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Carbohydrates- di and polysaccharides

Let's explore two types of carbohydrates: disaccharides and polysaccharides. Explore how monosaccharides link to form disaccharides through glycosidic linkages, and how these expand into polysaccharides. Understand the role of common disaccharides like lactose, maltose, and sucrose, and polysaccharides like cellulose, starch, and glycogen in our daily lives. Created by Ryan Scott Patton.

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  • leaf green style avatar for user Aqib Riaz
    why sucrose is non reducing sugar?
    (8 votes)
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    • male robot hal style avatar for user AkbarAmin
      Reducing capability is defined by the presence of free or potential aldehyde or ketone group.

      All monosaccharides have free ketone or aldehyde group. this means that they are all reducing sugars.

      Maltose and sucrose are disaccharides, which means that they are made up of two monosaccharides.

      Maltose is made up of two glucose units while sucrose is made up of glucose and fructose. The reducing ability of disaccharides is defined by the presence of a potential aldehyde or ketone group.
      In the ring structures of sucrose and maltose, you have an anomeric carbon. this is the carbon which was hydrolyzed in the straight-chain structure. This is also the carbon that can open up the ring structure and reduce a metal ion.

      Maltose's anomeric carbon is "free" and can therefore open up the ring and reduce the metal ion.

      On the other hand, sucrose's anomeric carbon is not "free" since this carbon is used to link fructose and glucose together. therefore, this anomeric carbon can't open up the ring structure and react with the reagent.
      (39 votes)
  • aqualine ultimate style avatar for user Aidan Kaye
    What is a reducing or non-reducing sugar?
    (7 votes)
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    • blobby green style avatar for user ilanag94
      A reducing sugar has the ability to keep bonding to more monosaccharides to make longer carbohydrate chains (make more glycosydic bonds). A non-reducing sugar is an acetal that can no longer keep bonding with other monosaccharides and make a longer chain. It is called reducing or non-reducing because a sugar making this bond is the sugar "reducing" in the context of the glycosidic bond reaction.
      (22 votes)
  • aqualine ultimate style avatar for user Felisha Wong
    What is a hemiacetal and acetal? How do we know what they are?
    (4 votes)
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    • leafers tree style avatar for user bernny felix
      Approximately after he starts to explain what they are. Listen when he points out the anomeric carbons on the sucrose. Compare the anomeric carbons of sucrose with the anomeric carbons of maltose and galactose. If you see, the anomercc carbons in sucrose have no free hydroxyl group to make a bond, therefore it is an acetal. In contrast, lactose and maltose have free hydroxyl groups on one of their anomeric carbons. I was confused as well. Hope my explanation helps.
      (8 votes)
  • winston default style avatar for user adinsx0001
    Lactose= Galactose + Glucose
    Maltose=Glucose + Glucose
    Sucrose= ? + Fructose
    What was the other carbohydrate that was paired with Fructose?
    (3 votes)
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  • old spice man green style avatar for user yadiramtz
    how can fructose make an acetal if it's a ketose?
    (4 votes)
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  • blobby green style avatar for user Ryan Legere
    The maltose example accidentally has a molecule of galactose on the left instead of glucose
    (5 votes)
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  • old spice man green style avatar for user FG
    If lactose also has a beta glycosidic linkage, how can we break it if we dont have the enzyme to break this kind of bond?
    (3 votes)
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  • starky seed style avatar for user Jamshed Arslan
    Which "genius" advised you to make this video with chair conformations rather than very simple Fisher or Haword projections? Latter would have made the learning much more easy (the goal of Khan Academy).
    (3 votes)
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  • piceratops tree style avatar for user Ashley Maitland
    At , which video is he referring to when he said he talked about naming in another video? Thanks.
    (2 votes)
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  • duskpin sapling style avatar for user karlamolina2412
    what is the linkage for sucrose?
    (2 votes)
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Video transcript

