Proteins are large, organic, nitrogen-containing molecules that are essential both to the structure and to the function of all living cells. Exceedingly complex themselves, the protein molecules have as their structural units much simpler compounds called amino acids. Typically, the amino acids in each molecule are strung together in chain-like fashion, with the longer chains folded into ribbons, spirals, and other three-dimensional forms.

The types of amino acids on the chain (over 22 varieties have so far been identified in animal protein), the number of them (a chain can contain a few amino acids or several thousand), and the particular configuration all help differentiate one protein from another. Because proteins have such a wide range of specific functions and characteristics, they can be classified in a number of ways. Probably the simplest way is by function. One of protein's main functions is to serve as "raw material" for the...