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AQA GCSE Biology

Enzymes and digestion — AQA GCSE Biology revision

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

How enzymes work

  • Enzymes are biological catalysts — proteins that speed up reactions without being used up.
  • The substrate fits the active site (lock and key idea).
  • Too hot or the wrong pH changes the active site shape — the enzyme denatures and stops working.

Digestive enzymes

  • Carbohydrase (e.g. amylase) breaks starch into simple sugars.
  • Protease breaks protein into amino acids.
  • Lipase breaks lipids into fatty acids and glycerol.
  • Bile is not an enzyme: it neutralises stomach acid and emulsifies fat to increase surface area.

Key terms

Catalyst
Speeds up a reaction without being used up.
Active site
The part of an enzyme where the substrate binds.
Denatured
The enzyme's shape has changed so the substrate no longer fits.
Emulsify
Break large fat drops into small droplets.
Enzyme
A biological catalyst, usually a protein, that speeds up reactions without being used up.
Substrate
The molecule an enzyme acts on.

Common exam traps

  • Saying enzymes are 'killed' — they are denatured, not alive.
  • Calling bile an enzyme.
  • Forgetting that optimum pH differs: stomach protease works in acid, small-intestine enzymes prefer alkaline.

Practice questions with answers

  1. 1. Which enzyme breaks down protein?

    • • Amylase
    • • Lipase
    • • Protease
    • • Catalase

    Answer: Protease

    Protease → amino acids.

  2. 2. Explain why an enzyme stops working at 70 °C.

    Answer: High temperature changes the shape of the active site, so the substrate no longer fits — the enzyme is denatured.

    Mention active site shape and substrate fit.

  3. 3. What are the two jobs of bile?

    • • Digest starch and protein
    • • Neutralise stomach acid and emulsify fats
    • • Absorb sugars and store glycogen
    • • Kill bacteria and make enzymes

    Answer: Neutralise stomach acid and emulsify fats

    Both effects speed up lipase activity.

  4. 4. Name the products of lipid digestion.

    Answer: Fatty acids and glycerol.

    Both needed for the mark.

  5. 5. What happens to an enzyme at very high temperatures?

    • • It works faster forever
    • • It denatures and stops working
    • • It turns into a substrate
    • • Nothing changes

    Answer: It denatures and stops working

    High heat changes the shape of the active site so the substrate no longer fits.

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