Enzymes
Enzymes are biological catalysts. They speed up chemical reactions in living organisms without being used up. An enzyme has an active site whose shape is complementary to the substrate. The lock-and-key model describes this fit, while the induced-fit model explains that the active site can adjust slightly around the substrate.
Factors affecting enzyme activity
Temperature and pH affect enzyme activity. Increasing temperature usually increases the rate until an optimum is reached. Above the optimum, bonds maintaining the enzyme shape can break and the enzyme becomes denatured. Each enzyme also has an optimum pH.
The digestive system
| Part | Function |
|---|---|
| Mouth | Chews food and mixes it with saliva; salivary amylase begins starch digestion. |
| Oesophagus | Moves the bolus to the stomach by peristalsis. |
| Stomach | Churns food and provides acidic conditions for protein digestion. |
| Small intestine | Completes digestion and absorbs soluble products through villi. |
| Large intestine | Absorbs water and forms faeces. |
Types of digestion
Mechanical digestion breaks food into smaller pieces without changing its chemical nature. Chemical digestion uses enzymes to break large insoluble molecules into small soluble molecules. Carbohydrates are digested to simple sugars, proteins to amino acids, and lipids to fatty acids and glycerol.
Revision questions
- Define an enzyme.
- Explain the effect of temperature on enzyme activity.
- Distinguish between mechanical and chemical digestion.
- State the function of the small intestine in digestion.
