An ecosystem is the term that describes all the interacting organisms in an area together with the non-living physical factors (abiotic components like temperature, soil, rainfall) of their surroundings.

Source: Chapter 13, Section 13.1 – Ecosystem: What Are Its Components?

The examiner expects the single word ecosystem along with a brief definition. Mentioning biotic (living) and abiotic (non-living/physical) components strengthens the answer. Avoid writing a long paragraph — one clear sentence is enough for 1 mark.

An ecosystem is a system where all living organisms (biotic components) and physical surroundings (abiotic components) interact with each other. In a pond ecosystem, fish is a biotic component and sunlight/water is an abiotic component.

Source: Chapter 13, Section 13.1

This question is worth only 1 mark, so one clean sentence covering the definition plus the two examples is sufficient. Examiners look for: (1) the idea of interaction between living and non-living components, (2) one correct biotic example (fish, aquatic plants, frogs, etc.), and (3) one correct abiotic example (water, sunlight, temperature, minerals, etc.). Do not write more than 2 lines.

Role of Decomposers:
Decomposers (bacteria and fungi) break down dead and decaying organic matter — dead plants, animals, and their waste — into simple inorganic substances like minerals and nutrients, which are returned to the soil. This makes nutrients available again for producers (plants), completing the nutrient cycle in the ecosystem.

If Decomposers Were Absent:
Dead organic matter would keep accumulating in the environment. Nutrients locked in dead organisms would never be recycled back into the soil. This would deprive producers of essential minerals, disrupting the entire food chain. The ecosystem would eventually collapse due to nutrient depletion and overwhelming accumulation of waste.

Source: Our Environment, Chapter 13

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• Examiners expect two clear parts: the role (nutrient/mineral recycling) and the consequence of absence (accumulation of dead matter + nutrient depletion).
• The source passage emphasises interdependence of ecosystem components and biodegradable waste — tie decomposers to biodegradable breakdown and nutrient cycling.
• Avoid vague phrases like "they help the environment." Be specific: break down → release minerals → absorbed by producers.
• At 3 marks, aim for ~2–3 focused points per part — do not over-write.

(B) So little usable energy remains after four trophic levels that supporting another level becomes impossible.

The 10% energy law states only ~10% of energy passes to the next trophic level; after four levels, energy is too little to sustain further consumers. Options A, C, and D are factually incorrect as per the textbook.

| Organism | Trophic Level | Type of Consumer |
|---|---|---|
| Grass | 1st (Producer) | — (Autotroph) |
| Grasshopper | 2nd | Primary consumer (Herbivore) |
| Frog | 3rd | Secondary consumer (Small carnivore) |
| Snake | 4th | Tertiary consumer (Larger carnivore) |

The autotrophs (grass) fix solar energy at the first trophic level. Each subsequent animal feeds on the organism below it, moving energy up the chain, with only ~10% transferred at each level.

Source: Chapter 13, Section 13.1.1 – Food Chains and Webs

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• Trophic level = each step in a food chain; examiners expect you to number them (1st, 2nd, etc.) AND name the level (producer, primary consumer, etc.).
• The textbook explicitly lists: herbivores = primary consumers (2nd), small carnivores = secondary consumers (3rd), larger carnivores = tertiary consumers (4th). Use these exact terms.
• A table format is clear and earns full marks efficiently for this type of question — it covers all three required organisms (animals) plus the producer without wasting words.
• Do not confuse "trophic level" with "consumer level" — grass is at the 1st trophic level but is not a consumer.

The remaining 90% is lost as heat to the environment, and some is used in digestion and doing work by the organisms at that trophic level.

Source: Our Environment, Chapter 13, Section 13.1.1

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The key phrase examiners look for is "lost as heat to the environment" plus mention of use in digestion/life processes. Avoid vague answers like "it is used up" — be specific that energy is dissipated as heat and used in metabolic activities, making it unavailable to the next level.

(C) Non-degradable chemicals accumulate progressively at each trophic level, so the final consumer inherits the combined load from all levels below.

