Which event describes the entry of pyruvate into the mitochondria and its conversion to acetyl-CoA?

• A. Glycolysis in cytoplasm
• B. Fermentation in mitochondria
• C. Conversion of pyruvate to acetyl-CoA by the pyruvate dehydrogenase complex
• D. Direct formation of CO2 and water from pyruvate

Answer: (C)

This question tests how pyruvate from glycolysis is prepared for entry into the citric acid cycle. After pyruvate is transported into the mitochondrial matrix, it is oxidatively decarboxylated by the pyruvate dehydrogenase complex. This reaction removes one carbon as CO2, attaches the remaining two-carbon fragment to coenzyme A to form acetyl-CoA, and reduces NAD+ to NADH. The acetyl-CoA then feeds into the citric acid cycle, continuing aerobic energy production.

Glycolysis happens in the cytoplasm, not inside mitochondria. Fermentation also occurs in the cytoplasm and does not produce acetyl-CoA for the citric acid cycle. The complete formation of CO2 and H2O from pyruvate occurs only after the acetyl-CoA enters the citric acid cycle and oxidative phosphorylation, not directly from pyruvate itself.