After glycolysis, pyruvate may enter the mitochondria to be further metabolized in which cycle if oxygen is present?

• A. Krebs Cycle
• B. Fermentation
• C. Glycolysis
• D. Electron Transport Chain

Answer: (A)

When oxygen is present, pyruvate from glycolysis is transported into the mitochondrion and converted to acetyl-CoA by the pyruvate dehydrogenase complex, releasing CO2 and generating NADH. Acetyl-CoA then enters the Krebs cycle (also called the citric acid cycle), where it combines with oxaloacetate to form citrate and proceeds through a series of steps that produce more NADH, FADH2, and GTP. The NADH and FADH2 feed electrons into the electron transport chain, which requires oxygen to regenerate NAD+ and FAD for continued cycling. Therefore, the cycle processing the pyruvate-derived carbon skeleton in the presence of oxygen is the Krebs cycle.