Lipids

Emulsion test:

• 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 cloud-white colour indicates the presence of a lipid
• As a control, repeat the procedures using water, the final solution should remain clear

The cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emmulsion.

Light passing through this emulsion is refracted as it passes from oil droplets to water droplets, making it appear cloudy.

Roles:

• They contain carbon, hydrogen and oxygen
• The proportion of oxygen to carbon and hydrogen is smallrer than in carbohydrates
• They are insoluble in water
• They are soluble in organic solvents such as alcohols and acetone

In the cell membrane -

• Conributes to the flexability of membranes and the transfer of lipid-soluble subtances across them

Source of energy -

• When oxidised, lipids provide more than twice the energy as the same mass of carbohydrate and release valuble water

Waterproofing -

• Lipids are insoluble in water and therefore useful as a waterproofing
• Both plants and insects have waxy, lipid cuticles that conserve water, while mammals produce an oily secretion from the sebaceous glands in the skin

Insulation -

• Fats are slow conductors of heat and when stored beneath the body surface help to retain body heat.
• They also act as electrical insulators in the myelin sheath around the nerve cells

Protection -

• Fat is often stored around delicate organs, such as the kidney

Triglycerides:

Structure -

• Three fatty acids…