- Alright, so 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. And I believe that I mentioned that sometimes the Alcohol comes in and is reduced, is actually another Carbohydrate. So let me kind of draw this in here. And it makes sense because what you see with Carbohydrates is that they're choke-full of Hydroxides, they're choke-full of these OH groups, and so, really they can function really similarly to Alcohols and reactions, and when this happens, the individual Monosaccharides are linked together to make an Acetal, and we call this linkage a Glycosidic Linkage. So this is a Glycosidic Linkage. Now, when two Monosaccharides are linked together in this fashion by Glycosidic Linkages we call the product a Disaccharide. Disaccharide. And 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 anomeric carbon, or C1, so remember this is the anomeric carbon, the C1, so over in our Glycoside here, be right here, just the same, we got a C1 of the first Sugar, and the 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, and that's the second Sugar. So we call this a 1,4 Glycosidic Linkage. And then just like we could have further break down our Monosccharides into Alpha and Beta based off the orientation of the anomeric Hydroxide group, we can more specifically call the 1,4 Linkage an Alpha or a Beta Linkage, again, based off what is now the orientation of the OR group on the anomeric carbon. So same rules apply, if the group is cis with respect to the sixth carbon it's Beta, and of course if it's trans it would be an Alpha Linkage. So, in this case, our OR group, which-- Our OR group is this whole Carbohydrate is cis with respect to the sixth carbon, so we have a Beta 1,4 Glycosidic Linkage. And if that bit of naming confused you a little bit I went over that in greater detail in a different video, but what I wanna confused on here are some of the common Disaccharides, so 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, and what we have here is a Disaccharide, you see two Carbohydrates, two Monosaccharides linked together, and this one happens to be Lactose which you might be familiar with. So, Lactose. And Lactose happens to be, really the principal Disaccharide found in milk and that's actually true for both human milk and for cow milk, and unlike really most Disaccharides Lactose isn't really appreciably sweet. It consists of one Galactose, so this one right here is Galactose, and one Glucose Carbohydrate, and their bound together by a 1,4 Glycoside Bond just like we saw before. So we've got the one and a four, and this is a Glycoside Bond, and this one happens to be in a Beta orientation, so Lactose is a Disaccharide made of Galactose and Glucose joined together by a Beta 1,4 Glycoside Bond. Now, next step we have Maltose, so let me write that in here, we've got Maltose. And Maltose is, again, a Disaccharide but this time it's made of two individual Glocuse units, so we got a Glucose right here and we've got a Glucose right here and they're bound together similarly by a 1,4 Glycoside. So we've got the one carbon right here, that's this one, and we've got the four carbon over here, so this is again a 1,4 Glycosidic Linkage, but as opposed to Lactose, up here this one is actually Alpha, you can see that this OR group, the second Carbohydrate which is functioning as the OR group is in a trans position with respect to the first Carbohydrate's sixth carbon over here, so this is an Alpha 1,4 Glycosidic Linkage unit, binds together two Glucose units. So that's Maltose, another pretty common Disaccharide. And then last but not least, let me pull in here for you, Sucrose. And Sucrose is actually probably the most common Disaccharide in all of nature and you deal with it quite frequently unimagined 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, and I wanna point out a couple of the key differences. So, in Lactose and Maltose, both of these appear, 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, and 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've got Fructose right here. So, Fructose... And what happens is, you have both of the Carbohydrates linked together by their anomeric carbons, so right here we've got two anomeric carbons linked together. Now, that's different that Maltose and Lactose, so for example right here is the anomeric carbon of both Maltose and Lactose, it's over here, that's the C4 that's bound, so these are both linked together by their anomeric carbons, and what happens is you have two Acetals that are formed, so we've got an Acetal right there, and remember an Acetal is when a carbon is linked to an OR group over here, and an OR group over here, and then you have a second Acetal at the Fructose's anomeric carbon. And so 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've got a Hemiacetal over kind of on the tail, on the second Glucose, that's a Hemi, a Hemiacetal. And then you can look at 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, so that makes Sucrose a non-reducing Sugar and then Lactose and Maltose are both reducing Sugars. And so, Lactose, Maltose and Sucrose are probably the three most common Disaccharides and they give a good basis for Disaccharides. And then, really, Polysaccharides are just an extension of these thoughts. So let me clear some space. And for those reducing Sugars like Maltose and Lactose that are let with a Hemiacetal group at the end, we can keep adding Sugar groups on to 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 an, another drawing here, let me make a little bit more room for it. And that's what I've shown you, I've shown just kind of the addition, a couple additions of extra Carbohydrates onto Disaccharides. So both of these have three Carbohydrates, and you could keep going, but that's what makes them Polysaccharides. So, this first one that I drew in is a Polysaccharide called Cellulose, and Cellulose is found in the cell walls of really nearly all plants, and it gives support and structure to wood and to plant stems, and really cotton is essentially just pure Cellulose. But Cellulose is a Polysaccharide made of repeating Glucose units that are joined together by Beta 1,4 Glycosidic Bonds. So all of these are Beta 1,4 Glycosidic Linkages. And, really this, it forms an unbranched, just kind of straight chain, and that's the Polysaccharide Cellulose. Now, down here I have another Polysaccharide which is also super common, and this is Starch. And you can see that, really this is made of repeating again, repeating Glucose units here. The difference is that these Linkages are Alpha, still 1,4. So still 1,4 Linkages but these are Alpha units. And really the functional difference here is that as humans we have the enzyme to break down these Alpha 1,4 Linkages and we can use Starch which, again, is found in a lot of plant products as a source of energy 'cause we can break these down into Glucose to undergo cellular respiration, but we lack the enzyme to break down the Beta 1,4 Glycosidic Linkages of Glucose. So we can't appreciably use Cellulose as an energy source. And 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 kind of branch off of the Starch every once in a while on the C6 carbon, so that's the C6 carbon right here. And you add on another Glucose, so I'll stop there, this is another, another Glucose. You can keep going with Alpha 1,4-- Yeah, excuse me, Alpha 1,4 Linkages again, and you can form essentially just branches, branches. So this one would go here, and then you can maybe down the line here have another one. And so these are, these are mostly Alpha 1,4 Linkages, every once in a while you get an Alpha 1,6 Linkage thrown in there which creates some significant branching. And so, if it's highly branched, we call it Glycogen, and that's a little bit, a little bit down of a concept but essentially that's what Glycogen is, it's a major Polysaccharide made of Alpha 1,4 Linkages with, that are heavily branched by this Alpha 1,4-- 1,6, excuse me, breaks. But Glycogen, it's significance for us, it's principally as a source of storage of energy. So 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. So, that's probably a good start for Polysaccharides as well, we've got Cellulose which is Beta 1,4 Linkages of Glucose in a straight chain. We've got Starch which is essentially kind of a chain of Alpha 1,4 Linkages of Glucose. And then we've got Glycogen which is really, really similar to Starch, except that there are Alpha 1,6 Linkage breaks in here that enable a form kind of chains of this Polysaccharide.