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(a) "In a food chain energy… (Class 10 Science 2025, Q33)

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ScienceCBSE Class 10 · Board exam 2025, set 31/3/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 33

Set in the CBSE Class 10 board exam of 2025, paper 31/3/1, section C, question 33, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q33 · 3 marks · Short answerOur Environment

(a) "In a food chain energy flow is unidirectional." Give two reasons for the given statement.

(b) If 10,000 J energy is available at the producer level, how much energy will be available to the secondary consumers? Give reason to justify your answer.

  1. (a)

    "In a food chain energy flow is unidirectional." Give two reasons for the given statement.

    2 marks
  2. (b)

    If 10,000 J energy is available at the producer level, how much energy will be available to the secondary consumers? Give reason to justify your answer.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/3/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Can energy eaten by a herbivore go back to the plant?

Hint 2

Apply the 10% law twice.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) Reason 1: energy captured by plants does not go back to the sun, and energy passed to herbivores does not come back to the autotrophs.

    1 mark

  2. Reason 2: as energy moves through successive trophic levels, much is lost as heat at each level, so it is no longer available to the previous level.

    1 mark

  3. (b) Producer 10000 J → primary consumer 1000 J → secondary consumer 100 J.

    ½ mark

  4. Reason: by the 10% law, only about 10% of the energy at one trophic level is transferred to the next.

    ½ mark

Answer: (a) Energy does not return to earlier levels and is lost at each level; (b) 100 J (10% law)

Where the marks are

  • Any two correct reasons for unidirectional flow (1 + 1 mark)
  • 100 J (½ mark)
  • Justification by 10% law (½ mark)

Revise the chapter

  • Chapter: Our Environment (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Our Environment, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/3/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.