Now that we have discussed the events of glycolysis and the citric acid cycle, we are ready to explore the electron transport chain and oxidative phosphorylation, the last step in cellular respiration. The first two rounds of aerobic respiration have produced only 4 ATP and a number of coenzymes. This final stage marks the ATP payoff, in which the NADH and FADH produced in earlier phases enter the inner membrane of the mitochondria where they are oxidized. As a result of the oxidation of NADH and FADH2, electrons are released. These electrons then pass through a series of four protein complexes called the electron transport chain. The energy produced from the flow of electrons drives oxidative phosphorylation in which ATP is synthesized via the addition of phosphor (phosphorylation) to ADP.

Oxidative phosphorylation marks the final stage of aerobic cell respiration. We have traced metabolism from food to glucose, through the citric acid cycle, and we will now see where the bulk of the ATP we use as fuel comes from. We will begin our discussion by reviewing some key features of the mitochondria. Next, we will discuss electron flow through the electron transport chain and ATP synthesis through oxidative phosphorylation.