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
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.