Carbohydrates or saccharides (Greek σάκχαρον meaning "sugar") are simple molecules that are straight-chain aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. Carbohydrates are the most abundant biological molecules, and fill numerous roles in living things, such as the storage and transport of energy (starch, glycogen) and structural components (cellulose in plants, chitin in animals). Additionally, carbohydrates and their derivatives play major roles in the working process of the immune system, fertilization, pathogenesis, blood clotting, and development.
The basic carbohydrate units are called monosaccharides, such as glucose, galactose, and fructose. The general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater. Monosaccharides can be linked together in almost limitless ways. Two joined monosaccharides are called disaccharides, such as sucrose and lactose. Carbohydrates containing between about three to six monosaccharide units are termed oligosaccharides; anything larger than this is a polysaccharide. Polysaccharides, such as starch, glycogen, or cellulose, can reach many thousands of units in length. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example, deoxyribose, a component of DNA, is a modified version of ribose; chitin is composed of repeating units of N-acetylglucosamine, a nitrogen-containing form of glucose.