Animals, too, use the same class of molecules for structure as
for energy storage. An animal stores its excess energy in the form
of fats. The closely related lipids are one of the two components
of all types of biological membranes, in both plant and animal cells.
The fatty-acid soap films discussed in Chapter 20 are a surprisingly
good model for simple membranes, although turned inside out.
In this chapter we shall look at two large families of carbon
compounds: lipids, which include membrane materials, fats,
and other organic hydrocarbons; and carbohydrates, which
include starches, cellulose, and the simple sugars that are found
in living organisms. "Lipid” is the general term for
any water-insoluble organic molecules that can be extracted from
cells by ethers, benzene, or other nonpolar solvents. The most important
lipids for our purposes are fats and the fat-derived molecules of
membranes. Other types include steroids, terpenes, and various small
hydrocarbon derivatives, which serve as membrane components, detergents,
some hormones, regulators, and light-gathering antennae. The carotenoids,
whose light-absorbing talents we have seen already, fall into this
11-trans-retinal, a photosensitive compund
used in the human eye.