Science (086) — AI-generated practice question

About 90% of the energy at each trophic level is lost to the environment as heat during metabolic processes such as digestion, respiration, and doing work, and some is used for the organism's own growth and reproduction. Only 10% is stored in body tissues and passed to the next level.

This makes a six- or seven-level food chain practically impossible because the energy diminishes drastically at each step. By the fourth trophic level, so little usable energy remains that it cannot support organisms at a fifth or sixth level. Hence, food chains are generally limited to three or four trophic levels.

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

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What examiners look for (3 marks):

• 1 mark – State where the 90% goes: lost as heat to the environment through metabolic activities (digestion, respiration, work).
• 1 mark – Explain the consequence: energy keeps diminishing at each successive level.
• 1 mark – Conclude: after 3–4 trophic levels, usable energy is negligible, so a 6–7 level chain is impossible.

Key tip: Always use the textbook phrase "lost as heat to the environment" — avoid vague terms like "wasted." The 10% law is the core concept here; state it clearly before explaining its consequence on food chain length.