Which stage of cellular respiration is defined as the breakdown of pyruvic acid to carbon dioxide in a series of energy-extracting reactions?

• A. Krebs cycle
• B. Glycolysis
• C. Electron transport chain
• D. Fermentation

Answer: (A)

The Krebs cycle oxidizes the acetyl group from acetyl-CoA to carbon dioxide in a series of energy-harvesting reactions. Pyruvate from glycolysis is first converted to acetyl-CoA in the link reaction, releasing CO2 and producing NADH. That acetyl-CoA then enters the Krebs cycle, where its two carbon atoms are released as CO2 across the cycle’s steps, while NADH and FADH2 are generated to fuel the electron transport chain for ATP production. So the full oxidation of pyruvate-derived carbons to CO2 through these successive steps is characteristic of the Krebs cycle. Other processes don’t complete this oxidation: glycolysis breaks glucose into pyruvate, fermentation proceeds without full oxidation in the absence of oxygen, and the electron transport chain uses NADH/FADH2 to make ATP and water rather than produce CO2.