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any member of a large class of chemical compounds that includes sugars, starches, cellulose, and related compounds. These compounds are produced naturally by green plants from carbon dioxide and water (see photosynthesis).
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A carbohydrate molecule composed of 3–20 monosaccharides (simple sugars). Generally, free oligosaccharides do not constitute a significant proportion of naturally occurring carbohydrates. Most carbohydrates that occur in nature are in the form of monosaccharides (such as blood sugar, or glucose), disaccharides (such as table sugar, or sucrose, and milk sugar, or lactose), and polysaccharides (such as starch and glycogen, polyglucose molecules, or chitin). See Glucose, Lactose, Monosaccharide, Polysaccharide
The monosaccharides of multiple sugar units such as oligosaccharides are connected with each other through bonds called glycosidic linkages. They are linked primarily to other sugars and to other molecules through aldehyde or ketone reducing groups.
Most naturally occurring oligosaccharides are linked either to proteins (glycoproteins) or to lipids (glycolipids). Glycoconjugates are present in essentially all life forms and particularly in cell membranes and cell secretions. Many hormones are glycoproteins, and an increasing number of enzymes have been shown to have sugars attached. Antigenic properties of the human red blood cell ABO blood group system are determined by glycolipid oligosaccharides. In fact, all the major protein components of blood serum, with the exception of serum albumin, are glycoproteins. See Blood groups, Cell membranes, Glycolipid, Glycoprotein
Many changes in the structures of oligosaccharides of glycoconjugates have been detected in cancer cells. Changes or differences in oligosaccharide structures are generally the result of differences in biosynthetic pathways or of degradative pathways. An understanding of glycoconjugates in normal biological systems and in certain disease states is currently of great importance.