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- [Voiceover] All right, cellular respiration includes the metabolic pathways of glycolysis, the Krebs cycle, and the electron transport chain, as represented in the figures. So we have the figures here of, glycolysis, the Krebs cycle, and the electron transport chain. If all of this looks completely foreign to you, I encourage you to watch the videos on Khan Academy on glycolysis, the Krebs cycle, and the electron transport chain, and on cellular respiration in general. All right, but let's tackle this problem. It's a nice review of all of those things. In cellular respiration, carbohydrates and other metabolites are oxidized, meaning electrons are taken away from them, and the resulting energy-transfer reactions support the synthesis of ATP. All right, using the information above, describe one contribution of each of the following in ATP synthesis. All right, the first one. Catabolism of glucose in glycolysis and pyruvate oxidation. The second was oxidation of intermediates in the Krebs cycle. And the third one is formation of a proton gradient by the electron transport chain. Now, each of these statements seem kind of intimidating, but they're really just saying, describe how glycolysis and pyruvate oxidation contribute to ATP synthesis. Describe how the Krebs cycle, because the Krebs cycle is essentially nothing but the oxidation of these intermediates to produce things that are useful for ATP synthesis. And then the formation of the proton gradient by the electron transport chain. Well that's what the electron trans-prit (laughs), that's what the electron transport chain does. It takes high energy electrons from NADH or FADH2, and then as those high energy electrons go to lower and lower energy states, it's pumping these hydrogen protons across the membrane and when they come back in, that's used to synthesize ATP. So let's just answer. We have to describe one contribution of each. So, let's first focus on glycolysis. So if we look here, there's more than one contribution. You see that it can phosphorylate these two ADPs to two ATPs. So that's one contribution we could list. We could say that it's producing these NADHs. It's producing these NADHs, which can provide both the hydrogen proton and, more importantly, the high-energy electrons for the electron transport chain later on. You see that right over here. Or you could say that, well, look, it's producing the Acetyl-CoA, which can enter the Krebs cycle, which is used to produce GTP or more NADHs or FADH2. So all of these are contributions, and since we only have to list one, I'll list the most direct and obvious one, although you could list any of these, is, I will list, so this is one contribution. I'll say the phosphorylation, phosphorylation of two ADPs, two ADPs, to two ATPs, to two ATPs. And I could list several more, but that's a good one right over there, we just have to have one contribution. All right, oxidation of intermediates in the Krebs cycle. So, the oxidation of the intermediates, that's just talking about each of these things keep getting oxidized, and as they get oxidized, we can use that to reduce other things, including NAD plus to NADH. So, when you reduce it, it's gaining electrons. Notice, NAD plus is positive and then it becomes NADH, which is neutral. And these NADHs are used later on in the electron transport chain to pump hydrogen protons across the membrane which are then used in oxidative phosphorylation to produce the ATP as they go back through the membrane. So, you could talk about the NADHs or FADH2s, or you could talk about the direct creation of GTPs which could be used to create ATPs. So any of that is fine. So I'll just list one of them. So, I'll write reduction of NAD plus to NAD, NADH, which is used in electron transport chain, in electron transport chain to pump hydrogen protons to get, to create proton gradient, to create proton gradient, gradient. And once again, I could talk about the GDP, the GT (laughs), the GTPs being created. I could talk about the FADH2 being created. Formation of a proton gradient by the electron transport chain. Well, as the protons flow with the gradient back into, back across the membrane, they power ATP synthase which creates ATP from ADP. So, as protons flow across membrane, flow with gradient across membrane, flow with gradient across membrane, and if you're actually taking the AP test, you might want to just do it below, so you might have more space, but I'll just fill it in here. As protons flow with gradient across membrane, they drive, they drive ATP synthase, ATP synthase, which, which takes, we can say oxidatively phosphorylates, so I'll just write phosphorylates, phosphorylates ADP, ADP to ATP. And once again, if everything I'm saying here sounds foreign, and if these diagrams don't make a lot of sense to you. If they don't trigger a pleasurable memory in your head (laughs), I encourage you to watch the videos (laughs) on Khan Academy, where hopefully you'll get a little bit more intuition for the things that I am talking about. All right, now let's see if we can tackle part b. Use each of the following observations to justify the claim that glycolysis first occurred in a common ancestor of all living organisms. So, nearly all existing organisms perform glycolysis. So, it's much more likely, so much more likely that this, in order for this to happen, maybe I'll write this. In order, so for this one right over here, in order for this to be the case, to be the case, it is much more likely, much more likely that this evolved in a common ancestor, in common ancestor and was passed down, and was passed down, or you could say selected for, even, and selected for, selected for, rather than independently coming about multiple times, than independently, independently arising in multiple branches of the evolutionary change, than independently arising in multiple branches, branches, of tree of life. Or let me say, multiple branches of evolutionary tree. Evolutionary, evolutionary tree. The fact that all, nearly, or they're saying nearly all, nearly all existing organisms perform glycolysis. In theory, it could have happened independently, just because it gets selected so for strongly, but if something happens in nearly all organisms, it's very likely that it evolved at a very, very early stage, at a kind of a primitive ancestor organism, and that was selected for, and it continues to be selected for and that's why we continue to see it. All right, glycolysis occurs under anaerobic conditions. So actually, let me create some space here so things don't get too jumbled up. So glycolysis occurs under anaerobic conditions. Well, early Earth, early Earth had little, had little oxygen little oxygen in atmosphere. Early life. So, early life, had to perform, had to, I guess you could say, produce ATP, or you could, early life had to metabolize, tab-ol-ize sugars or carbohydrates in anaerobic environment. In anaerobic, so an environment that doesn't have oxygen. Anaerobic, anareobic environment. So that seems to, once again, justify the claim that glycolysis first occurred in a common ancestor of all living organisms. Glycolysis only occurs in cytosol. So, so most, so, earliest, earliest life didn't have membrane-bound organelles, membrane-bound organelles, so the fact that it only occurs in cytosol, so fact that occurs in cytosol consistent with it be, that it's possible that early life did it. Is consistent that earl, with early life with it first occurring, occurring in common ancestor. First, with it first occurring in common, early ancestor. I'll then tackle parts c and d in the next video.