What two carbons are lost as CO2 during the complete oxidation of glucose in the Citric Acid Cycle?

The correct answer is that the two carbons lost as CO2 during the complete oxidation of glucose in the Citric Acid Cycle come from acetyl-CoA.

When glucose is metabolized through glycolysis, it is ultimately converted into two molecules of pyruvate, which then enter the mitochondria and are transformed into acetyl-CoA. This molecule consists of a two-carbon unit that feeds into the Citric Acid Cycle.

Once acetyl-CoA enters the cycle, it combines with oxaloacetate to form citrate. As the cycle progresses, citrate undergoes a series of transformations, during which two carbon atoms from acetyl-CoA are released as carbon dioxide. This process is part of the decarboxylation steps, specifically during the conversion of isocitrate to alpha-ketoglutarate and then from alpha-ketoglutarate to succinyl-CoA, where carbon atoms are removed as CO2. Therefore, the two carbons initially present in acetyl-CoA are the source of the carbon dioxide that exits the cycle.

Understanding this progression highlights the crucial role of acetyl-CoA in the Citric Acid Cycle, as it is the primary starting point for carbon oxidation, leading to energy production in