Explain how the use of an oxygen isotope helped elucidate the chemistry of photosynthesis.

Photosynthesis is a process by which certain organisms such as plants and algae utilize solar energy to obtain organic compounds. During photosynthesis, plants use water and carbon dioxide to produce organic compounds such as carbohydrates in the presence of sunlight.

The chemical reaction involved in photosynthesis can be summarized as follows:

\({\rm{6C}}{{\rm{O}}_{\rm{2}}}\,{\rm{ + 12}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\,{\rm{ + Sunlight}}\, \to \,\,{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}\,{\rm{ + }}\,{\rm{6}}{{\rm{O}}_{\rm{2}}}\,{\rm{ + }}\,{\rm{6}}{{\rm{H}}_{\rm{2}}}{\rm{O}}\)

Prior to the discovery of water splitting during photosynthesis, it was thought that carbon dioxide was the source of oxygen molecules during photosynthesis. However, this hypothesis was challenged by Van Niel.

He noticed that certain photosynthetic bacteria generated carbohydrates but did not release oxygen when they were investigated. Instead of water, another set of bacteria employed hydrogen sulfide (H₂S) for photosynthesis.

As a result, he concluded that photosynthetic organisms require a hydrogen supply to produce carbohydrates. He believed that water is the source of oxygen generated during photosynthesis based on his findings.

Researchers conducted two tests utilizing a heavy isotope of oxygen (¹⁸O) to test the Van Niel theory.

Water was labeled ¹⁸O in one experiment, and oxygen generated during photosynthesis was also labeled ¹⁸O. Carbon dioxide was labeled with ¹⁸O in another experiment, and it was discovered that the oxygen released did not include ¹⁸O.

The research findings proved that the water released during photosynthesis came from water rather than carbon dioxide. Scientists were able to confirm the Van Niel hypothesis by employing a heavy isotope of oxygen.