Although living cells are primarily made up of water, a number of other molecules are also abundant. Gigantic molecules, called macromolecules, populate a cell and provide it with important functions for life. For example, macromolecules provide structural support, a source of stored fuel, the ability to store and retrieve genetic information, and the ability to speed biochemical reactions. Four major types of macromolecules—proteins, carbohydrates, nucleic acids, and lipids—play these important roles in the life of a cell.
In terms of overall structure, all macromolecules except the lipids are considered polymers. A polymer is a chain of similar subunits, or monomers, that are linked together by covalent bonds. In proteins, the monomers are amino acids; in carbohydrates, the monomers are sugars; and in nucleic acids, the monomers are nucleotides. Lipids are a diverse group of molecules, which come in a variety of nonpolymeric forms.