Polysaccharides | Richard Crooks's Website

Long chained carbohydrates which have many repeating units are known as polysaccharides. Although all carbohydrates make up of multiple monosaccharide units joined together are polymers (and thus polysaccharides), by convention carbohydrates with only two units are known as disaccharides, while those with three to ten units are known as oligosaccharides, and only those with greater than ten units are known as polysaccharides. However this is not a rigid distinction, and the basic chemistry which underpins all of them is identical.

A picture of molecular models of polysaccharides showing the formation of straight and branched polysaccharides. — Figure 1: Two example sections of polysaccharides where one chain (A) only features 1-4 glycoside bonds, and thus the four monosaccharides in this sections form a linear structure. By contrast the section in (B) features 1-4 glycoside bonds and a 1-6 glycoside bond, giving this section of the molecule a branched structure, where only three of the monosaccharides are in the linear chain, and a fourth monosaccharide has started a new branch from the linear chain from which further 1-6 bonds, and 1-4 bonds may form.

Polysaccharides can form condensation bonds (known as glycoside bonds) between the 1 and 4 carbon atoms, as well as between the 1 and 6 carbon atoms (Figure 1). Bonds between the 1 and 4 carbon atoms lead to extending the carbohydrate chain in a straight line, while those bonds between the 1 and 6 carbon atoms lead to branching of the polypeptide chain, and often occur in addition to bonds between the 1 and 4 carbon atoms, and both can occur on the same monosaccharide unit. The amount of branching varies between different polypeptides (Table 1).

Table 1: Prominent polysaccharides derived from glucose found in nature, which are energy storage molecules found in plants or animals. Plants have two glucose based energy storage molecules whereas animals only have one. What in dietary terms are called complex carbohydrates are these polypeptides found mostly in plant based foods, whereas muscle glycogen, an important energy source in endurance sports is a form of polysaccharide found in animal cells. There are also other polysaccharides composed of non-glucose molecules found in nature which have important biochemical properties which are not included here. Branching is defined as the number of monosaccharide units between each branch, with a lower number indicating a greater degree of branching.
Polysaccharide	Occurence	Branching (%)
Amylose	Plants	None
Amylopection	Plants	~25
Glycogen	Animals	11-15

The most prominent use of polysaccharides is as a long term energy storage medium. Glucose is metabolised to produce cellular energy, so the ability to store glucose as a stable molecule for future use is an important one. High glucose concentrations in solution cause problems, as glucose affects the osmolality of solutions which it is, and also diffuses into and out of cells. Thus a glucose storage medium like a polysaccharides, that stores the glucose as one stable molecule is advantageous, as an organism can regulate where glucose is stored, and how and when that glucose is used. Both plants and animals both have this ability, though the type of polysaccharide that is stored, and where it is stored differs (Table 1).

Another common polysaccharide is cellulose, which is made from a structural variant of glucose known as beta glucose, and is what gives plants their rigid structure. DNA and RNA both incorporate sugar molecules, and polymerisation in their structures, however they also incorporate non-sugar molecules, so are not considered polysaccharides, even if the monosaccharide molecules in those biological macromolecules have important biochemical roles.