Enzymes are biological catalysts that speed up reactions, and each one acts on a particular substrate. The lock and key model explains this: the substrate fits the active site like a key fits one lock.
This is one of the core ideas in biological molecules and enzymes, and the temperature and pH lessons build on it.
How does the model explain specificity?
An enzyme is a protein with a folded chain. The folding creates a small region, the active site, with a particular three-dimensional shape.
A substrate molecule with a complementary shape fits into the active site. An enzyme-substrate complex forms, the reaction happens, the products leave, and the enzyme is free to act again.
A substrate with a different shape does not fit. No complex forms, so no reaction takes place.
Steps for a model-based explanation
- Name the enzyme and substrate in the question.
- Describe the active site as having a specific shape.
- State whether the substrate is complementary to that shape.
- Give the outcome: complex forms, or does not form.
- Say what happens to the enzyme afterwards: it is unchanged and can be reused.
Worked example
Amylase breaks down starch but has no effect on protein. Explain why, using the lock and key model.
Step 1: amylase has an active site with a specific shape.
Step 2: starch molecules have a shape complementary to that active site, so they fit and form an enzyme-substrate complex.
Step 3: protein molecules have a different shape and do not fit into the active site, so no complex forms.
Step 4: after starch is broken into smaller molecules the amylase is released unchanged, so it can work again.
Model answer: amylase has an active site whose shape is complementary to starch but not to protein. Starch fits and is broken down. Protein does not fit, so it is not acted on.
The mistake to watch for
A common answer says “the enzyme and substrate have the same shape”. Two things go wrong here.
First, complementary is not the same as identical. A key and a lock have different shapes that match. Second, it is only the active site of the enzyme, not the whole enzyme, that has to match.
The correction: “the active site is complementary in shape to the substrate”.
Check yourself
1. Lipase digests fats but not starch. Explain using the model.
Show answer
Lipase has an active site complementary to the shape of fat molecules, which fit and form an enzyme-substrate complex. Starch has a different shape, does not fit and is not acted on.
2. A student says the enzyme is “used up” once the products form. Correct this.
Show answer
The enzyme is not used up. It is released unchanged when the products leave the active site and can bind another substrate molecule.
3. State one way the lock and key model is not a perfect picture of a real enzyme.
Show answer
Any one of: real enzymes are not rigid, the model shows no movement, or it does not show the forces involved. It is a simplified diagram for explaining shape matching.
Where this leads next
Once specificity is clear, see how conditions change enzyme rates in interpreting temperature and pH response graphs. A reminder of the building blocks is in distinguishing a monomer from a polymer.
A written explanation is easy to write but hard to check on your own. Our teachers look at this kind of answer in online one-to-one Biology tuition.