The final stage!!!!!
In the electron transport chain, molecular oxygen is reduced to water at the end and electrons from NADH or FADH2 passes through the system via carrier molecules. In the electron transport chain another reaction is taking place simultaneously and that is the formation of ATP from ADP.
In the inner membrane of the mitochondrion there are large protein complexes that act as electron carriers. They are : flavoproteins, quinones and cytochromes. In the membrane they are arranged in order of their increasing electron affinity, which means their attraction to the electrons.
In the beginning of the chain NADH will lose electrons, and pass them to the first carrier and then return to its normal oxidised state. which is NAD+
NADH + H+ ———-> NAD+ + 2H+ + 2e-
When received in carrier 1 it passes to carrier 2 going down the chain. When the electrons reach to the end of the chain accepted by molecular oxygen along with the protons that were produced by the oxidation of the NADH in the beginning of the chain. It occurs at the end of the chain, an enzyme called cytochrome oxidase.
During this process electrons are being passed along the chain, they lose energy in this way, the energy that they lose is actually used to pump protons through the inner membrane of the mitochondrion to the space between the two membranes of the mitochindrion. The electron carriers molecules can also act as pumps. This results in a proton concentration gradient, that is set up across the membrane. This acts as a storage of energy and it could be released if the protons wanted to pass back. However there are other protein channels that allow this ‘pass’ to occur, they have the enzyme ATP synthase and when the protons re-enter through the protein channel the energy is used to synthesis ATP from ADP and inorganic phosphate, catalyzed by the ATP synthase.