Polysaccharide

Polysaccharides are long carbohydrate molecules composed of many monosaccharide units linked by glycosidic bonds. They are a major class of carbohydrates and can be linear or highly branched. Depending on the monomer composition, they are classified as homopolysaccharides, containing only one type of sugar, or heteropolysaccharides, containing two or more different sugars. They are typically high molecular weight and vary in solubility and physical properties.

Common examples include starch and glycogen, which serve as energy storage in plants and animals, respectively; cellulose and chitin, which provide structural support in plants, fungi, and arthropods; and a variety of other polysaccharides such as pectin, agar, carrageenan, and the glycosaminoglycans (for example, hyaluronic acid and chondroitin sulfate) that occur in extracellular matrices and connective tissues. Many polysaccharides are used industrially as thickeners, stabilizers, or gelling agents in foods and pharmaceuticals.

Functions of polysaccharides are diverse. Energy-storage polysaccharides are typically highly branched to allow rapid release of monosaccharide units. Structural polysaccharides have rigid, extended chains that confer strength and protection. In biology, degradation and remodeling of polysaccharides are catalyzed by specific enzymes, such as amylases, cellulases, and pectinases, whereas structural integrity often depends on unique linkages, such as beta-glycosidic bonds, which are not digestible by all organisms.

Biosynthesis generally proceeds via glycosyltransferases that assemble activated sugar nucleotides (for example, UDP-glucose or ADP-glucose) into polysaccharide chains. Degradation occurs through hydrolysis by hydrolytic enzymes. The physical properties of polysaccharides—solubility, viscosity, and gelation—are central to their biological roles and commercial applications.