Temperature, pH and enzyme activity

Structure and functions in living organisms · Biological molecules and enzymes · note 4 of 5

Spec 2.11, 2.13Download all Biological molecules and enzymes notes (PDF)Download all notes (PDF)7 pages

Temperature, pH and enzyme activitySpec 2.11, 2.13

In short

Enzymes work fastest at their optimum temperature and optimum pH. Raising the temperature increases collisions between enzyme and substrate, so the rate rises, but above the optimum the active site changes shape, the substrate no longer fits and the enzyme is denatured. Extreme pH also denatures enzymes. Many human enzymes work best at about 37 °C.

Temperature

As temperature increases, the particles move faster and have more kinetic energy. Enzyme and substrate collide more often, so the rate of reaction increases. The rate is highest at the optimum temperature, which is around 37 °C for many human enzymes.

Above the optimum temperature, the enzyme's structure is damaged and the shape of the active site changes. The substrate no longer fits, so the enzyme is denatured and the rate falls quickly towards zero. Denaturing is permanent.

pH

Each enzyme has an optimum pH where it works fastest. If the pH is higher or lower than the optimum, the shape of the active site is altered, so the substrate fits less well and the rate decreases. At extreme pH the active site changes shape so much that the substrate cannot fit: the enzyme is denatured.

Different enzymes have different optimum pH values. For example, amylase in the mouth works best at about pH 7 (neutral), while pepsin, a protease in the acidic stomach, works best at about pH 2.

Summary of the effects on enzyme activity
Factor	Below the optimum	At the optimum	Above the optimum
Temperature	Rate increases as particles collide more often	Highest rate	Active site changes shape, enzyme denatured, rate falls
pH	Rate decreases as pH moves away from the optimum; denatured at extreme pH	Highest rate	Rate decreases as pH moves away from the optimum; denatured at extreme pH

Graph of rate of reaction against temperature rising to an optimum temperature then falling steeply where the enzyme is denatured; bell-shaped graph of rate against pH with an optimum pH; below, a substrate fitting an active site and a denatured enzyme whose changed active site the substrate cannot fit. — Rate rises to the optimum then falls. Above the optimum temperature, or at extreme pH, the active site changes shape and the enzyme is denatured.

Exam tip:

When a graph falls after a peak, always give the reason: the active site has changed shape and the enzyme has been denatured. Do not say the enzyme has been 'killed'. Enzymes are not alive.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

How do enzymes speed up reactions?

Enzymes speed up reactions by acting as biological catalysts. The substrate fits into the enzyme's active site because their shapes are complementary, forming an enzyme-substrate complex. The reaction takes place at the active site, then the products leave and the enzyme is free to be used again, because it is not used up.

Why do enzymes denature at high temperatures?

Enzymes denature at high temperatures because the heat damages the enzyme's structure, so the shape of the active site changes. The substrate is no longer complementary to the active site and cannot fit, so the rate of reaction falls quickly towards zero. Denaturing is permanent, and enzymes are not alive, so they are not 'killed'.

What happens to enzymes at low temperatures?

At low temperatures enzymes work slowly but are not denatured. The enzyme and substrate particles have less kinetic energy and move more slowly, so they collide less often and fewer enzyme-substrate complexes form. As the temperature rises towards the optimum, around 37 °C for many human enzymes, collisions increase and the rate rises.

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