K E Y C O N C E P T S 5.1 Macromolecules are polymers, built from monomers 5.2 Carbohydrates serve as fuel and building material 5.3 Lipids are a diverse group of hydrophobic molecules 5.4 Proteins include a diversity of structures, resulting in a wide range of functions 5.5 Nucleic acids store, transmit, and help express hereditary information O V E R V I E W The Molecules of Life Given the rich complexity of life on Earth, we might expect organisms to have an enormous diversity of molecules. Re- markably, however, the critically important large molecules of all living things?from bacteria to elephants?fall into just four main classes: carbohydrates, lipids, proteins, and nucleic acids. On the molecular scale, members of three of these