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What are enzymes, and how do temperature and pH affect how well they work?

Describe enzymes as biological catalysts, explain how they speed up reactions, and describe the effect of temperature and pH on enzyme activity

A focused N(A)-Level answer on enzymes. Enzymes as biological catalysts, why each enzyme is specific, and how temperature and pH change enzyme activity including denaturing.

Generated by Claude Opus 4.87 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 enzymes as biological catalysts, to explain how they speed up reactions, and to describe how temperature and pH affect how well they work. The central idea is that enzymes are special proteins with a particular shape that controls almost every reaction in living things.

The answer

What enzymes are

An enzyme is a biological catalyst: a protein that speeds up a chemical reaction in a living thing without being used up itself. Because the enzyme is not used up, a small amount can keep working again and again. Enzymes control reactions such as digestion, respiration and photosynthesis.

Why enzymes are specific

Each enzyme works on only one type of substance, called its substrate. This is because the enzyme has a particular shape that fits its substrate, often described as a lock and key: only the right key (substrate) fits the lock (enzyme). This is why one enzyme breaks down starch and a different one breaks down protein.

The effect of temperature

Temperature has a strong effect on enzyme activity:

as the temperature rises towards the optimum (best) temperature, around body temperature, the activity increases because the particles move and collide more often,
above the optimum, the enzyme is denatured: its shape changes so the substrate no longer fits, and it stops working. High temperature damage cannot be undone.

The effect of pH

Each enzyme also has an optimum pH at which it works best. If the pH is too high or too low, the enzyme's shape changes and it is denatured, so it works more slowly or stops. This is why digestive enzymes in the acidic stomach are different from those in the more neutral small intestine.

Explaining an enzyme rate graph

A student measures how fast an enzyme works as the temperature increases from $0\ ^\circ\text{C}$ to $60\ ^\circ\text{C}$ . The rate rises, peaks at about $40\ ^\circ\text{C}$ , then falls sharply. Explain the graph.

Step 1: Explain the rising part

From $0\ ^\circ\text{C}$ upward, raising the temperature gives the particles more energy, so they collide more often and the reaction speeds up.

Step 2: Explain the peak

At about $40\ ^\circ\text{C}$ the enzyme reaches its optimum temperature, where it works fastest.

Step 3: Explain the falling part

Above the optimum, the high temperature denatures the enzyme: its shape changes so the substrate no longer fits, and the rate drops.

Step 4: State the key point

The peak is the optimum temperature; the fall is due to denaturation, which is permanent. This is why most human enzymes work best near body temperature.

Examples in context

Example 1. Why a fever is dangerous. Body enzymes work best near $37\ ^\circ\text{C}$ . A very high fever can begin to denature enzymes, stopping vital reactions, which is why a dangerously high temperature must be brought down.

Example 2. How biological washing powders work. Biological detergents contain enzymes that break down stains such as food and grease at warm (but not hot) temperatures. Washing too hot would denature the enzymes, so these powders are used at lower temperatures where the enzymes stay active.

Try this

Cue. Define a catalyst. A substance that speeds up a reaction without being used up.
Cue. State what happens to an enzyme above its optimum temperature. It is denatured (its shape changes) and stops working.
Cue. Explain why an enzyme only works on one substrate. Its shape fits only that substrate, like a key fitting one lock.

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.

Original4 marks(a) Define an enzyme. (b) Describe what happens to an enzyme's activity as the temperature rises from

10\ ^\circ\text{C}

to body temperature, and then to

70\ ^\circ\text{C}

Show worked answer →

(a) An enzyme is a biological catalyst, a protein that speeds up a chemical reaction in living things without being used up.

(b) From $10\ ^\circ\text{C}$ up to body temperature (about $37\ ^\circ\text{C}$ ), the activity increases as the particles move and collide more. Above this, by $70\ ^\circ\text{C}$ , the enzyme is denatured (its shape changes) and stops working.

What markers reward: enzyme defined as a biological catalyst that is not used up, activity rising to an optimum, then falling because the enzyme is denatured at high temperature.

Original3 marks(a) Explain why an enzyme only works on one type of substance. (b) State what happens to an enzyme in very acidic conditions if it normally works at a neutral pH.