In cellular respiration, an organic or inorganic substrate is oxidized (e.g., glucose to CO2) and another substrate (the final electron acceptor) is reduced (e.g., 1/2O2 to H2O). The electrons acquired from the oxidized substrate are transferred through a chain of electron carriers of increasingly positive affinity for electrons (redox potential) on their way to the final electron acceptor. These electron carriers include small molecules (e.g., FADH2, NADH, ubiquinone) and proteins (e.g., cytochromes, Fe-S proteins). The electron flow through the ETS (Figure 1) is usually tightly coupled with unidirectional proton pumping across a membrane, thus establishing a proton gradient which is used to drive ATP synthesis (chemios-mosis; Figure 2).
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