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Calculating ATP produced in cellular respiration

Video transcript

all right so if we were going to go on the ambitious task of telling up how much ATP was produced in one cycle of cellular respiration or just to be super clear here I mean how much each EP was produced per the oxidation or breakdown of one molecule of glucose in cellular respiration we might start off by just getting ourself organized and reminding ourselves that there are two kind of main ways that we produce ATP and cellular respiration so the first minor contribution comes from something called substrate level phosphorylation and remember that this is exactly what it sounds like we have a substrate or a molecule I'm just going to say R and remember that in the context of cellular respiration this is usually we think of this as a kind of metabolites and intermediate metabolites of glucose so somewhere along glucose is oxidation we get a metabolite and we activate this metabolite with a phosphate group and from this phosphate group we can actually donate it directly to ADP to produce ATP and of course our molecule also gets modified in the process usually gaining a hydroxyl group but the details aren't entirely important except to realize that this phosphorylation is occurring at the level of a substrate this is in contrast of course to oxidative phosphorylation which is where we get the bulk of our ATP and this oxidative refers to the fact that this process requires oxygen and in fact the importance of oxygen here is that this oxygen is reduced my electron carrier molecules and something called the electron transport chain so remember that we have electron carrier molecules called nadh and fadh2 that are produced at various stages of cellular respiration in glycolysis the oxidation of pyruvate the Krebs cycle and it's basically storing up all of that energy from the glucose molecule and it's going to donate it into with the electron transport chain and of course the final electron acceptor is oxygen which is then reduced to water but the important part here is that this flow of electrons is able to power something essentially fuel something called ATP synthase which is an enzyme that is in the mitochondrial membrane that produces the bulk of our ATP now the next point I want to make here is that it's actually been possible for us to calculate the exact number of ATP producing substrate level phosphorylation and we've also nailed down the amount of nadh and fadh2 molecules that are produced in this process as well but for quite a while it was difficult to nail down the exact number of ATP molecules that were produced in oxidative phosphorylation and for this reason actually and I'll get back to kind of why we're unable to you know kind of nails in a number here but for this reason you might often see quite a range of predictions for how much ATP is actually produced in one cycle of cellular respiration just to give you an idea that you know when I look at some textbooks you can see a range of anywhere from 30 to 38 molecules of ATP that are predicted to be produced for the oxidation of one molecule of glucose so of course to get back to this kind of elusive calculation of ATP researchers have done controlled studies in which they basically take a known amount of nadh or fadh2 and they have mitochondria available in the lab and they basically allow the mitochondria to oxidatively phosphorylate these molecules and essentially measure how much ATP is produced but kind of to their surprise at first they found that for NADH for one molecule of NADH they calculated there was not a whole number of ATP produced in fact they found that there was somewhere between two to three ATP molecules produced for every one NADH molecule and for fadh2 also found that there was no whole integer number of ATP but rather there was a range somewhere between 1 to 2 ATP produced now for the longest time researchers kind of looked at these results and said you know whole numbers are a lot easier to deal with and so why don't we just assume for the sake of assumption that we can kind of round up and we'll say that for every one molecule of NADH let's say that we have 3 molecules of ATP produced and for every molecule of fadh2 rather we have 2 molecules of ATP produced and so using these kind of estimations they calculated essentially the upper range of ATP so these calculations were ultimately used to calculate kind of this number of 38 ATP produced in cellular respiration kind of the upper limit of ATP produced but of course we still have this range and in fact it's worth kind of pausing to stop and think about for a second if it is surprising that we have this range in the first place and so to think about this a little bit further I want to go ahead and kind of just draw out without getting too detailed kind of a depiction of what's going on in the electron transport chain so remember that the electron transport chain is taking place in the mitochondria and the mitochondria has two membranes we have the inner mitochondrial membrane which I'm gonna label here as I and we have the outer mitochondrial membrane and along the inner mitochondrial membrane we have a series of proteins that are known as protein complexes and you know these all have specific names but just for our purposes it's important to recognize there are kind of just four main protein complexes and in some textbooks people will act really call ATP synthase which I'm going to go ahead and draw here in yellow as complex number five so let's let me go ahead and label these one through five just so we remember that so these four represent the protein complexes that shuttle electrons and of course five represents ATP synthase now recall that the basic premise here is that these reduced electron carriers donate electrons to the electron transport chain and in fact specifically NADH donates two electrons to protein complex number one and fadh2 donates two electrons to protein complex number two now the second important point is that as these electrons are kind of