Biology: Concepts and Investigations 2nd Edition by Mariëlle Hoefnagels

Exercise 37

In 1941, biologists exposed photosynthesizing cells to water containing a heavy oxygen isotope, designated 18O. The "labeled" isotope appears in the O₂ gas released in photosynthesis, showing that the oxygen came from the water. Where would the 18O have ended up if the researchers had used 18O-labeled CO₂ instead of H₂ O?