Starch vs Cellulose

 
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We mentioned earlier that starch and cellulose could be considered to be condensation polymers of glucose. But starch and cellulose have such different properties. How can we make two very different polymers out of the same monomer?

Let's look at the condensation reaction for the production of a glucose dimer we discussed before:


A possible condensation reaction to generate a glucose dimer (a cellulose-like oligomer)

Depending on how the two molecules are oriented, this is called an alpha-1,4 or a beta-1,4 bond. Starch is made up of alpha-1,4 linked glucose units; cellulose of beta-1,4 linked glucose units. And that's basically the only difference between them.


structure of starch and cellulose dimers

structure of starch and cellulose

Can you think of a reason why the properties of starch and cellulose should be so different?

It is also possible to form links between the 1-hydroxy group of one glucose and the 6 hydroxy group of another, giving a branch point; this does not happen in cellulose. Highly branched starch is called amylopectin, and starch with few or no branches is called amylose.

The structures of amylopectin and amylose.

So, the way you join the monomer units up can have a very big effect on how the polymer behaves.