Phospholipids – the basis of biological membranes:
Essentially, a phospholipid molecule is almost identical to a triglyceride molecule. It consists of a glycerol molecule with fatty acid molecules bonded by condensation reaction to produce ester bonds.
In phospholipids, the third fatty acid is not added to the glycerol molecule. Instead, a phosphate group is covalently bonded to the third OH group on the glycerol. The bonding of the phosphate group occurs by condensation reaction and so a water molecule is released.
The phosphate ‘head’ of the molecule is hydrophilic, but the hydrocarbon chain fatty acid ‘tails’ are hydrophobic. The majority of phospholipid molecule is insoluble in water, like all lipid molecules. The water-solubility of the head group gives phospholipids their characteristics in terms of the capacity to form membranes.
Phospholipids and membrane fluidity:
The fatty acids that make up a phospholipid may be saturated or unsaturated. Organisms can control the fluidity of membranes using this feature. For example, organisms living in colder climates have an increased number of unsaturated fatty acids in their phospholipid molecules. This ensures that membranes remain fluid, despite the low temperatures.
Lipids and respiration:
Respiration of lipids first requires the hydrolysis of the ester bonds holding the fatty acids and glycerol together (the reverse of the condensation reaction which joins them). Both the glycerol and fatty acids can then be broken down completely to carbon dioxide and water. This releases energy, which is used to generate ATP molecules.
The respiration of 1 gram of lipid gives out about x2 as much energy as the respiration of 1 gram of carbohydrate. Because lipids are insoluble in water, they can be stored in a compact way and they do not affect the water potential of the cell contents. These features make triglyceride and excellent energy storage molecule.
The respiration of a lipid gives out a great deal more water than the respiration of carbohydrate. This metabolic water is…