BIOL 115 – Carbohydrates
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BIOL 115 – Carbohydrates

October 3, 2019


[MUSIC PLAYING] Today, we’re going to discuss
my favorite macromolecule, carbohydrates. By the end of this
prelecture, I hope that you’ll come away with one,
an understanding of the three functions of carbohydrates. Two, a knowledge of the chemical
formula of carbohydrates. Three, the main
functional groups that we find in
all carbohydrates. And fourth, that you can
identify a simple sugar or a monosaccharide. So let’s start with the three
functions of carbohydrates. One, they are used for energy. Fortunately, they
are also very tasty. So the carbohydrates
that are used for energy include molecules like
glucose and starch. The second function of
carbohydrates is for structure. And the major macromolecules
carbohydrates use for structure include cellulose, which
is a component of the cell wall in plants. And chitin, which is found in
the exoskeleton of organisms like insects and crabs. Not so tasty. And the third major
function of carbohydrates is identification. So on the surface of cells– I’m drawing a cell here– we find molecules
called glycoproteins. And these are a combination
of proteins and carbohydrates. And they’re useful for cells to
be able to determine what’s me and what’s not me. And if that cell is not me,
to be able to get rid of it. So very useful for
identification. So you may be
scratching your head or wondering wow, that’s
a lot of functions for one simple macromolecule. How do they do it? Incredible carbohydrates. Well, it’s because carbohydrates
can be put together in many different ways. And because they
can be put together in a diversity of ways, they can
have many different functions. So now let’s look at
the chemical formula of a carbohydrate. And the key to understanding
the chemical formula is really in the words. We have carbo,
which means there’s carbon in carbohydrates, and
hydrate, which means water. Now that’s not to say that
in carbohydrates, water is linked to carbon. That’s not it. But carbon is always found in
a ratio of one to one with H2O. Sorry. I’m drawing an N there. That’s a messy N. Where
N can be anything, can a number between
three up to over 1,000. But again, don’t think
that in carbohydrates we find water linked to carbon. Rather, the important
functional groups that we find in
carbohydrates include number one, hydroxyl groups. Remember, that’s that OH. The second important functional
group we find in carbohydrates are carbonyl. So let’s look in more detail
at the structure and chemical formula of carbohydrates. And specifically,
we’re going to start by talking about
monosaccharides, which are the building
blocks of all carbohydrates. And literally this
means one sugar. Let’s start with a carbohydrate
that has three carbons. And that would be
called a triose. It would have a
carbonyl group and it would have lots
of hydroxyl groups as well as carbon
hydrogen bonds. Again, there’s our carbonyl,
and we have two hydroxyl groups. Let’s be sure I have the
molecule drawn correctly with the correct
chemical formula. We have three carbons. We should have six
hydrogens then. Three times two is six. 1, 2, 3, 4, 5, 6. Got that right. And we should have
three oxygens. 1, 2, 3. Perfect. How about a six
carbon carbohydrate? That would be called a hexose. 1, 2, 3, 4, 5, 6. There’s our carbonyl group. And we can say lots of hydroxyl
groups, fill in our carbons with hydrogens. And again, this is
referred to as a hexose. And specifically, what
I’ve drawn here is glucose. Now carbohydrates differ by
the position of the carbonyl and also, the orientation
of the hydroxyl group. Let me show you. So we can take this
hydroxyl group right here and flip it to the other
side, and we end up with a different molecule
called galactose. Completely different
carbohydrate. Carbonyl is in the same place. Everything else is
in the same place. But we’ve simply moved
that hydroxyl group to the other side. Moved it over here and
now we have galactose. Totally different
tasting carbohydrate. Different structure,
slightly different function. We can also alter the
position of the carbonyl. So if we move the
carbonyl down one carbon, but leave everything else the
same as we did in glucose, I should have six
so 1, 2, 3, 4, 5, 6. So now we’ve put the
carbonyl at what’s called the second carbon. I should have mentioned that
the carbons are numbered in carbohydrates starting
with the carbon that’s closest to the carbonyl group. So this would be carbon
1, 2, 3 4, 5, and 6. Here, we’ve put the carbonyl
group on the second carbon, and in this position,
instead of being an aldehyde, it’s now a ketone
or a keto sugar. And instead of being
galactose, it’s a carbohydrate called fructose. So again, to summarize,
carbohydrates can differ in the
number of carbons. They differ in the position
of the carbonyl group. And they differ
in the orientation of the hydroxyl group. So next, in lecture, we’ll
talk about how we put together these simple
monosaccharides to build more complex
polysaccharides that have many different functions.

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