The electron transport system is the place in the cell where electrons generated by oxidation are transferred. Passage of the electrons through the system generates potential energy that is used to make ATP in oxidative phosphorylation.
Components of electron transport chain
Components of ETC are arranged in the inner mitochondrial membrane involved in the process of electron transport. In biological system, fuel molecules are oxidized by losing electrons, which are transferred through the carrier molecules like NAD and FAD. Finally electrons are donated to oxygen to reduce it to a water molecule. Coenzyme Q is a respiratory electron carrier. Cytochromes are a group of heme containing proteins. The major respiratory cytochromes are b, c and c1 and a or a3.
Electron Transport – Carriers and arrangement of carriers into complexes
Integral membrane proteins embedded in the inner mitochondrial membrane are arranged into complex I, II, III and IV. They accept or donate electrons from preceding carrier to the following in sequence.
Pathway of Electron Transfer through the Carriers
I NADH dehydrogenase, II Succinate dehydrogenase, III CoQ – Cytochrome c oxidoreductase, and IV Cytochrome oxidase are the four complexes. Electrons are transferred through these complexes. Through complex I, III and IV, protons are pumped out of mitochondrial matrix creating proton gradient.
Proton Motive Force
Coupling of ATP synthesis to electron transfer via an electrochemical H+ gradient across a membrane is called chemiosmosis. Energy from electron transport drives an active transport system which pumps protons out of the mitochondrial matrix into inter- membrane space. An electrochemical gradient of protons is created, with a lower pH value outside the inner mitochondrial membrane than inside. The protons on the outside have a thermodynamic tendency to flow back in, so as to equalize pH on both sides of the membrane.
When protons from inter-membrane space do flow back into the matrix, the free energy arising from the gradient is used to drive the synthesis of ATP. ATP synthase also called complex V is the site where ADP is phosphorylated to ATP.