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Exam questions · Biology · Cycles in Ecosystems

The Carbon Cycle

  • 7 exam questions
  • 21 marks
  • 8 quick checks
  1. 1 Name [3 marks]

    Figure 1 shows the carbon cycle. Name the processes shown at A, B and C.

    The carbon cycle with three processes marked A, B and C.
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    Model answer

    A is photosynthesis. B is respiration. C is combustion.

    Mark scheme

    • A: photosynthesis — 1 mark
    • B: respiration — 1 mark
    • C: combustion — 1 mark
  2. 2 State [2 marks]

    State two processes that return carbon dioxide to the air.

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    Model answer

    Any two of: respiration, decay (by decomposers), combustion.

    Mark scheme

    • Process 1 — 1 mark
    • Process 2 — 1 mark
  3. 3 Describe [3 marks]

    Describe how carbon dioxide in the air becomes part of a rabbit.

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    Model answer

    Plants take in carbon dioxide by photosynthesis and use it to make glucose and other compounds, which become part of the plant. The rabbit eats the plant. The carbon compounds in the plant become part of the rabbit's body.

    Mark scheme

    • Photosynthesis takes carbon dioxide into plants — 1 mark
    • The rabbit eats the plant — 1 mark
    • Carbon compounds become part of the rabbit — 1 mark
  4. 4 Explain [3 marks]

    Explain why the decay of a dead animal releases carbon dioxide.

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    Model answer

    Decomposers, such as bacteria and fungi, feed on the dead animal. The decomposers respire. Respiration releases carbon dioxide into the air.

    Mark scheme

    • Decomposers feed on the dead animal — 1 mark
    • The decomposers respire — 1 mark
    • Respiration releases carbon dioxide — 1 mark
  5. 5 Explain [4 marks]

    Explain why the level of carbon dioxide in the air stays fairly constant in a natural ecosystem.

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    Model answer

    Plants remove carbon dioxide from the air by photosynthesis. Plants, animals and decomposers return carbon dioxide to the air by respiration. Decomposers also return carbon dioxide when they break down dead material. The amount removed is about the same as the amount returned.

    Mark scheme

    • Photosynthesis removes carbon dioxide — 1 mark
    • Respiration returns carbon dioxide — 1 mark
    • Decay returns carbon dioxide — 1 mark
    • The amounts removed and returned are about equal — 1 mark
  6. 6 Explain [3 marks]

    Explain why burning fossil fuels increases the amount of carbon dioxide in the air.

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    Model answer

    Fossil fuels contain carbon that was locked up from the remains of plants and animals millions of years ago. When they are burnt in combustion, the carbon combines with oxygen and is released as carbon dioxide. This adds carbon dioxide to the air faster than photosynthesis removes it.

    Mark scheme

    • Fossil fuels contain locked-up carbon — 1 mark
    • Burning releases carbon dioxide — 1 mark
    • It is added faster than photosynthesis removes it — 1 mark
  7. 7 Explain [3 marks]

    Explain why the carbon cycle is important to living organisms.

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    Model answer

    Living things are made of carbon compounds such as carbohydrates, proteins and fats, so they need a supply of carbon. The carbon cycle means that the same carbon is used again and again. It also keeps the level of carbon dioxide in the air available for photosynthesis.

    Mark scheme

    • Living things need carbon for their compounds — 1 mark
    • The carbon is reused again and again — 1 mark
    • Carbon dioxide is available for photosynthesis — 1 mark

Quick check

  1. 1

    Which process removes carbon dioxide from the air?

    1. ARespiration
    2. BPhotosynthesis
    3. CCombustion
    4. DDecay
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    B: Photosynthesis

    Photosynthesis takes carbon dioxide from the air and makes glucose.

  2. 2

    How does carbon pass from a plant to a rabbit?

    1. ABy respiration
    2. BBy combustion
    3. CBy feeding
    4. DBy photosynthesis
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    C: By feeding

    Carbon passes along the food chain by feeding.

  3. 3

    Which of these organisms release carbon dioxide by respiration?

    1. AOnly animals
    2. BOnly plants
    3. COnly decomposers
    4. DPlants, animals and decomposers
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    D: Plants, animals and decomposers

    Plants, animals and decomposers all respire.

  4. 4

    What is released when coal is burnt?

    1. ACarbon dioxide
    2. BOxygen
    3. CNitrogen
    4. DGlucose
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    A: Carbon dioxide

    Burning releases the carbon in the coal as carbon dioxide.

  5. 5

    Why do decomposers release carbon dioxide?

    1. AThey carry out photosynthesis
    2. BThey burn the dead material
    3. CThey respire
    4. DThey eat carbon dioxide
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    C: They respire

    Decomposers respire as they break down dead material.

  6. 6

    Which process locks carbon up for millions of years?

    1. ARespiration
    2. BThe formation of fossil fuels
    3. CFeeding
    4. DDecay
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    B: The formation of fossil fuels

    The formation of fossil fuels from the remains of living things locks carbon up for millions of years.

  7. 7

    What is the main store of carbon in the air?

    1. AMethane
    2. BGlucose
    3. COxygen
    4. DCarbon dioxide
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    D: Carbon dioxide

    Carbon is in the air as carbon dioxide.

  8. 8

    What happens to the carbon dioxide level if fossil fuels are burnt faster than photosynthesis removes it?

    1. AIt rises
    2. BIt falls
    3. CIt stays the same
    4. DIt disappears
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    A: It rises

    More carbon dioxide is added than removed, so the level rises.