What you'll learn
Enzymes are biological catalysts that control the reactions of life, and they are central to digestion. For AQA GCSE Biology you need to understand what enzymes are, the "lock and key" model of how they work, the factors that affect their activity, and the digestive enzymes that break down our food. This guide covers enzyme structure and action, the effects of temperature and pH, the digestive enzymes and their products, and the role of bile. By the end you should be able to explain how enzymes work, how conditions affect them, and how they digest food.
Key terms and definitions
Enzyme — A biological catalyst that speeds up a reaction without being used up.
Catalyst — A substance that increases the rate of a reaction without being changed itself.
Substrate — The molecule an enzyme acts on.
Active site — The part of the enzyme where the substrate binds; its shape is complementary to the substrate.
Lock and key model — The idea that the substrate fits the active site like a key fits a lock.
Denatured — When an enzyme's shape (and active site) is permanently changed so it no longer works.
Optimum — The temperature or pH at which an enzyme works fastest.
Bile — An alkaline liquid, made in the liver, that emulsifies fats and neutralises acid.
Core concepts
What enzymes are and how they work
Enzymes are biological catalysts: they speed up chemical reactions in living things without being used up, so they can be used again and again. Each enzyme has a specific region called the active site, whose shape matches a particular substrate. The substrate fits into the active site, the reaction takes place, and the products are released. This is described by the lock and key model — the substrate (key) fits the active site (lock).
Because the shape of the active site must match the substrate, each enzyme is specific to one type of reaction or substrate. Enzymes both break large molecules into smaller ones (as in digestion) and build large molecules from smaller ones.
The effect of temperature
Temperature affects enzyme activity. As temperature increases, the rate of reaction increases, because the enzyme and substrate molecules move faster and collide more often. But above the optimum temperature, the enzyme becomes denatured: the heat changes the shape of the active site so the substrate no longer fits, and the reaction stops. Human enzymes have an optimum of around 37 °C (body temperature). So the rate rises to a peak at the optimum, then falls sharply as the enzyme denatures.
The effect of pH
Each enzyme also has an optimum pH at which it works fastest. If the pH is too high or too low, the shape of the active site changes and the enzyme is denatured, so it no longer works. Different enzymes have different optimum pH values to suit where they work — for example, the enzyme in the stomach works best in acidic conditions, while enzymes in the small intestine work best in alkaline conditions.
Digestive enzymes and their products
Digestion uses enzymes to break large, insoluble food molecules into small, soluble ones that can be absorbed. The three main types are:
- Amylase (a carbohydrase) — breaks down starch into sugars (like maltose). Made in the salivary glands, pancreas and small intestine.
- Protease — breaks down proteins into amino acids. Made in the stomach, pancreas and small intestine.
- Lipase — breaks down lipids (fats) into fatty acids and glycerol. Made in the pancreas and small intestine.
Each enzyme is specific to its substrate, which is why amylase only breaks down starch and protease only breaks down protein.
The role of bile
Bile is made in the liver and stored in the gall bladder, then released into the small intestine. It has two functions:
- It is alkaline, so it neutralises the acid that arrives from the stomach, providing the alkaline conditions the enzymes in the small intestine need to work.
- It emulsifies fats, breaking large fat droplets into many small droplets, which increases the surface area for lipase to work on, speeding up the digestion of fat.
Bile is not an enzyme — it does not chemically digest fat, but it helps lipase to work faster.
Why digestion matters
Digestion breaks food into small, soluble molecules that can be absorbed into the blood and used by the body — glucose for respiration, amino acids to build proteins, and fatty acids and glycerol for membranes and energy storage. Enzymes make this possible at body temperature, which is why they are essential to life.
Where digestion happens
It helps to know where each stage of digestion takes place. Digestion begins in the mouth, where teeth break food into smaller pieces (increasing the surface area) and salivary amylase starts to break down starch. In the stomach, protease enzymes work in acidic conditions (provided by hydrochloric acid), which also kills bacteria, and the food is churned. In the small intestine, enzymes from the pancreas and the intestine itself complete digestion, helped by bile from the liver, and the small soluble products are absorbed. Knowing this sequence, and that different enzymes work best in the different conditions of the stomach and small intestine, helps explain why each enzyme has its own optimum pH.
Calculating and comparing rates of reaction
Enzyme experiments often produce data on the rate of reaction, and you should be able to work with it. The rate can be found by measuring how much product is made, or how much substrate is used up, in a given time — for example, the volume of gas produced per second, or the time taken for a colour change. Rate is calculated as the amount of change divided by the time taken. By measuring the rate at different temperatures or pH values and plotting a graph, you can find the optimum, shown as the peak of the curve. Being able to read such a graph — rate rising to a peak at the optimum, then falling as the enzyme denatures — is a common exam skill that ties the theory to real data.
Worked examples
Example 1: The lock and key model
Explain how the lock and key model describes enzyme action. The substrate fits into the enzyme's active site like a key fits a lock, because their shapes are complementary. The reaction then takes place and the products are released. Because the shape must match, each enzyme is specific to one substrate.
Example 2: Explaining denaturation
Explain why an enzyme stops working at high temperatures. Above the optimum temperature, the heat changes the shape of the enzyme's active site, so the substrate no longer fits. The enzyme is denatured, and because the substrate cannot bind, the reaction stops.
Example 3: Matching enzyme to product
Which enzyme breaks down fats, and what are the products? Lipase breaks down lipids (fats) into fatty acids and glycerol.
Example 4: The role of bile
Explain the two functions of bile in digestion. Bile neutralises the stomach acid, providing the alkaline conditions that enzymes in the small intestine need. It also emulsifies fats, breaking large droplets into small ones to increase the surface area for lipase to act on.
Common mistakes and how to avoid them
A common error is saying an enzyme is "killed" at high temperature. Enzymes are not alive — they are denatured, meaning their shape is permanently changed. Use the correct term.
Students often say denaturation only happens with heat. Extremes of pH also denature enzymes by changing the active site's shape, so mention pH as well as temperature.
Another mistake is describing the active site as "melting" or "breaking". It changes shape, so the substrate no longer fits — this is the phrasing that earns marks.
When naming digestive products, learn them: starch → sugars, protein → amino acids, fats → fatty acids and glycerol. Forgetting glycerol is a common slip.
Finally, remember bile is not an enzyme. It neutralises acid and emulsifies fats to help lipase; the chemical breakdown of fat is done by lipase itself.
Exam technique for "Enzymes and digestion"
Be ready to describe the lock and key model and to explain why enzymes are specific. For temperature and pH questions, explain the rise to an optimum and the fall due to denaturation, always in terms of the active site changing shape.
A table linking each enzyme to its substrate, product and site of production is the most useful thing to memorise. For bile, always give both functions (neutralising acid and emulsifying fats) and explain why each matters.
Use precise terms — enzyme, substrate, active site, denatured, emulsify — throughout, and link the digestion of each food group to the enzyme responsible and the product formed.
Quick revision summary
- Enzymes are biological catalysts with a specific active site; the substrate fits like a key in a lock (lock and key model).
- Rate rises with temperature to an optimum (~37 °C in humans), then falls as the enzyme is denatured; each enzyme also has an optimum pH.
- Denaturation is a permanent change in the shape of the active site, so the substrate no longer fits.
- Amylase → sugars; protease → amino acids; lipase → fatty acids and glycerol.
- Bile (from the liver) is alkaline — it neutralises stomach acid and emulsifies fats to increase surface area for lipase. It is not an enzyme.
- Digestion produces small, soluble molecules that can be absorbed and used by the body.