I have a biology question I am stuck on regarding isotopes?

Question

Animals obtain energy through a series of chemical reactions in which water and oxygen gas are reactants. This process produces water and carbon dioxode as waste products. How might you use a radioactive isotope to find out whether the oxygen in carbon dioxide comes from sugar or oxygen gas?

Answer 1

Design an experiment where an animal is fed sugar that contains O-18 (an isotope of oxygen), and water that contains normal oxygen (O-16). You also design a tandem experiment where the animal is given water containing O-18, and sugar with normal oxygen. Test the CO2 given off by both animals: If the first animal's CO2 contains O-18, then the sugar contributed an O to the formation of CO2. If the second animal's CO2 contains O-18, then the O in the CO2 comes from water.