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Exam questions · Biology · Material Cycles

The Carbon Cycle

  • 8 exam questions
  • 25 marks
  • 8 quick checks
  1. 1 Name [2 marks]

    Name two processes that release carbon dioxide into the air.

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    Model answer

    Any two of: respiration, decay (decomposition), combustion (burning).

    Mark scheme

    • First correct process — 1 mark
    • Second correct process — 1 mark
  2. 2 State [1 mark]

    Name the process by which plants take carbon dioxide from the air.

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    Model answer

    Photosynthesis.

    Mark scheme

    • Photosynthesis — 1 mark
  3. 3 Describe [3 marks]

    Describe the role of decomposers in the carbon cycle.

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    Model answer

    Decomposers, which are bacteria and fungi, break down dead plants and animals and waste. They release enzymes that digest the dead material and they absorb the products. They respire, which releases carbon dioxide into the air so that it can be used again by plants.

    Mark scheme

    • Bacteria and fungi break down dead material and waste — 1 mark
    • They respire — 1 mark
    • Carbon dioxide is released into the air, ready to be used by plants again — 1 mark
  4. 4 Explain [3 marks]

    Explain why a forest does not become buried under dead leaves and fallen trees.

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    Model answer

    Decomposers such as bacteria and fungi feed on the dead leaves and wood. They break the material down with enzymes. This releases carbon dioxide and mineral ions, which are returned to the air and the soil, so the material does not build up.

    Mark scheme

    • Decomposers (bacteria and fungi) feed on the dead material — 1 mark
    • They break it down using enzymes — 1 mark
    • Carbon dioxide and mineral ions are returned to the environment — 1 mark
  5. 5 Explain [4 marks]

    Explain how burning fossil fuels affects the carbon cycle and why this matters.

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    Model answer

    Fossil fuels contain carbon that was stored over millions of years. Burning them releases it as carbon dioxide into the air in a very short time. This is faster than photosynthesis can remove it, so the level of carbon dioxide in the air rises. Carbon dioxide is a greenhouse gas, so this contributes to global warming.

    Mark scheme

    • Fossil fuels store carbon locked away for millions of years — 1 mark
    • Combustion releases carbon dioxide into the air — 1 mark
    • Carbon dioxide is released faster than photosynthesis can remove it, so the level rises — 1 mark
    • Carbon dioxide is a greenhouse gas that contributes to global warming — 1 mark
  6. 6 Calculate [2 marks]

    Before the Industrial Revolution the air contained about 280 parts per million of carbon dioxide. By 2023 it contained 420 parts per million. Calculate the percentage increase.

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    Model answer

    Increase = 420 − 280 = 140. \(140 \div 280 \times 100 = 50\%\).

    Mark scheme

    • \(140 \div 280 \times 100\) — 1 mark
    • 50% — 1 mark
  7. 7 Describe [4 marks]

    A food chain is grass, rabbit, fox. Describe how a carbon atom in the carbon dioxide in the air becomes part of the fox.

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    Model answer

    The grass takes in carbon dioxide from the air by photosynthesis and uses it to make glucose, so the carbon becomes part of carbohydrates, proteins and fats in the grass. The rabbit eats the grass, and the carbon compounds pass to the rabbit. The fox eats the rabbit, so the carbon compounds pass to the fox and become part of its body.

    Mark scheme

    • Grass takes in carbon dioxide by photosynthesis — 1 mark
    • Carbon is made into carbohydrates, proteins and fats in the plant — 1 mark
    • The rabbit eats the grass — 1 mark
    • The fox eats the rabbit, so carbon compounds pass to the fox — 1 mark
  8. 8 Explain [6 marks]

    Explain the importance of the carbon cycle.

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    Model answer

    The carbon cycle recycles carbon, so there is always a supply of carbon to make the carbohydrates, proteins and fats of living things. Plants take carbon dioxide from the air by photosynthesis, and the carbon passes to animals by feeding. The carbon returns to the air by respiration, and by decomposers that break down dead organisms and waste and release carbon dioxide. These processes balance, so the carbon dioxide level in the air stays fairly steady. The decay also returns mineral ions to the soil, so plants can keep growing. If people burn fossil fuels or cut down forests, the balance is upset, carbon dioxide rises and global warming results.

    Mark scheme

    • A continuous supply of carbon for new organisms — 1 mark
    • Photosynthesis removes carbon dioxide and feeding passes carbon along food chains — 1 mark
    • Respiration returns carbon dioxide to the air — 1 mark
    • Decomposers break down dead material and waste and release carbon dioxide — 1 mark
    • Decay returns mineral ions to the soil, so plants continue to grow — 1 mark
    • Human activity such as burning fossil fuels upsets the balance, raising the carbon dioxide level and causing global warming — 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 in carbon dioxide and uses it to make glucose.

  2. 2

    How does carbon pass from a plant to a rabbit?

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

    Carbon compounds in the plant pass to the rabbit when it eats the plant.

  3. 3

    Which of these is a decomposer?

    1. AA fungus
    2. BA green plant
    3. CA herbivore
    4. DA carnivore
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    A: A fungus

    Decomposers are bacteria and fungi, which break down dead material.

  4. 4

    What do decomposers release into the air when they respire?

    1. AOxygen
    2. BNitrogen
    3. CGlucose
    4. DCarbon dioxide
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    D: Carbon dioxide

    Respiration releases carbon dioxide, which plants can then use in photosynthesis.

  5. 5

    How were fossil fuels such as coal and oil formed?

    1. AFrom rocks heated by volcanoes
    2. BFrom dead remains that did not fully decay
    3. CFrom carbon dioxide dissolved in the sea
    4. DFrom the waste of decomposers
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    B: From dead remains that did not fully decay

    Dead remains that were buried without oxygen did not fully decay, and became fossil fuels over millions of years.

  6. 6

    Which of these processes puts carbon dioxide into the air?

    1. APhotosynthesis
    2. BEvaporation
    3. CCombustion
    4. DFossilisation
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    C: Combustion

    Combustion of fuels, like respiration and decay, releases carbon dioxide into the air.

  7. 7

    Why is the carbon dioxide level in the air rising?

    1. AMore carbon dioxide is released than is removed
    2. BPlants have stopped respiring
    3. CDecomposers make oxygen
    4. DAnimals no longer respire
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    A: More carbon dioxide is released than is removed

    Burning fossil fuels and deforestation add carbon dioxide faster than it is removed.

  8. 8

    Why do the carbon atoms in a plant eventually return to the air?

    1. ABecause plants break down carbon dioxide
    2. BBecause carbon dioxide dissolves in the soil
    3. CBecause the Sun converts carbon to gas
    4. DBecause respiration and decay release carbon dioxide
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    D: Because respiration and decay release carbon dioxide

    Carbon is released by respiration of the plant, of the animals that eat it and of the decomposers that break it down when it dies.