Transcription
Oh, hello friends. In this video, we'll be discussing about electron transport chain and oxidative phosphorylation.
First of all, let's recap the things we have been discussing about cellular respiration in the previous videos. We discussed about glycolysis and the citric acid cycle. We saw the oxidation of glucose molecule to pyruvate constitutes as the glycolysis, and in that, we also get a reduction of NAD+ molecule into NADH. Although ATPs are also getting produced here, but we'll be more focused towards electron carrying molecules, NADH and FADH2, because this is what the ETC is all about.
Then, pyruvate is first converted into acetyl-CoA, which is further oxidized in cyclic operations called the citric acid cycle or simply Krebs cycle. So, eventually, what we get from these biochemical reactions besides the production of ATP molecules, we also get a reduction of NAD+ and FADH2. In glycolysis, the NAD+ is reduced to NADH, thus gaining the electrons here. And furthermore, in the conversion process of pyruvate to acetyl-CoA, we get a reduction of NAD+ to NADH also. And finally, in the citric acid cycle or Krebs cycle, NAD+ and FAD are reduced to NADH and FADH2 respectively. So, you can see in the end, we get these two electron carrying molecules in the name of NADH and FADH2, and are called electron carriers.
And these electron carriers are subjected to oxidation in the electron transport chain. Well, these are oxidized back to its original form, that's NAD+ and FAD. They lose their high-energy electrons, and energy in these electrons is used to pump the protons from the matrix to the intermembrane space. When electrons transit from one complex molecule to the other in the electron transport chain of the inner mitochondrial membrane, they lose their energy, and this energy is used to pump the protons into the intermembrane space. So, this is how the electron transport chain works in the mitochondria.
Now, let's see in detail how electron transport chain drives in the inner mitochondrial membrane. First of all, we will see the electron transport chain for NADH electrons. How the electrons from NADH are drawn into the electron transport chain. You know, the electron transport chain drives in the inner mitochondrial membrane, with which we get matrix side and intermembrane space as shown in the diagram. And for NADH electrons to be transported into the chain, we have four protein complex molecules present in the membrane. First, we have NADH dehydrogenase, also called as complex I molecule. Then, we have cytochrome c reductase, also called as complex III molecule. After that, we have cytochrome c oxidase, also called as complex IV molecule. Here, in this case, complex II is missing because this complex II is not used in NADH electrons, rather it is used for FADH2 electrons. That's why complex II is not present here.
And not only these three molecules are present for NADH electrons to be carried into the chain, but here also we have a mobile electron carrier protein, which is known by the name of cytochrome c complex, which transports one electron at a time from complex III to complex IV, as shown in the diagram. It takes the electrons from complex III and delivers it to the complex IV. Now, what's the work to be done by the electrons that will be delivered into this transport process? So, we know in the matrix, we have a high concentration of protons, and these protons need to be transported out of the matrix to create an electrochemical proton gradient, so that we can produce the ATPs from proton motive force.
So, let's start the electron transport chain. Now, here, first of all, NADH is oxidized back to NAD+ and H+ with the help of complex I, that's NADH reductase enzyme. A total of two electrons are lost by NADH molecule, and with which flavin mononucleotide is first to reduce it before gives electron to the iron-sulfur clusters, and finally, the two electrons are received by ubiquinone molecule, which is further reduced to ubiquinol. And in between these processes, four protons are pumped into the intermembrane space, as shown in the diagrams. And after that, ubiquinol loses these accepted electrons, and in that process, additional two protons are pumped into the intermembrane space. And finally, the ubiquinol is again oxidized back into the ubiquinone, and the two electrons are accepted by the iron-sulfur cluster first, with which another two protons are pumped into the intermembrane space. That's a total of four protons pumped at complex III site.
Now, from here, two electrons which are with the Fe-S protein are first accepted by cytochrome b, and from here, cytochrome c complex, a mobile carrier, accepts a single electron at a time and carries it with a complex IV. So, it must be kept in mind that two electrons are not carried simultaneously by cytochrome c complex, rather only a single electron is carried. And after delivery of one electron to the complex IV, it jumps back to the complex III and carries another electron. Then, within this complex IV, the electrons are transferred again one at a time, first to a pair of copper ions called CuA, then to cytochrome a and other cytochrome molecules, and finally, the electrons are accepted by oxygen molecule, the ultimate electron acceptor, yielding us water molecule in the end. And in that last phase, two protons are pumped into the intermembrane space. So, this completes our electron transport chain for the NADH electrons.
So, looking at the diagram, we can see a total of four protons pumped at complex I site, then four protons at complex III site, and finally, the last two protons are pumped at complex IV site. So, we can say from one NADH molecule to the ETC, or we can say electron transport chain, a total of ten protons into the intermembrane space.
Now, let's see the electron transport chain for the electrons of FADH2 molecules. In this process of electron transport chain, the complex I is skipped, and here we have complex II, succinate dehydrogenase, in action. First of all, we see succinate dehydrogenase enzyme oxidizes a molecule of succinate to fumarate, and remember this step is from the citric acid cycle. So, why we have shown it here? It is because the succinate dehydrogenase enzyme is an integral component of succinate dehydrogenase complex, that's why I have shown it here. The two electrons released in the conversion of succinate to fumarate in the citric acid cycle are transported first to FAD, which gets converted into FADH2. Then, this FADH2 molecule loses two electrons, then iron-sulfur cluster, and finally to CoQ, which gets reduced to ubiquinol. And in that process, four protons are pumped into the intermembrane space from the matrix, as you can see in this diagram. And eventually, we also get CoQ back into the Q cycle.
From here, all the reactions are same as we have seen for NADH electrons. So, we know that complex I is not available in action here, or we can say complex I molecule is not getting involved here. You know, in ETC of NADH molecule, it pumps four protons, so that means four protons will not be pumped here for FADH2 electrons because it's missing here. So, a total of six protons are getting pumped by FADH2 oxidation: four at complex III and two at complex IV. So, this concludes our ETC for both NADH and FADH2 molecules.
Now, at the end, if we calculate how many protons are getting pumped by the electron transport chain, we see one NADH pumps ten protons to the intermembrane space, while as FADH2 molecule pumps six protons, only a short of four protons from the NADH. So, we need an electrochemical proton gradient across the membrane, and now we have it. So, the next process will be the oxidative phosphorylation, while we will briefly see how protons will generate the ATP production.
In the oxidative phosphorylation, we see the involvement of another important complex molecule known by the name of ATP synthase, or complex V. Considering the proton gradient across the inner mitochondrial membrane, the ATP synthase molecule has got an ion channel present in it, and it's through this ion channel the protons flow back to the matrix. That's the chemiosmosis movement of ions across a semipermeable membrane down their electrochemical gradient, and the rotation of F1 subunit driven by the proton movement through F0 powers ATP synthesis. So, this is how the oxidative phosphorylation is coupled with the electron transport chain.
Now, a thing to remember: one ATP molecule is getting generated by the flow of four protons through the ATP synthase from the intermembrane space to the matrix. So, if we recall, one NADH pumps ten protons to the intermembrane space, and when these ten protons will flow back to the matrix, that time this will be equal to 2.5 ATPs from oxidative phosphorylation, as four protons gives us one ATP. And for FADH2 molecule, which pumps six protons, that equals to 1.5 ATPs from oxidative phosphorylation. So, this is how the NADH and FADH2 differ in the ATP production.
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