Unsaturated fatty acids are those that possess a double bond within their structure. A monounsaturated fatty acids therefore has a single double bond, whereas the prefix ‘poly’ signifies multiple double bonds. Unsaturated oils are those that contain a predominance of unsaturated fatty acids, and these oils are important nutritionally. In addition, unsaturation of fatty acids can occur endogenously via enzyme catalysed reactions. Both endogenous and exogenous unsaturated fatty acids have properties that are important to cell function in addition to their role in eicosanoid formation. One thing that becomes immediately apparent to those investigating the chemical and physical properties of fatty acids is that the unsaturated oils are liquids at room temperature. This relates to their structure, in so far as the presence of a double bond prevents tight packaging of the molecules and this lowers their melting point. It is the lower melting points of unsaturated oils that gives them unique properties in the regulation of cells structure.
The currently accepted model of the plasma membrane is a double layer of phospholipids made up of fatty acid ‘tails’ bonded to phosphate ‘heads’. Within this phospholipid membrane are located proteins which ‘float’ on the membrane thus creating the fluid mosaic model. The fluidity of the membrane is regulated by the ratio of unsaturated to saturated fatty acids in the phospholipid molecules, as well as by regulation of the cholesterol content of the membrane bilayer. Decreasing the unsaturated fatty acid to saturated fatty acid ratio and increasing the bilayer content of cholesterol stiffens the membrane and decreases fluidity. In this way cells can regulate the fluidity of their membranes depending on the physiological milieu. That cells regulate the fatty acid content of their membranes rapidly depending on the environmental temperature supports the theory that they use unsaturated fatty acids to regulate fluidity. In a similar way cells must also regulate the degree of saturation of triglyceride stores in cells to prevent damage to plasma membranes.
This property of cells to regulate the liquidity of stored fatty acids is important because tripalmitin, the main storage triglyceride in humans, has a melting point of 65°C which means that at body temperature the fat is solid. Solidification of stored fat droplets could lead to cell damage and so this must be regulated. Tripalmitin is used as a storage form of fats because storing unsaturated fatty acids is inefficient due to their relative oxidised state. This oxidation results in a lower enthalpy for oxidation, therefore decreasing the efficiency of stored body fat. Increasing the unsaturated fatty acid content of the triglycerides decreases the melting point of the fats and allows them to remain liquid, but maintains efficiency by permitting palmitate storage. This process is demonstrated by the inverse association between the unsaturated fatty acid content of triglycerides in fat stores and the body or environmental temperatures in animals and plant cells, respectively. Therefore unsaturated fatty acids have useful cellular properties beyond their ability to regulate eicosanoid production.
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