IGCSE Biology / Life processes

Temperature, pH and denaturing revision notes

Edexcel International GCSE Biology (4BI1), chapter 1: Life processes, specification point 2.11, 2.13. Free lesson.

Watch the video lesson · IGCSE Biology past papers

Temperature and pH affect enzyme activity by changing how often enzyme and substrate molecules meet, and by changing the enzyme’s shape. Knowing why the curves rise and fall helps you explain enzyme activity and denaturing.

How temperature changes the reaction rate

A temperature graph has a gentle rise to a peak, followed by a sharp fall. The peak is the optimum temperature, where the enzyme works fastest. For most human enzymes, this is about 37 °C, close to body temperature. As temperature rises towards the optimum, molecules gain kinetic energy and move faster. Enzyme and substrate molecules collide more often, so more enzyme-substrate complexes form each second. The reaction rate rises. Above the optimum, heat breaks bonds that hold the enzyme’s shape. Its active site changes shape, so the rate falls sharply.

How pH changes the reaction rate

A pH graph is hill-shaped, with the highest rate at the enzyme’s optimum pH. The rate falls on either side because a pH away from the optimum disturbs the bonds holding the enzyme’s shape. Pepsin, which works in the stomach, has an optimum of about pH 2 and barely works by pH 5. Amylase, which works in the mouth and small intestine, has an optimum of about pH 7. Different enzymes have different optima, which match the conditions where they work in the body.

How denaturing changes the active site

Denaturing is a lasting change in an enzyme’s shape. When heat goes above the optimum, or pH moves away from the optimum, the active site changes shape and the substrate no longer fits. The enzyme-substrate complex cannot form, so the reaction rate falls. Denaturing is permanent. Cooling a denatured enzyme or returning the pH to normal does not restore its active site. This permanent change helps explain why the temperature curve falls more sharply than it rises.

How to explain the rising part of the temperature curve

If asked to explain the rise, give the cause, not just what the graph shows. Saying that enzymes work faster only describes the change in rate. The explanation links increased kinetic energy to collisions and then to more enzyme-substrate complexes.

More lessons in Life processes