IGCSE Biology glossary
Lock-and-key hypothesis
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.
Definition
A model explaining enzyme specificity: the enzyme's active site (the lock) has a shape complementary to the substrate (the key). Only the correct substrate can fit into the active site, so each enzyme catalyses only one type of reaction.
What to include in an exam answer
Use the word 'complementary' not 'matching'. State that the substrate fits into the active site like a key fits a lock. This model is frequently tested in 3-mark explain questions.
Example in context
Use the lock-and-key hypothesis to explain why amylase breaks down starch but not protein (3 marks): The active site of amylase has a shape complementary to starch. Protein has a different shape that is not complementary to the active site, so protein cannot fit into the active site of amylase.
Related syllabus topic
Enzymes