We return now to the idea I began with at the start of this chapter—the notion that people and all other organisms are built and run by molecules. It isn't a metaphorical or a metaphysical idea—it's literally true. To convince you of this, I'll introduce you to protein molecules.

Protein is one of the things listed on my boxes of breakfast cereal, but to a molecular biologist, proteins are the very foundation of living systems. Virtually every process and product in living cells depends on proteins. They do everything from activating essential chemical reactions, to carrying messages between cells, to fighting infections, to making cell membranes, tendons, muscles, blood, bone, and other structural materials.

Since proteins are responsible for practically all of a cell's distinctive properties, we can say that proteins make an organism what it is. Proteins make the differences between, say, a hormone-secreting cell in your