• Lipids have a much lower proportion of water than other molecules such as Carbohydrates.
  • insoluble
  • Long carbon chains
  • They are made from two molecules: Lipids can exist as fats, oils and waxes.
    • Fats and oils are very similar in structure (triglycerides).
  • At room temperature, fats are solids and oils are liquids.
    • Fats are of animal origin, while oils tend to be found in plants.
  • Waxes have a different structure (esters of fatty acids with long chain alcohols) and can be found in both animals and plants.
  • They are a diverse group of:
    • Fats
    • Phospholipids
    • Steroids
  • They do not form polymers, they have large molecules (unlike proteins and carbohydrates which are polymers)
  • They have a greater ratio of Oxygen and hydrogen than H2O
  • Lipids perform many functions, such as:
  1. Storage – lipids are non-polar and so are insoluble in water.
  2. High-energy store – they have a high proportion of H atoms relative to O atoms and so yield more energy than the same mass of carbohydrate.
  3. Production of metabolic water – some water is produced as a final result of respiration.
  4. Thermal insulation – fat conducts heat very slowly so having a layer under the skin keeps metabolic heat in.
  5. Electrical insulation – the myelin sheath around axons prevents ion leakage.
  6. Waterproofing – waxy cuticles are useful, for example, to prevent excess evaporation from the surface of a leaf.
  7. Hormone production – steroid hormones. Oestrogen requires lipids for its formation, as do other substances such as plant growth hormones.
  8. Buoyancy – as lipids float on water, they can have a role in maintaining buoyancy in organisms.

Testing for Lipids: Emulsion Test


  • A clean test tube
  • 2cm3 of the sample being tested
  • 5cm3 Ethanol
  • 5cm3 water


  1. Put 2cm3 of the sample being tested into the test tube
  2. Add 5cm3 of ethanol to the test tube
  3. Add 5cm3 of water and shake the mixture gently
  4. For a control, use water as the sample in a new test tube. Repeat instructions.


If the solution turns cloudy white, this indicates lipids are present.