The test for lipids is known as the emulsion test and is carried out as follows:
- Take a completely dry and grease free test tube.
- To 2cm^(3) of the sample being tested, add 5cm^(3) of ethanol.
- Shake the tube thoroughly to dissolve any lipid in the sample.
- Add 5cm^(3) of water and shake gently.
- A cloudy-white colour indicates the presence of a lipid.
- As a control, repeat the test using water instead of sample, and the solution should remain clear and colourless.
Lipids - varied group of substances that share the following characteristics:
- Contain carbon, hydrogen and oxygen.
- Proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.
- Insoluble in water.
- Soluble in organic solvents such as alcohols and acetone.
Main groups of lipids are triglycerides, phospholipids and waxes.
Roles of lipids:
- The main role of lipids is in plasma membranes, as phospholipids contribute to the flexibility of membranes and transfer of lipid soluble substances.
- An energy source, as when oxidised lipids provide more than twice the energy as the same mass of carbohydrate.
- Waterproofing, as lipids are insoluble in water and therefore are useful to keep water out of the cells.
- Insulation, as fats are slow conductors of heat and help to retain body heat.
- Protection, as fat is often stored around delicate organs.