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 CH 2OHCHOHCHO, 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.

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