Polysaccharides

Polysaccharides are the complex carbohydrates which contain tens, hundreds, or even thousands of monosaccharides linked together through glycosidic linkages. Polysaccharides contain a single hemiacetal group at one end of the long chain (negligible as compared to acetal groups) hence, they are termed as non-reducing sugars and they do not show any noticeable mutarotation. The most common naturally occurring polysaccharides are starch, cellulose, and glycogen.

Cellulose:

Cellulose is the main structural element of plants. It constitutes about 90% composition of cotton and wood contains about 50% of cellulose. Cellulose is made up of repeating units of glucose molecules joined by a 1🡪4-β-glycosidic linkages. The β-glycosidic bonds create long unbranched chains that are stacked in sheets. Due to extensive intermolecular hydrogen bondings between the hydroxyl groups only a small number of -OH groups are available to interact with external systems hence, making cellulose insoluble in polar solvents like water.

The hydrolysis of cellulose yields both α-D-glucose and β-D-glucose.

In cells the hydrolysis of cellulose is accomplished by an enzyme called β-glucosidase. This enzyme is absent in humans hence, humans are unable to digest cellulose. On the other hand, the digestive system of ruminant animals has bacteria containing β-glucosidase hence, allowing them to get nutritional benefits from eating plant parts containing cellulose.

Starch:

Starch is found in potatoes, flour, corn, beans, rice, and peas. Starch is a mixture of two different polysaccharides – amylopectin (∼80%) and amylose (∼20%). Amylose is a long unbranched chain made up of repeating D-glucose units joined together via α-1,4′-glycosidic bonds.

Whereas amylopectin  is a long branched chain polysaccharide made up of repeating D-glucose units joined together via α-1,4′-glycosidic bonds and it also consists of α-1,6′-glycosidic bonds.

Both amylopectin and amylose are hydrolyzed into glucose molecules by an enzyme called α-glucosidase. The  α-glucosidase enzyme is present in the human digestive system hence, starch is readily digested by the human digestive system.

Glycogen:

Glycogen is the polysaccharide which serves as an energy storage function in animals. The surplus dietary carbohydrates which are not in need for instant energy are converted into glycogens by the animal body for long-term use. Like amylopectin, glycogens also contain α-1,4′-glycosidic and α-1,6′-glycosidic linkages but the branching in glycogen is far more extensive. Due to a highly branched system, the glucose units present at the end of branches are cleaved when the body needs them for energy. Following figures represent the structure of glycogen.