The name carbohydrate comes from the dark days of science when all they knew
was that the chemical formula added up to the same number of H20s as Cs,
thus a "Hydrated" Carbon.
 
Actually, a carbohydrate is one or more sugars in a chain.  The simplest
sugar, a monosaccharide is a circle of carbons with hydroxy (-OH) groups
attached to each carbon.  Most of the sugars in our foods are hexoses (six
carbon) sugars.  Glucose and galactose are hexoses and very easily broken
down by the glycolytic enzymes in our body.  The next level is a
disaccharide, two sugars.  Glucose+galactose is sucrose (I think, I don't
have any reference books in front of me.)  Lactose and Fructose are also
disaccharides and your body must use a special enzyme to break the bond
between the two sugar rings before the free monosaccharides can enter the
glycolytic pathway in the body.  (Imagine a chain of two rings, each ring
being made up of six beads.  In order to get the beads, you have to separate
the links, then take the beads apart one by one.) Starches have dozens of
sugar units and enzymes are required to break these chains down into
individual sugar links.  Breads, grains, pasta, rice, all have starches in
them that can be broken down by human beings into the component sugars and
digested.  Yet another type of carbohydrate is cellulose--a plant
polysaccharide.  We can't break cellulose down into simple sugars because we
lack the enzyme to break the bonds between the sugars.  The monosaccharides
in starch are connected by a different bond than the monosaccharides in
cellulose, thus, the cellulose passes through our system essentially
unchanged.  Cows culture microorganisms in their guts that break up the
cellulose for them so they can get the simple sugars out of it.  However,
when we eat celery, we get very few calories out of it.
 
An enzyme is a special protein that does work for you in your body by
catalyzine chemical reactions.  What we can use for food an cannot is
dictated by our enzymes, which differ from ferret enzymes and bacterial
enzymes, etc.
 
In sum, carbohydrates are all kinds of sugar, from the very simplest, to
the long, complex chains of sugars which we call "fiber."
 
-Catherine the off-topic biochemist
[Posted in FML issue 1911]