Fats and Lipids

     When organisms store glucose molecules for long periods, they usually store them as fats rather than as carbohydrates. Fats are large molecules made up of carbon, hydrogen, and oxygen, as are the carbohydrates, but their hydrogen-to-oxygen ratio is higher than 2;1. For this reason, fats contain more energy-storing carbon-hydrogen bonds than carbohydrates. In addition, fats are nonpolar, insoluble molecules, so they work well as storage molecules.
      Fats are only one kind of lipid. Lipids include a wide variety of molecules, all of which are soluble in oil but insoluble in water. This insolubility results because almost all the bonds in lipids are nonpolar carbon-carbon or carbon-hydrogen bonds. Three important categories of lipids are (1) oils, fats, and waxes; (2) phospholipids; and (3) steroids.
      Lipids are composite molecules; that is, they are made up of more than one component. Oils and fats are built from two different kinds of subunits:

1. Glycerol: Glycerol is a three-carbon molecule with each carbon bearing a hydroxyl (^-OH) group. The three carbons form the backbone of the fat molecule.
   
2. Fatty acids: Fatty acids have long hydrocarbon chains (chains consisting only of carbon and hydrogen atoms) ending in a carboxyl (^-COOH) group. Three fatty acids are attached to each glycerol backbone (Figure 13). Because there are three fatty acids, the resulting fat molecule is called a triglyceride:

H                 
|                 
H--C--O--Fatty acid

H--C--O--Fatty acid

H--C--O--Fatty acid
|                 
H                

Most dietary fat is in the form of triglycerides, an abundant type of lipid.

      The difference between fats and oils has to do with the number of double bonds in their fatty acids. As Figure 14 shows, a fatty acid with only single bonds between its carbon atoms can hold more hydrogen atoms than a fatty acid with double bonds between its carbon atoms. A fatty acid that carries as many hydrogen atoms as possible, such as the fatty acid in Figure 14a, is saturated. Fats composed of fatty acids with double bonds are unsaturated because the double bonds replace some of the hydrogen atoms. If a fat has more than one double bond, it is polyunsaturated. Polyunsaturated fats (Figure 14b) have low melting points and are therefore liquid fats, or oils.

Figure 13 Structure of a triglyceride. Triglycerides are composite molecules, made up of three fatty acid molecules bonded to a single glycerol molecule. This bonding takes place by dehydration synthesis.

     The fatty acids of most plant triglycerides such as vegetable oils are unsaturated. (Exceptions are the tropical oils.) Animal fats, in contrast, are often saturated and occur as hard fats. Human diets with large amounts of saturated fats may contribute to clogged arteries and raise the risk of developing diseases of the circulatory system.

    Waxes, which are used by land plants and some animals as a waterproofing material, differ from fats and oils by having a chemical backbone slightly different from glycerol. Phospholipids also differ from oils in that one of their fatty acids is replaced by a phosphate group attached to a nitrogen-containing group. Phospholipids play a key role in the structure of cell membranes. Membranes often contain steroids, a lipid having a structure very different from oils. Steroids are composed of four carbon rings. Most of your cell membranes contain the steroid cholesterol. Male and female sex hormones are also steroids.

Figure 14 Saturated and polyunsaturated fats. (a) Palmitic acid, a fatty acid with only single bonds between its carbon atoms, has a maximum of hydrogen atoms and is a saturated fat. (b) Linolenic acid, with three double bonds and thus fewer than the maximum number of hydrogen atoms bonded to the carbon chain, is a polyunsaturated fatty acid.