Electron Transport/Oxidative Phosphorylation
Mitocohondria are called the power plants of the cell because most of a cell’s ATP is produced there, in a process referred to as oxidative phosphorylation. The mechanism by which ATP is made in oxidative phosphorylation is one of the most interesting processes in all of biology. It has three primary considerations. The first is electrical – electrons from reduced energy carriers, such as NADH and FADH2, enter an electron transport system via protein complexes containing iron. As seen in the figure on the following page,electrons move from one complex to the next, not unlike the way they might move through an electrical circuit.
The next consideration arises as a secondary phenomenon. When electrons pass through complexes I, III, and IV, protons are moved from the mitochondrial matrix (inside of mitochondrion) and deposited in the intermembrane space (between the inner and outer membranes of the mitochondrion). The effect of this redistribution is to increase the electrical and chemical potential across the membrane. Students may think of the process as “charging the battery.”
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Just like a charged battery, the potential arising from the proton differential across the membrane can be used to do things. This is the third consideration. In the mitochondrion, the “thing” that the proton gradient does is create ATP from ADP and Pi (inorganic phosphate). This process requires energy and is accomplished by movement of protons through a protein complex in the inner mitochondrial membrane. The protein complex is an enzyme that has several names, including Complex V, PTAS (Proton Translocating ATP Synthase), and ATP Synthase. Central to its function is the movement of protons through it (from outside back into the matrix). Protons will only move through ATP Synthase if their concentration is greater outside the inner membrane than in the matrix.
In summary, the electron transport system charges the battery for oxidative phosphorylation by pumping protons out of the mitochondrion. The intact inner membrane of the mitochondrion keeps the protons out, except for those that re-enter through ATP Synthase. The ATP Synthase allows protons to re-enter the mitochondrial matrix and harvests their energy to make ATP.
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