Final answer:
For every glucose molecule undergoing cellular respiration, four carbons enter the Krebs cycle. With 13 original glucose molecules, 52 carbons (13x4) from these glucose molecules enter the Krebs cycle.
Step-by-step explanation:
In the presence of oxygen, Krebs cycle processes the pyruvate molecules produced by glycolysis. For each glucose molecule, glycolysis generates two pyruvate molecules. Each pyruvate contributes 3 carbons to the process, but one carbon is lost as CO2 when pyruvate is converted into acetyl CoA, leaving two carbons to enter the Krebs cycle per pyruvate. This step occurs twice as there are two pyruvates per glucose, so four carbons enter from one glucose molecule. Therefore, for 13 glucose molecules, a total of 52 carbons (13 glucose molecules x 4 carbons/glucose) originally from glucose enter the Krebs cycle. However, it's important to remember that all six carbon atoms from each glucose molecule will eventually be released as CO2 during the cellular respiration process.