NCERT Solutions for Class 10 Science Chapter 13 Our Environment

A trophic level is each step of a food chain — the textbook puts it plainly: “each step or level of the food chain forms a trophic level” (NCERT, p. 210). The producers are the first trophic level, and each consumer above them forms the next.

An example chain, built fresh rather than copied from the book: grass → grasshopper → frog → snake.

• Grass — producer, first trophic level
• Grasshopper — primary consumer (herbivore), second trophic level
• Frog — secondary consumer (carnivore), third trophic level
• Snake — tertiary consumer, fourth trophic level

Figure 13.1 below shows real chains from a forest, a grassland and a pond; the trophic-level diagram in the key-concepts section above stacks the same levels in order.

Decomposers — bacteria and fungi — break down the dead remains and waste products of organisms into simple inorganic substances (NCERT, p. 209). These substances return to the soil and are taken up once more by producers. That is what makes an ecosystem self-sustaining: producers make food, consumers eat it, and decomposers recycle the leftovers so producers can start again.

In their absence, dead animals, dead plants and garbage would simply pile up, and the natural replenishment of the soil would stop. The nutrients locked in dead matter would never come back to the producers.

Biodegradability depends on whether the enzymes of bacteria and fungi can break a substance down. Enzymes are specific in their action — a particular enzyme acts on a particular substance, which is exactly why we cannot digest coal (NCERT, p. 213). Many human-made materials like plastics have no matching enzyme, so no microorganism can decompose them.

Physical processes such as heat and pressure may wear these materials down, but under ordinary environmental conditions they persist for a long time. So the dividing line is simple: substances broken down by biological processes are biodegradable; those that are not are non-biodegradable.

Biodegradable substances return to nature through decay, but in excess they still disturb it. Two distinct effects the board expects:

• When large amounts decay together, they release foul gases and breed harmful organisms such as flies and disease-causing microbes.
• Dumped in one place, they add so many nutrients that they pollute water bodies — overloading the water instead of enriching it.

Decay is natural, but the volume and location of the waste decide how damaging it becomes.

Non-biodegradable substances do not decay, so they accumulate. Two distinct effects follow:

• They persist in the environment for a long time, occupying land as they pile up in dumps, and choking drains and water bodies.
• Animals may ingest them and be harmed; being inert and persistent, they keep harming members of the ecosystem over long periods (NCERT, p. 213).

The damage is long-lasting precisely because nothing biological can remove these materials.

Ozone is a molecule made of three oxygen atoms (\( O_3 \)), formed high in the atmosphere (NCERT, p. 212). There it performs a vital function: it shields the Earth’s surface from ultraviolet (UV) radiation from the Sun, which is highly damaging to organisms and causes skin cancer in human beings.

Ozone forms when high-energy UV radiation splits molecular oxygen into free oxygen atoms, which then combine with oxygen molecules:

\[ O_2 \xrightarrow{\text{UV}} O + O \] \[ O + O_2 \rightarrow O_3 \]

(NCERT, p. 213). If CFCs deplete this shield, more UV reaches the ground. That harms the producers at the base of every food chain, so the damage travels upward through the whole ecosystem — a drop in plant life starves every consumer above it.

Waste disposal is a problem of volume and mix. Two methods a Class-10 student can actually use:

• Segregate waste at home — separate biodegradable matter from non-biodegradable. This lets the biodegradable part be composted and treated, and stops non-biodegradable materials from contaminating the whole lot.
• Compost kitchen refuse — vegetable peels and spoilt food break down in a compost pit, returning nutrients to the soil instead of adding to landfill.

Each method works because it reduces the amount of waste that reaches the dump and keeps the biodegradable part cycling back to nature.

Correct answer: (a). Run every item against the definition: biodegradable means broken down by biological processes of microorganisms (NCERT, p. 213). Grass, flowers and leather all decay, so (a) is the group the textbook accepts.

