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(i) It was estimated… (Class 12 Biology SQP 2024-25, Q21A)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2024-25: question 21(A)

From the official CBSE sample question paper for 2024-25, section B, question 21(A), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(A) · 2 marks · Very short answerCompetency-basedEcosystem

(i) It was estimated that if an evergreen forest has a GPP of 400 J/m²/day and 150 J/m²/day worth of carbon dioxide flows out of that forest, what is the NPP in that forest?
(ii) Explain why pyramids of energy must always be upright.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

The CO₂ released by the forest comes from respiration.

Hint 2

Net productivity is what remains after respiration is subtracted.

Hint 3

For (ii), think about the 10% law and heat loss.

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. (i) The CO₂ flowing out represents respiration losses (R).

    Formula: NPP = GPP − R

    Substitution: NPP = 400 J/m²/day − 150 J/m²/day

    NPP = 250 J/m²/day

    1 mark

  2. (ii) Energy flows one way, from each trophic level to the next. At every level a large part of the energy is lost as heat in respiration, and only about 10% is passed on. Each higher level therefore always has less energy than the one below it, so the pyramid of energy is always upright and can never be inverted.

    1 mark

Answer: (i) NPP = 250 J/m²/day; (ii) Energy is lost as heat at each trophic level, so higher levels always have less energy.

Where the marks are

  • Uses NPP = GPP − R and gets 250 J/m²/day (1 mark)
  • Explains that energy is lost as heat at each trophic level, so the pyramid is always upright (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Ecosystem, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: 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.