When organisms store glucose molecules for long periods, they usually store them as fats rather than as 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. The three carbons form the backbone of the fat molecule.
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.