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Enzymes Cambridge IGCSE Biology (9–1) revision

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In plain words

The chemical reactions that keep you alive would be far too slow on their own. Enzymes are proteins that speed them up. Each one has a pocket shaped to fit one particular molecule, like a lock that takes only one key, so each enzyme does only one job.

Three things to know

  1. Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up.
  2. The substrate fits into the enzyme's active site because their shapes are complementary. That is why each enzyme works on only one substrate.
  3. An enzyme works fastest at its optimum temperature and pH. Too hot, or the wrong pH, and the active site changes shape so the substrate no longer fits: the enzyme is denatured.

Tips and tricks

  • Enzymes are denatured, not killed. They were never alive.
  • Low temperature only slows an enzyme down (fewer collisions). High temperature denatures it, and that can't be undone.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Enzymes: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What kind of molecule is an enzyme?
    • a carbohydrate
    • a fat
    • a protein (the answer)
    • a mineral

    All enzymes are proteins.

  2. What is the part of an enzyme where the substrate fits?
    • the nucleus
    • the active site (the answer)
    • the membrane
    • the product

    Its shape is complementary to the substrate.

  3. What happens to an enzyme at a very high temperature?
    • It works faster and faster.
    • It is killed.
    • It is denatured. (the answer)
    • It becomes a substrate.

    The active site changes shape and the substrate can no longer fit.

  4. Why does each enzyme work on only one substrate?
    • Enzymes are very small.
    • The active site fits only one shape of molecule. (the answer)
    • Enzymes are used up.
    • Substrates are all the same.

    This is called specificity.

  5. What happens to an enzyme at the end of a reaction?
    • It is used up.
    • It becomes the product.
    • It is unchanged and can be used again. (the answer)
    • It is destroyed.

    That is what makes it a catalyst.

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