Figure

Crowded cytoplasm. This scale drawing, which shows only the macromolecules, gives a good impression of how crowded the cytoplasm is. RNAs are shown in blue, ribosomes in green and proteins in red. (Adapted from D.S. Goodsell, Trends Biochem. Sci. 16:203–206, (more...)

It is at first sight difficult to accept the idea that each of the living creatures described in the previous chapter is merely a chemical system. The incredible diversity of living forms, their seemingly purposeful behavior, and their ability to grow and reproduce all seem to set them apart from the world of solids, liquids, and gases that chemistry normally describes. Indeed, until the nineteenth century it was widely accepted that animals contained a Vital Force—an “animus”—that was uniquely responsible for their distinctive properties.

We now know there is nothing in living organisms that disobeys chemical and physical laws. However, the chemistry of life is indeed of a special kind. First, it is based overwhelmingly on carbon compounds, whose study is therefore known as organic chemistry. Second, cells are 70 percent water, and life depends almost exclusively on chemical reactions that take place in aqueous solution. Third, and most importantly, cell chemistry is enormously complex: even the simplest cell is vastly more complicated in its chemistry than any other chemical system known. Although cells contain a variety of small carbon-containing molecules, most of the carbon atoms in cells are incorporated into enormous polymeric molecules—chains of chemical subunits linked end-to-end. It is the unique properties of these macromolecules that enable cells and organisms to grow and reproduce—as well as to do all the other things that are characteristic of life.