The textbook (Ch. 13) explicitly states that pesticides "are not degradable" and therefore "get accumulated progressively at each trophic level," with the maximum concentration reaching top consumers — this phenomenon is called biological magnification. Options A, B, and D are factually incorrect as per the chapter.

Biological magnification is the increase in the concentration of harmful chemicals (like pesticides) at each successive trophic level in a food chain. This happens because these chemicals are non-biodegradable and accumulate in the body of organisms.

The top consumer (e.g., humans) in a food chain has the highest concentration of pesticides.

Reason: Since these chemicals cannot be broken down, they keep accumulating at each trophic level. By the time they reach the top consumer, the concentration becomes the highest, as the top consumer eats many organisms from the lower levels over its lifetime.

Source: Our Environment, Chapter 13

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• 3 marks = 3 points: Define biological magnification (1 mark) + name the correct organism (1 mark) + give a valid reason (1 mark).
• Examiners expect the term "non-biodegradable" in the reason — it is the key word from the chapter.
• "Top consumer / top carnivore / humans" are all accepted; pick any one.
• Don't write a long paragraph — three tight points are enough for full marks.

Biodegradable substances are broken down by biological processes (microorganisms). Example: vegetable peels.
Non-biodegradable substances are not broken down biologically and persist in the environment. Example: plastic.

Source: Our Environment, Section 13.2.2

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Since this is a 1-mark question, examiners expect both definitions in one or two compact lines with one example each. No elaboration needed. Key terms to use: "biological processes / microorganisms" for biodegradable, and "not broken down / persist" for non-biodegradable. Common examples: biodegradable — fruit peels, food waste; non-biodegradable — plastic, glass, DDT. Avoid writing lengthy paragraphs.

Bacteria and decomposers break down substances using specific enzymes. Just as enzymes in our body are specific to particular substances, no natural enzyme exists that can break down the chemical structure of plastics (human-made materials). Since plastics lack a matching biological enzyme, they cannot be decomposed by bacteria or other saprophytes, making them non-biodegradable.

Source: Chapter 13, Section 13.2.2 – Managing the Garbage we Produce

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• The key concept here is enzyme specificity — examiners want this term used explicitly.
• Mention that plastic is a human-made (synthetic) material, for which no corresponding enzyme exists in nature.
• Avoid vague answers like "bacteria can't digest plastic" without explaining why (enzyme specificity is the reason).
• Two marks = two clear points: (1) enzymes are specific, (2) no enzyme exists for plastic → non-biodegradable.

(C) Empty plastic medicine strips, synthetic packaging material, plastic bags — all are non-biodegradable as they are human-made materials not broken down by biological processes.

The passage (Activity 13.5) lists empty medicine strips/bubble packs and broken footwear as examples of waste, and states that human-made materials like plastics are non-biodegradable. Options A and D contain biodegradable items (vegetable/fruit peels, tea leaves, cotton, dry leaves, paper). Option B includes spoilt food (biodegradable). Only Option C has all non-biodegradable items.

Modern lifestyle has led to increased generation of non-biodegradable waste in the following ways:

1. Change in packaging: Changes in packaging have resulted in much of our waste becoming non-biodegradable. Products are increasingly packed in plastics and synthetic materials that cannot be broken down by biological processes.

1. Use of disposable items: Changes in attitude have made more and more things disposable (e.g., disposable plastic cups, wrappers, medicine strips/bubble packs). These single-use items are made of human-made materials like plastics, which are not broken down by bacteria or other saprophytes and persist in the environment for a long time.

Source: Chapter 13, Section 13.2.2 — Managing the Garbage we Produce

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• The question asks for two ways, but it is 3 marks — so both points need some elaboration (not just one-word answers).
• The textbook explicitly states: "Changes in packaging have resulted in much of our waste becoming non-biodegradable" and "more and more things we use becoming disposable" — use these exact ideas.
• Avoid copying unrelated content (like CFCs or ozone); the question is specifically about non-biodegradable waste generation from lifestyle changes.
• Examiners look for: correct identification of the lifestyle change + linking it to non-biodegradable waste. Both points must be distinct.