All photosynthetic eukaryotic cells contain chloroplasts that use the radiant energy of sunlight to convert carbon dioxide and water into carbohydrates. As a byproduct of photosynthesis, oxygen gas is also released into the atmosphere through tiny openings in the leaves called stomata. The carbohydrates produced by photosynthesis provide us with an important energy source, while the oxygen is a critical component of the air we breathe.
The reactants and products of photosynthesis have been known since the early 1800s. At first, it was generally assumed that the oxygen released as a byproduct of photosynthesis came from the carbon dioxide. However, the question was not definitely answered until the early 1940s, and the result was surprising to many.
One of the first uses of an isotopic tracer in biology was in the determination of the source of the oxygen produced by photosynthesis. Using a heavy isotope of oxygen, 18O, to label one of the two reactants in the photosynthetic process, Samuel Ruben and collegues at UC Berkeley were able to determine that the oxygen gas came not from carbon dioxide, but from water.
Textbook Reference: Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy