Polysaccharides: Occurrence, Significance, and Properties

Abstract

Polysaccharides are present in all living organisms where they carry out one or more of their diverse functions. While there is no specific category or definition of a complex polysaccharide, most are structurally complex. Polysaccharides contain 1-5 different monosaccharide (sugar) units. The different sugar units may have different anomeric configurations and/or be joined by different glycosidic linkages. Polysaccharides may be linear or branched. Branches may be short saccharide units on a linear backbone or the molecule may have a branch-on-branch structure; in either case, the branches may be isolated or clustered. Polysaccharides may contain non-carbohydrate groups. Esters or cyclic acetal groups, when present, can be removed by appropriate treatments. All polysaccharides are polydisperse, i.e., are present in a range of molecular weights rather than having a single molecular weight. Most are polymolecular, i.e., differ in fine structure from molecule to molecule. So most polysaccharides can be said to be structurally complex. They may be attached to protein molecules or to other polysaccharide molecules. They are solvated by water. Most dissolve in aqueous systems, especially if they are alkaline. Polysaccharides can be depolymerized by acids and heat, specific enzymes, and high pH systems following oxidation. Their hydroxyl groups can be esterified (acylated), etherified (alkylated), and oxidized. Amino groups can be acylated (and deacylated). Carboxyl groups can be converted into esters, amides, and amines. Structural modification makes the molecules even more complex and polymolecular and, perhaps, polydisperse.