Carbohydrates (Sugars)

We know very well what sugars taste like and smell like.
Let's take a few minutes to see what they actually look
like. I mean their chemical structure. Hopefully you're
already familiar with looking at the chemical structure
of amino acids and proteins, so let's look at the atoms
that come together in nature to form the molecules we call sugars.

Here are some examples:

This is Glucose
First, let's explain the 'alpha' and 'beta' terms. You'll notice that the diagram
shows the alpha form of glucose and the beta form of glucose. Simply, the alpha form
has the -OH on carbon #1 pointed downward, while the beta form of glucose has the -OH on
carbon #1 pointed upward. Remember that in the real world, this is a 3-dimensional molecule
and so it will make a difference 3-dimensionally which way the -OH group points.

It is a sugar that has 6 carbons. Each one of these carbons numbered 1-6, starting
with the one at the rightmost tip, and moving clockwise, so that the top one that is outside
the loop is carbon #6. You'll also notice, if you look closely, that each carbon
has a -H and a -OH group attached to it. Well, almost every carbon, actually you'll see
that carbon #5 lost the 'H' of the -OH and used this bond to connect to carbon #1.
And you might be wondering about carbon #6, with the CH2OH. It simply
follows our sort-of rule of having each carbon having a -H and -OH,
but since carbon #6 is outside the loop, it must have four
covalent bonds and so we simply add another -H.


This is Fructose
It, too, has 6 carbons, each with a -H and -OH attached. The only difference between
it and glucose is that you'll see if you look closely that carbon #5 used it's oxygen to bind to
carbon #2 instead of carbon #1 like glucose does. Notice carbon #1 is outside the loop.
Both of these examples will help us illustrate the rule of sugars (carbohydrates), and that
rule is that they will obey this chemical formula: CNH2NON
where the 'N' can be replaced with any whole number.
For glucose and fructose, since they both have 6 carbons, their chemical formulas will be:
C6H12O6.


Let's check this out with a 5-carbon sugar. If we count the carbons, hydrogens, and oxygens
we'll see that this 5-carbons sugar has the formula:
C5H10O5.
And it's true of the 3-carbons sugar too. If we add up all the atoms, we'll see that the
formula is: C3H6O3.

You, hopefully, know how important glucose is to the body for ATP production.
Let's mention here the important, but simple, difference between ribose and deoxyribose.
DNA gets it's name from the fact that the 5-carbon sugar it uses in it's
'---sugar-phosphate-sugar-phosphate---' backbone is the deoxyribose 5-carbon sugar.
RNA uses just good ol' plain ribose, hence the name ribonucleic acid. Otherwise the two
sugars are very similar, but with or without the oxygen changes the name of the nucleic acid.

Let's now see how we can attach one sugar to another. In other words, how we can
chemically link them together, how we can attach them with a covalent bond.
Take a look at the left side of the diagram below.

You'll notice shaded in blue an -OH and -H. If we were to remove the -OH from one glucose
and the -H from the other glucose, by breaking each covalent bond, we would then
have a free -OH and free -H that can combine into H-O-H or water (H2O). The two broken
covalent bonds left behind on the two glucose molecules can then also combine, thereby
linking these two separate glucoses into a disaccharide of glucose called maltose.

You'll notice on the right of this diagram that the two glucoses are linked by
an '-O-' pointed downward. Remember that when the -OH on carbon #1 of glucose
points downward, we say that this glucose is an 'alpha glucose'. So the pointing
downward '-O-' that links the two glucoses together now forms what's called
an alpha-linkage. Not so hard is it. Just some biochemistry terms.
Think of it this way, if you were going to learn how to fix your car, you'd have
to learn the names of the parts of your engine like the alternator, fuel pump,
fuel injector, and so on. They're just names of things. Well, it is the same thing here.
We're just learning the names of molecules and their reactions in the cell, that's all.

So on the subject of naming thing, as you might guess, we have to name this type
of reaction. The reaction where we covalently link to sugars together and
have a water molecule left over as a harmless by-product. This type of reaction
is called a dehydration synthesis. This is a good name for it.
It's a synthesis, or creation, of something new, in this case, a new disaccharide.
And we essentially pulled water off of these two glucoses, so we dehydrated it.
So there you have it, a dehydration synthesis. You may even remember that
we called the linking of two amino acids together while making a protein a
dehydration synthesis also. They are very similar reactions.

Now, we do not have to always link glucose. We can use other forms of sugar in
our dehydration reactions. The diagram below shows some other examples of
disaccharides using 'glucose and fructose' and 'galactose and glucose'.

Now, the body will store glucose by linking them together
to form a branching chain of glucoses. This is glycogen. You'll
notice from the diagram below that all the linkages are of
the 'alpha' type. The middle of the diagram below shows starch.

This is the sugar we eat in our fruits and vegetables.
You'll notice it consists of a linear chain of glucoses,
all in the 'alpha' linkage. This string of glucoses actually
coils around itself forming a helix. The bottom of the diagram
below shows cellulose. This too is made up of glucoses, however
you'll notice that all the glucoses are linked in a 'beta' linkage.
Our digestive system does not make an enzyme that will recognize,
or react, with glucoses linked in a 'beta' linkage (one upside
down compared to the next). So we cannot digest cellulose.