IGCSE Biology glossary
Optimum pH
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Definition
The pH at which an enzyme works at its maximum rate. Different enzymes have different optimum pH values: pepsin works best at pH 2 (acidic, in the stomach), while trypsin works best at pH 8 (alkaline, in the small intestine).
What to include in an exam answer
A common exam application is explaining why pepsin does not work in the small intestine. The pH is too high (alkaline), so the active site changes shape and the substrate cannot fit.
Example in context
Explain why pepsin does not work well in the small intestine (2 marks): Pepsin has an optimum pH of 2. The small intestine is alkaline (pH 8), which is far from the optimum, so the active site of pepsin changes shape and the substrate cannot bind.
Related syllabus topic
Enzymes