During cellular respiration, which stage breaks down pyruvic acid into carbon dioxide in a series of energy-extracting reactions?

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

Answer: (A)

The stage that oxidizes carbon atoms to carbon dioxide while extracting energy is the citric acid cycle. After glycolysis, pyruvate is moved into the mitochondrion and converted to acetyl-CoA, releasing one CO2 and forming NADH. The acetyl-CoA then enters the cycle, where its carbons are fully oxidized to CO2 through a series of steps that also generate NADH and FADH2. Those carriers feed the electron transport chain to produce most of the ATP. The other stages don’t release CO2 in this way: glycolysis happens in the cytoplasm and makes a modest amount of ATP and NADH; the electron transport chain uses NADH/FADH2 to make ATP but doesn’t release CO2; fermentation occurs under anaerobic conditions and yields far less energy, without completing the full oxidation to CO2.