7.6 Complex Carbohydrates

The most abundant polymer on Earth is cellulose, a polysaccharide consisting of glucose units bound in β(1→4) linkages. The letter beta (β) refers to the precursor glucose's tautomer: its C1 has an equatorial oxygen. The two numbers mean the C1 of one monomer is connected (via an oxygen atom) to the C4 of the next monomer. When the two monomers are brought together, the 1- and 4- hydroxy groups are dehydrated to link up the glucose units. It's that simple.

A short strand of cellulose.

The structure of cellulose means adjacent strands can form strong hydrogen bonds with each other. The strands have a stiffness to them and bundle together into tough fibers.

Starch is formed differently, but not much differently. Starch is made up of amylose and amylopectin, and amylose consists of glucose units in α(1→4) linkages. The only difference from cellulose is that the glucose subunits are in the alpha configuration, with a polar oxygen on C1. But what a difference it makes. Amylose tends to coil up into helices, which makes it stretchier and more flexible compared to rigid cellulose. Amylose is also soluble in water.

A short strand of amylopectin.

Amylopectin is similar to amylose but has a branched structure, incorporating α(1→6) linkages on some of its monomers.

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