Final answer:
Atmospheric molecular oxygen is the source of oxygen that combines with protons and electrons to form water during oxidative phosphorylation. The oxidation of pyruvate to acetyl CoA is the step in cellular respiration that does not produce any ATP.
Step-by-step explanation:
In the process of oxidative phosphorylation, the oxygen used to make water comes from atmospheric molecular oxygen. This oxygen acts as the final electron acceptor in the electron transport chain, combining with electrons and hydrogen ions to form water. The step of cellular respiration that does not produce ATP is the oxidation of pyruvate to acetyl CoA.
Oxidative phosphorylation is a process that produces ATP through chemiosmosis and occurs within the mitochondria of eukaryotic cells and the plasma membrane of prokaryotic cells. During this process, electrons are passed through a series of protein complexes in the inner mitochondrial membrane, which leads to the pumping of protons and the creation of an electrochemical gradient. This gradient powers ATP synthase to phosphorylate ADP into ATP. The molecular oxygen from the atmosphere is essential because without oxygen, the electrons would not move through the transport chain, stopping the production of ATP. The citric acid cycle does contribute to the overall production of ATP, but it generates a lesser amount of ATP than that produced by oxidative phosphorylation.