Any member of a very abundant and widespread class of natural organic compounds that includes sugar s, starch , and cellulose .
They are commonly classified as monosaccharide s (simple sugars; e.g., glucose , fructose ), disaccharides (2-unit sugars; e.g., sucrose , lactose ), oligosaccharide s (3–10 or so sugars), and polysaccharide s (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 n (H 2 O) n . The carbon (C) atoms are bonded to hydrogen atoms (―H), hydroxyl groups (―OH; see functional group ), and carbonyl groups (―C=O), whose combinations, order, and geometric arrangement lead to a large number of isomer s 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; glycoside s, compounds of sugars with other molecules containing a hydroxyl group; and amino sugars, sugars with an amino group (see amino acid ).