Final answer:
When 18O labeled glucose is given to a human, the 18O label is most likely to appear in the exhaled CO2. This is due to the metabolism of glucose in cellular respiration, where oxygen atoms are used to form water and carbon dioxide as by-products.
Step-by-step explanation:
The question involves understanding the metabolism of glucose, specifically the isotopic labeling of glucose with oxygen-18 (18O) and tracking its fate within human cellular respiration. Given that glucose is metabolized to produce ATP, NADH, FADH2, and carbon dioxide (CO2), and considering that a significant portion of oxygen in glucose will be used to form water (H2O) and carbon dioxide (CO2), the 18O label from glucose will predominantly end up in the carbon dioxide (CO2) exhaled by the lungs.
Thus, if 18O labeled glucose is given to a human, the label is most likely to appear in exhaled CO2.
This is because during the process of cellular respiration, specifically in the Krebs cycle and electron transport chain, oxygen atoms from glucose are transferred to form water and carbon dioxide as by-products. The label from 18O-labeled glucose is therefore released as part of carbon dioxide when glucose is fully oxidized for energy production.