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AQA GCSE Biology

Respiration and exercise — AQA GCSE Biology revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 4 minutes to read.

Aerobic vs anaerobic

  • Aerobic: glucose + oxygen → carbon dioxide + water. Releases much more energy.
  • Anaerobic in muscle: glucose → lactic acid. Anaerobic in yeast: glucose → ethanol + carbon dioxide (fermentation).
  • Respiration is exothermic and happens in every living cell, all the time.

Exercise response

  • Heart rate, breathing rate and breath depth increase to supply oxygen and remove CO₂.
  • Oxygen debt: extra oxygen needed after exercise to oxidise the lactic acid that built up.

Key terms

Exothermic
Transfers energy to the surroundings.
Oxygen debt
The extra oxygen needed after exercise to break down lactic acid.
Metabolism
The sum of all reactions in a cell or body.
Aerobic respiration
Respiration using oxygen, releasing lots of energy from glucose.
Anaerobic respiration
Respiration without oxygen, releasing less energy and producing lactic acid (animals) or ethanol+CO2 (yeast/plants).
Fermentation
Anaerobic respiration in yeast, producing ethanol and carbon dioxide.

Common exam traps

  • Saying respiration 'produces energy' — energy is transferred/released, not created.
  • Mixing up respiration with breathing.
  • Thinking anaerobic respiration releases more energy than aerobic — it releases much less per glucose molecule.

Practice questions with answers

  1. 1. What is produced by anaerobic respiration in human muscle?

    • • Ethanol
    • • Lactic acid
    • • Carbon dioxide only
    • • Water

    Answer: Lactic acid

    Yeast makes ethanol; muscle makes lactic acid.

  2. 2. Explain why breathing rate stays high after exercise finishes.

    Answer: To repay the oxygen debt — extra oxygen is needed to oxidise the lactic acid that built up.

    Mention oxygen debt and lactic acid.

  3. 3. Write the word equation for aerobic respiration.

    Answer: Glucose + oxygen → carbon dioxide + water

    This is essentially photosynthesis reversed.

  4. 4. Which type of respiration releases the most energy per glucose molecule?

    • • Anaerobic
    • • Aerobic
    • • Fermentation
    • • None release energy

    Answer: Aerobic

    Aerobic respiration fully breaks down glucose using oxygen, releasing much more energy.

  5. 5. Write the word equation for aerobic respiration.

    Answer: Glucose + oxygen → carbon dioxide + water (energy released).

    This is the main energy-releasing reaction in most living cells.

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