sugar, compound of carbon, hydrogen, and oxygen belonging to a class of substances called carbohydrates. Sugars fall into three groups: the monosaccharides, disaccharides, and trisaccharides. The monosaccharides are the simple sugars; they include fructose and glucose. The disaccharides are formed by the union of two monosaccharides with the loss of one molecule of water. Disaccharides include lactose, maltose, and sucrose. Less well known are the trisaccharides; raffinose is a trisaccharide present in cottonseed and in sugar beets. Sugars belong to two families denoted by the letter d - or l - written before the name of a sugar. The families are related to glyceraldehyde CH2OHCHOHCHO, which can exist in two three-dimensional forms that are mirror images of each other. The isomer of glyceraldehyde that rotates plane polarized light clockwise is labeled d -glyceraldehyde; all natural sugars can be derived from this substance and thus belong the the d family. Although l -sugars can be prepared in the laboratory, they cannot be utilized by animals.