5.7. Polysaccharides
Long saccharide polymers (polysaccharides) are very common in nature. Recall that a polysaccharide is an n-sugar carbohydrate polymer where n can vary substantially (10 to 10,000 typically). Polysaccharides are classed this way because often the exact number of monomers in the chain does not matter and varies over time. Most polysaccharide compounds were identified and characterized before the IUPAC system was developed. As a result, the use of trivial names for polysaccharides remains commonplace. In principle a specific polysaccharide molecule may also be described using formalized IUPAC naming. Because of several factors, primarily the variability in chain length, this is not done in practice (the exact name is not meaningful). However, the polymers are often described using similar terminology; they are typically repeating units of specific monosaccharides connected through specific glycosidic bonds.
Cellulose is a common polysaccharide in nature (Figure 5.32). It is structurally rigid and typically used in plants for this purpose (e.g. cell walls). It is a linear polymer of β-D-glucopyranose. All of the glycosidic bonds are β-1,4.
Figure 5.32 – Structure of the Polysaccharide Cellulose.
Amylose is a common polysaccharide in nature (Figure 5.33). It is easily but slowly digested and typically used in plants for this purpose (e.g. long-term energy storage). It is a linear polymer of β-D-glucopyranose and one of the two polysaccharides that make up “starch”. All of the glycosidic bonds are α-1,4.
Figure 5.33 – Structure of the Polysaccharide Amylose.
Amylopectin is a common polysaccharide in nature (Figure 5.34). It is easily and (semi-)quickly digested and typically used in plants for this purpose (e.g. short-term energy storage). It is a branched polymer of β-D-glucopyranose and one of the two polysaccharides that make up “starch”. Most of the glycosidic bonds are α-1,4 but approximately every 25th saccharide also has an α-1,6 bond. The branches then continue with additional β-D-glucopyranose monomers connected by α-1,4 bonds. Functionally, the α-1,6 glycosidic bonds connect multiple chains together.
Figure 5.34 – Structure of the Polysaccharide Amylopectin.
Glycogen is a common polysaccharide in nature (Figure 5.35). It is almost identical to amylopectin. It is easily and quickly digested and typically used in animals for this purpose (e.g. quickly accessed short-term energy storage). It is a branched polymer of β-D-glucopyranose. Most of the glycosidic bonds are α-1,4 but approximately every 12th saccharide also has an α-1,6 bond. The branches then continue with additional β-D-glucopyranose monomers connected by α-1,4 bonds. The higher degree of branching results in more locations on the molecule for digestive enzymes to quickly break off a glucose monomer, resulting in (much) faster access to energy.
