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Video transcript

- In the video on dehydration synthesis, we saw how we could start with a glucose molecule, and through dehydration synthesis form a bond with another glucose molecule. And just by doing that, you'd form the disaccharide maltose if these were both glucose molecules. But then you could keep going, and you could form longer chains of glucose molecules. And to these things, where you would take a monosaccharide, glucose is the most common example of that, and you create chains of these, we call these polysaccharides. Polysaccharides, this is a polysaccharide. And there's all sorts of interesting examples of polysaccharides all around you, especially polysaccharides of glucose, or things that are derived from glucose. This right here this is a bowl of mashed potatoes, which is mostly starch. Which is mainly just chains of glucose. So this right over here, that is starch. The shell of a lot of insects and things like lobsters, and the wings of these insects right over here, that's made of something called chitin. And chitin is also a polysaccharide. It's made of chains, a modification of glucose chains of that, that's chitin right over there. Very similar to starch, in our muscles we have glycogen, which is our store of energy in our muscles. You have cellulose, which is probably all around you right now. Cellulose are things like-- Because this is something that's all around you, and you don't even realize it. Cellulose, this is what constitutes things like paper and wood. It's involved in the cell walls of plants. This right over here is a picture of cotton, cotton in its natural form. And cotton is actually one of the purest forms of cellulose, it's roughly 90 percent cellulose. And if you take a zoom in on a cotton fiber, actually a fiber of cellulose, you'll see chains of glucose molecules. So you see this right over here, that is a glucose molecule. Then you see another glucose molecule. And this chain has been formed through dehydration synthesis. And difference between starch and cellulose, for the main difference, in terms of how this bonding has. With starch, the glucose molecules just keep forming the way that you saw in the video on dehydration synthesis. While in cellulose, they get flipped over. So you can see here, this oxygen is pointing that way, this oxygen is pointing that way, that oxygen is pointing that way. And you could look up more about cellulose. But it's really interesting, what gives it its structure are these hydrogen bonds that form between the partially negative, the very electronegative oxygens on one strand. And the partially positive hydrogens on another strand, and that's actually what give its structure. So really, really interesting things, these polysaccharides. The question is, how do you actually break these things down? If I were to eat these mashed potatoes, how do I eventually turn this thing into glucose, so I could use it for energy? And the way that happens is through hydrolysis. And you could break down this word. The "hydro", if you see hydro, the prefix hydro, that's a good clue that it has something to do with water. And then if you see "lysis", if you're lysing something, this means that you're gonna break it down. So this is breaking down something using water. And that's exactly what happens with hydrolysis. If you have this polysaccharide, and let's throw a water molecule in there, this water molecule is going to be able to break one of these bonds. So we might end up with something like-- This chain could keep going in both directions, but we could end up with something that looks like this. That looks something like that. So half of this water molecule gets broken up, essentially to break this bond. It's the opposite of dehydration synthesis. So let's see if we can understand, get an overview of exactly how that happens. So this right over here, this is maltose right over here. It's disaccharide, it's just two glucose molecules attached to each other. If we kept doing this, if this kept going, if this guy had bond to another glucose molecule, and this guy had a bond to another glucose molecule, then we'd be dealing with starch. Or we could be dealing with glycogen. If this was flipped over, and they kept flipping over and over, then we could be talking about cellulose. But let's just thing about how this is the mechanism. The mechanism by which this bond can actually be broken. It's really just the reverse of dehydration synthesis. This is going to just be an overview of it. This oxygen right over here, it's got two lone pairs. There's always a chance that if it bumps into something in just the right way, it could nab a hydrogen proton that is just sitting out there in the fluid. We're assuming that this is happening in an aqueous solution, it's happening in water. So it can just grab a hydrogen proton from a passing hydronium molecule. And so if it does that, it would form a covalent bond, and have a positive charge. And now relative to actually both carbons, but let's focus on this carbon right over here. This guy would be, what we call in organic chemistry, a good leaving group. So these electrons, the oxygen might want to just take these back, because it's got a positive charge, oxygen is really electronegative, so things just bump in exactly the right way. If things interact in exactly the right way, you might have another water molecule. And this is where that extra water molecule is valuable in our hydrolysis. So let's say this is just another water molecule just passing by in exactly the right way. This could form a bond with that carbon right over there. And just as it forms a bond with that carbon, the carbon says, "Okay, I'm getting to share "some other electrons, let me let go of these electrons." So it lets go of these electrons. And what do you have left? Well, we can go over here, and so now this carbon has, let me color code it, so this bond that was just forming, that is this bond right over here. This oxygen is this oxygen right over there. It actually has another hydrogen attached to it, so let me do that. So right when it makes the bond, it will have a positive charge. And then, this bond right over goes back to this oxygen. This oxygen right over here, is that oxygen right over there. Now when it started off, this guy grabbed-- The hydrogen proton that I grabbed, I showed in orange, that's this one. That's this one right over here. Now, this one grabbed a hydrogen proton, and now this one can actually give back a hydrogen proton through solution. If a water molecule passing by could just grab this hydrogen proton and then become a hydronium molecule. So it took a hydrogen proton, it's giving it back, and so what we are left with-- It took up this water molecule right over here to break the bond. And so this is a positive charge. It could be a passing hydronium molecule, and it'll just hand it off to that. And there you have it, we have two standalone glucose molecules right over there. We have broken the bond. And these could be parts of chains, in which case, we've just broken the chain. Or, if we're just dealing with maltose, now we've broken it down into the individual glucose molecules. And the example here is with glucose, but it could've been the case with maltose, and it could've been the case with sucrose where we break sucrose down using hydrolysis into a glucose molecule and a fructose molecule. So it's a very important reaction in biology.
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