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

Explain that enzymes are biological catalysts, describe the lock and key model, and explain the effects of temperature and pH on enzyme activity

A scaffolded answer to the N(A)-Level Biology outcome on enzymes. Enzymes as biological catalysts, the lock and key model, and how temperature and pH change enzyme activity, including denaturing, with a worked rate example.

Generated by Claude Opus 4.88 min answer

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

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

This outcome wants you to explain what enzymes are (biological catalysts), how they work using the lock and key model, and how temperature and pH change how fast they work. The most important ideas are that each enzyme has a special shape that fits only its substrate, and that too much heat or the wrong pH changes that shape so the enzyme stops working, which is called denaturing.

The answer

What an enzyme is

An enzyme is a biological catalyst. A catalyst is a substance that speeds up a chemical reaction without being used up itself, so the same enzyme can be used over and over. Enzymes are proteins, and they control nearly all the chemical reactions in living things, such as digestion and respiration.

The lock and key model

Each enzyme has a special shape with a part called the active site. The substrate (the substance the enzyme acts on) has a shape that fits exactly into the active site, like a key fitting into a lock. Because of this exact fit, each enzyme only works on one particular substrate. This is called being specific. Once the substrate is in the active site, the reaction happens, the products are released, and the enzyme is free to work again.

The effect of temperature

As temperature rises towards the optimum (the best temperature, about 37 degrees Celsius for human enzymes), the rate of reaction increases. This is because the molecules move faster and collide more often. Above the optimum, the heat changes the shape of the active site so the substrate no longer fits. The enzyme is then denatured and stops working, so the rate falls sharply.

The effect of pH

Each enzyme has an optimum pH at which it works best. Moving away from this pH, in either direction, slows the enzyme down. A pH that is too far from the optimum changes the shape of the active site and denatures the enzyme. For example, the stomach enzyme pepsin works best in acidic conditions (low pH), while many other enzymes work best around neutral.

Examples in context

Example 1. Why a fever is dangerous. Human enzymes work best at about 37 degrees Celsius. A very high fever can push the body's temperature high enough to start denaturing enzymes, which is why a dangerously high temperature must be brought down. It shows why the optimum matters in real life.

Example 2. Enzymes in washing powder. Many washing powders contain enzymes that break down food and blood stains (made of proteins and fats). They work best at warm, not boiling, temperatures, because boiling water would denature them, which is why some are labelled for use at lower wash temperatures.

Try this

Q1. Define the term biological catalyst. [2 marks]

  • Cue. A substance (an enzyme) that speeds up a chemical reaction in a living organism without being used up itself.

Q2. Explain why each enzyme works on only one substrate. [2 marks]

  • Cue. The substrate must fit exactly into the enzyme's active site, like a key in a lock; only the matching shape fits, so the enzyme is specific.

Q3. Explain what happens to an enzyme when it is heated well above its optimum temperature. [2 marks]

  • Cue. The shape of the active site changes so the substrate no longer fits; the enzyme is denatured and stops working, so 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.

Original4 marksExplain what happens to the rate of an enzyme reaction as the temperature is raised from 10 ∘C10\ ^\circ\text{C} to 60 ∘C60\ ^\circ\text{C}.
Show worked answer →

From about 10 ∘C10\ ^\circ\text{C}, as the temperature rises towards the optimum (around body temperature, 37 ∘C37\ ^\circ\text{C} for human enzymes), the rate increases. This is because the enzyme and substrate molecules move faster and collide more often, so more reactions happen.

Above the optimum, the rate falls sharply. The high temperature changes the shape of the enzyme's active site, so the substrate no longer fits. The enzyme is denatured and stops working. By 60 ∘C60\ ^\circ\text{C} most enzymes are denatured, so the rate is very low or zero.

What markers reward: the increase up to the optimum (more collisions), the optimum temperature, the fall above it, and the word denatured linked to a change in the shape of the active site. Saying the enzyme is killed loses the mark, because enzymes are not alive.

Original4 marksDefine the term enzyme and describe, using the lock and key model, how an enzyme breaks down its substrate.
Show worked answer →

Definition: an enzyme is a biological catalyst, a protein that speeds up a chemical reaction in a living organism without being used up itself.

Lock and key model: the substrate has a shape that fits exactly into the enzyme's active site, like a key fitting a lock. The substrate binds to the active site, the reaction takes place, and the products are released. The enzyme is unchanged and can be used again.

What markers reward: biological catalyst (and not used up) in the definition, and the lock and key description with the substrate fitting the active site, the reaction occurring, and products released. The point that each enzyme is specific to one substrate (because of the shape) is often credited too.

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