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Oxidation and reduction in cellular respiration

Video transcript

now that we have a little bit of a review of oxidation and reduction under our belts let's see if we can apply what we now may be re-understand to cellular respiration so cellular respiration for every mole of glucose is c6h12o6 we combine that maybe that's in an aqueous state it's dissolved in water we combine that with six moles of molecular oxygen and then our cells perform cellular respiration in a whole series of steps and I'll do more videos on that performs cellular respiration let's abbreviate it and then we end up with we end up with six moles of carbon dioxide we have to breathe this oxygen in in order to perform cellular respiration and then we have to breathe this carbon dioxide out because this is the byproduct of cellular respiration six moles of carbon dioxide six moles of water and then the whole point the whole point of cellular respiration is plus some energy some energy is generated by this reaction and our bodies store this energy well some of it is just turns into heat but the whole point of cellular respiration is to store it as 38 ATP's which we've learned already is the energy currency of biological systems and then our bodies or biological systems in general can use these ATP's to contract muscles or generate nerve impulses or grow cells or divide cells or whatever else that a biological system has to do in the last video we learned a little bit about oxidation and reduction so let's apply those ideas here now we saw in the last video that a chemist a chemist would say let me write it this way a chemist would say that oxidation means losing electrons or being able to hog them not being able to hog them losing electrons while a chemist will tell you that reduction reduction is gaining electrons gaining electrons and if you need you know if you have trouble remembering you know oxidation is losing that's kind of the oil that's the mnemonic oxidation is losing electrons and reduction is gaining or rig so oil rig this is what you learn in chemistry class now biologist or biochemist will say oh well you know I like to define a little bit different let me write this a biologist will say that oxidation is losing hydrogen atoms hydrogen atoms and they'll say reduction is gaining gaining hydrogen hydrogen atoms and we saw in the last video that this definition is actually hard when you're actually playing it to hydrogen because not like a hydrogen atom can lose itself or gain itself and the reason why we said these two ideas are consistent is because if I'm talking about a carbon and a carbon is losing a hydrogen so you know let's say I have some compound that looks like this maybe it's connected to a bunch of other things someplace else and then later on the carbon you know let's say have carbon that looks like that and then I have an oxygen that's maybe bound to another oxygen I'm doing it very kind of a hand wavy explanation here and this mean that oxygen is bonded to something else if this is what I start off with and then I on the other side of this equation I end up with something that looks like this where the carbon is bonded to an oxygen and maybe that other oxygen is bonded to this hydrogen the biologists will say the bio to say oh this carbon has been oxidized because it lost its hydrogen right the hydrogen went from here do it in a different color went from this carbon to this oxygen and the biologists would also say that this oxygen has been reduced it's been reduced because it gained hydrogen's with the reality or maybe the chemical the chemists definition which I like a little bit more is over here the carbon was because carbon is more electronegative right we see carbon is much more electronegative than hydrogen and oxygen is even more electronegative than carbon when any of these guys bond with hydrogen they're going to hog the electron so here carbon got to hog the electron so here carbon hogs carbon hogs electron while here carbon gets its electrons hogged by oxygen so here oxygen hogs oxygen hogs so by losing the hydrogen the carbon actually lost its opportunity to hog electrons and since it ended up bonding with an oxygen it actually not only can't hog hydrogen's electrons but then it gets its electrons hogged by an even more electronegative atom so that's why these two definitions are consistent same thing with the oxygen here it's bonding with another oxygen not hogging anything but when it gains the hydrogen when it gains the hydrogen it's able to hog hydrogen's electrons because it's so much more electronegative or you could say that gaining electrons so that's why these two definitions are somewhat consistent although sometimes they fall apart if we're not dealing with hydrogen the chemistry definition applies more consistently to everything but sometimes the biologists definition is easier to kind of glance at or you'll actually see it written in textbooks so let's go back to cellular respiration and try to figure out what's being oxidized and reduced so if we look over here over here well we have our glucose and actually I drew there I've copied and pasted from Wikipedia a glucose molecule and actually there's one air in here maybe I should edit it on the capito there should be another hydrogen bonded to that carbon right there but as you see all of the hydrogen's there either