Science — CBSE Class 10 board question

Trophic levels: Each step or level of a food chain at which organisms feed is called a trophic level.

Autotrophs at first trophic level: Autotrophs (green plants) fix solar energy and convert it into chemical food energy, making it available for all other organisms. Since they are the primary source of energy for every food chain, they occupy the first trophic level.

Limited trophic levels: Only about 10% of energy is transferred from one trophic level to the next; the rest is lost as heat. So after three or four trophic levels, the energy available becomes too little to support any further level, limiting food chains to 3–4 steps.

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

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• Examiners expect three clear parts: definition of trophic levels, reason for autotrophs at T1, and reason for limited levels.
• The 10% law is the key concept for the third part — always mention that energy loss is so great that very little usable energy remains after four levels.
• Avoid long sentences; state each point directly and concisely.