lipids. Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. They make up most of the organic matter on earth because of their multiple roles in all forms of life. First, carbohydrates serve as energy stores, fuels, and metabolic intermediates. Starch in plants and glycogen in animals are polysaccharides that can be rapidly mobilized to yield glucose, a prime fuel for the generation of energy. ATP, the univer- sal currency of free energy, is a phosphorylated sugar derivative, as are many coenzymes. Second, ribose and deoxyribose sugars form part of the structural framework of rna and DNA. The conformational flexibility of these sugar rings is important in the storage and expression of genetic information. Third, polysaccharides are structural elements in the cell walls of bacteria and plants, and in the exoskeletons of arthropods. In fact, cellulose, the main constituent of plant cell walls, is one of the most abundant organic compounds in the biosphere. Fourth, carbohydrates are linked to many proteins and lipids. For example, the sugar units ofgl-onorin, an integra/ membrane protein, give red blood cells a highly polar anionic coat