NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text Questions
What are trophic levels? Give an example of a food chain and state the different trophic levels in it.
Answer
Each step in a food chain where transfer of energy occurs is called a trophic level. Organisms at each level feed on those at the level below and are eaten by those at the level above.
Example: Grass
⇒ Grasshopper
⇒ Frog
⇒ Snake
⇒ Hawk
T1 (Producers): Grass | T2 (Primary consumers/herbivores): Grasshopper | T3 (Secondary consumers): Frog | T4 (Tertiary consumers): Snake | T5 (Quaternary consumers): Hawk
What is the role of decomposers in the ecosystem?
Answer
Decomposers (bacteria and fungi) break down the dead remains and waste products of organisms into simple inorganic substances (nutrients) that are returned to the soil, water, and air. This process is essential because: (i) it recycles nutrients, making them available to producers for reuse, (ii) it cleans the environment of dead organic matter, (iii) without decomposers, dead bodies and waste would accumulate, and the nutrient cycle would stop, eventually halting life.
Why are some substances biodegradable and some non-biodegradable?
Answer
Biodegradable substances (like food waste, paper, cotton) are broken down by biological processes — enzymes produced by microorganisms (bacteria, fungi) can act on their chemical bonds because these substances are naturally occurring organic compounds. Non-biodegradable substances (like plastics, glass, metals, pesticides like DDT) cannot be broken down by microorganisms because their chemical structures are synthetic/man-made and microorganisms have not evolved enzymes to decompose them. They persist in the environment for long periods.
Give any two ways in which biodegradable substances would affect the environment.
Answer
(i) Excessive accumulation of biodegradable waste (like sewage or animal waste) in water bodies causes eutrophication — excessive growth of algae that depletes oxygen, killing aquatic life. (ii) Decomposing biodegradable waste in landfills produces methane (a greenhouse gas) and foul-smelling gases like hydrogen sulphide, contributing to air pollution and global warming.
Give any two ways in which non-biodegradable substances would affect the environment.
Answer
(i) Non-biodegradable substances like plastics persist in the environment for hundreds of years, accumulating in soil and water, choking drains, and harming wildlife that ingests them. (ii) Pesticides like DDT undergo biological magnification — their concentration increases at each trophic level of the food chain, causing reproductive failures and organ damage in top predators including humans.
Section B: NCERT Exercise Questions
Which of the following groups contain only biodegradable items?
Answer
(a) Grass, flowers and leather (b) Grass, wood and plastic (c) Fruit peels, cake and lime juice (d) Cake, wood and grass
Ans. (a), (c), and (d) all contain only biodegradable items. (b) is incorrect because plastic is non-biodegradable. Grass, flowers, leather, fruit peels, cake, lime juice, wood — all are biodegradable.
Which of the following constitute a food chain?
Answer
(a) Grass, wheat and mango (b) Grass, goat and human (c) Goat, cow and elephant (d) Grass, fish and goat
Ans. (b) Grass
⇒ Goat
⇒ Human. This is a valid food chain where each organism feeds on the previous one. (a) has three producers, (c) has three herbivores, (d) doesn't show a natural feeding relationship between fish and goat.
Which of the following are environment-friendly practices?
Answer
(a) Carrying cloth bags for shopping (b) Switching off unnecessary lights and fans (c) Walking to school instead of going by car (d) All of the above
Ans. (d) All of the above. Cloth bags reduce plastic pollution, switching off lights saves electricity (reduces fossil fuel burning), and walking reduces vehicle emissions. All three practices reduce environmental impact.
What will happen if we kill all the organisms in one trophic level?
Answer
Removing all organisms at one trophic level would severely disrupt the food chain and ecosystem: (i) The organisms at the trophic level above would lose their food source and decline or die. (ii) The organisms at the trophic level below would multiply uncontrollably due to the absence of predators, potentially exhausting their own food source. (iii) This cascading effect would destabilise the entire ecosystem. For example, killing all frogs would cause insect populations to explode while snake populations would decline.
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?
Answer
Yes, the impact varies. Removing producers (T1) would be most catastrophic — all life depends on them for food and oxygen. Removing top predators (T4/T5) would cause less immediate damage but would lead to overpopulation of prey species. No trophic level can be removed without causing damage — every level plays an essential role in maintaining the balance of the ecosystem. Even decomposers are critical for nutrient recycling.
What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?
Answer
Biological magnification (biomagnification) is the progressive increase in the concentration of non-biodegradable toxic chemicals (like DDT, mercury, lead) at each successive trophic level of a food chain. This happens because these chemicals cannot be metabolised or excreted and keep accumulating in the body tissues of organisms.
