There are three principal kinds of carbohydrates and each are classified according to the number of sugar molecules they contain. Monosaccharides, such as ribose, glucose, and fructose, contain only one sugar molecule. Disaccharides consist of two sugar molecules linked covalently. Familiar examples are sucrose (table sugar), maltose (malt sugar), and lactose (milk sugar). Polysaccharides, such as cellulose and starch, contain many sugar molecules linked together. Monosaccharides are organic compounds composed of carbon, hydrogen, and oxygen. They can be described by the chemical formula (CH2O)n, where n may be as small as 3 or as large as 8. They are characterized by hydroxyl groups and an aldehyde