commonly used in thermodynamics.) However, fat storage is long term. The energy contained in fats is not released as rapidly as with the polysaccharides starch or glycogen. Other functions of fats are for insulation for animals exposed to cold, and for flotation in marine animals. The blubber of sea mammals is an example of both of these functions.
The degree of fat saturation in the human diet is linked to human health. A small amount of unsaturated fat is required in the diet, because humans cannot form fatty acids with double bonds. One of these in particular, linoleic acid, can be converted into all the other fatty acids needed by humans. For this reason, it and several others are placed in a dietary group called the essential fatty acids.
A mammalian diet containing a high proportion of saturated fats is associated with cardiovascular disease, via its connection with cholesterol and blood lipoproteins. (Cholesterol is another type of lipid, described below. Lipoproteins are hybrid compounds, also described below.) Among the unsaturated fats, monounsaturated fats, those with only one double bond, seem to be even more healthful than polyunsaturated fats, those with more than one double bond. Table 3.5 shows how various fats compare in these dietary constituents. Olive oil has the highest monounsaturated fat content of all. This has lent support to the "Mediterranean diet,'' in which olive oil replaces butter and other animal fats in recipes, to the extent of placing a plate of oil on the table for dipping bread instead of a butter dish. Lest one should think that only animal fats are suspect, notice the high saturated fat content of coconut oil. High polyunsaturated fat content also seems to beneficially lower blood lipid concentration. Safflower oil is best in this regard, followed by corn, peanut, and cottonseed oils.
Phospholipids are particularly critical to life, as they form the basic structure of cell membranes. Glycerol can form ester linkages with inorganic as well as organic acids. Phospholipids consist of ester linkages of glycerol with two fatty acids and with phosphoric acid in the third position. Usually, another of the phosphate hydroxyl groups form, in turn, another ester bond with still another organic molecule, called a variable group. The variable group often contains nitrogen, adding to the polar character of the lipid.
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Phospholipid a mwhim^im m > l\ l\ > i >)) >\ !>) mi (hh 11) >
This structure results in a surfactant, a molecule that has a hydrophilic part and a hydrophobic part. Similar to soaps and detergents, if there are enough molecules in solution, they will form aggregates called micelles, a sort of "circling the wagons'' in which the polar ends face the water and the hydrophobic ends form a separate phase in the interior of the micelle (Figure 3.9). One of the phospholipids is lecithin, which is important in metabolism of fats by the liver. Egg yolks are rich in lecithin, and its detergent nature helps maintain the emulsion between oil and vinegar in mayonnaise.
Biological lipids show an important difference in behavior from other surfactants. Under the right conditions, they form micelles with multilayered structures that can become vesicles (water-filled cavities) bounded by a lipid bilayer, as also shown in Figure 3.9. This important structure is the basis for the cell membrane, and therefore for the cell itself. The bilayer membrane forms a barrier to the uncontrolled passage of water-soluble constituents into and out of the cell. However, its lipophilicity makes it the site of action of many lipophilic pollutants, such as some industrial organic solvents.
Other lipids: Since the definition of lipids is based on physicochemical properties and not chemical structure, it is not surprising that the group is very diverse. A variety of other biological compounds fit the category. Waxes are long-chain fatty acids combined with long-chain alcohols other than glycerol. They are formed by plants to produce protective and water-conserving layers on their surfaces and by animals such as the honeybee, or for ear canal protection. Terpenes are long-chain hydrocarbons based on units similar to the compound isoprene. They include essential oils from plants, among other compounds.
One important group of compounds that is in this catch-all class of lipids is the steroids. Steroids consist of four fused rings, three with six carbons and one with five, and with a hydroxide at one end and a hydrophobic "tail" at the other. Steroids are important regulatory chemicals in plants and animals. They are able to move through cell membranes formed by lipid bilayers. The sex hormones estrogen and testosterone are steroids, as are the fat-soluble vitamins A, D, and E. The most abundant steroid in animals is cholesterol. It forms an essential part of the cell membrane, affecting its fluidity. However, factors related to diet and heredity can cause cholesterol to form obstructive deposits in blood vessels, causing strokes or heart attacks.
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