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How do temperature and pH change how fast an enzyme works?

Describe and explain the effects of temperature and pH on the rate of enzyme-controlled reactions

A focused answer to the O-Level Biology outcome on enzyme activity. The effect of temperature and pH on the rate of reaction, the meaning of the optimum, and what denaturing does to an enzyme's active site.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to describe and explain how temperature and pH affect the rate of an enzyme-controlled reaction. You should be able to read a rate-against-temperature or rate-against-pH graph, identify the optimum, and explain the rising part by collisions and the falling part by denaturation, using the idea of the active site changing shape.

The answer

Effect of temperature

As temperature increases from low values, the rate of an enzyme-controlled reaction rises. This is because the enzyme and substrate molecules gain kinetic energy and move faster, so they collide more often and more substrate fits into active sites.

The rate is highest at the optimum temperature (around $37\ ^\circ\text{C}$ for human enzymes). Above the optimum, the rate falls sharply, because the high temperature makes the enzyme lose its precise shape: the active site changes shape, so the substrate no longer fits. The enzyme is then said to be denatured, and once denatured it cannot work again.

Effect of pH

Each enzyme has an optimum pH at which it works fastest. Moving the pH away from the optimum, more acidic or more alkaline, slows the reaction, because the change in pH alters the shape of the active site so the substrate fits less well. A pH far from the optimum can denature the enzyme.

Different enzymes have different optimum pH values to suit where they work: pepsin in the acidic stomach has an optimum around pH 2, while amylase in the neutral mouth has an optimum around pH 7.

What denaturing means

Denaturing is the permanent change in the shape of an enzyme (caused by high temperature or an extreme pH) so that its active site no longer fits the substrate. A denatured enzyme cannot catalyse its reaction. Note that the enzyme is not killed: it was never alive; its shape is simply changed.

Explaining a rate-against-temperature graph

A graph shows the rate of an enzyme reaction rising to a peak at $40\ ^\circ\text{C}$ then falling to zero by $60\ ^\circ\text{C}$ . Explain it in stages. [4 marks]

Step 1: Explain the rising part

From low temperatures up to $40\ ^\circ\text{C}$ , the molecules gain kinetic energy and move faster, colliding more often, so more enzyme-substrate complexes form and the rate rises.

Step 2: Identify the peak

At about $40\ ^\circ\text{C}$ the rate is at its maximum; this is the optimum temperature.

Step 3: Explain the falling part

Above the optimum, the heat causes the enzyme to lose its shape: the active site changes shape, so the substrate no longer fits. Fewer complexes form and the rate falls. This is denaturation.

Step 4: Explain why it reaches zero

By $60\ ^\circ\text{C}$ all the enzyme molecules are denatured, so no substrate can bind and the rate drops to zero. A full answer links the rise to collisions and the fall to denaturation, naming the optimum.

Examples in context

Example 1. Why a fever is dangerous. A very high body temperature can begin to denature human enzymes, which work best near $37\ ^\circ\text{C}$ . If key enzymes lose their shape, vital reactions slow or stop, which is why a high fever is treated quickly.

Example 2. Enzymes in washing powder. Biological washing powders contain enzymes that digest stains. They work best at warm, not boiling, temperatures, because very hot water would denature the enzymes and the powder would lose its cleaning power.

Try this

Q1. Define the optimum temperature of an enzyme. [1 mark]

Cue. The temperature at which the enzyme works fastest (has its highest rate of reaction).

Q2. Explain why the rate of an enzyme reaction increases as temperature rises toward the optimum. [2 marks]

Cue. The molecules gain kinetic energy and move faster, so enzyme and substrate collide more often and more complexes form.

Q3. Explain what happens to an enzyme above its optimum temperature. [2 marks]

Cue. The heat changes the shape of the active site (denatures the enzyme), so the substrate no longer fits and the rate falls.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksAn experiment measures the rate of an enzyme-controlled reaction at temperatures from

10

60\ ^\circ\text{C}

. The rate rises to a peak at about

40\ ^\circ\text{C}

and then falls sharply to zero by

60\ ^\circ\text{C}

. Explain the shape of this graph.

Show worked answer →

As temperature rises from $10\ ^\circ\text{C}$ toward $40\ ^\circ\text{C}$ , the enzyme and substrate molecules gain kinetic energy and move faster, so they collide more often and the rate of reaction increases.

At about $40\ ^\circ\text{C}$ the rate is highest; this is the optimum temperature.

Above the optimum, the high temperature causes the enzyme to lose its shape: the active site changes shape (the enzyme is denatured), so the substrate no longer fits and fewer enzyme-substrate complexes form. The rate falls and, once all the enzyme is denatured, drops to zero.

Markers reward more frequent collisions up to the optimum, naming the optimum, and denaturation (the active site changing shape so substrate no longer fits) above it. Denaturation must not be described as the enzyme being killed.

Original4 marksThe enzyme pepsin works in the stomach and the enzyme amylase works in the mouth. Pepsin has an optimum pH of about

2

and amylase about

7

. Explain what is meant by optimum pH and why pepsin would not work well in the mouth.

Show worked answer →

The optimum pH is the pH at which an enzyme works fastest (has its highest rate of reaction).

Pepsin works best at about pH 2, the acidic conditions of the stomach. In the mouth the pH is about 7 (neutral), which is far from pepsin's optimum. At this pH the shape of pepsin's active site is altered, so the substrate fits less well and the reaction is much slower; pepsin may be denatured.

Markers reward the definition of optimum pH (fastest rate) and the explanation that a pH far from the optimum changes the active site shape, slowing the reaction.