Option (b) clearly fails because plastic is non-biodegradable — no enzyme of any microorganism acts on it, so it persists. Options (c) and (d) also contain biodegradable materials such as fruit-peels and wood, and are close calls; the cleanest group whose members all clearly rot away naturally is (a), which is why it is the intended answer.

Correct answer: (b). A food chain must run from producer to consumer, with each organism eaten by the next (NCERT, p. 210). In (b), grass is the producer, the goat is a primary consumer that eats the grass, and the human eats the goat — a genuine chain.

The others fail: in (a) wheat and mango are also producers, not eaters of grass; in (c) all three are consumers with no producer, so no one feeds anyone; in (d) fish do not eat grass directly, so no real feeding link exists. Figure 13.3 shows how real chains interlock into a food web.

Correct answer: (d) All of the above. Each habit reduces waste or energy use. Cloth bags replace single-use plastic bags, so less non-biodegradable waste is generated. Switching off lights and fans saves electricity. Walking to school cuts fuel use and emissions. All three are environment-friendly.

Every trophic level depends on the one below it for food, and the level above depends on it.

Remove one level and the chain breaks in both directions (NCERT, p. 210): the organisms of the level above lose their food source and may starve, while the organisms of the level below, no longer hunted or eaten, multiply excessively and overuse their own food.

The result is a collapse of the whole chain, because the 10% law means each level’s energy supply depends entirely on the level beneath it. The trophic-level diagram in the key-concepts section shows this dependence in order.

Yes, the impact differs by level. If the producers go, every consumer above them loses its food instantly and the whole ecosystem collapses. If a top predator goes, its prey may overgrow and then over-eat the level below — serious, but slower to reach every level.

No level can be removed without damage. Energy flow is unidirectional: energy captured by producers never returns to them, and once it moves on it is no longer available to the previous level (NCERT, p. 211). Every step depends entirely on the one below, so removing any step starves what sits above it.

Biological magnification is the progressive accumulation of non-degradable chemicals as they pass up a food chain (NCERT, p. 211). Pesticides wash into soil and water, plants absorb them, herbivores eat the plants, and carnivores eat the herbivores. Because the chemicals are not degradable, each consumer stores the doses from all the prey it ate, so the concentration builds at every step.

The levels of magnification are different at different trophic levels — they rise as you go up the chain. Plants hold the smallest amount, the top carnivore the largest, and human beings, at the top, accumulate the maximum concentration. Figure 13.4 shows why: energy flowing up the chain is one-way, and the non-degradable chemicals ride along with it, concentrating as they go.

Non-biodegradable waste does not decay, so it accumulates, and the problems follow from that persistence (NCERT, p. 213). It occupies land as it piles up in dumps, chokes drains and water bodies, and stays in the soil for long periods. Animals may ingest pieces of it and be harmed.

Because nothing biological can break it down, the damage it causes is long-lasting rather than temporary.

No — biodegradable does not mean harmless. The impact lessens but never disappears. If huge volumes of biodegradable waste are dumped in one place, it still decays, and that decay releases foul gases and breeds harmful organisms. Dumped in large amounts into water, it adds enough nutrients to pollute the water body.

So even fully biodegradable waste causes problems when the quantity overwhelms the natural decay process.

Ozone (\( O_3 \)) shields the Earth from ultraviolet (UV) radiation from the Sun, which is highly damaging to organisms and causes skin cancer in humans (NCERT, p. 212). Damage to this shield lets more UV through, harming producers at the base of every food chain and, through them, the whole ecosystem.

Ozone is depleted by synthetic chemicals called chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. To limit the damage, the United Nations Environment Programme (UNEP) forged a 1987 agreement to freeze CFC production at 1986 levels. It is now mandatory for manufacturing companies worldwide to make CFC-free refrigerators (NCERT, p. 213).

Ozone itself forms when UV splits oxygen molecules into free atoms, which join oxygen molecules:

\[ O_2 \xrightarrow{\text{UV}} O + O \] \[ O + O_2 \rightarrow O_3 \]