Topic 2 of 8 · Form 3
Biological molecules and enzymes
Iodine for starch (blue-black), Benedict's for reducing sugars (brick-red on heating), biuret for protein (purple), ethanol emulsion for fats (milky white), DCPIP for vitamin C (decolourises).
Notes for this topic 1 file, in order
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Biological molecules
- Carbohydrates: glucose units form starch, glycogen and cellulose.
- Fats and oils: fatty acids and glycerol.
- Proteins: chains of amino acids.
- DNA: two strands in a double helix with paired bases (A–T, C–G).
Food tests
Iodine for starch (blue-black), Benedict's for reducing sugars (brick-red on heating), biuret for protein (purple), ethanol emulsion for fats (milky white), DCPIP for vitamin C (decolourises).
Enzymes
Proteins that act as biological catalysts. The substrate fits the complementary active site (lock and key model).
- Temperature: activity increases to an optimum, then the enzyme denatures as the active site changes shape.
- pH: each enzyme has an optimum pH; extremes denature it.
Uses of enzymes
Biological washing powders, fruit-juice extraction (pectinase), lactose-free milk (lactase), and in industrial processes.
Worked example: Explain why amylase activity falls sharply above 45 °C.
High temperatures break bonds holding the enzyme's shape, so the active site changes and the substrate no longer fits – the enzyme is denatured.
Check yourself
- Which reagent tests for protein?
- What are the subunits of a protein?
- Name the bases that pair in DNA.
Show answers
- Biuret reagent.
- Amino acids.
- Adenine with thymine; cytosine with guanine.
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