Glucose (C6H12O6) is an important molecule in biology. The six carbon atoms, 12 hydrogen atoms and six oxygen atoms can be combined in different arrangements to make the different "isomers" of glucose. Each of these isomers act differently in the body. The first two isomers that were discovered were named alpha glucose and beta glucose.
The only chemical differences between alpha glucose and beta glucose is the way that the hydrogen, carbon and oxygen atoms are attached to each other. It is as if the two different arrangements (alpha and beta) were two differently shaped interlocking plastic blocks. The alpha glucose molecules snap together in such a way that they are compact, portable and easily taken apart again. The beta glucose molecules snap together in such a way that they are very stable, structurally solid and hard to take apart.
Biologically, the difference between chains of alpha glucose and chains of beta glucose is the difference between starch and cellulose. Starch, made up of chains of alpha glucose, is an excellent way to store sugar. It is relatively stable but also easy to break down into simple sugars (individual alpha glucose molecules) for use. On the other hand, cellulose, comprised of chains of beta glucose molecules, is hard to break down--but an excellent building material. The hard parts of plants are usually made of cellulose and the softer, tastier parts of plants are made of starch.
Plants use starches for sugar storage and cellulose for structural material. No vertebrates have the enzymes to break down cellulose. Some animals, such as cows, horses and termites, manage to break down cellulose with the help of microscopic organisms that live in their guts. When we eat plants, we digest the starch and the cellulose passes through our systems unchanged. This does not mean that cellulose is unimportant. In nutritional science cellulose is called "fiber" and it is important for the proper functioning of the gastrointestinal tract. Millions of years of evolution and millions of ancestors who ate plants have left us with bodies that need both the digestible sugars from the links in the alpha glucose chains and the indigestible chains of beta glucose.
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