bonded to a a oxygen or a carbon over here right on the left hand side there either bonded to an oxygen or carbon if we were to write its oxidation state in every case it's bonded to something that's more electronegative so it's going to be giving up its electron so it'll have a plus 1 oxidation state an oxygen in every case is either bonded to a carbon or a hydrogen and so oxygen if it's bonded to a carbon our hydrogen is going to hog an electron from either one of those guys so in every in every situation on glucose oxygen has a two minus or a minus two oxidation state and carbon since this whole thing is neutral one would think that carbon would be would have a neutral oxidation state and if you go through this you'll actually find that most of these carbons do have neutral oxidation state me Circle a few so for example this carbon right here it's hogging an electron from this hydrogen but then it gets an electron hog by this oxygen and then of course it does nothing with the carbon so that's neutral this is neutral for the same reason this is neutral for the same reason this one is also neutral for the same reason it's bonded with two carbons it has an electron hog by oxygen but then it hogs an electron from hydrogen so it's neutral so four of these carbons are neutral this carbon right here this carbon right here has two electrons hogged by oxygens and then it gets to take one back from the hydrogen so it has a plus one oxidation state and then this one's the opposite it has two hydrogen's that it hogs from then it has to give one away to the oxygen so this has a minus one and so these two cancel out so on average you can say that the carbons in glucose have a neutral oxidation state and I'm dealing with the chemists definition I'm going to show you that they're essentially equivalent here all of the oxygens have no oxidation state because they're just bonded let me do it in a better color no oxidation state or neutral oxidation state because they're double bonded with oxygen no one's hogging from any one there obviously equally electronegative if we look at the products carbon dioxide looks like this so in either of these cases oxygen this oxygen is hogging two electrons from this carbon so it has a minus two oxidation state this oxygen is hogging two electrons from carbon so it has a minus two oxidation state and this carbon is getting all of its valence electrons all four hog by the oxygens so it has a plus or oxidation state it's lost four electrons you can imagine because it's getting honked so that's carbon so we could write this as four plus for the carbon and then each oxygen has a two minus and we could do the math later on to figure out what the total is and then if we look at the water and we've looked at this before the oxygen is hogging two electrons form each from one from each hydrogen so two minus and then each of the hydrogen's have a plus one oxidation state so if you want to do a half reaction for cellular respiration and that's in kind of the chemists sense of things just dealing with electrons you can well you can immediately say a that Lloyd I start with 12 hydrogen's I start with 12 hydrogen's on this side let me just write it this way so H 12 on this side and they all have a plus 1 oxidation state and then cellular respiration occurs and now I have 12 hydrogen's I can write the 12 a little bit differently here but they still have a plus 1 each of them still has a plus 1 oxidation state so nothing from an oxidation reduction point of view happens to the hydrogen now if we do the carbon if we do the carbon on the left-hand side of the equation we have 6 carbons they have a neutral oxidation state but then on the right-hand side of the equation what happens I now have 6 carbons written a little bit differently but I have 6 carbons and they each have a +4 oxidation state which means that they have lost 4 electrons or their hypothetical charge by losing those 4 electrons has gone up by 4 because they're losing negatively charged atoms or actually negatively charged electrons so the carbon the the 6 carbons after cellular respiration end up with 6 oxidized carbons with +4 oxidation states plus so each of these got lost 4 electrons we have 6 of them 4 times 6 is 24 electrons these are the electrons that the carbon lost so we see in cellular respiration that the carbon is oxidized carbon oxidized right oxidation is losing electrons we see in cellular respiration we draw the half reaction carbon is losing the 6 carbons are losing a collective 24 electrons and then finally if I were to do the oxygen on this side I've lost my equation up here so over here I have well I have two oxygens and I'm going to draw them a little bit separate so I have these six oxygens here I have these six oxygens here that have a minus two oxidation state on the left-hand side so I'll draw it like this they have a minus two oxidation state and then I have these twelve oxygens that are completely neutral and then I have we do that those twelve oxygens that are completely neutral so won't even write an oxidation state or oxidation number there and then after we perform cellular respiration what happens well now I have in the carbon dioxide I have 12 carbons 12 carbons that have a minus 2 oxidation state right 6 times o2 so let me write that down from the carbon dioxide so I have 602 s that I'll have a 2 minus oxidation state and then I have