Sugar, sweet, soluble compound of carbon, hydrogen, and oxygen. There are 3 groups of sugars. Monosaccharides (such as fructose and glucose) are called simple because they contain a single chain of carbon atoms. Disaccharides (such as lactose and sucrose) contain 2 monosaccharide units joined by an oxygen bridge. Their chemical and physical properties are similar to those of monosaccharides. Trisaccharides (such as raffinose) are composed of 3 monosaccharide units.