Radioactive Isotopes are the study of reaction mechanism
Mechanism of photosynthesis in plants
A small quantity of Radioactive CO2 containing radioactive oxygen O18 is mixed with ordinary carbon dioxide and the process is carried out. It has been found that oxygen gas evolved along with sugar formation is non-radioactive.
Therefore O2 produced comes from water and not from carbon dioxide. So the correct mechanism is as follows.
6CO2 + 6H2O → C6H12O6 + 6O2
Study of hydrolysis of ester
By labeling oxygen, the mechanism of ester hydrolysis can be studied by using water labeled with O18. The hydrolysis of an ester by water enriched with radioactive oxygen is indicated as:
Therefore it is the acid and not alcohol produced which is radioactive confirming the above mechanism.