Starch and cellulose are two very similar polymers. In fact, they are both made from the same monomer, glucose, and have the same glucose-based repeat units.
There is only one difference; in starch, all the repeat units are oriented in the same direction, so a starch molecule looks like a ‘uuuuuuuuuuuuuuu’. It is thus easier to break the link between two glucose units in starch.
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| Starch |
Cellulose polymer looks like a ‘ununununununununun’, with each successive glucose unit rotated 180 degrees around the axis of the polymer backbone chain, relative to the last repeat unit.
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Cellulose |
This seemingly small change creates a big difference! The most important one is the way the two polymers behave: We can eat and digest starch but not cellulose. Our body contains enzymes that will break starch down into glucose which is then converted to energy. But we don’t have enzymes that can break down cellulose. Some animals, however, do have such enzymes, such as termites, who eat wood, or cattle, who eat grass and break down the grass cellulose in their four-chambered stomachs. Cellulose, which is strong enough to make fibres from, and hence rope, clothing, is alot stronger than starch, which is practically useless as a material.
Cellulose doesn’t dissolve in water the way starch will, and doesn’t break down as easily. Breaking down or dissolving in water just would be a little too inconvenient for something we use to make clothes.
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