Ans: A food chain is a linear sequence showing the transfer of food energy from one organism to another. Example: Grass → Grasshopper → Frog → Snake → Eagle. Each level is called a trophic level. 10% Law (Lindeman, 1942): Only 10% of the energy at one trophic level is transferred to the next trophic level; 90% is lost as heat, used in metabolic processes, or not consumed. Example: If grass has 1000J of energy → grasshopper gets 100J → frog gets 10J → snake gets 1J → eagle gets 0.1J. This is why food chains rarely exceed 4–5 trophic levels and why it is more efficient for humans to eat plants directly than to eat animals.

Ans: Ozone (O₃) is a molecule of three oxygen atoms, forming a layer in the stratosphere (15–35 km above Earth). It absorbs harmful UV (ultraviolet) radiation from the sun, acting as a protective shield. Depletion: CFCs (chlorofluorocarbons) — used in refrigerators, air conditioners, aerosol sprays — rise to the stratosphere. UV light breaks CFC molecules → releases chlorine atoms → each Cl atom destroys thousands of O₃ molecules. The ozone hole is most severe over Antarctica. Consequences of UV reaching Earth: skin cancer, cataracts, weakened immunity, damage to marine ecosystems, reduced crop yields. Solution: Montreal Protocol (1987) — international treaty phasing out CFC production.

Ans: Biological magnification (biomagnification): the progressive increase in the concentration of harmful, non-biodegradable chemicals as they move up the food chain, because these substances are not broken down and accumulate in fatty tissues. Example: DDT (pesticide) sprayed on crops → absorbed by plants (low concentration) → taken in by insects eating plants (higher) → concentration increases in frogs eating insects → even higher in snakes eating frogs → very high in eagles at top. Humans, being top consumers, accumulate the highest concentrations. Effects: reproductive failure in birds, cancer in humans, developmental disorders. Biomagnification of heavy metals (mercury, lead) and pesticides (DDT, BHC) is a serious environmental concern.

Ans: Biodegradable waste: can be broken down by microorganisms (bacteria, fungi) into simpler harmless substances. Examples: food scraps, vegetable peels, paper, cotton, wood, dung, leaves. Can be composted. Non-biodegradable waste: cannot be broken down by microorganisms and persists in the environment for very long periods. Examples: plastics, metals, glass, pesticides (DDT), synthetic fibres, nuclear waste. Non-biodegradable waste causes long-term pollution, enters food chains (biomagnification), and is difficult to manage. Solution: reduce use of non-biodegradable materials, recycle, and dispose of safely.