AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    Students should be able to explain how temperature, water and availability…

    Full specification wording

    Decomposition (biology only): Students should be able to explain how temperature, water and availability of oxygen affect the rate of decay of biological material.

    Decay is the breakdown of biological material by microorganisms such as bacteria and fungi.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 1 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. Their enzymes digest the material, and the rate depends on conditions. Warmth increases enzyme activity and microbial growth, so decay is faster up to an optimum; very high temperatures denature enzymes and slow or stop decay. Moisture is needed because microorganisms and their enzymes work in solution, so damp conditions speed up decay while dry conditions slow it. Oxygen is needed for aerobic respiration by decomposers, so plenty of oxygen increases the rate; waterlogged or sealed conditions limit oxygen and slow aerobic decay. These principles explain why food is refrigerated, dried, salted or vacuum-packed to preserve it.

    What earns the marks

    • Warm temperatures increase enzyme activity and microbial growth, so decay is faster up to an optimum temperature.
    • Very high temperatures denature the enzymes of decomposers, so the rate of decay falls.
    • Moist conditions allow microorganisms and their enzymes to work, so decay is faster in damp material than in dry material.
    • Availability of oxygen supports aerobic respiration in decomposers, so more oxygen increases the rate of decay.
    • Limited oxygen, for example in waterlogged or sealed conditions, slows aerobic decay and can favour anaerobic decomposers.

    Common mistakes and how to put them right

    • Saying higher temperature always increases decay.

      Put it right: Decay increases up to an optimum, then enzymes denature and the rate falls.

    • Thinking water is a reactant that is used up.

      Put it right: Water provides the moist conditions needed for microbial enzymes to work.

    • Ignoring oxygen and assuming decay is the same in all conditions.

      Put it right: Oxygen availability strongly affects the rate of aerobic decomposition.

    Exam tips

    • Use the word 'optimum' when describing temperature and explain what happens above it.
    • Link each factor to the microorganisms: enzymes, respiration or growth, rather than treating decay as a simple chemical reaction.
    • When explaining food preservation, name the factor that is controlled and state how it slows decay.

    Try the lesson

    The start of the Decomposition (biology only): Students should be able to explain how temperature, water and availability of oxygen affect the rate of decay of biological material. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which condition would slow down the decay of biological material?

    Listen

    Decay is the breakdown of biological material by microorganisms such as bacteria and fungi. Their enzymes digest the material, and the rate depends on conditions. Warmth increases enzyme activity and microbial growth, so decay is faster up to an optimum; very high temperatures denature enzymes and slow or stop decay. Moisture is needed because microorganisms and their enzymes work in solution, so damp conditions speed up decay while dry conditions slow it. Oxygen is needed for aerobic respiration by decomposers, so plenty of oxygen increases the rate; waterlogged or sealed conditions limit oxygen and slow aerobic decay. These principles explain why food is refrigerated, dried, salted or vacuum-packed to preserve it. A strong answer earns marks for these points. Warm temperatures increase enzyme activity and microbial growth, so decay is faster up to an optimum temperature. Very high temperatures denature the enzymes of decomposers, so the rate of decay falls. Moist conditions allow microorganisms and their enzymes to work, so decay is faster in damp material than in dry material. Availability of oxygen supports aerobic respiration in decomposers, so more oxygen increases the rate of decay. A common slip: Saying higher temperature always increases decay. Instead, decay increases up to an optimum, then enzymes denature and the rate falls. Exam tip: Use the word 'optimum' when describing temperature and explain what happens above it.

    See it

    A top-down flowchart with three starting nodes. The first, Temperature, branches two ways: one arrow leads to Warmth increases enzyme activity and microbial growth, which leads to Decay faster up to an optimum; the other arrow leads to Very high temperatures denature enzymes, which leads to Rate of decay falls. The second starting node, Water, leads to Moist conditions let microorganisms and enzymes work, which leads to Decay faster in damp material than dry. The third starting node, Oxygen, branches two ways: one arrow leads to Supports aerobic respiration in decomposers, which leads to More oxygen increases rate of decay; the other arrow leads to Limited oxygen, e.g. waterlogged or sealed, which leads to Slows aerobic decay.
    Shows how temperature, water and oxygen availability each affect the rate of decay of biological material.

    Key words

    Say what each word means before you turn the card.

    Decomposition and decay

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    Enzyme activity and temperature

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    Aerobic respiration and oxygen

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    Food preservation

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    See every definition
    Decomposition and decay
    The breakdown of biological material by microorganisms such as bacteria and fungi, whose enzymes digest the material.
    Enzyme activity and temperature
    Warmth increases enzyme activity and microbial growth so decay is faster up to an optimum, but very high temperatures denature enzymes and slow or stop decay.
    Aerobic respiration and oxygen
    Oxygen is needed for aerobic respiration by decomposers, so plenty of oxygen increases the rate of decay, while waterlogged or sealed conditions limit oxygen and slow aerobic decay.
    Food preservation
    Refrigerating, drying, salting or vacuum-packing food to preserve it, explained by controlling the factors that affect decay.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart with three starting nodes. The first, Temperature, branches two ways: one arrow leads to Warmth increases enzyme activity and microbial growth, which leads to Decay faster up to an optimum; the other arrow leads to Very high temperatures denature enzymes, which leads to Rate of decay falls. The second starting node, Water, leads to Moist conditions let microorganisms and enzymes work, which leads to Decay faster in damp material than dry. The third starting node, Oxygen, branches two ways: one arrow leads to Supports aerobic respiration in decomposers, which leads to More oxygen increases rate of decay; the other arrow leads to Limited oxygen, e.g. waterlogged or sealed, which leads to Slows aerobic decay.

    • 1Box 1

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    • 3Box 3

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