The two classes of organic compounds that we have been examining in this chapter - lipids and carbohydrates - play similar roles in living organisms. Both classes have some members that serve as important components of structure: cellulose in plants, and membrane lipids in both plants and animals. Both classes also are used in energy storage: starch and glycogen in plants and animals, and fats in animals.

The high energy-to-weight ratio in fats is offset by the involved procedures required to get the energy in and out of storage. Fats are insoluble in water, so prior to digestion they must be brought into suspension with bile detergents such as cholic acid. They then are cleaved to glycerol and fatty acids with the enzyme lipase, and finally are broken down into two-carbon acetate units, which can be fed into the energy-extracting machinery. In contrast, digestion of starch is a simple process requiring only one enzyme and acid.

This is why glycogen is used for quick-access energy storage in amimals even though fats are the primary energy storage.