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Exam questions · Biology · Cycles in Ecosystems

Cycling of Materials and Decomposers

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

    Figure 1 shows how materials are cycled in an ecosystem. Name the groups of organisms shown at A, B and C.

    A materials cycle with three boxes marked A, B and C.
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    Model answer

    A is the producers (plants). B is the consumers (animals). C is the decomposers (fungi and bacteria).

    Mark scheme

    • A: producers — 1 mark
    • B: consumers — 1 mark
    • C: decomposers — 1 mark
  2. 2 State [1 mark]

    State two materials that are cycled through ecosystems.

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    Model answer

    Any two of: carbon, nitrogen, water.

    Mark scheme

    • Two materials — 1 mark
  3. 3 Explain [2 marks]

    Explain why materials have to be cycled in an ecosystem.

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    Model answer

    The supply of materials on Earth is limited. If they were not cycled, the producers would run out of the materials that they need.

    Mark scheme

    • The supply is limited — 1 mark
    • Producers would run out of materials — 1 mark
  4. 4 Describe [3 marks]

    Describe how decomposers break down a dead leaf.

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    Model answer

    Bacteria and fungi release enzymes onto the leaf. The enzymes break the leaf down into small, soluble molecules. The microorganisms absorb these molecules.

    Mark scheme

    • Bacteria and fungi release enzymes — 1 mark
    • The enzymes break down the leaf into soluble molecules — 1 mark
    • The microorganisms absorb the molecules — 1 mark
  5. 5 Explain [3 marks]

    Explain why decomposers are important to plants.

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    Model answer

    Decomposers break down dead material and waste. This releases mineral ions, such as nitrates, into the soil. The plants take up the mineral ions through their roots, and use them to grow.

    Mark scheme

    • Decomposers break down dead material and waste — 1 mark
    • Mineral ions are released into the soil — 1 mark
    • Plants take up the ions to grow — 1 mark
  6. 6 Explain [3 marks]

    A farmer spreads manure on a field before planting a crop. Explain how the manure helps the crop to grow.

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    Model answer

    Decomposers in the soil break down the manure. They release mineral ions, such as nitrates, into the soil. The crop plants take up the ions through their roots and use them to make proteins and to grow.

    Mark scheme

    • Decomposers break down the manure — 1 mark
    • Mineral ions are released into the soil — 1 mark
    • The crop takes up the ions to grow — 1 mark
  7. 7 Suggest [3 marks]

    Earthworms feed on dead leaves in the soil. Suggest how earthworms help the decomposers to break down the leaves.

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    Model answer

    The earthworms break the leaves into smaller pieces. The smaller pieces have a larger surface area. This lets the bacteria and fungi release their enzymes onto more of the leaf, so the leaf decays faster.

    Mark scheme

    • Earthworms break the leaves into smaller pieces — 1 mark
    • A larger surface area — 1 mark
    • Microorganisms can act on more of the leaf, so it decays faster — 1 mark

Quick check

  1. 1

    Which of these is an abiotic part of an ecosystem?

    1. AA bacterium
    2. BA tree
    3. CWater
    4. DA fungus
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    C: Water

    Water is non-living, so it is abiotic.

  2. 2

    Which group of organisms breaks down dead plants and animals?

    1. AProducers
    2. BDecomposers
    3. CHerbivores
    4. DPredators
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    B: Decomposers

    Decomposers, mainly bacteria and fungi, break down dead material.

  3. 3

    What do decomposers release onto dead material?

    1. AEnzymes
    2. BHormones
    3. CChlorophyll
    4. DSugar
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    A: Enzymes

    They release enzymes that break the material down into small, soluble molecules.

  4. 4

    Which gas do decomposers release when they respire?

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

    Respiration releases carbon dioxide.

  5. 5

    Why do materials have to be cycled?

    1. ABecause decomposers make new atoms
    2. BBecause plants make nitrogen
    3. CBecause the supply of materials is limited
    4. DBecause animals do not need them
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    C: Because the supply of materials is limited

    The supply of materials on Earth is limited, so they must be used again.

  6. 6

    What do plant roots take up from the soil that decomposers have released?

    1. AMineral ions
    2. BGlucose
    3. COxygen
    4. DStarch
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    A: Mineral ions

    Plants take up mineral ions such as nitrates.

  7. 7

    What do nitrogen-fixing bacteria do?

    1. AThey make carbon dioxide
    2. BThey turn nitrogen from the air into usable compounds
    3. CThey eat plant roots
    4. DThey make glucose
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    B: They turn nitrogen from the air into usable compounds

    They turn nitrogen from the air into compounds that plants can use.

  8. 8

    Which of these is a decomposer?

    1. AA rabbit
    2. BA grass plant
    3. CA hawk
    4. DA fungus
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    D: A fungus

    Fungi, such as mushrooms, feed on dead material.