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1.5 Lipids (ROLES OF LIPIDS (Lipids are found in the cell membrane and…
1.5 Lipids
ROLES OF LIPIDS
Lipids are found in the cell membrane and membranes surrounding membrane bound organelles. They make membranes more flexible. They allow lipid-soluble substances to cross the cell membrane.
Lipids are a source of energy. - when oxidised lipids provide more than twice the amount of energy as the same amount of carbon dioxide and release water as a byproduct.
Lipids are insoluble in water and therefore useful as waterproofing. Both plants and insects have waxy lipid cuticles that conserve water, while mammals produce an oily secretion from sebaceous glands in the skin.
Fats are slow conductors of heat and when stored beneath the body surface help to provide insulation. They also act as electrical insulators in the myelin sheath around nerve cells.
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TRIGLYCERIDES
Triglycerides are 3 fatty acids combined with glycerol. Each fatty acid forms an ester bond with glycerol in a condensation reaction.
Hydrolysis of a triglyceride therefore produces glycerol and 3 fatty acids.
They have a high ratio of carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.
They are large non-polar molecules and therefore insoluble in water - they dont affect the water potential in cells (no osmosis)
Release water when oxidised and therefore are an important source of water especially for organisms living in dry deserts.
They also have a low mass to energy ratio making them a good storage molecule because more energy can be stored in a smaller volume.
PHOSPHOLIPIDS
Fatty acid molecules repel water (hydrophobic) whereas phosphate molecules attract water (hydrophilic). Therefore a phospholipid is made up of two parts:
- a hydrophilic head - which interactswith water but not with fat
- a hydrophobic tail - which turns away from water but mixes with fat.
This makes a phospholipid a polar molecule. (it has two poles with different characteristics)
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When a phospholipid is placed in water, the position themselves so that the hydrophilic heads are as close to the water as possible and the hydrophobic tails are as far away from the water as possible.
Phospholipids form a bilayer within cell-surface membranes due to their polar properties. This creates a hydrophobic barrier between the outside and the inside of the cell.
The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.
FATTY ACIDS
All variations in lipids come from different fatty acids. There are over 70 different fatty acids and all have a carboxyl
(-COOH) group with a hydrocarbon chain attached.
Saturated fatty acids - no carbon double bonds. All the carbon atoms are linked to the maximum number of hydrogen atoms.
Unsaturated fatty acids
- mono-unsaturated = there is one carbon double bond.
- polyunsaturated = there is more than one carbon double bond.
TEST FOR LIPIDS
EMULSION TEST
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6) as a control, repeat the procedures using water instead of the sample. 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 emulsion. Light passing through this emulsion is refracted as it passes from oil droplets to water droplets making it appear cloudy.
- Lipids are made up of carbon, hydrogen and oxygen.
- The are insoluble in water
- They are soluble in organic solvents such as alcohols and acetone.
- The main groups are triglycerides and phospholipids.
- Fats are solid at room temperature whilst oils are liquid.