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Disaccharides and polysaccharides

Created by Ryan Scott Patton.

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Video transcript

- [Voiceover] All right. In a previous video, I talked about how cyclic monosaccharides, like this green cyclic glucose, can react with alcohols, like this pink alcohol, to form acetals and ketals. I believe that I mentioned that sometimes the alcohol that comes in and is reduced is actually another carbohydrate. Let me draw this in here. It makes sense, because what you see, with carbohydrates, is that they're chock full of hydroxilate groups. They're chock full of these OH groups. So, really, they can function really similarly to alcohol in reactions. When this happens, the individual monosaccharides are linked together to make an acetal. We call this linkage a glycosidic linkage. This is a glycosidic, a glycosidic linkage. Now, when two monosaccharides are linked together in this fashion, by glycosidic linkages, we call the product a disaccharide. A disaccharide. We have "di," which means two, and "saccharide," which means sugar. So sugar. So two monosaccharides linked together, they're called a disaccharide. Now, with disaccharides, most commonly the glycosidic linkage forms between the anomaric carbon, or C1 ... Remember, this is the anomeric carbon. That's C1. Over in our glycosite here, it'd be right here, just the same, we got C1 of the first sugar. Then, C4 of the second sugar, so right here would be C4, and it's just the same over here. So right here we have C4. That's the second sugar. So we call this a one, four glycosidic linkage. Then, just like we could further break down our monosaccharides into alpha and beta based off the orientation of the anomeric hydroxyl group, we can more specifically call the one, four linkage an alpha or a beta linkage, again, based off what is now the orientation of the OR group on the anomeric carbon. Same rules apply. If the group is cis with respect to the sixth carbon, it's beta. Of course if it's trans, it would be an alpha linkage. In this case, our OR group, which the OR group is this whole carbohydrate, is cis with respect to the C6 carbon. So we have a beta one, four glycosidic linkage. If that bit of naming confused you a little bit, I went over that in greater detail in a different video. What I wanna focus on here are some of the common disaccharides. Let me clear some space. Let me give us some room. I'm gonna go and fade in a drawing that I did a little bit ago to save just a bit of time. What we have here is a disaccharide. You see two carbohydrates, two monosaccharides, linked together. This one happens to be lactose, which you might be familiar with. Lactose. Lactose happens to be really the principal disaccharide found in milk. That's actually true for both human milk and for cow milk. Unlike, really, most disaccharides, lactose isn't really appreciably sweet. It consists of one galactose. This one right here is galactose. Then one glucose carbohydrate. They're bound together by a one, four glycoside bond, just like we saw before. So we've got the one and the four. This is a glycoside bond, and this one happens to be in the beta orientation. So lactose is a disaccharide made of galactose and glucose, joined together by a beta one, four glycoside bond. Now, next up we have maltose. Let me write that in here. We've got maltose. Maltose is, again, a disaccharide. But this time, it's made of two individual glucose units. So we've got a glucose right here and we've got a glucose right here. They're bound together similarly by a one, four glycoside, so we've got the one carbon right here, that's this one, and we've got the four carbon over here. This is, again, a one, four glycosidic linkage. But, as opposed to lactose up here, this one's actually alpha. You can see that this OR group, this second carbohydrate, which is functioning as the OR group, is in the trans position with respect to the first carbohydrate's six carbon over here. So this is an alpha one, four glycosidic linkage and it binds together two glucose units. So that's maltose, another pretty common disaccharide. Then, last but not least, let me pull in here for you sucrose. Sucrose is actually probably the most common disaccharide in all of nature, and you deal with it quite frequently, I'd imagine, because sucrose is the principal disaccharide of table sugar, which comes from sugar cane. So sucrose is actually quite sweet. But it's different, substantially so, from maltose and lactose. I wanna point out a couple of the key differences. In lactose and maltose, both of these up here, you have two pyranoses. Remember, pyranoses are six-membered carbohydrate rings. I went over that in a previous video. But we have two six-membered rings bound together by this glycoside. In sucrose, that's different. We've got a six-membered glucose right here ... This is glucose ... Bound to a five-membered, or a furanose, fructose. So we got fructose right here. Fructose. And