Yes, the levels differ — concentration increases at higher trophic levels. For example, if water has DDT at 0.003 ppm, zooplankton may have 0.04 ppm, small fish 0.5 ppm, large fish 2 ppm, and fish-eating birds 25 ppm. Top predators have the highest concentration and suffer the most damage.
What are the problems caused by the non-biodegradable wastes that we generate?
Answer
(i) They persist in the environment for very long periods (plastics for hundreds of years), causing land and water pollution. (ii) They choke drainage systems and water bodies, causing floods and stagnant water (mosquito breeding). (iii) Animals and marine life ingest plastic, leading to choking, starvation, and death. (iv) Toxic chemicals undergo biomagnification in food chains. (v) Burning non-biodegradable waste releases toxic fumes (dioxins, furans). (vi) They occupy valuable landfill space. (vii) Microplastics contaminate soil and water, entering the human food chain.
If all the waste we generate is biodegradable, will this have no impact on the environment?
Answer
No, even if all waste were biodegradable, it would still impact the environment if generated in excessive amounts: (i) Large quantities of biodegradable waste decomposing simultaneously would produce methane and CO₂, contributing to greenhouse effect and global warming. (ii) Excessive organic waste in water bodies causes eutrophication — oxygen depletion and death of aquatic life. (iii) Decomposing waste produces foul odours and attracts disease-carrying vectors (flies, rats). (iv) Rapid decomposition can release nutrients too quickly, disrupting soil chemistry. However, the impact would be far less severe and shorter-lasting than non-biodegradable waste.
Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?
Answer
The ozone layer (in the stratosphere, 20-50 km above Earth) absorbs harmful ultraviolet (UV) radiation from the Sun. Damage to this layer allows more UV rays to reach Earth's surface, causing: (i) skin cancer, (ii) cataracts and eye damage, (iii) weakened immune systems, (iv) damage to crops and reduced agricultural productivity, (v) harm to marine phytoplankton (affecting oceanic food chains).
Steps taken: The Montreal Protocol (1987) — an international agreement to phase out the production and use of ozone-depleting substances (ODS), mainly CFCs (chlorofluorocarbons) used in refrigerators, ACs, aerosol sprays, and fire extinguishers. Alternative chemicals (HFCs, which don't harm ozone) have been developed. As a result, the ozone layer has started recovering.
Section C: Important Extra Questions
What is an ecosystem? Explain its components.
Answer
An ecosystem is a self-sustaining functional unit of nature where living organisms interact with each other and with their physical (non-living) environment.
Components
(i) Abiotic (non-living): Sunlight, temperature, water, air, soil, minerals, humidity, pH.
(ii) Biotic (living): Producers (green plants — make food via photosynthesis), Consumers (animals — herbivores, carnivores, omnivores), Decomposers (bacteria and fungi — break down dead matter).
Examples: Forest ecosystem, pond ecosystem, desert ecosystem, grassland ecosystem.
Explain the 10% law of energy transfer.
Answer
The 10% law (Lindeman's rule) states that only about 10% of the energy available at one trophic level is transferred to the next trophic level. The remaining 90% is used by the organism for its own life processes (respiration, movement, growth, reproduction) and is lost as heat to the environment. For example, if producers capture 10,000 J of solar energy, herbivores get 1,000 J, secondary consumers get 100 J, and tertiary consumers get only 10 J. This is why food chains rarely have more than 4-5 trophic levels, and why there are fewer top predators than herbivores.
What is the difference between a food chain and a food web?
Answer
Food chain: A linear, single pathway showing the transfer of energy from one organism to the next. E.g., Grass
⇒ Deer
⇒ Lion. Simple and shows one feeding relationship.
Food web: An interconnected network of multiple food chains in an ecosystem, showing that most organisms eat and are eaten by more than one species. E.g., Grass is eaten by deer, rabbits, and grasshoppers; grasshoppers are eaten by frogs and birds. Food webs are more realistic and stable — if one species declines, others can substitute.
How can we manage waste effectively? Explain the 3R approach.
Answer
The 3R approach — Reduce, Reuse, Recycle — minimises waste and environmental damage:
Reduce: Use less — avoid unnecessary purchases, use both sides of paper, switch off lights, use cloth bags instead of plastic.
Reuse: Use items again without processing — refill water bottles, donate old clothes, use containers for storage, repair items instead of discarding.
Recycle: Reprocess waste into new products — recycle paper, glass, metals, and certain plastics. Separate biodegradable and non-biodegradable waste for proper processing. Compost organic waste to produce manure.