The study of biochemistry specifically explores the structure and function of important molecules found in living organisms. All important biomolecules are organic, that is, based on the carbon atom. Such molecules are often called macromolecules, because they are relatively large in comparison with the molecules studied by traditional, inorganic chemistry. Biological macromolecules are often polymers -- molecules formed by the stepwise addition of smaller subunits (monomers). Four major classes of biological molecules have been identified, each with unique structural properties and different roles.