In the presence of oxygen. The way that a cell produces ATP molecules that are used as the energy source in that cell is via certain types of processes that together are known as aerobic cellular respiration. And this includes glycolysis, pyruvate decarboxylation, the citric acid cycle, as well as the electron transport chain. Now, glycolysis breaks down our glucose into two pyruvate molecules as well as a net of two ATP molecules. And in the presence of oxygen, those two pyruvate molecules will then go into the mitochondrial matrix where they will undergo a series of processes that eventually produce NADH molecules. And those NADH molecules then go onto the electron transport chain, give off their electrons onto the enzymes of the electron transport chain, and the NADH molecules are oxidized back into the NAD plus molecules.