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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Lipids are made of carbon, hydrogen and oxygen (with a higher proportion of hydrogen than carbohydrates). They include fats (solid at room temperature) and oils (liquid at room temperature). Roles include energy storage, insulation, protection and forming cell membranes.
Lipids store more than twice the energy per gram compared to carbohydrates, making them the body's most efficient energy reserve. This guide covers lipid structure, the differences between fats and oils, their biological roles, and the ethanol emulsion test.
Structure of Lipids (Supplement)
Lipids are made from glycerol and three fatty acid chains. This structure is called a triglyceride.
Each fatty acid joins to the glycerol by a condensation reaction, releasing one molecule of water. Three fatty acids = three water molecules released.
The bond formed between glycerol and a fatty acid is called an ester bond.
Saturated fats: no double bonds between carbon atoms in the fatty acid chains. Tend to be solid at room temperature (e.g. Butter, lard). Found mainly in animal products.
Unsaturated fats: one or more double bonds between carbons. Tend to be liquid at room temperature (oils, e.g. Olive oil, sunflower oil). Found mainly in plants and fish.
Biological Roles of Lipids
Energy storage: lipids release more than twice the energy per gram (37 kJ/g) compared to carbohydrates (17 kJ/g). They are stored under the skin and around organs.
Thermal insulation: a layer of subcutaneous fat beneath the skin reduces heat loss. Marine mammals like whales have thick blubber.
Protection: fat cushions organs like the kidneys and heart, protecting them from physical damage.
Cell membranes: phospholipids (modified lipids with a phosphate group) form the bilayer of cell membranes.
Waterproofing: waxy lipids on leaf surfaces (cuticle) reduce water loss by evaporation.
Buoyancy: fat is less dense than water, helping aquatic mammals float.
The Ethanol Emulsion Test for Lipids
Crush or dissolve the food sample in ethanol (alcohol).
Shake well to dissolve any lipids present.
Pour the ethanol solution into a test tube of cold water.
A cloudy white emulsion forms if lipids are present.
If no lipids are present, the solution remains clear.
Why does this work? Lipids dissolve in ethanol but not in water. When the ethanol solution is added to water, the lipids come out of solution and form tiny droplets: creating the cloudy emulsion.
Try a focused question (3 marks)
Explain why lipids are a better long-term energy store than carbohydrates. (3 marks)
Show the answer and marking guidance
Lipids release more than twice the energy per gram compared to carbohydrates.
Lipids are insoluble in water, so they do not affect osmosis / water potential of the cell.
Lipids are compact and can be stored in large quantities under the skin and around organs.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying the emulsion test uses "oil": you dissolve the sample in ethanol, then add to water.
Confusing lipids with carbohydrates for cell wall composition: lipids make up membranes, cellulose makes up plant cell walls.
Forgetting that all fats and oils are lipids: fats are solid lipids, oils are liquid lipids.
Practical connection
Food Tests: Reagents, Methods and Colour Changes
Use Benedict’s plus heat for reducing sugar, iodine for starch, biuret for protein, ethanol then water for fat, and DCPIP decolourisation for vitamin C. State both the reagent and the positive colour change.