In the previous chapter we learned that all matter is composed of atoms. The atoms of living organisms are arranged into large, complex macromolecules. Life is built upon these macromolecules. They store energy for later use, form membranes, provide structural support, help control chemical reactions within the organism, and store the hereditary information that is used to direct every aspect of the organism's life and is passed on to the next generation. The four major categories of macromolecules are carbohydrates, lipids, proteins, and nucleic acids.