Biology Mock Exam

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.

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