How many carbons of the original glucose molecule have been released as carbon dioxide by the time both pyruvate molecules have become acetyl groups?

1. Removing a carboxyl group from one pyruvate releases one carbon as carbon dioxide. 2. Glycolysis makes two pyruvate molecules from each glucose, so the step proceeds twice. 3. Two of the six carbons of the original glucose have therefore been removed at the end of both steps. 4. Answering one forgets that the six-carbon sugar was split into two halves earlier. 5. Four carbons are the ones that go on into the next cycle, not the ones lost at this point. 6. All six are only released later, over further turns of the cycle. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.3 Oxidation of Pyruvate and the Citric Acid Cycle_