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Carbohydrates – naming and classification | Chemical processes | MCAT | Khan Academy

October 8, 2019


– [Voiceover] The term ‘carbohydrate’ refers to a chemical compound made up of carbon atoms
that are fully hydrated. So ‘carbo’, for carbon, and ‘hydrate’, for hydration or water. And because these biological molecules are hydrates of carbon, you can find them fitting
into the general formula; C, so a number of carbon atoms, n for kind of a generic
number of carbon atoms, and then a matching
number of water molecules, so H2O is usually the exact same number as your carbon atoms. Because all of these carbons kind of have an associated water, you can think of this as being essentially a
one-to-two-to-one ratio of carbon, hydrogen, and oxygen. Now, when we have one of
these carbohydrate molecules, we call it a ‘monosaccharide’. Monosaccharide essentially
means ‘one saccharide’ and saccharide is just a
synonym for carbohydrate. So saccharide is actually derived from the Greek word for sugar, so you might hear a single carbohydrate referred to as a simple sugar. But in all these instances, we’re talking about the
same kind of molecule. And these molecules that
we’re calling carbohydrates, they do some pretty incredible and pretty hugely necessary
things in our bodies and in most living
things, for that matter. Maybe one of the most
familiar of these tasks is the fulfillment of
our body’s energy source. Carbohydrates fulfill
our body’s energy needs. The main energy source for
metabolism in our bodies is glucose, and I bet you’ve
heard of glucose before. You might’ve heard it in the context of “checking blood glucose levels” for people with diabetes. But glucose is a monosaccharide
made of six carbons. You might also be familiar
with the structural, rigidity of cell walls in plants. That rigidity comes from
the rigid carbon backbone of several carbohydrates linked together to form the polysaccharide. So, polysaccharide is several saccharides, several carbohydrates linked together, and we call that ‘cellulose’. Cellulose is the polysaccharide,
the carbohydrate, that makes up the structural
backbone of cell walls. Then, of course, one of the most beautful carbohydrate roles, in my
mind, is the use of ribose, which is a five carbon sugar that supports the transcribed
products of our genes in RNA. So you might have caught on that in all three of the carbohydrates
that I just mentioned, we see the ending ‘ose’. We see that in glucose,
cellulose, and in ribose. That’s because ‘-ose’ is
the suffix for sugars. There are actually two prefixes that help us further break down the naming of these compounds. The first prefix that
we’re gonna consider is how many carbons are in the chain. So the number of carbons
that are in the chain for this molecule. For example, I’m gonna
draw glyceraldehyde, which is generally considered to be the simplest carbohydrate,
and it looks like this. And if this carbonyl group, right up here, was just another hydroxyl group, this would be glycerin, three carbons with three hydroxyl groups. But instead it’s an aldehyde. So this molecule is, for that reason, named ‘glyceraldehyde’. The aldehyde is our functional group, so if we’re gonna count how many carbons are in this molecule, we’ll start with our
functional group carbon, this carbonyl carbon up here. We’ve got one, two, three
carbons in glyceraldehyde. So there are three carbons, and for that reason we
would call this a ‘triose’. ‘Tri’ for three and again,
‘-ose’ as our suffix for sugar. And if we added a fourth carbon, we would called it a ‘tetrose’. So four carbons in a
carbohydrate chain is a tetrose. And we add a fifth carbon and
that would be a ‘pentose’. So pentose for five. If we added an additional carbon, we would have six carbons, and that would give us a ‘hexose’. ‘Hex’ being the prefix for six. And I mentioned before
when I was talking about the energy source of our body, that glucose is actually
a six carbon carbohydrate. So as an example of a hexose,
I’ll draw glucose here. So this carbon chain, there’s six carbons, we’ve got one, two, three,
four, five, six carbons, and this is a hexose called ‘glucose’. Now, in the case of glucose, the functional group is an aldehyde, just like our glyceraldehyde. So the functional group
up here is an aldehyde. But what if we make it a ketone? You see, we can actually make it a ketone and still retain that
one-to-two-to-one ratio. It brings us another pretty
popular hexose called fructose. Another hexose, it still
has that one-to-two-to-one carbon, hydrogen, oxygen ratio, but instead of having the
aldehyde functional group, it has a ketone functional
group right here. So that brings up kind of
the second naming prefix. We have to indicate whether we’re working with an aldehyde or a ketone. So glucose would be more
accurately referred to as an aldohexose. That ‘aldo’ is a
reference to the fact that the functional group in this
carbohydrate is an aldehyde. Fructose, on the other hand, lemme write fructose down, fructose is a ketohexose. And again, the ‘keto’ is a reference to the fact that the functional
group here is a ketone. And then if we want to
kind of just exhaust this second prefix idea, going back up to glyceraldehyde,
which we said was a triose, because the functional
group is an aldehyde, this would be an aldotriose. So aldotriose. So we name based on length
of the carbon chain, the number of carbons
that are in the chain, and the functional group
that’s in our carbohydrate. The last major component of naming is the stereochemistry of the highest numbered chiral center. So again, we start with a carbonyl carbon and if we use a Fischer Projection
like we did with glucose, then we go to the highest chiral center, which would be this last one, and we decide the stereochemistry
of that chiral carbon. Just as a shortcut, with
Fischer Projections, if the highest subsituent, in
this case, the hydroxyl group is on the right-hand side, then it’s an R-stereochemistry. If it’s on the left side, it
would be an L-stereochemistry. Lemme try to make that
a little bit easier. I’ll kind of re-draw glyceraldehyde, a nice, small molecule,
as a Fischer Projection. So we’ve got our aldehyde and then we’ve got our next
two carbons in this chain, three carbons, and we
have one chiral center in this carbohydrate. This one right in the middle here. And our -OH group, you can
see, is on the right-hand side. So this is an R configuration. For carbohydrates, a lot
of the naming is associated with the guy, Fischer, who
invented these Fischer diagrams. He decided that since it was an R, and the Latin for
right-handed is ‘dexter’, we assign a D to this configuration. If we kind of drew this in a mirror image, and we drew the enantiomer of it, and we had our aldehyde carbon up here and our last two carbons
and their hydroxyl groups, now we see that the highest
numbered chiral carbon has it’s primary
subsituent on the left side at the bottom of this Fischer Projection. This would be assigned an ‘L’
which is a little bit easier. So as an example, I guess,
going back to the glucose, this would be a D-aldohexose because the hydroxyl group
is on the right-hand side of this molecule. Again, with our fructose, same thing. The last chiral carbon has the -OH group on the right-hand side, so this is a D-ketohexose. Now, before I move on, I want to allow you to review Fischer Diagrams because I know I just
kind of blazed through it. Really, they’re important,
especially with carbohydrates, because stereochemistry
becomes quite important in the biological
implication of carbohydrates. So I included a great video
by Jay with Khan Academy on Fischer Projections. It’s actually the video I
used to learn about them. So I’d encourage you to
pause here for a second and give that video a watch
on Fischer Projections and get real comfortable with absolute configuration, and then we’ll move forward with the discussion of carbohydrates.

