Enzyme Rate Graphs IGCSE Biology 0610

Enzyme Rate Graphs: Temperature, pH and Denaturation

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 concept

Enzyme rate rises as temperature increases because molecules move faster and collide more often, reaches an optimum, then falls sharply when high temperature changes the active-site shape. Unsuitable pH can also alter the active site and reduce activity.

Students often describe the line on an enzyme graph but fail to explain it. Full answers must connect the condition to molecular movement or active-site shape and then to successful enzyme–substrate interactions.

Temperature below the optimum

As temperature increases, enzyme and substrate molecules gain kinetic energy and move faster. They collide more frequently, so successful collisions and enzyme–substrate complexes form more often. The reaction rate increases.

At low temperature the enzyme is not denatured. The reaction is simply slower, so warming it can restore the rate.

The optimum and the sharp fall

The optimum is the temperature at which the measured rate is highest. Above it, heat disrupts bonds that maintain the enzyme’s shape. The active site changes permanently, the substrate is no longer complementary and fewer complexes form. The enzyme is denatured.

Never say the enzyme is “killed”. Enzymes are proteins, not living organisms.

How pH changes enzyme activity

Each enzyme has an optimum pH. Moving away from it changes interactions that maintain the active-site shape, reducing the rate; extreme pH can denature the enzyme. Different enzymes can have different optima because their environments and structures differ.

When comparing two graph curves, quote values where possible and separate description from explanation.

Planning a fair enzyme-rate investigation

Change one independent variable, such as temperature, and measure a defined endpoint, such as the time for starch to disappear. Keep enzyme concentration, substrate concentration, volumes and pH constant. Repeat each condition and calculate a mean.

For rate, use 1 ÷ time when the same amount of substrate is converted each time.

Try a focused question (4 marks)

An enzyme reaction is fastest at 40 °C and almost stops at 65 °C. Explain the difference in rate.

Show the answer and marking guidance

At 40 °C, molecules have enough kinetic energy for frequent successful collisions.
The active site retains a complementary shape near the optimum.
At 65 °C, high temperature changes the active-site shape.
The substrate no longer fits, so few or no enzyme–substrate complexes form.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying the enzyme is killed.
Describing the graph without explaining collisions or active-site shape.
Claiming low temperature denatures the enzyme.
Changing both temperature and pH in the same fair-test investigation.

Practical connection

Enzyme Rate Investigation With Amylase and Starch

Mix amylase and starch under controlled conditions, sample at timed intervals into iodine and record when starch is no longer detected. Change one factor, repeat each value and calculate rate as 1 ÷ time.

Review the practical method →