants and fish, an incomplete food web, too few decomposers and no natural inflow or outflow of water, and the fish are fed from outside. Uneaten food, fish excreta and dead matter therefore collect at the bottom, ammonia and other harmful substances build up, the water turns foul and cloudy and the dissolved oxygen falls, which would kill the fish. So the aquarium has to be cleaned and its water changed by hand — we do for it the work that decomposers and flowing water do for a pond by themselves.

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