GCSE Biology · Edexcel 1BI0 · Topic 1B
Enzymes and food tests
Key points
- Enzymes are proteins that speed up reactions without being used up. The substrate fits the active site (the lock and key model).
- High temperatures or extreme pH change the shape of the active site so the substrate no longer fits: the enzyme is denatured. Each enzyme has an optimum temperature and pH.
- Rate rises with substrate concentration until all active sites are busy, then levels off.
- Digestive enzymes break large molecules into small ones: carbohydrases (e.g. amylase: starch to sugars), proteases (proteins to amino acids), lipases (lipids to fatty acids and glycerol). Other enzymes build large molecules from small ones.
- Food tests: iodine turns blue-black with starch; Benedict's reagent, heated, goes green, yellow or brick-red with reducing sugars; biuret goes purple with protein; the ethanol emulsion test gives a cloudy white layer with fats.
- Calorimetry: burn food under water and use the temperature rise to estimate the energy released.
- Rate calculation: or change ÷ time.
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Enzymes are the reason life's chemistry happens fast enough at body temperature. This sub-topic explains how enzymes work, what changes their activity and how to measure it, plus the chemical tests that identify the main nutrients in food.
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