placement of a hydroxyl group?

Question

Explain why the simple placement of a hydroxyl group in the building blocks of a polysaccharide can determine the digestibility of polysaccharides.

Answer 1

I think you refer to the fact that the ring form of a sugar like glucose has a C-O-C(OH)-C linkage in "front", and that the -OH may be either *alpha* ("down") or *beta* ("up"). Next the -OH undergoes ether formation with a "rear" -OH group of another glucose, forming a -C-O-C link to the second glucose. If glucose molecules combine with one another head-to-tail with all their head O's *alpha,* then the result is starch, and animals can digest starch. But if they combine all *beta,* the result is cellulose, and only plants and bacteria have enzymes that can degrade cellulose.