Final answer:
Pyruvate's fate in human cells depends on oxygen availability: it is converted into acetyl CoA during aerobic respiration when oxygen is present, and into lactic acid during anaerobic respiration when oxygen is absent.
Step-by-step explanation:
The fate of pyruvate in human cells differs depending on the availability of oxygen. When oxygen is available, aerobic respiration occurs, and pyruvate enters the mitochondria to be converted into acetyl CoA, which enters the citric acid cycle resulting in the production of approximately 36-38 ATP per molecule of glucose. In contrast, in the absence of oxygen, pyruvate undergoes anaerobic respiration or fermentation, being converted into lactic acid in muscle cells or ethyl alcohol in yeast, yielding only about 2 ATP per molecule of glucose, as well as allowing glycolysis to continue by recycling NADH back into NAD+.