IGCSE Biology: enzymes and pH is one of the most searched topics because enzymes appear in almost every Biology paper. You need to explain how enzymes work, how pH and temperature affect them, and how to interpret graphs and practical results.

Key Concepts for Enzymes

• Lock and key (and induced fit) – Active site and substrate fit; enzyme catalyses the reaction.
• Optimum – Each enzyme has an optimum pH and temperature; above or below, rate decreases (denaturation at high temperature or extreme pH).
• Graphs – Sketch or interpret rate vs pH / rate vs temperature; identify optimum and explain shape.

Use precise language: “active site”, “denatured”, “optimum”, “substrate”.

How to Answer “Explain” and “Describe” Questions

• Describe: State what happens (e.g. “as pH increases, rate increases to a maximum then decreases”).
• Explain: Give the biological reason (e.g. “at low pH the active site is denatured so substrate cannot bind”).
• Practical: Identify variables (independent, dependent, control); suggest how to make the experiment fair.

Tutopiya’s Biology tutors can help you with enzyme theory, graph interpretation, and exam wording for full marks.

Exam-Style Questions and Common Mistakes

Describe questions: State what happens (e.g. “as pH increases from 1 to 7, rate increases to a maximum at the optimum pH; above the optimum, rate decreases”). Explain questions: Give the biology (e.g. “at low pH the active site is denatured, so the substrate no longer fits and the rate decreases”). You may be asked to draw or interpret a graph of rate vs pH; label the optimum and explain the shape. Common mistakes: Saying “enzyme stops working” instead of “denatured”; not linking pH to active site shape and substrate binding; forgetting that both high and low pH can denature. Use the terms “active site”, “substrate”, “denature”, and “optimum” precisely.

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