Final answer:
Four carbon atoms from each glucose molecule enter the Krebs cycle when oxygen is present, as each glucose molecule is broken down into two acetyl CoA molecules during glycolysis and conversion of pyruvate.
Step-by-step explanation:
The question asks how many carbon atoms from each glucose molecule enter the Krebs cycle in the presence of oxygen. During glycolysis, one molecule of glucose is broken down into two molecules of pyruvate. These two pyruvate molecules are then converted into two molecules of acetyl CoA. Each acetyl CoA molecule enters the Krebs cycle and contains two carbon atoms. Therefore, a total of four carbon atoms (2 from each acetyl CoA) enter the Krebs cycle from one glucose molecule. The citric acid cycle, which is another name for the Krebs cycle, runs twice for each glucose molecule because each glucose yields two acetyl CoA molecules. During this process, all six carbon atoms of the original glucose molecule will eventually be released as carbon dioxide, along with the generation of energy carrier molecules such as NADH and FADH2.