Structure of saccharides

It is estimated that approximately 4x10 tons of carbohydrates are biosynthesized each year on earth by plants and photosynthesizing bacteria. The majority of these carbohydrates are produced as polysaccharides. Polysaccharides are macromolecules consisting of a large number of monosaccharide residues. They are sometimes also called glycans. (The term 'glycan' is also used for the saccharide component of a glycoprotein even though the chain length may not be large; cf. chapter 6). For polysaccharides which contain a substantial proportion of amino sugar residues the term glycosaminoglycan is a common one. Polysaccharides which consist of only one kind of monosaccharide are called homopolysaccharides (homoglycans); when they are built up of two or more different monomeric units they are named heteropolysaccharides (heteroglycans). In the latter type, the monosaccharide units are usually linked to each other in a definite pattern, rather than randomly. Certain sequences of monomeric building blocks are often found to be regularly repeated as so–called repeating units. Homo as well as heteropolysaccharides can be linear or branched. For polysaccharides, as for every polymer, it is not possible to attribute one distinct molecular weight as they are polydisperse molecules, which are characterized by an average molecular weight. The number of monosaccharide units in a polysaccharide is termed degree of polymerization or d.p. The size of a polysaccharide varies between approximately 16,000 and 16,000,000 daltons (Da). Polysaccharides exist in an enormous structural diversity as they are produced by a geat variety of species, including microbes, algae, plants and animals. Among these are fructans, xanthans, fucans, bacterial gel polysaccharides, capsule polysaccharides of bacteria, or agar, which is a mixture of two polysacccharides and is obtained from red–purple seaweeds. The most well–known polysaccharides are starch, glycogen, cellulose and chitin.