MSH | B5 Enzymes

B5.1

Enzymes as biological catalysts

Success criteria: explain active-site action and specificity; describe how temperature and pH affect activity; interpret experimental data using collision frequency, shape/fit and denaturation.

Watch: how enzymes work

Open on YouTube if embedding is restricted.

Specificity challenge

An enzyme has an active site complementary to substrate A. Which statement best explains specificity?

Sequence: substrate enters active site → enzyme–substrate complex forms → reaction occurs → product(s) leave → enzyme can be reused.

Interactive model

Temperature and pH simulator

Move the sliders. The model gives a relative activity score for a typical enzyme with optimum ≈ 40 °C and pH 7.

Temperature: 37 °CpH: 7

Relative activity

At low temperature: lower kinetic energy → fewer effective collisions. Above the optimum: bonds maintaining the active-site shape are disrupted → denaturation → substrate no longer fits.

Investigation planner

You are testing the effect of temperature on amylase activity. Select the best variable plan.

Original Cambridge-style practice

Paper 2, Paper 4 and Paper 6 practice

[1]1. What is an enzyme?

[4]2. Explain why enzyme activity increases from 20 °C to 40 °C.

Particles gain kinetic energy; enzyme and substrate move faster; collision frequency increases; more collisions are effective / more enzyme–substrate complexes form per unit time.

[4]3. Explain why activity falls sharply at very high temperature.

Enzyme denatures; active-site shape changes; substrate is no longer complementary / does not fit; fewer or no enzyme–substrate complexes form.

[5]4. Design an investigation of pH on catalase activity. Include the independent variable, dependent variable and two control variables.

Use buffer solutions of different pH; measure oxygen produced per unit time (or time to collect a fixed volume); control temperature, enzyme concentration/volume, substrate concentration/volume, total volume; repeat and calculate a mean.

Sources and teacher note

Aligned to the Cambridge IGCSE Co-ordinated Sciences (Double Award) 0654 syllabus for examinations in 2025–2027. The 0973 course has the same subject content. Embedded video links were checked during creation on 14 September 2026.

Video: Cognito, “What are Enzymes?”.

This independent learning resource is not endorsed by Cambridge International. Cambridge-style questions are original and are not copied from past papers.