Macromolecules are large molecules made up of thousands of covalently bonded atoms. Many important ones are things known as polymers - materials made of many repeating subunits. These subunits are known as monomers - building blocks that react together with other monomers to form the larger polymers. By combining a variety of monomers, each with their own functional groups, molecules with more versatile or complex chemical properties can be created.

The four main macromolecules in living things, also known as biomolecules, are carbohydrates, lipids, proteins, and nucleic acids. These are organic molecules and are thus primarily organized around carbon (with hydrogen atoms bound to it). A carbon atom can form four bonds with other atoms, and the biomolecules are typically derivatives of hydrocarbons, where the hydrogens bound to the carbon are replaced by other atoms or functional groups. A functional group is a specific arrangement of atoms that gives a molecule certain properties and reactivity, such as a hydroxyl (-OH) group adding polarity and the ability to form hydrogen bonds.