Enzymes are biological catalysts

An enzyme is a biological catalyst: a protein that speeds up the rate of a reaction without being used up, so it can be reused again and again.

• Enzymes let reactions happen quickly at the low temperatures of the body (around 37 °C) instead of needing lots of heat.
• Each enzyme has a specially shaped part called the active site, where the reacting molecule — the substrate — fits and reacts.
• Enzymes are specific: one enzyme works on one type of substrate, because the active site only fits that molecule's shape.

Watch out: because they are proteins, enzymes have a precise 3-D shape. If that shape is destroyed the enzyme can no longer work.

The lock-and-key model

The lock-and-key model explains enzyme specificity. The substrate (the key) fits exactly into the enzyme's active site (the lock). They join to form an enzyme–substrate complex, the reaction happens, then the products are released and the enzyme is free to work again.

Key idea: only a substrate with the complementary shape fits the active site — this is why each enzyme is specific to one reaction.

What is an enzyme?

Effect of temperature

As temperature rises, enzyme-controlled reactions get faster — molecules move more and collide more often. But above an enzyme's optimum temperature the rate falls sharply.

Above the optimum, the heat breaks the bonds holding the enzyme's shape together. The active site changes shape, the substrate no longer fits, and the enzyme is denatured. Denaturing is permanent — it does not reverse on cooling.

Misconception: a denatured enzyme is not "killed" — enzymes are not alive. Its shape is changed so the active site no longer fits the substrate.

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 enzyme denatures — again the active site changes shape.

Link it up: the stomach makes hydrochloric acid to give pepsin its acidic optimum and to kill microbes; bile later neutralises this acid so the small intestine's enzymes have their alkaline optimum.

What happens to the enzyme?

Tap a condition, then the box that describes the enzyme's activity.

Your turn — rate of reaction

Your turn — rate from a graph reading

The human digestive system

Digestion breaks large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. Food passes along the gut in order:

• Mouth — teeth chew food; salivary amylase begins starch digestion.
• Oesophagus — a muscular tube that pushes food to the stomach.
• Stomach — muscular walls churn food with hydrochloric acid and protease (pepsin).
• Small intestine — digestion is completed; small food molecules are absorbed into the blood.
• Large intestine — water is absorbed from the remaining material, forming faeces.

The digestive enzymes

Three groups of digestive enzymes break the three main food groups into small, soluble products:

• Amylase (a carbohydrase) breaks starch → sugars (e.g. glucose/maltose). Made in the salivary glands and pancreas.
• Protease breaks proteins → amino acids. Pepsin is the stomach protease that works in acid; more protease is made by the pancreas.
• Lipase breaks lipids (fats) → fatty acids + glycerol. Made in the pancreas and small intestine.

Match each enzyme to what it does

Tap a statement on the left, then its matching enzyme on the right.

Which products?

Bile — not an enzyme

Bile is made in the liver, stored in the gall bladder, and released into the small intestine. Bile is alkaline and it is not an enzyme — but it has two important jobs:

• Neutralises the acid that arrives from the stomach, making conditions alkaline — the optimum pH for enzymes in the small intestine.
• Emulsifies fats: it breaks large fat droplets into many tiny droplets, giving a much bigger surface area for lipase to work on. This speeds up fat digestion.

Exam trap: bile does not digest fat — lipase does the chemical breakdown. Bile only emulsifies (physical change) and neutralises acid.

What does bile do?

Absorption in the villi

The digested food is absorbed in the small intestine, whose inner lining is covered in millions of tiny finger-like villi (singular: villus). Villi are beautifully adapted for fast absorption:

• Large surface area — millions of villi (with micro-villi) mean more room for absorption.
• Thin walls — only one cell thick, so molecules diffuse across a short distance.
• Good blood supply — capillaries carry absorbed glucose and amino acids away, keeping a steep concentration gradient.

Link it up: the same adaptations (big surface area, thin wall, short diffusion path) make any good exchange surface efficient — like the alveoli in the lungs.

Why villi?

The big ideas to know

You've covered enzymes, digestion and absorption for CCEA GCSE Biology. Press Finish to see your score.