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 point
Master the lock-and-key model, temperature/pH effects on activity, denaturation (permanent shape change of active site), and how to read enzyme rate graphs.
Enzymes are biological catalysts and feature in nearly every exam paper. Focus on the lock-and-key model, how temperature and pH affect enzyme activity, denaturation, and interpreting rate-of-reaction graphs. This topic also connects to digestion, respiration, and photosynthesis.
Priority 1: Lock-and-key model
Enzymes have a specific active site that fits one specific substrate (like a lock and key). The substrate binds to the active site, the reaction occurs, and products are released. Enzymes are specific: each enzyme catalyses one reaction.
Priority 2: Temperature and pH effects
Temperature: as temperature increases, rate increases (more kinetic energy, more collisions) until the optimum temperature. Above the optimum, the enzyme denatures: the active site changes shape and the substrate can no longer fit. Rate drops to zero. Denaturation is permanent.
pH: each enzyme has an optimum pH. Moving too far from this pH changes the shape of the active site (denaturation). Pepsin works best at pH 2 (stomach); amylase works best at pH 7 (mouth/small intestine).
Priority 3: Interpreting graphs
Be able to describe and explain the shape of enzyme activity vs temperature and enzyme activity vs pH graphs. Identify the optimum, explain the rising portion (kinetic energy), and explain the falling portion (denaturation).
Frequently asked questions
Is denaturation the same as killing the enzyme?
Enzymes are not alive, so they cannot be "killed." Denaturation means the active site has permanently changed shape due to high temperature or extreme pH, so the substrate can no longer fit.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.