Polysaccharides and why carbohydrates matter

Polysaccharides are long chains of monosaccharide units held by glycosidic linkages. They are the carbohydrates met most often in nature and serve mainly for food storage or structure.

Starch, the main storage polysaccharide of plants and the chief dietary source for people, is a polymer of α-glucose found richly in cereals, roots, tubers and some vegetables. It has two parts.

Amylose, about 15–20% of starch, dissolves in water. It is an unbranched chain of 200–1000 α-D-(+)-glucose units joined C1–C4.

Amylopectin, about 80–85% of starch, is insoluble in water. Its chains are joined C1–C4 and it branches through C1–C6 links.

Cellulose is found only in plants, is the most abundant organic substance in the plant kingdom and makes up much of plant cell walls. It is an unbranched chain of β-D-glucose units joined C1 to C4.

Starch and cellulose are both glucose polymers linked C1–C4. The difference is the anomer: α-glucose in starch, β-glucose in cellulose.

Glycogen is how animals store carbohydrate, hence 'animal starch'. It resembles amylopectin but is more highly branched, occurs in liver, muscles and brain (and in yeast and fungi), and enzymes break it down to glucose when the body needs it.

Uses: carbohydrates are a large part of food, and honey has long served Vaids in ayurveda as instant energy. Cellulose makes the cell walls of bacteria and plants, and reaches us as wood for furniture and cotton for clothes. Textiles, paper, lacquers and breweries use carbohydrates as raw material.

D-ribose and 2-deoxy-D-ribose, both aldopentoses, are the sugars of nucleic acids. Carbohydrates also occur bound to proteins and lipids.