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 the tests that reveal what a food contains.

Why digest? 🏭

Food is made of **large, insoluble** molecules — too big to be absorbed. The **digestive system** breaks them into **small, soluble** molecules that can pass into the blood. Organs like the stomach, small intestine, pancreas and liver each play a part in this production line.

Biological catalysts 🔑

**Enzymes** are **biological catalysts** — proteins that speed up reactions without being used up. Each enzyme has an **active site** with a specific shape. Only a matching **substrate** fits into it — the **lock-and-key** model. That is why each enzyme works on **one** kind of molecule.

One key, one lock

In the lock-and-key model, why does each enzyme only work on one type of substrate?

• Its active site has a specific shape that only that substrate fits
• Enzymes are too small to hold more than one substrate
• Each enzyme is used up after one reaction
• Each enzyme only works at one temperature

Heat and acidity 🌡️

Enzymes are fussy about their conditions: • **Temperature** — the rate rises as it warms, up to an **optimum** (about 37 °C in humans). Too hot and the enzyme **denatures**. • **pH** — each enzyme has an **optimum pH**; too acidic or alkaline also denatures it. **Denature** means the **active site changes shape**, so the substrate no longer fits — it does NOT mean the enzyme "dies".

Read the curve

On a graph of enzyme rate against temperature, the rate rises to a peak at about 37 °C, then falls **sharply**. Why does it fall after the peak?

• The enzyme denatures — its active site changes shape, so the substrate no longer fits
• The enzyme dies
• It runs out of substrate
• It gets too cold to react

Three enzymes, three jobs 🧫

Three types of digestive enzyme break down the three big food groups: • **Carbohydrases** (e.g. **amylase**) → break **starch** into **sugars**. • **Proteases** → break **proteins** into **amino acids**. • **Lipases** → break **lipids** (fats) into **fatty acids and glycerol**.

Match the products

Name the products

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

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: • It is **alkaline**, so it **neutralises** the acid from the stomach — giving enzymes their optimum pH. • It **emulsifies** fats — breaking large fat droplets into small ones, increasing the **surface area** for lipase to work faster.

Why emulsify?

Bile emulsifies fats. How does that speed up digestion?

• It breaks fat into small droplets, increasing the surface area for lipase
• It digests the fat itself into fatty acids
• It makes the small intestine more acidic
• It turns the fat into sugar

RP5: pH and amylase

An interactive activity.

Food test colours