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Biomass of a standing… (Class 12 Biology SQP 2023-24, Q21a)

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BiologyCBSE Class 12 · Sample paper, session 2023-24 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2023-24: question 21(a)

From the official CBSE sample question paper for 2023-24, 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

Biomass of a standing crop of phytoplankton is 4 kg/m which supports a large standing crop of zooplankton having a biomass 11 kg/m. This is consumed by small fishes having biomass 25 kg/m which are then consumed by large fishes with the biomass 37 kg/m.

Draw an ecological pyramid indicating the biomass at each stage and also name the trophic levels. Mention whether it is an upright or inverted pyramid.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Put the producers at the base and draw each bar in proportion to its biomass.

Hint 2

Compare the biomass at the base with the biomass at the top. Which is larger?

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. Pyramid of biomass, drawn from bottom to top (each bar's width proportional to biomass):

    Level (top to bottom)OrganismsBiomass
    T4 – Tertiary consumers (top carnivores)Large fishes37 kg/m² (widest bar, at top)
    T3 – Secondary consumersSmall fishes25 kg/m²
    T2 – Primary consumers (herbivores)Zooplankton11 kg/m²
    T1 – ProducersPhytoplankton4 kg/m² (narrowest bar, at base)
    1½ marks

  2. The base (producers) is the smallest bar and the bars widen upwards. So it is an inverted pyramid of biomass, which is typical of aquatic ecosystems because the small standing crop of phytoplankton turns over rapidly.

    ½ mark

Answer: Pyramid with phytoplankton (producers) 4 → zooplankton (primary consumers) 11 → small fish (secondary consumers) 25 → large fish (tertiary consumers) 37 kg/m²; it is an inverted pyramid of biomass.

Where the marks are

  • Correct pyramid drawn with biomass values at each level (1 mark)
  • Trophic levels correctly named (0.5 mark)
  • States it is inverted (0.5 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 2023-24: 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.