Lipids

Roles of Lipids

Lipids have many roles, one role of lipids is in the cell membranes (cell-surface membranes and membranes around organelles).
Phospholipids contribute to the flexibility of membranes and the transfer of lipid-soluble substances across them.
Other roles of lipids include:
Source of Energy: when oxidised, lipids provide more than twice the energy as the same mass of carbohydrate and release valuable 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 nerve cells.
Protection: Fat is often stored around delicate organs, such as the kidney.
Fats are solid at room temperature (10-20c) whereas oils are liquid.

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Triglycerides are so called because they have three (tri) fatty acids combined with glycerol (glyceride).
Each fatty acid forms an ester bond with glycerol in a condensation reaction.
Hydrolysis of a triglyceride therefore produces glycerol and three fatty acids.
As the glycerol molecule in all triglycerides is the same, the differences in the properties of different fats and oils come from variations in the fatty acids.
There are over 70 different fatty acids and all have a carboxyl (-COOH) group with a hydrocarbon chain attached.
If this chain has no carbon-carbon double bonds, the fatty acid is then described as saturated, because all the carbon atoms are linked to the maximum possible number of hydrogen atoms, in other words they are saturated with hydrogen atoms.
If there is a single double bond, it is mono unsaturated.
If more than one double bond is present, it is polyunsaturated.

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Triglycerides have a high ratio of energy storing carbon hydrogen bonds to carbon atoms and are therefore an excellent source of energy.
Triglycerides have low mass to energy ratio, making them good storage molecules because much energy can be stored in a small volume. This is especially beneficial to animals as it reduces the mass they have to carry as they move around.
Being large, non polar molecules, triglycerides are insoluble in water. As a result their storage does not affect osmosis in cells or the water potential of them.
As they have a high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised and therefore provide an important source of water, especially for organisms living in dry deserts.

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Phospholipids are similar to lipids except that one of the fatty acid molecules is replaced by a phosphate molecule.
Whereas fatty acid molecules repel water (are hydrophobic), phosphate molecules attract water (are hydrophilic).
A phospholipid is therefore made up of two parts:
A hydrophilic 'head', which interacts with water (is attracted to it) but not with fat.
A hydrophobic 'tail', which orients itself away from water but mixes readily with fat.
Molecules that have two ends (poles) that behave differently in this way are said to be polar.
This means that when these polar phospholipid molecules are placed in water they 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.

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Phospholipids are polar molecules, having a hydrophilic phosphate head and a hydrophobic tail of two fatty acids. This means that in an aqueous environment, phospholipid molecules form a bilayer within cell surface membranes. As a result, a hydrophobic barrier is formed between the inside and outside of a cell.
The hydrophilic phosphate 'heads' of phospholipid molecules help to hold at the surface of the cell surface membrane.
The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell surface membrane. These glycolipids are important in cell recognition.

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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 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.

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