Enzyme Action

Enzyme Action

The sandwich you ate is far too big to enter your blood. To use it, your body must **break it down**, and the tools that do it are **enzymes**. Learn how enzymes work, why heat and acidity make or break them, and how to explain the graph the examiners always print.

Biological catalysts

Food is made of **large, insoluble** molecules, far too big to be absorbed. Digestion breaks them into **small, soluble** ones that can pass into the blood, and enzymes are what do the breaking. The words the exam uses:

Mark the claims

Five statements about enzymes. Tap the TWO that are WRONG.

• An enzyme speeds up a reaction without being used up in it.
• Each enzyme is used up after catalysing one reaction, so the body must keep making more.
• An enzyme is specific because only a matching substrate fits its active site.
• Enzymes are specific because each one only works at a single temperature.
• Enzymes are proteins.

Heat and acidity

Enzymes are fussy about their conditions, and both temperature and pH work the same way: there is an **optimum**, and either side of it the rate falls. **Denature** means the **active site changes shape**, so the substrate no longer fits. It does NOT mean the enzyme "dies" (enzymes were never alive) and, crucially, **it cannot be undone**: cooling a denatured enzyme back to 37 °C does not restore it.

Plot the rate

An enzyme experiment gave these results. The x-axis is temperature in units of 10 °C (so x = 3 means 30 °C); the y-axis is the rate of reaction. Plot the five points: 10 °C → 1, 20 °C → 3, 30 °C → 6, 40 °C → 8, 50 °C → 2.

Cool it back down

A sample of enzyme is heated to 60 °C, well past its optimum, and the rate collapses. The sample is then cooled carefully back to 37 °C. What happens to the rate?

• It stays low, because denaturing permanently changed the shape of the active site
• It returns to its previous maximum, because 37 °C is the optimum temperature
• It rises even higher than before, because the heat has activated the enzyme
• It falls to exactly zero, because the enzyme has died

Three enzymes, three jobs

Three types of digestive enzyme break down the three big food groups: - **Carbohydrases** (for example **amylase**) break **starch** into **sugars**. - **Proteases** break **proteins** into **amino acids**. - **Lipases** break **lipids** (fats) into **fatty acids and glycerol**. Where they work matters as much as what they do. Amylase is made in the salivary glands and the pancreas and works around neutral pH; protease in the stomach works in strong acid; lipase from the pancreas works in the small intestine, where bile has made conditions alkaline.

Name the products

Amylase breaks starch into _____; protease breaks protein into _____; lipase breaks lipids into _____.

Why not in the stomach?

Amylase begins digesting starch in the mouth and finishes the job in the small intestine, but it does no work at all while the food is in the stomach. Why not?

• The stomach is strongly acidic, far below amylase's optimum pH, so the amylase is denatured
• The stomach is too cold for amylase to work
• There is no starch left in the food by the time it reaches the stomach
• The amylase was used up while the food was still in the mouth

Bile: the fat-buster

**Bile** is made in the **liver**, stored in the **gall bladder**, and released into the small intestine. It does two jobs, and it is an enzyme for neither of them: - It is **alkaline**, so it **neutralises** the acid arriving from the stomach, giving the enzymes of the small intestine their optimum pH. - It **emulsifies** fats, breaking large fat droplets into many small ones. That increases the **surface area**, so lipase can work far faster. Bile does not digest anything. It makes the conditions right and it makes the target bigger; lipase does the actual breaking down.

Bile at work

Bile is made in the _____ and stored in the gall bladder before being released into the small intestine. It is _____, so it neutralises the acid arriving from the stomach and restores the optimum pH for the enzymes there. It also emulsifies fat into small droplets, which does not digest the fat at all: it increases the _____ area, so that _____ can work on it faster.

RP5: pH and amylase

RP5 investigates how pH affects amylase. Put the method into the correct order.

• Mix amylase and starch together at a set pH
• Every 30 seconds, drop a sample onto iodine solution
• Watch until the iodine no longer turns blue-black
• Record the time taken to break down the starch
• Repeat the whole test at different pH values

Explain the graph

A student measures the rate of an enzyme-controlled reaction at temperatures from 10 °C to 60 °C. The rate rises to a peak at about 37 °C and then falls sharply. Explain the shape of this graph.

• Explain why the rate RISES as the temperature increases up to 37 °C
• Name what happens above the optimum, using the correct term
• Say what physically changes about the enzyme, and why that stops the substrate reacting
• Say what would happen if the enzyme were cooled back to 37 °C, and why

In the exam

Enzymes are biological _____ that speed up reactions without being used up. Each has an active site whose shape fits only one _____, which is the lock-and-key model. Above the optimum temperature, or at the wrong pH, the active site changes shape and the enzyme is _____, which cannot be undone. Bile digests nothing: it neutralises stomach acid and emulsifies fat, increasing the _____ area for lipase.