Enzyme-Substrate Diagram IGCSE Biology 0610

Enzyme-Substrate Diagram: Lock and Key Model

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.

Key diagram

The lock and key diagram shows how the substrate fits into the enzyme's active site, forming an enzyme-substrate complex. The active site has a specific shape complementary to only one substrate, explaining enzyme specificity.

The enzyme-substrate diagram, often called the lock and key model, is central to IGCSE Biology enzyme questions. Students must draw it clearly, use the correct terminology and connect it to explanations of specificity and denaturation.

Drawing the lock and key model

Draw the enzyme as a large irregular shape with a clearly defined notch or cleft on one side: this is the active site. Draw the substrate as a smaller shape whose outline exactly matches the active site, like a key fitting a lock. Show three stages: (1) enzyme and substrate separate, (2) substrate bound in the active site forming the enzyme-substrate complex, (3) products released and enzyme unchanged.

Key terminology

The active site is the region of the enzyme where the substrate binds. Its shape is complementary to the substrate: use this word instead of "fits". The enzyme is specific: each enzyme catalyses only one reaction because only one substrate shape matches the active site. After the reaction, the products leave and the enzyme can be reused.

Do not say the enzyme and substrate have the "same shape". They have complementary shapes: one is the inverse of the other.

Denaturation

At high temperatures or extreme pH, the enzyme's shape changes. The active site is no longer complementary to the substrate, so the substrate cannot bind and the reaction stops. This permanent change in shape is called denaturation. Draw a version of the enzyme with a misshapen active site and the substrate unable to fit.

Students often lose marks by writing that the enzyme is "killed" or "destroyed". Enzymes are proteins, not living things. The correct phrase is "the enzyme is denatured".

Interpreting enzyme rate graphs

Temperature graphs show rate increasing to an optimum, then falling sharply as the enzyme denatures. PH graphs show a peak at the optimum pH with reduced activity on either side. When interpreting these graphs, state the optimum value, describe the trend on each side, and explain using active-site shape and denaturation.

Label checklist

Check these labels are present and correctly positioned before finishing your drawing.

✓ Enzyme
✓ Substrate
✓ Active site
✓ Enzyme-substrate complex
✓ Products
✓ Denatured enzyme (misshapen active site)

Try a diagram question (3 marks)

Using the lock and key model, explain why enzymes are specific. [3]

Show the answer and marking guidance

The active site of the enzyme has a specific shape.
The shape is complementary to only one substrate.
Other substrates cannot fit into the active site, so the enzyme catalyses only one reaction.

Original Cambridge-style practice written for this site.

Common diagram mistakes

Check these against your own drawing before moving on.

Writing that the substrate and active site have the "same shape" instead of "complementary shape".
Saying the enzyme is "killed" or "destroyed" by heat instead of "denatured".
Drawing the active site as a simple hole rather than a shaped cleft.
Forgetting to show the enzyme unchanged after the products are released.