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Overview of the Electron Transport Chain. Created by Sal Khan.
Video transcript
After being done with glycolysis and the Krebs Cycle we're left with 10 NADHs and 2 FADH2s. And I told you that these are going to be used in the electron transport chain and they're all sitting in the matrix of our mitochondria and I said that they're going to be used in the electron transport chain in order to actually generate ATP. So that's what I'm going to focus on in this video: the electron transport chain. And just so you know, a lot of this stuff is known, but some of the details are actually current areas of research. People have models and they're trying to substantiate the models, but things are happening at such a small scale here, that people can just look at the evidence, some of which is indirect, and say this is probably what's happening. Most of this is very well established, but some of the exact mechanisms, for example, how exactly some of the proteins work, aren't completely understood. So I think it's very important for you to understand that this is kind of at the cutting edge. You're already there. So the basic idea here is that the NADHs, and that's where I'll focus, FADH2 is kind of the same idea, although its electrons are at a slightly lower energy state, so they won't produce quite as many ATPs. Each NADH, maybe I'll write it here, each NADH is going to be, as you'll see, indirectly responsible for the production of three ATPs, and each FADH2, in a very efficient cell, in both of these cases, will be indirectly responsible for the production of 2 ATPs. And the reason why this guy produces fewer ATPs is because the electrons that he has going into the electron transport chain are at a slightly lower energy level than the ones from NADH. So in general, I just said indirectly, how does this whole business work? Well in general, NADH, when it gets oxidized... So, NADH... remember oxidation is the losing of electrons or the losing of hydrogens that happen to have electrons, we can write its half reaction like this, its oxidation reaction like this. You'll have some NAD+, which you can then go and use back in the Krebs cycle and in glycolysis you have some NAD+, you'll have a proton, right? A positive hydrogen ion is just a proton, and then you'll have two electrons. This is the oxidation of NADH. It's losing these two electrons Oxidation is losing electrons. OIL RIG Oxidation is losing electrons or you can imagine it's losing hydrogens from which it can hog electrons Either one of those is the case. Now, this is really the first step of the electron transport chain. These electrons are transported out of the NADH Now, the last step of the Electron Transport Chain is you have two electrons two electrons plus two hydrogen protons and obviously if you just add these two together you're gonna have two hydrogen atoms which is just a proton and an electron plus one oxygen atom, so I could say one half of molecular oxygen that's the same thing as saying one oxygen atom, and you're going to produce, If I have one oxygen and two complete hydrogens I'm left with water, and you can view this, we're adding electrons or we're gaining electrons to oxygen. OIL RIG - Reduction Is Gaining electrons So, this is the reduction of oxygen to water This is the oxidation of NADH to NAD+. Now, these electrons that are popping out of, these electrons right here, that are popping out of this NADH, when they're in NADH, they're in a very high energy state, and what happens over the course of the electron transport chain is that these electrons get transported to a series of transition molecules. But these transition molecules, as the electrons go from one to the other, they go into slightly lower energy states and I won't even go into the details of these molecules... One is coenzyme Q, and cytochrome C, and then they eventually end up right here and they're used to reduce your oxygen into water. Now, every time an electron goes from a higher energy state to a lower energy state, and that's what it's doing over the course of this electron transport chain, it's releasing energy. Energy is released from when you go from a higher state to a lower state. When these electrons were in NADH, they were in a higher state than they are when they bond to coenzyme Q. So they release energy, and then they go to cytochrome C to release energy. Now that energy is used to pump protons across the cristae, across the inner membrane of the mitochondria, I know this is all very complicated sounding and you know, this is the cutting edge so it maybe should sound a little complicated. So, let me draw a small mitochondria just so you know where we're operating, that's its outer membrane, and then its inner membrane, or its crista, will look like that. Now let me zoom in on the membrane. So let's say if I were to zoom in right there, that box will look like this: You have your cristae here, I'm going to draw it thick, color it in with green just like that. And then you have your outer membrane and this outer membrane, I could do it up here, just color it in. You don't even have to see the outside of the outer membrane. Right here, this base right here, this is the outer compartment and then we learned in the last video, this base right here is the matrix. This is where our Krebs cycle occurred and where a lot of our NADH or really all of our NADH is sitting. So what happens is everytime NADH gets oxidized to NAD+, and the electrons keep transferring from one molecule to another, it's occurring in these big protein complexes and I'm not going into details on this, so each of these protein complexes span where this first oxidation reaction is occurring and releasing energy, and then say there's another protein complex here where the second oxidation reaction is occurring and releasing energy. And these proteins are able to use that energy to essentially pump hydrogens into the outer membrane. It actually pumps hydrogen protons into the outer membrane, and every one of these reactions pump out a certain number of hydrogen protons. So by the end of the electron transport chain, if we just followed one set of electrons, by the time that they have gone from their high energy state in NADH to their lower energy state in water, by the time they have done that, they have supplied the energy to these protein complexes that span our cristae, to pump hydrogen from the matrix into the outer membrane. So, really the only byproduct of the oxidation of NADH into eventually water, or the oxidation of NADH and the reduction of oxygen into water, isn't ATPs yet. It's just this gradient where we have a lot higher hydrogen proton concentration in the outer compartment than we do in the matrix. And then the outer compartment becomes a lot more acidic. Remember acidity is just hydrogen proton concentration. So the byproduct of all of this energy is really used to just move these protons into the outer membrane. The outer membrane becomes more acidic than the matrix inside, so we call that basic.