The molecules that give cells and entire organisms their shape as well as their ability to move, grow, and reproduce are the proteins. Although they come in an almost infinite variety of shapes and sizes, they have all been designed by the process of evolution to serve a defined and useful function in the processes of life. Some proteins, like actin and collagen, help to give a cell its physical shape. Other proteins, like lactase and pepsin, help in the digestion of food. Others transport signals between cells, help us fight off disease, or repair damaged DNA. For almost every job in a cell, there is a protein designed to do it.

Schematic diagram of protein primary structure. Amino acids are linked head to tail, so that at one end there is a free amino group, and at the other a free carboxyl group. Proteins are...