Science — CBSE Class 10 board question

(a) Energy flow in a food chain is unidirectional because:

1. Energy flows from producers → herbivores → carnivores, and cannot flow back to the previous level; it is lost as heat at each step.
2. At each trophic level, only 10% of energy is transferred to the next level; the remaining 90% is used up in metabolism and lost as heat, making reverse flow impossible.

(b) Energy available to secondary consumers = 100 J

Reason: According to the 10% Law, only 10% of energy is transferred from one trophic level to the next.

• Producer → Primary consumer: 10% of 10,000 J = 1,000 J
• Primary consumer → Secondary consumer: 10% of 1,000 J = 100 J

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• The 10% Law (Lindeman's Law) is the core concept here — always apply it step by step per trophic level.
• For part (a), the two reasons examiners want are: (i) one-way flow (producers → top carnivores only) and (ii) energy is lost as heat at each trophic level, so it cannot be recycled or reversed.
• Avoid vague answers like "energy decreases" — state clearly that heat loss makes reverse flow impossible.
• In part (b), show the two calculation steps to earn full marks.