Wax
- Insoluble in water but soluble in organic solvents
- Behave as water hating molecules ( hydrophobic molecules)
- They are relatively large molecules
- Wax is a solid at room temperature.
Naturally occurring fats, oils and wax are esters formed by condensation reaction between glycerol (an alcohol) and organic acids known as fatty acids (Stearic acid) with the expulsion of 3 water molecules.
Alcohol + Organic Acid → Ester + Water
(Glycerol) (Fatty acid) (Triglyceride)
Glycerol has the formula of C3H8O3. All of the three hydroxyl (OH) groups in glycerol can take part in the condensation reaction with fatty acid. The resulting ester is called a triglyceride (or triacylglycerol).
Fatty acid molecules are much larger than those of glycerol and consist of long non-polar hydrocarbon chains with carboxyl group (COOH) at the end. Fatty acids are divided into 2 groups, saturated (single bond carbon atoms) and unsaturated (double bond between carbon atoms) fatty acids. Fatty acids play an important role in cells. Fatty acids can be broken down and oxidized to release energy; they can also be converted into phospholipids, which are important constituents of cell membranes.
Triglycerides are the commonest lipids in living organisms and their primary importance is as energy stores. As I have said they are compact and insoluble and can be stored at very high concentrations in cells (they do not have any osmotic effect on the cell), in cells they occur as small oil droplets suspended in the cytoplasm. Triglycerides release twice the amount of energy as per gram of carbohydrates, but also when triglycerides are broken down metabolic water is produced which is very important for animals that live in arid countries. (Metabolic water is water produced when organic molecules are oxidised in a reaction. Oils are the major food stores in many seeds (sunflower, rape) and fruits (palm, olive).
Phospholipids are same in certain way to lipids but the difference is that they are esters of fatty acids and glycerol, but one of the fatty acid chain is replaced by a hyper polar group associated with a phosphate molecule. The polar group (phosphate heads) is very soluble in water, whereas the non polar hydrocarbon tails (chains) of fatty acids are not.
At an air-water, or oil-water interface, the phospholipids molecules orient themselves so that their polar heads are in the water (amphotheric molecule), below is a diagram of a phospholipid.
There are three types of lipids that are found in the cell membranes of cells:
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Phospholipids, which contain a phosphate group.
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Glycolipids, which have a branching carbohydrate (polysaccharide) molecule
- Cholesterol.
In both phospholipids and glycolipids, one part of the molecule will have an affinity, or attraction, for water (hydrophilic) and the other part will not mix with water (hydrophobic). The hydrophilic portion is said to be polar and the hydrophobic portion is said to be Non-polar. Glycolipids are less common than phospholipids in the cell surface membrane, but where they are present they occur in the outer layer with the carbohydrate portion, forming glycocalyx, and extending outwards into the intercellular space.
Cholesterol molecules have different structure, but do have polar and non-polar regions.
Cholesterol molecules are arranged in bilayer with their polar groups close to the polar groups of the other lipid molecules.
Waxes are esters of fatty acids and long chain alcohols. In the living world wax is used as waterproofing for plants and animals, honey bees produce wax to make honey combs and the cells in which the eggs are to be laid and where the development of larvae takes place.
The following are the uses of lipids in the living world
- Food stores in seeds ---lipids are stored as food in seeds of plants i.e. sunflower
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Insulation ---the adipose layer--- lipids insulate the body of animals (humans) from very cold weather.
- Protect vital organs--- lipids tend to cover the vital organs of animals i.e. if there was an accident the vital organs will be saved (kidneys, heart)
- Buoyancy in aquatic animals---lipids help aquatic animals (mammals i.e. see lions, whales, penguins and all kind of fishes) to float on water.
