The free energy decrease (positive cell potential) driving oxidative phosphorylation is free energy of oxidation of NADH or FADH2 by O2. NADH and FADH2 are excellent electron carriers. While normally an oxidation process whereby a nitrogen atom loses electron control would not be particularly favored, with NADH and FADH2, oxidation leads to a marked increase in resonance stabilization, the nicotinamide ring of NAD+ actually becoming aromatic through the process, so the typical love of nitrogen for electrons is counterbalanced by resonance so that the nitrogens of NADH and FADH2 will give the electrons they carry to the respiratory chain without the need for significant increase to the internal energy of the system. The electrons begin at high energy, and as they fall towards the deep potential energy well represented by strong covalent bonds with oxygen in the final state, the crank is turned in the cytochromes, pumping protons into the outer compartment, storing the energy which drives ATP synthesis in chemiosmosis.
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