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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 that just by doing that you would form the disaccharide maltose if these were both glucose molecules but then you could keep going and you could form and you could form longer chains of glucose molecules and these things where you take a monosaccharides glucose is the most common example of that and you create chains of these we call these polysaccharides poly 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 let me do that in a color that's actually visible so that is starch that is starch the shell of a lot of a lot of insects and and things like lobsters the end the wings of these insects right over here that's made of something called chitin and chitin and chitin is also a polysaccharide it's made of chains of essentially 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 you have cellulose which is actually probably all around you right now cellulose they are things like so let me write this down because this is something that is all around you you don't even realize it cellulose this is things 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 of cotton cotton in its natural form and cotton is actually one of the purest forms of cellulose it's roughly 90% cellulose and if you take a zoom in on a on a cotton fiber or actually a of the fiber of cellulose you will see chains you will see chains of glucose molecules so you see this right over here that is a glucose molecule then you see another you see another glucose molecule and these this change has been formed through dehydration synthesis and the dystrophy and starch and cellulose are 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 and dehydration synthesis while in cellulose they get flipped over so you can see here this oxygen is pointing this point oxygen is pointing that way this oxygen is pointing that way that oxygen is pointing that way and you could you can look up more about cellulose what it's really interesting what gives it its structure are these hydrogen bonds that form between the partially negative very electronegative oxygens on one strand and the partially positive hydrogen's on another strands and that's actually what give its structure so really really interesting things these polysaccharides but a 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 can use it for energy and the way that that happens the way that that happens is through is through hydrolysis and you can break down this word the hydro this is if you see hydro the prefix hydro that's a good that's a good clue that it has something to do with water and then if you see Lissa so if you're lysine something this means that you're going to 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 say you eat let's throw a water molecule in there we are going to add up where this water molecule is going to be able to break it's going to be able to break one of these bonds so we might end up with something like we would end up this chain can keep going in both directions but we could end up with something that looks like this with that looks something 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 a get an overview of exactly how that happens so this right over here this is a this is a maltose hi 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 if this guy had a bond to another glucose molecule and this guy had a bond to another glucose molecule then we'd be dealing with dealing with starch or we could be dealing with glycogen if this was flipped over in the chain and they kept flipping over and over then we can be talking about cellulose but let's just think about how this the mechanism the mechanism by which this bond can actually be broken it's really just the reverse of dehydration the reverse of dehydration synthesis so we can that's just going to be an overview of it so we could start this one this oxygen right over here it's got two it's got two lone pairs there's always a chance and if it bumps into something in just the right way it could nab it couldn't have a hydrogen proton that is just sitting out there that is just sitting out there in the fluid we're assuming that this is happening in in an aqueous solution it's happening in water so it can just grab a hydrogen proton from from maybe a passing hydronium molecule and so if it does that it would form it would form 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 an organic chemistry a good leaving group a good leaving group so these these electrons these electrons the oxygen might want to just take these back because hey it's got a positive charge oxygen is really electronegative and so things just bump into 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 this could form a bond 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 I'm getting to share some other electrons let me go let me let go of these electrons so let's go of these electrons and then what do you what do you have left well we can go over here and so now this carbon has let me do color-coded so bond that was just forming that is this bond that is this bond right over here this oxygen 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'll have a positive charge and then and then this bond this bond right over here goes back to this oxygen this oxygen right over here is that oxygen right over there now when it started off this guy grabbed a this guy well and this guy and this let me actually and the hydrogen proton that I grabbed I've shown an orange that's this one that's this one right over here that's that one right over here now this one grabbed a hydrogen proton and now this one can actually give back a hydrogen protons in solution so if a water molecule passing by I can just grab this hydrogen proton and then become and then become a hydronium molecule and so it took a hydrogen proton as giving it back and so what we are left with but net-net it took up it took up this water to break 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 then it'll just hand it off it'll just hand it off to that and there you have it we have our we have two standalone glucose molecules right over there we have broken the bond and these 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 into the individual glucose molecules and the example here is with glucose but it could have been the case or with maltose and it could have 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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