Polysaccharides: Starch and Cellulose

Starch and cellulose are both natural polysaccharides built entirely from repeating D-glucose units joined together by glycosidic bonds, yet they differ enormously in their physical properties and biological roles. The reason lies in one structural detail: the stereochemistry of the glycosidic linkage that joins their glucose units together.

Starch is the storage polysaccharide of plants (found abundantly in rice, wheat, potato and other staple foods) and is itself a mixture of two distinct glucose polymers. Amylose, which typically makes up roughly 15-20% of starch, is the unbranched component: a long, single chain of D-glucose units joined exclusively by $\alpha$-1,4-glycosidic linkages (a bond from C1 of one glucose unit to C4 of the next, with the linkage oxygen on the "alpha", or below-the-ring, face). This uniform $\alpha$-1,4 linkage makes the amylose chain naturally coil up into a helix. Amylopectin, the remaining 80-85% of starch, is the branched component: its main chains are also joined by $\alpha$-1,4-glycosidic linkages, but at intervals -- roughly every 24 to 30 glucose units -- an additional $\alpha$-1,6-glycosidic linkage branches off a new side chain, giving amylopectin a bushy, highly branched overall shape rather than one long single strand.

Cellulose, by contrast, is the structural polysaccharide that forms the rigid cell walls of plants (and is the chief constituent of cotton fibre and wood). Like amylose, cellulose is a completely unbranched, linear chain of D-glucose units -- but the glycosidic linkage joining them is a $\beta$-1,4-glycosidic linkage rather than an $\alpha$-1,4 one (the linkage oxygen sits on the "beta", or above-the-ring, face at each anomeric carbon). This single stereochemical difference has a dramatic structural consequence: whereas the $\alpha$-linked chain of amylose curls into a compact helix, the $\beta$-linked chain of cellulose is forced to lie almost perfectly straight, which allows very many such straight chains to line up side by side and hydrogen-bond extensively to their neighbours, producing the strong, rigid, fibrous bundles (microfibrils) that make cellulose such an effective structural material. …