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24 Comments

  • Reply meroy saleel January 30, 2015 at 3:13 pm

    thanx a lot
    شكرا جزيلا

  • Reply Ari K. March 15, 2015 at 11:20 pm

    this video was all over the place. please make an outline.

  • Reply Michael Persaud July 9, 2015 at 11:28 pm

    Great video, it was very well aligned with my kaplan 2015 MCAT biochemistry book. Thank you

  • Reply J H November 17, 2015 at 11:57 pm

    Its ok but I fucking hate fischer projections. worked for fischer sucks for me

  • Reply Yancy Aranas December 2, 2015 at 12:04 am

    why you did not explain tetrose and pentose? you directly went to hexose. and also you did not much elaborate this topic. i am disappointed, i cannot understand well.

  • Reply Matt Smith May 8, 2016 at 2:11 pm

    So with the Fischer projections were you assigning priority to the aldehyde on top or the alcohol on the bottom?

  • Reply peeli bhai August 1, 2016 at 3:55 am

    man! carbohydrates are not hydrates of carbon…but earlier this waS assume to be but later on various chemical test shows that h2o exist as ketone or aldehyde group… not as hydrate

  • Reply Reverend Al August 31, 2016 at 4:35 am

    Eat carbohydrates, avoid hydrocarbons.

  • Reply Abeer B September 3, 2016 at 12:20 am

    i think you mixed fructose with glucose!!!!

  • Reply Dallin Layton February 9, 2017 at 12:01 pm

    Don't leave off the one more Carbon with Fructose as was done in the video. If you count, there are only 5, but he names it a hexose.

  • Reply Matt Peters April 14, 2017 at 8:17 pm

    I thought that a keytone is contained within the carbon skeleton where as an aldehyde is attached to the end of a carbon skeleton?

  • Reply Tahlil Zoman June 19, 2017 at 6:14 pm

    did you miss a hydrogen on the fructose?

  • Reply Taylor Mae September 29, 2017 at 6:12 am

    This guy has nice hand writing

  • Reply Keyword MCAT October 28, 2017 at 7:28 pm

    great, very clear explanation. many thanks!

  • Reply Zakir Hossain November 13, 2017 at 12:49 pm

    this is really helpful

  • Reply Zakir Hossain November 24, 2017 at 2:45 pm

    that is awesome

  • Reply Happy Ineffable January 28, 2018 at 6:35 pm

    Simple

  • Reply Sina Mostafavi May 14, 2018 at 7:10 pm

    Great explanation! However, you forgot to mention that 'R'/right stereochemistry notation is often referred to as 'D'. D & L come from the Greek/Latin words for dextro and levo, which mean right and left.

  • Reply Nantasha Sonia July 8, 2018 at 11:21 am

    Thank you

  • Reply phantomkiller197 July 23, 2018 at 9:52 pm

    U mean D not R

  • Reply Jenny Konde November 12, 2018 at 1:57 pm

    Very helpful thank you

  • Reply Мисс NIC November 13, 2018 at 8:56 pm

    Thank you.

  • Reply Scragho :v April 10, 2019 at 10:49 pm

    what app is?

  • Reply / Jojo / July 7, 2019 at 7:32 pm

    Thank you a lot! This was actually not as hard as I thought. But I feel like now I'm on solid ground to be able to go on to the more complicated stuff with carbohydrates.

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