The electron transport chain (ETC) takes place in the inner mitochondrial membrane. During this process, the electrons donated by the electron carriers (such as NADH) pass through a chain of protein complexes embedded in the inner mitochondrial membrane. These electron carrieres were reduced during processes occuring before the ETC, for instance glycolysis and citric acid cycle.
(the ETC takes place in the inner membrane)
The movement of electrons initiates pumping of protons to the intermembrane space. This way, a proton gradient is created, and thus, the protons from the intermembrane space will flow back to the matrix through the ATP synthase, which is embedded in the inner mitochondrial membrane. When protons flow through the ATP synthase, ATP is made.
The image above gives us a pretty detailed picture of the process. Let's now examine it:
As you can see, there are four protein complexes embedded in the inner mitochondrial membrane. The ATP synthase, which you can see on the right side of the image, isn't considered a part of the ETC. Let's look what happens in each complex:
(PLEASE LOOK AT THE PICTURE ABOVE FOR REFERENCE. I ALSO INVITE YOU TO WATCH MY VIDEO ABOUT THIS TOPIC HERE)
Complex I: The NADH donates its two electrons to complex I. These electrons are first accepted by FMN, and then flow to the iron-sulfur center, which is also an electron acceptor. Next, these two electrons are transferred to a mobile protein called ubiquinone (or coenzyme Q). Coenzyme Q then moves through the phospolipid bilayer and donates these two electrons to complex III (yes, to complex III, not to complex II). This leads to pumping of four protons (4 H+) to the intermembrane space.
But let's hold on for a second and see what happens in complex II:
Complex II: This is where succinate comes in and reduces FAD to form FADH2. This way, it donates two electrons. These two electrons are going to be transferred to the iron-sulfur center and later to ubiquinone.
Complex III: The ubiquinone reduces the cytochrome b by giving away the two electrons. Then, cytochrome b is oxidized and the electrons travel to the iron-sulfur center. From there, they are passed to the cytochrome c1. Next, the cytochrome c1 captures these two electrons. Since cytochrome c1 can move from complex III to complex IV, it moves and thus carries these electrons to complex IV. When electrons are transferred from complex III to cytochrome c, four protons are also pumped from the matrix to the intermembrane space.
Complex IV: First of all, after cytochrome c has arrived, it gives the two electrons that it carries. These electrons are given to reduce cytochrome a. Then, they are passed to cytochrome a3, and later, to oxygen, the final electron acceptor. When oxygen accepts electrons (and two protons from the matrix), water is formed. When the electrons are transferred in complex IV, only two protons are pumped into the intermembrane space.
ATP synthesis:
(the awesome picture below, which I found online, for educational purposes only, illustrates the process)
The movement of electrons provided energy for pumping protons to the intermembrane space. Next, these protons are going to flow through the ATP synthase back to the matrix. As they flow through ATP synthase, they initiate movement of the top part of the ATP synthase (the one with C10 subunits). This again makes the gamma subunit spin. Since the lowest part of the ATP synthase doesn't move, the spinning of gamma subunits causes conformational changes in the beta subunits. These beta subunits are the subunits that synthesize ATP out of ADP and Pi, which are already present in the matrix.
(the ETC takes place in the inner membrane)The movement of electrons initiates pumping of protons to the intermembrane space. This way, a proton gradient is created, and thus, the protons from the intermembrane space will flow back to the matrix through the ATP synthase, which is embedded in the inner mitochondrial membrane. When protons flow through the ATP synthase, ATP is made.
The image above gives us a pretty detailed picture of the process. Let's now examine it:
As you can see, there are four protein complexes embedded in the inner mitochondrial membrane. The ATP synthase, which you can see on the right side of the image, isn't considered a part of the ETC. Let's look what happens in each complex:
(PLEASE LOOK AT THE PICTURE ABOVE FOR REFERENCE. I ALSO INVITE YOU TO WATCH MY VIDEO ABOUT THIS TOPIC HERE)
Complex I: The NADH donates its two electrons to complex I. These electrons are first accepted by FMN, and then flow to the iron-sulfur center, which is also an electron acceptor. Next, these two electrons are transferred to a mobile protein called ubiquinone (or coenzyme Q). Coenzyme Q then moves through the phospolipid bilayer and donates these two electrons to complex III (yes, to complex III, not to complex II). This leads to pumping of four protons (4 H+) to the intermembrane space.
But let's hold on for a second and see what happens in complex II:
Complex II: This is where succinate comes in and reduces FAD to form FADH2. This way, it donates two electrons. These two electrons are going to be transferred to the iron-sulfur center and later to ubiquinone.
Complex III: The ubiquinone reduces the cytochrome b by giving away the two electrons. Then, cytochrome b is oxidized and the electrons travel to the iron-sulfur center. From there, they are passed to the cytochrome c1. Next, the cytochrome c1 captures these two electrons. Since cytochrome c1 can move from complex III to complex IV, it moves and thus carries these electrons to complex IV. When electrons are transferred from complex III to cytochrome c, four protons are also pumped from the matrix to the intermembrane space.
Complex IV: First of all, after cytochrome c has arrived, it gives the two electrons that it carries. These electrons are given to reduce cytochrome a. Then, they are passed to cytochrome a3, and later, to oxygen, the final electron acceptor. When oxygen accepts electrons (and two protons from the matrix), water is formed. When the electrons are transferred in complex IV, only two protons are pumped into the intermembrane space.
ATP synthesis:
(the awesome picture below, which I found online, for educational purposes only, illustrates the process)
