Polysaccharide

Polysaccharides (; from Ancient Greek πολύς (polús) 'many, much' and σάκχαρ (sákkhar) 'sugar') are "compounds consisting of a large number of monosaccharides linked glycosidically". They are the most abundant carbohydrates in food. Their structures range from linear to highly branched polymers. Examples include storage polysaccharides such as starch, glycogen, and galactogen and structural polysaccharides such as hemicellulose and chitin. The term "glycan" is synonymous with polysaccharide, but often glycans are discussed in the context of glycoconjugates, i.e. hybrids of polysaccharides and proteins or lipids. Polysaccharides are often heterogeneous, containing slight modifications of the repeating unit. They may be amorphous (e.g. starch) or insoluble in water (e.g. cellulose). Saccharides are generally composed of simple carbohydrates called monosaccharides with the general formula (CH2O)n where n is three or more. Examples of monosaccharides are glucose, fructose, and glyceraldehyde. Polysaccharides, meanwhile, have a general formula of Cx(H2O)y where x and y are usually large numbers between 200 and 2500. When the repeating units in the polymer backbone are six-carbon monosaccharides, as is often the case, the general formula simplifies to (C6H10O5)n, where typically 40 ≤ n ≤ 3000. Polysaccharides are highly branched. In fact, starch holds I2iodine molecules in the helical portion. The starch-I2 is dark blue in colour. Cellulose does not contain complex helices and hence cannot hold I2; as a result, a colour change is not observed when placed in an I2-dye. This colour-changing property is a unique character test for polysaccharides As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides contain three to ten monosaccharide units, but the precise cutoff varies according to the convention. Polysaccharides are an important class of biological polymers. Their function in living organisms is usually either structure- or storage-related. Starch (a polymer of glucose) is used as a storage polysaccharide in...