what happens is, you have both of the carbohydrates linked together by their anomeric carbons. Right here, we've got two anomeric carbons linked together. That's different than maltose and lactose. For example, right here is the anomeric carbon of both maltose and lactose. It's over here. That's the C4 that's bound. These are both linked together by their anomeric carbons. What happens is, you have two acetals that are formed. So we've got an acetal right there. Remember, an acetal is when a carbon is linked to an OR group over here and an OR group over here. Then you have a second acetal at the fructose's anomeric carbon. With maltose, and the same thing with lactose, you have hemiacetals that are formed. So you've got an acetal right here and then you got a hemiacetal over on the tail, on the second glucose. That's a hemiacetal. You can look up and it's the exact same thing for lactose. But what happens here is, remember that with a hemiacetal, you can add on a second OH group to form another acetal. A hemiacetal can be further reduced into an acetal. But once you have an acetal, you can't further reduce it. That makes sucrose a non-reducing sugar. Then, lactose and maltose are both reducing sugars. Lactose, maltose and sucrose are probably the three most common disaccharides. They give us a good basis for disaccharides. Then, really, polysaccharides are just an extension of this thought. Let me clear some space. For those reducing sugars, like maltose and lactose, that are left with a hemiacetal group at the end, we can keep adding sugar groups onto the chain. That's kind of this reducing characteristic. They can keep growing, which ends up making more acetal groups, but always leaving a hemiacetal group on the end. So I've kind of pre-drawn in another drawing here. Let me make a little bit more room for it. That's what I've shown here. I've shown just the addition, a couple additions, of extra carbohydrates onto disaccharides. Both of these have three carbohydrates. You could keep going. But that's what makes them polysaccharides. This first one that I drew in is a polysaccharide called cellulose. Cellulose is found in the cell walls of really nearly all plants. It gives support and structure to wood and to plant stems, and, really, cotton is essentially just pure cellulose. But cellulose is a polysaccharide and made of repeating glucose units that are joined together by beta one, four glycosidic bonds. All of these are beta one, four glycosidic linkages. And really, it forms an unbranch, just kind of straight change, and that's the polysaccharide cellulose. Now, down here, I have another polysaccharide which is also super common. This is starch. You can see that, really, this is made up of repeating, again, repeating glucose units here. The difference is that these linkages are alpha. Still one, four. Still one, four linkages, but these are alpha units. Really, the functional difference here is that, as humans, we have the enzyme to break down these alpha one, four linkages and we can use starch, which again is found in a lot of plant products, as a source of energy, because we can break these down into glucose to undergo cellular respiration. But we lack the enzyme to break down the beta one, four glycosidic linkages of glucose, so we can't appreciably use cellulose as an energy source. Then, one last ... Excuse me. One last polysaccharide that I wanna show you is really very similar to starch. I'm gonna use the starch as a little basis here. But if you branch off of the starch, every once in a while on a C6 carbon, so that's the C6 carbon right here, and you add on another glucose. I'll start there. This is another glucose. You can keep going with alpha one, four ... Alpha one, four linkages again, and you can form essentially just branches. So this one would go here, and then you could maybe down the line here have another one. These are mostly alpha one, four linkages. Every once in a while, you get an alpha one, six linkage thrown in there, ' which creates some significant branching. If it's highly branched, we call it glycogen. That's a little bit dumbed down of a concept. But essentially, that's what glycogen is. It's a major polysaccharide made of alpha one, four linkages that are heavily branched by these alpha one, six breaks. But glycogen, it's significance for us is principally as a source of storage of energy. We can build glycogen stores in our body and it creates a really functional store of glucose because, with all these branches, we have a lot of tails of glucose that can be chopped off pretty quickly to get a fast glucose source. That's probably a good start for polysaccharides, as well. We've got cellulose, which is beta one, four linkages of glucose in a straight chain. We've got starch, which is, essentially, a chain of alpha one, four linkages of glucose. Then we've got glycogen, which is really, really similar to starch, except that there are alpha one, six linkage breaks in here that enable us to form chains of this polysaccharide.