A-Level Biology May/June 2024 Q3(a): Draw labelled and annotated diagrams in the space provided to show how the lock and key…

1 answer

• ▲0
  accepted ✓

  Lock and Key Mechanism of Lysozyme

  Stage 1: Enzyme and Substrate

     (Lysozyme Enzyme)                 (Peptidoglycan Substrate)
    +-----------------+                +-----------------------+
    |                 |                |                       |
    |    /\           |                |          /\           |
    |   /  \_         |                |         /  \          |
    |  |     | Active |                |        |    |         |
    |   \   /  Site  |                |         \  /          |
    |    \_/          |                |          \/           |
    |                 |                |                       |
    +-----------------+                +-----------------------+
  

  • Annotation: The lysozyme enzyme has an active site with a specific 3D shape. The peptidoglycan substrate has a shape that is complementary to the active site.

  ---

  Stage 2: Enzyme-Substrate Complex Formation

     (Enzyme-Substrate Complex)
    +-------------------------+
    |                         |
    |    /\ /\                |
    |   /  \/  \_              |
    |  |    |    |             |
    |   \  /\   /             |
    |    \/  \_/              |
    |                         |
    +-------------------------+
  

  • Annotation: The peptidoglycan substrate binds to the active site of the lysozyme, forming a temporary enzyme-substrate complex.

  ---

  Stage 3: Product Formation and Release

     (Lysozyme Enzyme)      (Products: NAM and NAG)
    +-----------------+         +--------+ +--------+
    |                 |         |        | |        |
    |    /\           |         |   /\   | |   /\   |
    |   /  \_         |         |  /  \  | |  /  \  |
    |  |     | Active |         | |    | | | |    | |
    |   \   /  Site  |         |  \  /  | |  \  /  |
    |    \_/          |         |   \/   | |   \/   |
    |                 |         |        | |        |
    +-----------------+         +--------+ +--------+
  

  • Annotation: A hydrolysis reaction occurs, breaking the glycosidic bond in peptidoglycan. The products (NAM and NAG) are released. The lysozyme enzyme is unchanged and its active site is free to bind to another substrate molecule.

  ---

  How the marks are awarded

  • B1 — The first diagram clearly shows the lysozyme molecule with a specifically shaped cleft, which is labelled as the 'Active Site'. This meets the requirement for drawing and labelling the active site.
  • B1 — The peptidoglycan substrate is drawn with a shape that is clearly complementary to the active site of the lysozyme, and it is labelled 'Peptidoglycan Substrate'. This satisfies the second marking point.
  • B1 — The second diagram shows the substrate fitting precisely into the enzyme's active site, and this entire structure is correctly labelled as the 'Enzyme-Substrate Complex'.

  Common mistakes

  • Drawing the active site changing shape to fit the substrate. This describes the induced-fit hypothesis, not the lock and key hypothesis requested.
  • Failing to show the enzyme is unchanged after the reaction. Some students draw the active site as being altered or the enzyme being 'used up'.
  • Using vague labels like 'enzyme' and 'substrate' instead of the specific terms 'lysozyme' and 'peptidoglycan' given in the question.
  • Forgetting to label key components, especially the 'active site' and the 'enzyme-substrate complex'.

  Examiner tip: Practice translating abstract biological models and processes into clear, sequential, and well-labelled diagrams to secure marks on descriptive questions.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.