Electron transport system is a series of enzymes and cytochromes operative in inner mitochondria membrane. The pairs of hydrogen atoms removed from respiratory intermediates by dehydrogenation reactions during glycolysis and Kreb’s cycle do not directly react with oxygen. These are used to reduce pH acceptor molecules such as NAD⁺ and FAD to NADH₂ and FADH₂. These reduced coenzymes releases the protons (H⁺) in the miitochondrial matrix, while the electrons (e^-) are channeled into electron- transport chain. (mitochondria respiratory chain). ETCs a set of seven electron – carries present in a specific sequence along inner mitochondrial membrane. These seven electron – carriers function in a specific sequence and are: Nicotinamide Adenine Dinucleotide (NAD). Flavin Mononucleotide (FMN). Co- enzyme Q or Ubiquinone. Cytochrome – b Cytochrome – c, Cytochrome –a and Cytochrome –a₃. It is also called cytochrome – system (CS), as four out of these seven carriers are cytochromes. Transport of electron – carriers is a down hill journey or a descending stairway. In this transport, the electrons tend to flow from electro- negative to electro- positive system, so there is a decrease in free energy and some energy is released and amount of energy with the electrons goes on decreasing. During electron transfer the electron – donor gets oxidised, while electron – acceptor gets reduced so these transfers involve redox – reaction and are catalysed by enzymes called reeducates. Oxidation and reduction are complimentary. This oxidation- reduction reaction over the ETC is called biological oxidation. Electron – donor.. e^- + electron – acceptorHere, electron – donor and electron acceptor form redox pair. During the electron transfer, the energy released at some steps is so high that ATP is formed by the phosphorylation of ADP in the presence of enzyme ATP synthetase present in head of F₂ – particles present on the miitochondrial crista. This process of ATP synthesis during oxidation of coenzymes is called oxidative Phosphorylation, so ETC is also called oxidative phosphorylation pathway. Role of oxygen. From the cytochrome a₃, two electrons are received by oxygen atom, which also receives two protons (H⁺) from the mitochondrial matrix and from water molecule. So the final acceptor of electrons is oxygen. So the reaction H₂ + ½ O₂ → H₂O (called metabolic water) is made is occur in many steps through ETC, so that most of the energy can be derived into a storage and usable form.