Chemical processes in the cell depend on just a few classes of carbon-based molecules. Proteins provide structural support and act as catalysts that facilitate chemical reactions. Nucleic acids encode and transmit genetic information. Carbohydrates provide a source of energy and make up the cell wall in bacteria, plants, and algae. Lipids make up cell membranes, store energy, and act as signaling molecules.

These molecules are all large, consisting of hundreds or thousands of atoms, and many are polymers, complex molecules made up of repeated simpler units connected by covalent bonds. Proteins are polymers of amino acids, nucleic acids are polymers of nucleotides, and carbohydrates such as starch are polymers of simple sugars. Lipids are a bit different, as we will see, in that they are defined by a property rather than by their chemical structure. The lipid membranes that define cell boundaries consist of fatty acids bonded to other organic molecules.

Building macromolecules from simple, repeating units provides a means of generating virtually limitless chemical diversity. Indeed, in macromolecules, the building blocks of polymers play a role much like that of the letters in words. In written language, a change in the content or order of letters changes the meaning of the word (or renders it meaningless). For example, by reordering the letters of the word SILENT you can write LISTEN, a word with a different meaning. Similarly, rearranging the building blocks that make up macromolecules provides an important way to make a large number of diverse macromolecules whose functions differ from one to the next.

In the following sections, we focus on the building blocks of these four key molecules of life, reserving a discussion of the structure and function of the macromolecules for later chapters.