The system through which electrons are transferred from one place to another and ATP and H2O are produced during the flow of electrons is called the electron transport system. It is also called respiratory chain. Energy is released at specific locations as electrons flow in this process. This energy combines ADP and Pi to form ATP. Thus the production of ATP with the help of the energy released in the oxidation process is called oxidative phosphorylation.
ADP + Pi → ATP
Several electron transport proteins combine to form multi-proteins. The inner membrane of mitochondria contains four multi-proteins (in order of strength) and transfer electrons. In this process NADH2, NADPH2, FADH2 and FADPH2 are oxidized to NAD, NADP, FAD and FADP and release energy. Under the influence of this energy, ATP is produced by combining ADP and Pi. Normally 2 of each FADH2 or FADPH2 and 3 of each NADH2 or NADPH2 are formed. [Currently, NADH2 = 2.5 ATP, FADH2 = 1.5 ATP]