Oxidative phosphorylation:

Oxidative phosphorylation is the name given to the synthesis of ATP (phosphorylation) which occurs when NADH and FADH₂ are oxidized (hence oxidative) through electron transport by the respiratory chain. Dissimilarly substrate level phosphorylation, it does not included phosphorylated chemical intermediates. Rather, an extremely hard mechanism   was proposed   via Peter Mitchell in the year of 1961 which is known as the chemiosmotic hypothesis.  This proposes which energy liberated through electron transport is used to build a proton gradient across the mitochondrial inner membrane and which it is this that is used to drive ATP synthesis. Therefore the proton gradient couples electron transport and ATP synthesis, not a chemical intermediate.  The  confirmation  is  overwhelming  in which  this  is  indeed  the way  which  oxidative  phosphorylation   works.  The real synthesis of ATP is carried out through an enzyme known as ATP synthase situated in the inner mitochondrial membrane shown in the figure.

In short, the process is as follows. The Electron transport down the respiratory chain from NADH oxidation causes H⁺ ions to be pumped out of the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space through the three H⁺ pumps; Complex I, III or IV. [Since FADH is reoxidized through ubiquinone its oxidation causes H⁺ ions to be pumped out only via Complex III or IV and so the amount of ATP formed from FADH₂ is less than from NADH.] The free energy modify in transporting an electrically charged ion across a membrane is associate both to its electrical charge and the concentration of the species.  The pumping  out of the H⁺ ions produces  a higher  concentration  of H⁺ions  in the intermembrane  space  and an electrical potential and with the side of the inner mitochondrial  membrane  facing the inter- membrane space being positive. Therefore, whole, an electrochemical proton

Figure:   The mechanism of oxidative phosphorylation.

gradient is created. The protons flow reverse into the mitochondrial matrix by the ATP synthase and this drives ATP synthesis. The ATP synthase is driven through proton-motive  force,  that  is  the  sum  of  the  pH  gradient example for  the  chemical gradient of H⁺ ions and the membrane potential and example for the electrical charge potential across  the inner mitochondrial  membrane.  There are several debates over the exact stoichiometry of ATP production; in past years it was believed in which 3 ATPs were produces per NADH and 2 ATPs per FADH₂ but some recent measurements have denotes in which the numbers of ATP molecules produced may be 1.5and 2.5, correspondingly.