Glycolysis and the Krebs’ Cycle together result in the formation of reduced coenzymes such as ten molecules of NADH + H+ ions and two molecules of FADH2, and four molecules of ATP. These reduced co-enzymes need to be oxidised to release and utilise the energy stored in them. This is made possible by the transport of protons and electrons from these co-enzymes to oxygen through electron carriers present in the inner mitochondrial membrane. This metabolic pathway of electron transport is called the Electron Transport System.
ETS serves three important functions in aerobic respiration. It regenerates the oxidised form of co-enzymes to be used in glycolysis and the Krebs’ Cycle. It transports 2 H+ and 2 electrons to oxygen, and it utilises the energy of the co-enzymes in the production of ATP. ETS comprises several electron carriers that include complex I, complex Q, complex II, complex III, cytochrome c, and complex IV. Electrons from NADH + H+ are transferred through complex I to ubiquinone, and protons are moved from the matrix of the mitochondria to the inter-membrane space. In the same way, electrons from FADH2 are transferred through complex II to complex Q, and protons are moved from the matrix of the mitochondria to the inter-membrane space, complex Q transfers the electrons to complex III, these transfers the electrons to complex IV through cytochrome c. Complex IV contains cytochrome a and cytochrome a₃. It transfers the electrons to the final electron acceptor oxygen. Oxygen, on receiving the electrons, reacts with 2 H+ ions and reduces to water and thereby drives the ETS. This reaction can be summarised as
2 H+ + 2 e+ + 1/2 O2 ---> H2O + energy
The electron transport chain is coupled to ATP synthesis.The electron transport and movement of protons creates a proton gradient across the mitochondrial membrane. The protons are pumped through a membrane protein called complex-V. The energy derived from the proton pumping is used for the synthesis of ATP. Oxidation of NADH + H+ results in 3 molecules of ATP, whereas oxidation of FADH2 results in 2 molecules of ATP. Aerobic respiration uses energy conserved in co-enzymes during oxidation-reduction reactions to create the proton gradient required for phosphorylation. The net gain of ATP produced during aerobic respiration from one molecule of glucose, during glycolysis, Krebs’ Cycle and ETS are 38 ATP. Fermentation, on the other hand, results in a net gain of 2 ATP molecules from the partial breakdown of the glucose molecule.