Metabolic respiration consists of three main processes. The final of these processes is the oxidative phosphorylation. There are two steps to this final stage of respiration, the oxidation of NADH and FADH2 and the process of phosphorylation.
Oxidation of NADH and FADH2
During the citric acid cycle, the net result is 3 NADH and 1 FADH2 molecules. These are used to transport high energy molecules to the electron transport chain. The NADH transfers its high energy molecules to protein complex I and the FADH2 transfers its high energy molecules to protein complex II. During this transfer, there is a loss of electrons through a process of oxidation.
When NADH is being oxidized, the protons are pumped through protein complex I into the inner membrane of the mitochondria. The electrons that were created are moved to an electron carrier called ubiquinone, or Q. The process with the protons is repeated, and this creates a buildup of protons in the inner membrane. This is how energy is stored in the cells.
The high energy electrons from the FADH2 do not follow the same route as the ones from NADH. Instead, they are transferred directly to Q and form QH2.
The next step involves transporting the electrons to protein complex 3. This occurs in the region UQH2. This is a non-polar region found in the phospholipid bilayer. The UQH2 delivers the electrons and also brings protons for pumping.
The cytochrome takes the electrons to protein complex 4. The energy released as result of this is used to move another proton to the inner membrane. The electrons seek out an electron receptor. They find the final electron receptor, oxygen, and water is formed. The water combines with the protons inside the cell.
Now that NADH and the FADH2 have gone through oxidation, the cell has a difference in the electrical charge of the two sides of the inner mitochondrial membrane. The outside of the membrane carries a positive charge due to the protons that have been pumped through it. The inner membrane has a negative charge due to the loss of protons. This creates a chemical concentration gradient that is used to store energy.
The high concentration of protons on the outer mitochondrial wall are pushed through ATP synthase. This causes the ATP synthase to spin. The spinning ATP synthase acts as a catalyst that allows for a phosphate to be added to ADP, creating ATP. Remember that the purpose of cellular respiration is to produce ATP, the cell's energy currency. There is a net gain of 2 ATP each for the other two stages of respiration, but oxidative phosphorylation alone produces 34!