Q2
1 markVery Short AnswerSection A

In the following food chain, 20,000 J of energy was available to the plants. How much energy would be available to man in this chain? Plants → Sheep → Man

Our Environment
Food Chains and Webs
Official Answer

According to the 10%10\% law of energy transfer in a food chain, only 10%10\% of the energy available at one trophic level is transferred to the next level.


  • Energy available to Plants = 20,000 J20,000\text{ J}
  • Energy transferred to Sheep (Primary Consumer) = 10%10\% of 20,000 J=2,000 J20,000\text{ J} = 2,000\text{ J}
  • Energy transferred to Man (Secondary Consumer) = 10%10\% of 2,000 J=200 J2,000\text{ J} = 200\text{ J}

Therefore, 200 J200\text{ J} of energy would be available to man.

10% lawenergy transfer200 Jtrophic levelprimary consumersecondary consumer

Marking Scheme

  • 1Identification of trophic levels and application of 10%10\% law: 0.5 marks
  • 2Correct calculation of energy available to man (200 J200\text{ J}): 0.5 marks

Hint

Apply the 10%10\% law of energy transfer successively from plants to sheep, and then from sheep to man.

Quick Oral Answer

According to the 10%10\% law, if plants have 20,000 J20,000\text{ J} of energy, the sheep gets 2,000 J2,000\text{ J}, and the man gets 200 J200\text{ J}.

Analysis & Explanation

The 10%10\% law, formulated by Raymond Lindeman, states that during the transfer of organic food energy from one trophic level to the next higher level, only about 10%10\% of the energy is turned into biomass and made available for the next level of consumers. The remaining 90%90\% is lost as heat to the environment, used in digestion, respiration, growth, and reproduction. This progressive and drastic loss of energy limits the length of food chains to generally three or four steps.

Common Mistakes

  1. 1Calculating 10%10\% of the initial energy directly for the final consumer, forgetting to apply it step-by-step for each intermediate trophic level.
  2. 2Writing incorrect units (e.g., writing kJ instead of J).

Interesting Facts

Because energy decreases so rapidly at each step, the biomass and number of individuals also generally decrease at higher trophic levels, which is why top predators are relatively rare in nature.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2017?

This question carries 1 mark in the CBSE Class 10 Science 2017 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Our Environment" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Food Chains and Webs" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2017 paper?

This is a Very Short Answer question from Section A in the CBSE Class 10 Science 2017 paper.