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Carbohydrates - naming and classification

Explore the world of carbohydrates, essential compounds for our bodies. Dive into the structure of monosaccharides like glucose and fructose, and learn how they provide energy. Discover the role of polysaccharides like cellulose in plant cell walls. Uncover the beauty of ribose in RNA. Understand the naming conventions based on carbon chain length and functional groups. Created by Ryan Scott Patton.

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Video transcript

- [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.