Any member of a very abundant and widespread class of natural organic compounds that includes sugars, starch, and cellulose. They are commonly classified as monosaccharides (simple sugars; e.g., glucose, fructose), disaccharides (2-unit sugars; e.g., sucrose, lactose), oligosaccharides (3–10 or so sugars), and polysaccharides (large molecules with up to 10,000 monosaccharide units, including cellulose, starch, and glycogen). Green plants produce carbohydrates by photosynthesis. In most animals, carbohydrates are the quickly accessible reservoir of energy, and oxidation (see oxidation-reduction) of glucose in tissues supplies energy for metabolism. Many (but by no means all) carbohydrates have the general chemical formula C(HO). The carbon (C) atoms are bonded to hydrogen atoms (H), hydroxyl groups (OH; see functional group), and carbonyl groups (CO), whose combinations, order, and geometric arrangement lead to a large number of isomers with the same chemical formula but different properties. The class is further enlarged because each isomer has various derivatives: uronic acids, sugars with an oxidized group; sugar alcohols, sugars with a reduced group; glycosides, compounds of sugars with other molecules containing a hydroxyl group; and amino sugars, sugars with an amino group (see amino acid).