Figure 3.4. This model illustrates the organization of the electron transport system, located within the inner membrane of the mitochondria. Electrons released during oxidative steps in glycolytic pathway and the tricarboxylic acid cycle (trapped as NADH and FADH2) move (shaded arrow) through a series of complexes to the terminal acceptor, oxygen. Four protein complexes participate in the process with complex I accepting energy from NADH and complex II from FADH2. The free energy released during electron transfer is coupled to the translocation of protons (H+) across the membrane, creating an electrochemical proton gradient. Protons on the exterior flow back through complex V, an ATP synthase complex that is coupled to the conversion of ADP + Pi to ATP, retrapping the free energy. When the alternative oxidase (AOX) is operative, only 1 proton is transferred from NADH. FAD reduction is associated with succinate dehydrogenase activity (i.e., succinate ? fumerate), and when the AOX is operative, energy in QH2 is transferred directly to oxygen, bypassing the formation of ATP. Thus, when the alternative pathway is operative, only 1 ATP equivalent is produced from NADH with the remaining energy being lost as heat..