FIG. 8.3

Does the oxygen released by photosynthesis come from H₂O or CO₂?

BACKGROUND The reactants in photosynthesis are water and carbon dioxide. Both contain oxygen, so it is unclear which one is the source of the oxygen that is produced in the reaction.

METHOD Most of the oxygen in the atmosphere is ¹⁶O, a stable isotope containing 8 protons and 8 neutrons. A small amount (0.2%) is ¹⁸O, a stable isotope with 8 protons and 10 neutrons. The relative abundance of molecules containing ¹⁶O versus ¹⁸O can be measured using a mass spectrometer. H₂O and CO₂ containing a high percentage of ¹⁸O can be used to determine whether the oxygen produced in photosynthesis comes from water or carbon dioxide.

EXPERIMENT

RESULT

CONCLUSION The percentage of ¹⁸O increases only when water contains ¹⁸O, but not when carbon dioxide contains ¹⁸O. This finding indicates that the oxygen produced in photosynthesis comes from water, not carbon dioxide.

FOLLOW-UP WORK Carbon also has several isotopes, and their measurements have been used to determine the source of increased CO₂ in the atmosphere today (Chapter 25).

SOURCE Adapted from Ruben, S., M. Randall, M. Kamen, and J. L. Hyde. 1941. “Heavy Oxygen (O¹⁸) as a Tracer in the Study of Photosynthesis.” Journal of the American Chemical Society 63:877–879.