Macromolecules are large molecules composed of many subunits or monomers linked together. There are four main types of macromolecules: carbohydrates which provide energy, proteins which make up cells and catalyze reactions, lipids which store energy efficiently, and nucleic acids which store genetic information. Each macromolecule type has characteristic monomer subunits that polymerize to form larger structures with distinct functions, such as starch, keratin, phospholipids, and DNA/RNA.