One of a group of organic compounds based on the general formula C_x(H₂O)_y. The simplest carbohydrates are the sugars (saccharides), including glucose and sucrose, and the oligosaccharides. Polysaccharides are carbohydrates of much greater molecular weight and complexity; examples are starch, glycogen, and cellulose. Carbohydrates perform many vital roles in living organisms. Sugars, notably glucose, and their derivatives are essential intermediates in the conversion of food to energy. Starch and other polysaccharides serve as energy stores in plants, particularly in seeds, tubers, etc., which provide a major energy source for animals, including humans. Cellulose, lignin, and others form the supporting cell walls and woody tissue of plants. Chitin is a structural polysaccharide found in the body shells of many invertebrate animals. Carbohydrates also occur in the surface coat of animal cells and in bacterial cell walls and serve as markers for the immune system of potentially pathogenic cells (see epitope).

http://themedicalbiochemistrypage.org/carbohydrates.php Summary of carbohydrate nomenclature and classification