Electron transport system and oxidative phosphorylation
Calculating ATP produced in cellular respiration
- [Instructor] Alright, so, if we were gonna go on the ambitious task of tallying up how much ATP was produced in one cycle of cellular respiration or, just to be super clear here, I mean how much ATP was produced per the oxidation or breakdown of one molecule of glucose in cellular respiration? We might start off by just getting ourselves organized and reminding ourselves that there are two kind of main ways that we produce ATP in 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 gonna say R. And remember that in the context of cellular respiration, this is usually, we think of this as a kind of metabolite, an intermediate metabolite 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 hydroxy 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 by electron carrier molecules and something called the electron transport chain so, remember that we have a electron carrier molecules called NADH and FADH two that are produced at various stages of cellular respiration, 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 gonna donate it into the electron transport chain, and of course the final electron acceptor is oxygen, which is then reduced to water. But the important 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 produced in substrate level phosphorylation and we've also nailed down the amount of NADH and FADH two molecules that are produced in this process as well. But for a 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 nail down a number here but for this reason, you might often see quite a range of predictions for how much ATP's actually produced in one cycle of cellular respiration, just to give you an idea of 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 FADH two 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 FADH, they also found that there was no whole integer number of ATP but rather, there was a range, somewhere between one to two 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 three molecules of ATP produced. "And for every molecule of FADH two, rather, "we have two 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 wanna 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, general label here is 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 actually call ATP synthase, which I'm gonna go ahead and draw here in yellow as complex number five, so 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 FADH two 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, it 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 FADH two 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 four plus four plus two, or ten protons are pumped out for every molecule of NADH. On the other hand, FADH two enters in complex number two, so it only contributes to the total pumping of six protons and so, we can say that there are six protons that are pumped for every molecule of FADH two. Alright, so, here's a good place to pause and kind of get to back to our original question, which was we have this kind of range of ATP molecules produced per molecule of NADH or FADH two 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 FADH two each contribute to this proton gradient but really, it's not NADH or FADH two that's directly donating anything to form ATP because remember, that it's this proton gradient that forms this intermembrane space here that essentially fuels this 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, of course, maybe the question we should really be asking is how many protons does it take, or how many protons need 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 up here that if we knew how many protons were necessary to produce one molecule of ATP, we would be able to calculate essentially the ratio of ATP to NADH or FADH two. 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 the state of the cells, the efficiency of this process is going to be different and might, you know, change moment to moment and so, maybe 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 two point five molecules of ATP. And for every molecule of FADH, we have six divided by four, where one point five molecules of ATP that are produced. So, remember, that even though it's kind of funky that we're talking about kind of two and a half ATP per molecule of NADH or per molecule of FADH two, really, what this is alluding to is the role of this chemi-osmotic coupling, or using the proton gradient to fuel to 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 FADH two. But with these ratios in mind, I actually wanna go ahead and calculate kinda the sum total of ATP that we produce in cellular respiration, so I've already gone ahead and kinda created a table here, and remember that we're talking about one cycle of cellular respiration, so, as a 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 is two ATP and two NADH, using our conversion factors, two times two point five, which is going to be five ATP. And then we have again, five ATP and two ATP here, forming substrate level phosphorylation is two ATP. And six NADH times two point five is going to yield 15. And two FADH two times one point five is going to yield three. And so, if we add all of this up, we get 32 ATP. Now, before I call it good, I wanna make one more last nitpicky point which is to realize that glycolysis, remember, takes place in the cytosol, so unlike the oxidation of pyruvate and the Krebs cycle, which take place in the mitochondria, the NADH that's produced in the glycolysis 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 body has 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 transport 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 gonna be different depending on which shuttle is used. So, I just wanna make that point and have you be aware of the fact that this number right here, this number here is actually a range, it can actually range from anywhere to three to five ATP produced per molecule of NADH. I'm gonna go ahead and kinda just adjust this to say that really the range here is 30 to 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.