Biology Mock Exam

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Emulsion Test

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.
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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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