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IGCSEBiology
Trial Class

Original practice · 5 questions · 15 marks · about 17 minutes

Enzyme Specificity and Lock-and-Key Practice Questions

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

These questions test your understanding of how enzymes work, including the lock-and-key hypothesis and why enzymes are specific to their substrates.

Before you open the answers

  • Attempt every question without notes.
  • Show working for calculations and write complete biological statements.
  • Use the suggested time as a guide, not a strict limit on the first attempt.

These are original practice questions. They do not reproduce an official Cambridge paper or official mark scheme.

  1. Question 1

    Paper 2 Both 3 marks Difficulty 2/5

    Define the term "enzyme" and state what is meant by enzyme specificity.

    Show model answer and marking guidance
    1. An enzyme is a biological catalyst that speeds up chemical reactions without being used up.
    2. Enzymes are proteins.
    3. Specificity means each enzyme only catalyses one type of reaction / acts on one substrate: due to the shape of its active site.

  2. Question 2

    Paper 4 Both 4 marks Difficulty 3/5

    Using the lock-and-key model, explain how an enzyme catalyses a reaction.

    Show model answer and marking guidance
    1. The substrate (key) has a shape complementary to the enzyme's active site (lock).
    2. The substrate binds to the active site to form an enzyme-substrate complex.
    3. The reaction occurs and products are released.
    4. The enzyme is unchanged / can be reused.

    Watch for: Saying the substrate and active site have the "same" shape: they are complementary, not identical.

  3. Question 3

    Paper 4 Extended 3 marks Difficulty 4/5

    Explain why amylase cannot digest protein.

    Show model answer and marking guidance
    1. Amylase has an active site with a specific shape.
    2. This shape is complementary to starch (its substrate), not to protein.
    3. Protein molecules cannot fit into the active site of amylase: no enzyme-substrate complex forms.

  4. Question 4

    Paper 2 Core 2 marks Difficulty 1/5

    What is the active site of an enzyme?

    Show model answer and marking guidance
    1. The region / part of the enzyme where the substrate binds.
    2. It has a specific shape complementary to the substrate.

  5. Question 5

    Paper 4 Extended 3 marks Difficulty 4/5

    Suggest why enzymes are described as "biological catalysts" and explain the advantage of using enzymes rather than high temperatures to speed up reactions in cells.

    Show model answer and marking guidance
    1. Biological because they are produced by living organisms / are proteins.
    2. Catalysts because they lower the activation energy / speed up reactions without being consumed.
    3. High temperatures would denature other proteins / damage cell structures: enzymes allow reactions at body temperature.

Mark the set consistently

Award a point only when the idea is clearly present in the written answer. Correct any missed point in a complete sentence, then test the same skill on a different question several days later.

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