Q. How many carbon dioxide (CO2) molecules are liberated after each cycle of the Krebs cycle? For a single glucose how many carbon dioxide molecules were already liberated by the aerobic respiration at that point?
Each round of the Krebs cycle liberates two carbon dioxide molecules.
At the end of the cycle all carbon atoms from the original glucose molecule degraded in glycolysis are already liberated incorporated into carbon dioxide a molecule that occurs because for each glucose two pyruvic acid molecules were made by glycolysis. Each of these two pyruvic acids then is converted into acetyl CoA with liberation of one carbon dioxide molecule (two in total) Since each of the two produced acetyl CoA cycles the Krebs cycle once, from the initial glucose two rounds of the Krebs cycle is generated and so four other carbon dioxide molecules are made.
All of the six carbons of the glucose molecule are then incorporated into six carbon dioxide molecules two made during acetyl CoA formation and four during the two cycles of the Krebs cycle.