Which molecule enters the Citric Acid Cycle after glycolysis?

• A. Pyruvate
• B. Acetyl CoA
• C. Glucose
• D. Oxygen

Answer: (B)

The main idea here is how glycolysis hands off its products to the Citric Acid Cycle. After glycolysis, pyruvate is produced in the cytosol and must be transported into the mitochondria. There, pyruvate is converted to Acetyl CoA by the pyruvate dehydrogenase complex, releasing CO2 and generating NADH in the process. This two-carbon Acetyl CoA then combines with the four-carbon oxaloacetate to form citrate, kicking off the Citric Acid Cycle. So the molecule that actually enters the cycle is Acetyl CoA. Pyruvate itself doesn’t enter the cycle directly, though it is the product of glycolysis. Glucose is the starting substrate for glycolysis, not a direct entrant to the cycle, and oxygen is required for efficient aerobic respiration but is not a substrate feeding directly into the Citric Acid Cycle.