Carbohydrates are organic compounds with a keto group and several hydroxyl groups. They are ubiquitous and may exist as monosaccharides (glucose), disaccharides (lactose, sucrose) or polysaccharides, such as starch (or glycogen) which serve for the storage of energy, and cellulose (or chitin) which are structural components. Their derivatives are also important biomolecules (ribose derivatives).