21. Lipids and Carbohydrates   Previous PageNext Page

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 class.

11-trans-retinal, a photosensitive compund used in the human eye.

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