flowing down these proteins for every two electrons that kind of flow by it's actually been calculated that protein complex number one pumps four protons into the intermembrane space protein complex three pumps also four protons and protein complex number four pumps two protons and protein complex number two doesn't really contribute now with these facts in mind we can go ahead and actually calculate how many protons are pumped for a molecule of fadh2 and how many protons are pumped for a molecule of NADH so let's go ahead and just quickly do that here so because NADH donates at the very first electron complex it contributes to a total of 4 plus 4 plus 2 or 10 protons are pumped out for every molecule of NADH on the other hand fadh2 enters in complex number 2 so it would only contribute to the total pumping of 6 protons and so we can say that there are six protons that are pumped for every molecule of fadh2 alright so here's a good place to pause and kind of get back to our original question which was we have this kind of range of ATP molecules produced per molecule of nadh or fadh2 and why is it that you know we don't have a whole number integer ratio between the amount of ATP and NADH that we have and the reason kind of that we might be able to justify this looking at our diagram here is that nadh and fadh2 each contribute to this proton gradient but really it's not nadh or fadh2 that's directly donating anything to form ATP because remember that it's this proton gradient that forms in this inter membrane space here that essentially fuels as ATP synthase remember that protons flow back through this ATP synthase molecule and in doing so they essentially power this pump to phosphorylate adp into ATP and so and so of course maybe the question we should really be asking is how many protons does it take or how many protons seem to flow through this ATP synthase to phosphorylate one molecule of ADP into ATP and so I'm actually gonna go ahead back to our ratios up here and write out here that if we knew how many protons weren't necessary to produce one molecule of ATP we would be able to calculate essentially the ratio of ATP to nadh or fadh2 and it's this calculation that I think researchers are actually still trying to you know nail down and you know I'm sure depending on the type of cell and state of the cells the efficiency of this process is going to be different and might you know changed moment-to-moment and so maybe maybe you know the expectation to have an exact number is not realistic but researchers are pretty confident with the number right now currently of four protons being necessary to produce one molecule of ATP so I'm gonna go ahead and just write that in here and with this number we can actually go ahead and calculate the ratio of ATP to NADH and so simply we have here for every molecule of NADH we have 2.5 molecules of ATP and for every molecule of fhh we have 6 divided by 4 or 1.5 molecules of ATP that are produced so remember that even though it's kind of funky that we're talking about kind of 2 and 1/2 ATP per molecule of NADH or per molecule of fadh2 really what this is alluding to is the role of this chemiosmotic coupling or using this proton gradient to fuel ATP synthase and because we're talking about protons now we need to factor in that we end up getting these non whole number ratios between ATP and NADH or fadh2 but with these ratios in mind I actually want to go ahead and calculate kind of the sum total of ATP that we produce in cellular respiration so I've already gone ahead and kind of created a table here and remember that we're talking about one cycle of cellular respiration so I'll say total ATP yield let's say per one molecule of glucose remember and so I've already kind of written out here how many ATP and electron-carrier molecules are produced in glycolysis and the oxidation of pyruvate and the Krebs and TCA cycles well now let's go ahead and using our ratios here let's go ahead and write out how much ATP we have so two ATP's to ATP and two NADH using our conversion factors 2 times 2 point 5 which is going to be 5 ATP and then we have again 5 ATP and 2 ATP here for min substrate level phosphorylation is 2 ATP and 6 nadh times 2.5 is going to yield 15 and to fadh2 times 1.5 is going to yield 3 and so if we add all of this up we get 32 ATP now before I call it good I want to make one more last nitpicky point which is to realize that glycolysis remember takes place in the cytosol so unlike the oxidation of pyruvate in the Krebs cycle which take place in the mitochondria the NADH that's produced in the glycolysis which must actually be shuttled somehow into the inner mitochondrial membrane in order to donate its electrons into the electron transport chain but for some reason it turns out that the inner mitochondrial membrane is actually not permeable to this molecule NADH so the bodies actually come up with something called shuttle transport systems to shuttle this NADH into the mitochondria and it turns out that depending on where the NADH is shuttled into the electron transfer chain so if we actually go back to our diagram here some of the electrons from the NADH produced in glycolysis can be shuttled into the first electron first protein complex and some of them are actually shuttled into this third protein complex here and so depending on whether it's you know shuttled earlier later on in the electron transport chain a different number of protons will be pumped into the proton gradient remember and so the conversion factor for the amount of ATP produced is going to be different depending on which shuttle is used so I just want to make that point and have you be aware of the fact that this number right here this number here is actually a range I can actually range from anywhere to 3 to 5 ATP produced per molecule of NADH I'm going to go ahead and kind of just adjust this to say that really the range here is 32 32 ATP produced per one cycle of cellular respiration and this right here is the generally accepted number for the amount of ATP produced in one cycle of cellular respiration