electron transport system
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electron transport system (ETS)a series of biochemical steps by which energy is transferred in steps from a higher to a lower level. Each step involves a specific electron carrier which has a particular energy level (or REDOX POTENTIAL), with the carriers organized in a sequence of decreasing energy.
Thus the oxidation of X releases more energy than is required to reduce Y. The released energy can be used to produce ATP, in MITOCHONDRIA and CHLOROPLASTS, probably by a process of chemiosmosis. This mechanism, proposed by P. Mitchell (1920- ), Nobel prize-winner in 1978), involves hydrogen ions (H+) being pumped in or out of the membranes containing the ETS, generating a , proton motive force that enables ATP synthesis from ADP and P.
- photosynthesis. Two ET systems are utilized during the LIGHT REACTIONS in the grana of the chloroplasts. One ETS enables the production of ATP by PHOTOPHOSPHORYLATION and the other enables the production of reduced NADP. See Fig. 145 .
- aerobic respiration. A molecule of NADH from GLYCOLYSIS or the KREBS CYCLE is passed to the cristae of mitochondria where it is oxidized in the respiratory ETS, the final products being water and three molecules of ATP (= OXIDATIVE PHOSPHORYLATION). See Fig. 146 . The free energy of NADH is approximately 220 kJ. Of this, about 102 kJ are stored in the three ATP molecules (3 × 34 kJ) with the remaining energy being lost as heat, a conversion efficiency of about 46%. FADH enters the ETS at a lower energy level than NADH, yielding only two ATP molecules, not three.