Flexi Says: The four main macromolecules in living things are proteins, lipids, nucleic acids, and carbohydrates. Of these, cells typically use lipids, carbohydrates, and proteins for energy. Lipids have the most calories per gram of the macromolecules. Lipids provide cells with 9 calories of energy per gram, while proteins and carbohydrates provide 4 calories per gram. Nucleic acids could provide about 1.5 calories per gram if metabolized.