How is pyruvate oxidized in aerobic respiration, and what are the products?

• A. Pyruvate is reduced to lactate, yielding lactate and NAD+.
• B. Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex, releasing CO2 and generating NADH.
• C. Pyruvate is fully oxidized to CO2 in the cytosol via a single-step reaction.
• D. Pyruvate is converted to oxaloacetate by pyruvate carboxylase, consuming ATP.

Answer: (B)

In aerobic respiration, pyruvate is oxidized in the mitochondrial matrix by the pyruvate dehydrogenase complex. This oxidative decarboxylation removes one carbon as CO2 and transfers the remaining two-carbon fragment to coenzyme A, forming acetyl-CoA. At the same time, NAD+ is reduced to NADH. This step is the crucial bridge between glycolysis and the citric acid cycle, supplying acetyl-CoA for the cycle and yielding NADH that will later feed into the electron transport chain to make ATP. The CO2 released is exhaled as a waste product, and the acetyl-CoA proceeds into the TCA cycle for further oxidation.

Other scenarios described don’t fit aerobic respiration: lactate formation occurs under low-oxygen conditions and regenerates NAD+, not producing acetyl-CoA or NADH for the TCA; a single-step oxidation of pyruvate in the cytosol isn’t how aerobic oxidation proceeds; and converting pyruvate to oxaloacetate by pyruvate carboxylase is anaplerotic—used to replenish TCA intermediates or for gluconeogenesis—not the normal entry point for oxidation in aerobic respiration.