Our Environment – NCERT Exercises

Solution: Biodegradable substances (food, cotton, paper) are made of natural polymers that micro-organisms possess the enzymes to break down. Non-biodegradable substances (plastics, DDT, glass, metals) have bonds or a molecular structure for which no natural enzyme exists, so microbes cannot act on them and they persist in the environment.

• If allowed to pile up, they release foul smell and breed flies, mosquitoes, rats that spread disease.
• Their decomposition releases gases like methane and CO₂, which add to the greenhouse effect.
• On the positive side, if composted they give excellent natural manure that replenishes soil nutrients.

• They accumulate as litter, clog drains and soil pores, kill stray animals that swallow plastic bags and pollute land and water for centuries.
• Pesticides like DDT enter food chains and undergo biomagnification, poisoning top consumers — including humans.

Ozone layer: a thin region of O₃ molecules in the stratosphere (about 15–35 km above the ground) that absorbs the Sun's harmful UV radiation.

Formation: UV splits O₂ into free O atoms; each O atom combines with another O₂ to give O₃.

Concern: CFCs release chlorine atoms that destroy ozone. A thinner ozone layer lets more UV reach the ground, causing skin cancer, cataracts, weakened immunity, reduced crop productivity and harm to phytoplankton — disturbing entire ecosystems.

• Segregate waste at home into wet, dry and hazardous bins so each can be treated correctly.
• Follow the 3 R's — reduce use of plastic and paper, reuse bottles/bags, recycle paper & metals.
• Compost kitchen and garden waste at home instead of sending it to a landfill.
• Carry cloth shopping bags and refuse single-use plastic cutlery.

Trophic level: each feeding step of a food chain is a trophic level.

Example chain: Grass → Grasshopper → Frog → Snake → Hawk

• T₁ Producer — Grass
• T₂ Primary consumer (herbivore) — Grasshopper
• T₃ Secondary consumer (small carnivore) — Frog
• T₄ Tertiary consumer — Snake
• T₅ Top carnivore — Hawk

• They break down the complex organic matter of dead plants and animals into simple inorganic substances (water, CO₂, minerals).
• These simple substances return to the soil and air, keeping the nutrient cycles (C, N, O) running so that producers can reuse them.
• They keep the environment clean by preventing accumulation of dead matter.

All the items in (a), (c) and (d) are biodegradable. Only (b) contains plastic, which is non-biodegradable. So the correct choices are (a), (c) and (d).

A food chain must contain a producer followed by at least one consumer, each level eating the previous one. Option (b) Grass → Goat → Human is a correct food chain (producer → primary consumer → secondary consumer). (a) has only producers; (c) has only consumers (none eats another); (d) has two herbivores and is unrealistic.

Environment-friendly practices save resources and reduce pollution. (a), (b) and (d) are friendly — they cut plastic use, save electricity and save water. (c) is NOT friendly — a scooter burns petrol and releases exhaust gases; walking or cycling for short distances is better.

Removing an entire trophic level breaks the food chain. The level below (its food) will grow unchecked, while the level above (which depended on it for food) will starve and decline. For example, killing all herbivores would leave carnivores with no prey and plants would grow out of control — the ecosystem would lose its balance and eventually collapse. A new equilibrium would take many years to establish.

Yes, the impact is different at different levels. Removing producers would starve every consumer above them — the ecosystem would collapse completely. Removing top carnivores leads to explosion of herbivores, overgrazing and eventual loss of plants. Removing decomposers stops the recycling of nutrients and the soil loses fertility.

No trophic level can be removed without damage. Every level — producer, consumer or decomposer — has a role in energy flow and nutrient recycling, so each is essential.

Biological magnification: the progressive increase in the concentration of harmful, non-biodegradable chemicals (like DDT, mercury) at each higher trophic level of a food chain.

Yes, levels are different. The concentration is very low in water and producers, moderate in primary consumers, and highest in top carnivores — because at every step the predator eats several prey, each already carrying the toxin, and the chemical itself does not get excreted. For example, DDT may be 0.00003 ppm in water but 25 ppm in a fish-eating bird.

• They pile up as litter for centuries because microbes cannot degrade them.
• Plastic bags choke drains, cause urban flooding, and kill stray animals and marine creatures that swallow them.
• Persistent pesticides (DDT) enter food chains and biomagnify into top consumers.
• Toxic chemicals from e-waste (lead, mercury, cadmium) leach into soil and ground water.
• Burning such waste (especially plastic) releases dioxins and other poisonous gases into the air.

Even if all waste were biodegradable, it would still have an impact if produced in large quantities.

• Decomposers take time to act; huge heaps would still smell foul and breed disease vectors.
• The decomposition process releases methane — a greenhouse gas — and CO₂.
• Large waste heaps could drain into rivers, lowering the dissolved oxygen and harming aquatic life (BOD rise).

So biodegradable waste must also be managed properly — preferably by composting so the nutrients return usefully to the soil.

Why a concern: A thinner ozone layer allows more UV-B to reach the Earth's surface, causing skin cancer, eye cataracts, lowered immunity, damage to DNA, reduced plant growth, and harm to phytoplankton — the base of oceanic food chains.

Steps being taken:

• The Montreal Protocol (1987) committed countries to freeze and then phase out CFCs.
• CFCs in refrigerators and air-conditioners have been replaced by HFCs/HCFCs with much lower ozone-depleting potential.
• Aerosol cans now use non-CFC propellants.
• India banned CFC production in 2010 and continues to follow the Protocol's roadmap.

Thanks to these measures, scientists expect the ozone layer to recover completely around 2066 — proof that international co-operation on the environment can work.