Enzymes and Digestion

Enzymes are biological catalysts that speed up chemical reactions in living organisms. In this lesson you will learn about enzyme structure and function, the factors that affect enzyme activity, the specific digestive enzymes required by the AQA GCSE Combined Science Trilogy specification (8464), and the required practical on food tests.

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What Are Enzymes?

Enzymes are large protein molecules that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up or changed in the process.

Key facts about enzymes:

• They are produced by living cells
• They are specific — each enzyme catalyses only one type of reaction
• They work by lowering the activation energy required for a reaction to take place
• They are not used up in the reaction and can be reused

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The Lock and Key Model

Enzymes work because of their specific three-dimensional shape. Each enzyme has a region called the active site, which has a unique shape that only the correct substrate (the molecule the enzyme acts on) can fit into — just like a key fits into a lock.

graph LR
    A["Substrate (the key)"] --> B["Fits into the active site (the lock)"]
    B --> C["Enzyme-substrate complex forms"]
    C --> D["Reaction occurs"]
    D --> E["Products released"]
    E --> F["Enzyme unchanged — ready to be reused"]

1. The substrate binds to the active site of the enzyme
2. An enzyme-substrate complex forms
3. The reaction takes place (the substrate is broken down or joined together)
4. The products are released
5. The enzyme is free to catalyse another reaction

Exam Tip: The lock and key model explains enzyme specificity. If the substrate does not fit the active site, no reaction occurs. This is why each enzyme only works on one type of substrate.

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Factors Affecting Enzyme Activity

Temperature

As temperature increases, enzyme activity increases because molecules have more kinetic energy and collide more frequently with the active site. However, above the optimum temperature, the enzyme begins to lose its shape.

• At very high temperatures, the bonds holding the enzyme's 3D shape break
• The active site changes shape — the substrate can no longer fit
• The enzyme is said to be denatured
• Denaturation is permanent — the enzyme cannot regain its shape

For most human enzymes, the optimum temperature is approximately 37°C (body temperature).

$$\text{Rate of reaction} \propto \text{temperature (up to the optimum)}$$Rate of reaction∝temperature (up to the optimum)

pH

Each enzyme has an optimum pH at which it works best. Moving away from this pH (in either direction) reduces enzyme activity and eventually causes denaturation.

Enzyme	Optimum pH	Location
Salivary amylase	~7 (neutral)	Mouth
Pepsin (protease)	~2 (acidic)	Stomach
Pancreatic lipase	~8 (slightly alkaline)	Small intestine

Exam Tip: If an exam question asks why pepsin works best in the stomach, explain that the stomach produces hydrochloric acid which creates a pH of about 2, and this is the optimum pH for pepsin. In the small intestine, bile neutralises the acid to create alkaline conditions for intestinal enzymes.

Substrate Concentration

• As substrate concentration increases, the rate of reaction increases because there are more substrate molecules to collide with active sites
• Eventually, all active sites are occupied (saturated) and increasing substrate concentration further has no effect — the rate plateaus

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Digestive Enzymes

Digestive enzymes are produced by specialised cells in glands and in the lining of the gut. They catalyse the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed.

The Three Main Groups

Enzyme Group	Substrate	Products	Where Produced	Where Active
Amylase	Starch	Sugars (maltose and then glucose)	Salivary glands, pancreas, small intestine wall	Mouth, small intestine
Proteases	Proteins	Amino acids	Stomach (pepsin), pancreas, small intestine wall	Stomach, small intestine
Lipases	Lipids (fats and oils)	Glycerol + fatty acids	Pancreas, small intestine wall	Small intestine

graph TD
    subgraph Carbohydrate Digestion
        A1["Starch (large, insoluble)"] -->|"Amylase"| A2["Maltose"] -->|"Maltase"| A3["Glucose (small, soluble)"]
    end
    subgraph Protein Digestion
        B1["Protein (large, insoluble)"] -->|"Proteases"| B2["Amino acids (small, soluble)"]
    end
    subgraph Lipid Digestion
        C1["Lipid / Fat (large, insoluble)"] -->|"Lipase"| C2["Glycerol + Fatty acids (small, soluble)"]
    end

The Role of Bile in Digestion

Bile is produced by the liver, stored in the gall bladder, and released into the duodenum.

Bile:

1. Emulsifies fats — breaks large fat droplets into many smaller droplets, dramatically increasing the surface area for lipase to work on
2. Neutralises stomach acid — bile is alkaline, which raises the pH in the small intestine to provide optimal conditions for pancreatic and intestinal enzymes

Exam Tip: Remember that bile is NOT an enzyme. It does not chemically break down fat molecules — it physically breaks large droplets into smaller ones (emulsification). The actual chemical digestion of fats is carried out by lipase.

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Required Practical: Food Tests

The AQA specification requires you to know how to test for the presence of different food groups using standard biochemical tests.

Test for Starch — Iodine Test

1. Add a few drops of iodine solution to the food sample
2. If starch is present, the solution turns from brown/orange to blue-black
3. If no starch is present, the iodine remains brown/orange

Test for Glucose (Reducing Sugars) — Benedict's Test

1. Add Benedict's reagent (blue) to the food sample in a test tube
2. Heat in a water bath at approximately 75°C for 5 minutes
3. If glucose (reducing sugar) is present, the colour changes from blue → green → yellow → orange → brick-red (depending on concentration)
4. If no reducing sugar is present, the solution remains blue

Test for Protein — Biuret Test

1. Add Biuret reagent (or sodium hydroxide followed by copper sulfate solution) to the food sample
2. If protein is present, the solution turns from blue to purple/lilac
3. If no protein is present, the solution remains blue

Test for Lipids — Sudan III Test (or Emulsion Test)

1. Shake the food sample with ethanol, then pour into water
2. If lipids are present, a white cloudy emulsion forms
3. Alternatively, add Sudan III stain — lipids will form a red-stained layer on top

Food Group	Reagent	Positive Result	Negative Result
Starch	Iodine solution	Blue-black	Brown/orange
Reducing sugars (glucose)	Benedict's reagent + heat	Green → orange → brick-red	Stays blue
Protein	Biuret reagent	Purple/lilac	Stays blue
Lipids	Ethanol + water (or Sudan III)	White cloudy emulsion (or red layer)	No emulsion / no red layer

Exam Tip: Learn these food tests thoroughly — they are frequently examined. Remember the colours, the reagents, and the conditions (Benedict's test needs heating). You may also be asked to describe how to make the test fair (same volumes, same concentrations, same time).

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