Question Bank — Ecosystem

NCERT Class 12 Biology · Chapter 12 · CBSE Board Pattern

Class 12 Biology Chapter 12 Question Bank CBSE Board Pattern

QUESTION BANK

Chapter: Ecosystem (NCERT Class 12 Biology)

Section A — MCQs (10 questions, 1 mark each)

  1. Which of the following is a man-made ecosystem?
    (a) Forest (b) Pond (c) Aquarium (d) Lake

  2. Net primary productivity is calculated as:
    (a) GPP + R (b) GPP – R (c) NPP + R (d) Secondary productivity

  3. In an ecosystem, the unidirectional flow of energy is in accordance with:
    (a) First law of thermodynamics only (b) Second law of thermodynamics only (c) Both laws (d) Neither law

  4. Which step of decomposition involves the downward movement of water-soluble nutrients?
    (a) Fragmentation (b) Leaching (c) Catabolism (d) Mineralisation

  5. The pyramid of energy in an ecosystem is always:
    (a) Upright (b) Inverted (c) Spindle-shaped (d) Variable

Assertion-Reason Questions

  1. Assertion (A): In a terrestrial ecosystem, a larger fraction of energy flows through the detritus food chain than through the grazing food chain.
    Reason (R): In terrestrial ecosystems, a major portion of the net primary productivity is not directly consumed by herbivores and instead enters the detritus pathway.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

  2. Assertion (A): The pyramid of biomass in an aquatic ecosystem is generally inverted.
    Reason (R): The biomass of zooplankton is much higher than the biomass of phytoplankton.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

  3. The rate of biomass production is expressed as:
    (a) gm⁻² (b) gm⁻² yr⁻¹ (c) kcal m⁻² (d) kcal

  4. Which of the following occupies the second trophic level in a lake ecosystem?
    (a) Phytoplankton (b) Zooplankton (c) Fishes (d) Benthos

  5. Humus is formed during which process of decomposition?
    (a) Fragmentation (b) Leaching (c) Humification (d) Mineralisation

Section B — Very Short Answer (6 questions, 2 marks each)

  1. Distinguish between gross primary productivity and net primary productivity.
  2. What is stratification? Give one example from a forest ecosystem.
  3. Why is the pyramid of energy always upright?
  4. Define secondary productivity. How is it different from primary productivity?
  5. Name the two types of food chains. Which one is the major conduit of energy flow in an aquatic ecosystem?
  6. What is detritus? Name two detritivores.

Section C — Short Answer (5 questions, 3 marks each)

  1. Describe the four basic components that show the functioning of an ecosystem as a unit.
  2. Explain the process of decomposition with the help of its five important steps.
  3. Differentiate between grazing food chain and detritus food chain (any three points).
  4. Why is the number of trophic levels in a grazing food chain limited? Explain with reference to the 10 per cent law.
  5. What are ecological pyramids? Name the three types of ecological pyramids.

Section D — Long Answer (3 questions, 5 marks each)

  1. Describe the structure and functioning of a pond ecosystem with the help of a labelled diagram. (Diagram description required)
  2. Explain the unidirectional flow of energy in an ecosystem. Describe the flow of energy through different trophic levels with a diagrammatic representation. (Diagram description required)
  3. What is primary productivity? Discuss the factors that affect primary productivity. How does primary productivity vary in different ecosystems?

Section E — Case/Source-Based (2 questions, 4 marks each)

Case 1

A small pond is a self-sustainable aquatic ecosystem. It contains water with dissolved inorganic and organic substances, soil at the bottom, solar input, phytoplankton, zooplankton, floating and submerged plants, fishes and decomposers (bacteria and fungi). The system shows conversion of inorganic matter into organic matter, consumption by heterotrophs, and decomposition of dead matter.

25.1 Identify the abiotic components of the pond ecosystem. (1)
25.2 Name the autotrophic and heterotrophic components. (1)
25.3 Which organisms perform the function of mineralisation in the pond? (1)
25.4 Why is energy flow in this ecosystem unidirectional? (1)

Case 2

In an ecosystem, less than 50 per cent of incident solar radiation is photosynthetically active radiation (PAR). Plants capture only 2–10 per cent of PAR. This energy flows from producers to consumers. When an organism dies, it becomes part of detritus. Energy trapped by producers is either passed to consumers or lost as heat.

26.1 What percentage of PAR is captured by plants? (1)
26.2 Why do ecosystems require a constant supply of energy according to the second law of thermodynamics? (1)
26.3 What happens to the energy when an organism dies? (1)
26.4 How are food chains interconnected in a natural ecosystem? (1)

▶ Answer Key Attempt all questions first,
then tap to reveal

Section A

  1. (c) Aquarium — 1 mark
  2. (b) GPP – R — 1 mark
  3. (c) Both laws — 1 mark
  4. (b) Leaching — 1 mark
  5. (a) Upright — 1 mark
  6. (a) — 1 mark
  7. (a) — 1 mark
  8. (b) gm⁻² yr⁻¹ — 1 mark
  9. (b) Zooplankton — 1 mark
  10. (c) Humification — 1 mark

Section B

  1. GPP is total organic matter produced during photosynthesis (1 mark); NPP = GPP – R (respiration losses) (1 mark).
  2. Vertical distribution of species in different layers (1 mark); e.g., trees, shrubs, herbs in forest (1 mark).
  3. Energy decreases at each trophic level due to loss as heat (1 mark); follows 10% law (1 mark).
  4. Rate of formation of new organic matter by consumers (1 mark); primary productivity is by producers (1 mark).
  5. Grazing food chain and detritus food chain (1 mark); Grazing food chain in aquatic ecosystem (1 mark).
  6. Dead organic matter (1 mark); earthworm, fungi/bacteria (1 mark).

Section C

  1. Productivity, decomposition, energy flow, nutrient cycling (1 mark each, any three with brief explanation).
  2. Fragmentation, leaching, catabolism, humification, mineralisation (½ mark each + ½ mark for any two examples).
  3. Any three differences: starting material, major organisms, energy flow proportion in terrestrial vs aquatic (1 mark each).
  4. 10% law — only 10% energy transferred to next level (2 marks); limits number of trophic levels (1 mark).
  5. Graphical representation of trophic relationships in terms of number/biomass/energy (1 mark); three types (1 mark each).

Section D

  1. Abiotic (water, soil, solar input) + biotic components (producers, consumers, decomposers) with pond diagram description (3 marks + 2 marks for correct labelling sequence).
  2. Sun → producers → consumers; unidirectional flow + energy loss as heat; trophic level diagram description (3 marks + 2 marks for diagram).
  3. Definition of primary productivity (1 mark); factors — plant species, nutrients, photosynthetic capacity, environment (3 marks); variation in ecosystems (1 mark).

Section E

25.1 Water, dissolved substances, soil, solar input, temperature (any two — 1 mark)
25.2 Autotrophs: phytoplankton, algae, plants; Heterotrophs: zooplankton, fishes, decomposers (1 mark)
25.3 Bacteria and fungi (1 mark)
25.4 Energy lost as heat at each step, never returns to sun (1 mark)

26.1 2–10% (1 mark)
26.2 To counteract increasing disorderliness / synthesise molecules (1 mark)
26.3 Converted to detritus, energy source for decomposers (1 mark)
26.4 Form food webs (1 mark)

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All questions are answerable from the NCERT chapter text. Reviewed by GFIS faculty.