Temperature, substrate concentration and pH

Key concepts in biology · Enzyme action and activity · note 3 of 5

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Temperature, substrate concentration and pHSpec 1.9

In short

Enzyme activity increases with temperature up to an optimum, around 37 °C for many human enzymes, because enzyme and substrate collide more often. Above the optimum the enzyme is denatured and the rate falls. Each enzyme also has an optimum pH. Increasing substrate concentration raises the rate until all the active sites are occupied, then the rate levels off.

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, the active site changes shape, the enzyme is denatured and the rate falls quickly towards zero.

Substrate concentration

At low substrate concentration, increasing it makes the rate increase, because collisions between substrate and active sites are more frequent. Eventually all the active sites are occupied at once. Adding more substrate then has no effect, so the rate levels off. The enzyme concentration is now limiting the rate.

pH

Each enzyme has an optimum pH where it works fastest. If the pH is higher or lower than the optimum the rate decreases, and at extreme pH values the active site changes shape and the enzyme is denatured. Different enzymes have different optimum pH values.

Three graphs of rate of reaction: against temperature it rises to an optimum then falls steeply to zero; against substrate concentration it rises then levels off when all active sites are occupied; against pH it is a bell-shaped curve with an optimum pH. — How temperature, substrate concentration and pH affect the rate of an enzyme-controlled reaction.

Summary of the effects on enzyme activity
Factor	Below the optimum / low	At the optimum	Above the optimum / high
Temperature	Rate increases as particles move faster and collide more often	Highest rate	Enzyme denatured, rate falls
pH	Rate decreases as pH moves away from the optimum, and the enzyme is denatured at extreme pH	Highest rate	Rate decreases as pH moves away from the optimum, and the enzyme is denatured at extreme pH
Substrate concentration	Rate increases	Not applicable	Rate stops increasing when all active sites are occupied

Exam tip:

When a graph falls after a peak, always give the reason: the enzyme has been denatured. When a graph levels off, say all the active sites are occupied.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

Frequently asked questions

How do enzymes speed up reactions?

Enzymes speed up reactions by acting as biological catalysts, without being used up. The substrate collides with the enzyme's active site and fits into it because their shapes are complementary, forming an enzyme-substrate complex. The reaction happens at the active site, the products leave, and the enzyme can be used again.

Why do enzymes denature at high temperatures?

Enzymes denature at high temperatures because the bonds that hold the enzyme in shape are broken. The enzyme and its active site change shape, so the substrate no longer fits and the reaction cannot be catalysed. This change is permanent, so above the optimum temperature the rate falls quickly towards zero.

What happens to enzymes at low temperatures?

At low temperatures enzymes work slowly, but they are not denatured. The particles have less kinetic energy and move more slowly, so the enzyme and substrate collide less often and the rate of reaction is lower. As the temperature rises towards the optimum, collisions become more frequent and the rate increases.

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