Polysaccharides may be linear or branched, and branched polysaccharides exhibit various branching modes that range from branches consisting of a single sugar unit to longer branches carrying other branches, as illustrated in Figure 1-12. In heteropolysaccha-rides composed of more than one kind of sugar, the sugar units may be linked in alternating sequence or in blocks of various length in which one sugar repeats itself. The physical properties of polysaccharides depend highly on their chemical and conformational structures. In general, polysaccharides that are highly branched are water soluble, whereas linear polysaccharides tend to be insoluble. However, linear polysaccharides possessing structural irregularities that hinder intermolec-ular hydrogen bonding may be soluble and give viscous solutions. Hyaluronic acid, a linear polysaccharide with two different sugars in alternating sequence, shows this mucilaginous characteristic. In contrast, glycogen, which is highly branched and very soluble, gives relatively nonviscous solutions. Many different polysaccharide structures are represented in the plant kingdom, whereas only a few have been identified in vertebrates. Bacteria synthesize many unusual sugars, which greatly increases the diversity of their polysaccharide antigens.
Polysaccharides are designated either by a trivial name, such as starch and glycogen, or by a systematic name constructed from the constituent sugar names and the suffix "an." Thus, (l^>4)-|3-D-glucan is the systematic name for cellulose.
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