Final answer:
The conversion of NAD to NADH and back again in the presence of oxygen is accomplished through glycolysis and the citric acid cycle, followed by the electron transport chain and chemiosmosis, resulting in oxidative phosphorylation.
Step-by-step explanation:
The conversion of NAD to NADH and then back to NAD when oxygen is present involves a pair of processes known as glycolysis and the citric acid cycle. Glycolysis produces pyruvate, which is converted to acetyl-CoA and enters the citric acid cycle. During the citric acid cycle, several redox reactions occur that result in the production of NADH. Following the citric acid cycle, NADH donates electrons to the electron transport chain, which operates in tandem with chemiosmosis to produce ATP through oxidative phosphorylation. This results in the regeneration of NAD from NADH as electrons are ultimately transferred to oxygen, producing water in the process.