How to Revise IGCSE Biology Topic 4: Biological Molecules
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key point
Memorise the four food tests (reagent + colour change). Know the building blocks of each molecule type. Understand starch vs glycogen vs cellulose as different forms of glucose polymer.
Topic 4 covers the molecules of life: carbohydrates, lipids, proteins, and their monomers. The food tests (Benedict's, iodine, biuret, ethanol emulsion) are frequently examined. Know the reagent, method, colour change, and what each test detects.
Priority 1: Four food tests
Reducing sugar (glucose): Benedict's reagent, heat in water bath → blue to orange/red. Starch: iodine solution → yellow-brown to blue-black. Protein: biuret reagent (sodium hydroxide + copper sulfate) → blue to purple/lilac. Fat/lipid: ethanol emulsion test → shake with ethanol, pour into water → cloudy white emulsion if fat present.
Priority 2: Molecules and their building blocks
Carbohydrates: made of simple sugars (monosaccharides like glucose). Starch (storage in plants), glycogen (storage in animals), cellulose (cell walls). Proteins: made of amino acids linked by peptide bonds. Lipids (fats and oils): made of fatty acids and glycerol.
Priority 3: The role of water
Water is the solvent for biochemical reactions, transport medium (blood plasma, xylem sap), reactant in some reactions (hydrolysis, photosynthesis), and provides a stable environment due to its high specific heat capacity.
Frequently asked questions
Do I need to know the chemical structures?
Not at IGCSE level. Know the building blocks (monomers), the polymers they form, and the food tests: not structural formulae.
Put the guide into practice
Questions and practical resources
Use a fresh question to check whether the idea can be applied without prompts.