- Lipids have a much lower proportion of water than other molecules such as Carbohydrates.
- 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:
- 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:
- Storage – lipids are non-polar and so are insoluble in water.
- 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.
- Production of metabolic water – some water is produced as a final result of respiration.
- Thermal insulation – fat conducts heat very slowly so having a layer under the skin keeps metabolic heat in.
- Electrical insulation – the myelin sheath around axons prevents ion leakage.
- Waterproofing – waxy cuticles are useful, for example, to prevent excess evaporation from the surface of a leaf.
- Hormone production – steroid hormones. Oestrogen requires lipids for its formation, as do other substances such as plant growth hormones.
- 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
- Put 2cm3 of the sample being tested into the test tube
- Add 5cm3 of ethanol to the test tube
- Add 5cm3 of water and shake the mixture gently
- 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.