another 6 oxygens that have a minus 2 oxidation state so plus 6 oxygens that have a minus 2 oxidation state so if you think about it over here I had a collective oxidation state on all of the oxygens these were neutral I have 6 times minus 2 that's a minus 12 you can kind of view it as collective charge of all six of them six times minus 2 here I have 6 times minus 2 which is minus 12 and then I have 6 times 2 oxygens per molecule so that's 12 times minus 2 that's minus 24 so to go from a minus 12 to a total oxidation or kind of hypothetical charge of minus 36 I must have gained 24 electrons and those 24 electrons that I gained that the oxygens gained are the same 24 electrons that the carbons law so that so from the chemistry point of view it's very clear carbon was oxidized and oxygen oxygen which gained electrons right it's gaining these electrons right here rig reduction is gaining oxygen is reduced and this is all a bit of review but it's nice to see it in the context of cellular respiration and this actually kind of answers one of the questions of where does this energy come from where does this energy come from in any of these chemical reactions when you see energy being produced it's because electrons are going from a higher energy state to a lower energy State right if I have an electron that's up here at a high energy state I and it is able to go to a more comfortable state a lower orbital a lower energy orbital so low energy or more stable energy state it'll generate energy in the form of heat or maybe this can do some work in some way help make ATP molecules and so when you see these half reactions you see these 24 electrons that are being lost by carbon carbon is being oxidized and they're going to oxygen they're going in a whole series of steps it's not just happening in kind of one huge explosion it's happening over a huge series of steps and as it does that it's entering lower and lower energy states and as these electrons enter the lower energy states essentially by going from the carbons and being pushed to the oxygens that's where the energy is coming from that's where the energy to make the 38 ATP's is coming from so so far we you know we talked a little bit about how a chemist views oxidation I punched in the beginning of the video how a biologist views oxidation and then we saw that cellular respiration from a chemists point of view is clearly showing that the carbon is being oxidized it's losing electrons and that the oxygen is being reduced it's gaining electrons it's being reduced that the electrons are going from this carbon and they're going essentially to these oxygens right here now how does the biology definition of oxidation hold up well here it holds up pretty well because you can imagine over here all of the hydrogen's in the equation are associated with glucose right and so they're either bonded if you look at B if you actually look at the structure of glucose the hydrogen's are either bonded to carbons or oxygens right so these are bonded to carbons and oxygens when we go on the right-hand side of the equation all of the hydrogen's all of the hydrogen's are only bonding with oxygen so net net carbon definitely lost hydrogen's and hydrogen's and oxygen definitely gained hydrogen's let me write that down we see in respiration carbon lost hydrogen's carbon lost hydrogen's and oxygen gained hydrogen's oxygen gained hydrogen's and that's consistent because we see that by losing hydrogen's we are being oxidized from a biologist's point of view and by gaining hydrogen's oxygen is being reduced and just so you can kind of make sense of this when you see this in you know when I start drawing out the mechanisms which I will hopefully not make too hairy this process of transferring these hydrogen's is facilitated by molecules like nad plus and FA D and we'll see that but it's really the if we just want to reconcile the two notions as the hydrogen's are being transferred from one electro negative atom to another electronegative atom what's really being transferred is the opportunity to hog electrons if carbon has a hydrogen it gets to hog the electrons but then if that hydrogen goes from the carbon and the whole atom not just the nucleus the whole atom goes to the oxygen now the oxygen has gained that electron that it can hog and carbon has lost the electron so carbon has been oxidized and oxygen has been reduced and I mentioned this in previous videos but probably the most confusing thing about oxidation is that you always want to say I that must have something to do with oxygen and it does the word really comes from what wood oxygen do to something so oxygen when it bonds with things it loses it takes away their electrons or in a reaction it'll often take away the hydrogen's and took away the hydrogen's from the carbon in this situation so that's where the term oxidation comes from but you don't have to have oxygen anywhere in your reaction for oxidation or reduction to occur anyway hopefully you found that reasonably useful this was actually a huge pain point for me when you know I learned I got comfortable with the chemistry definition of oxidation reduction and then all of a sudden you open up your biology book and they start talking about losing and gating hydrogen's as opposed to electron then it was it took me a while to really reconcile these two notions
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