Enzymes and Digestion

Enzymes are biological catalysts produced by living organisms. They speed up chemical reactions in the body without being used up themselves. In AQA GCSE Biology, understanding how enzymes work — and how they are involved in digestion — is essential. This lesson covers the structure and function of enzymes, factors affecting enzyme activity, and the specific digestive enzymes you need to know.

---

What Are Enzymes?

An enzyme is a large protein molecule that acts as a biological catalyst. A catalyst is a substance that speeds up a chemical reaction without being changed or used up in the process. Enzymes are produced inside living cells and control all the metabolic reactions in the body.

Key Properties of Enzymes

Property	Explanation
Proteins	Enzymes are made of long chains of amino acids folded into a specific 3D shape
Specific	Each enzyme only catalyses one particular reaction — this is called enzyme specificity
Reusable	Enzymes are not used up in reactions and can be used again and again
Biological catalysts	They speed up reactions that would otherwise happen too slowly to sustain life

Exam Tip: The examiners will expect you to state that enzymes are proteins that act as biological catalysts. This two-part definition should be memorised precisely.

---

The Lock and Key Model

The lock and key model is used to explain how enzymes work. Each enzyme has a region called the active site. This is a specific area on the surface of the enzyme molecule where the substrate (the molecule the enzyme acts on) binds.

graph LR
    A[Substrate] -->|Fits into| B[Active Site of Enzyme]
    B --> C[Enzyme-Substrate Complex]
    C --> D[Products released]
    D --> E[Enzyme unchanged and ready to reuse]

1. The substrate has a complementary shape to the enzyme's active site
2. The substrate binds to the active site, forming an enzyme-substrate complex
3. The enzyme catalyses the reaction, converting the substrate into products
4. The products are released from the active site
5. The enzyme is unchanged and is free to catalyse the next reaction

This model explains specificity: only a substrate with the correct complementary shape can fit into a particular enzyme's active site, just like only the right key fits a particular lock.

Exam Tip: Always use the precise terminology: active site, substrate, enzyme-substrate complex, complementary shape. Avoid saying the substrate "fits like a jigsaw" — use "complementary shape" instead.

---

Factors Affecting Enzyme Activity

Enzyme activity is affected by temperature and pH. Both factors can change the rate of enzyme-catalysed reactions.

Effect of Temperature

Temperature Change	Effect on Enzyme Activity
Low temperature	Molecules move slowly, fewer collisions between enzyme and substrate, so the rate of reaction is low
Increasing temperature	Molecules gain kinetic energy, collisions increase, rate of reaction increases
Optimum temperature	The temperature at which the enzyme works fastest; for most human enzymes this is approximately 37 degrees C
Above the optimum	The enzyme begins to denature — the active site changes shape so the substrate can no longer fit
Very high temperature	The enzyme is completely denatured and the reaction stops permanently

Effect of pH

Each enzyme has an optimum pH at which it works best. Moving away from this optimum (in either direction) causes the active site to change shape, reducing and eventually stopping enzyme activity.

Enzyme	Optimum pH	Location
Pepsin (protease)	pH 2 (acidic)	Stomach
Salivary amylase	pH 7 (neutral)	Mouth
Lipase (with bile)	pH 8 (slightly alkaline)	Small intestine
Pancreatic amylase	pH 7-8 (slightly alkaline)	Small intestine

Denaturation

When an enzyme is denatured, the bonds holding its 3D shape together are broken. The active site changes shape so that the substrate can no longer bind. This is a permanent change — the enzyme cannot recover. Denaturation can be caused by temperatures above the optimum or by extremes of pH.

Exam Tip: A very common misconception is that enzymes "die" at high temperatures. Enzymes are not alive — they are proteins. The correct term is denatured. Always say the active site "changes shape" rather than "breaks down."

---

Digestive Enzymes

The human body uses three main types of digestive enzyme. Each type breaks down a different food molecule:

Enzyme Type	Substrate (breaks down)	Products	Where Produced	Where It Works
Amylase (a carbohydrase)	Starch	Sugars (maltose and other simple sugars)	Salivary glands, pancreas	Mouth, small intestine
Protease	Proteins	Amino acids	Stomach (pepsin), pancreas	Stomach, small intestine
Lipase	Lipids (fats and oils)	Glycerol and fatty acids	Pancreas	Small intestine

graph TD
    A[Large Insoluble Molecules] --> B[Starch]
    A --> C[Proteins]
    A --> D[Lipids]
    B -->|Amylase| E[Simple Sugars]
    C -->|Protease| F[Amino Acids]
    D -->|Lipase| G[Glycerol + Fatty Acids]
    E --> H[Small Soluble Molecules — Can Be Absorbed]
    F --> H
    G --> H

Why Is Digestion Necessary?

Large food molecules such as starch, proteins and fats are insoluble — they cannot dissolve in water and therefore cannot pass through the walls of the digestive system into the blood. Digestive enzymes break these large molecules into small, soluble molecules that can be absorbed into the bloodstream and transported to cells.

---

The Role of Bile

Bile is not an enzyme, but it plays a crucial role in fat digestion:

Feature	Detail
Produced by	The liver
Stored in	The gall bladder
Released into	The small intestine (duodenum)
Function 1	Emulsifies fats — breaks large fat droplets into smaller droplets, increasing the surface area for lipase to work on
Function 2	Neutralises stomach acid — bile is alkaline, so it raises the pH of food entering the small intestine, creating the slightly alkaline conditions that intestinal enzymes work best in

Exam Tip: Bile is a common topic in exam questions. Remember: bile is NOT an enzyme. It emulsifies fats (increases surface area) and neutralises acid. These are two separate functions and both may be required for full marks.

---

Enzymes in Industry and Everyday Life

Although the main focus at GCSE is on enzymes in the digestive system, you should be aware that enzymes are used widely: