Radioactive isotopes (radioisotopes) of elements are commonly used in biological experiments as tracers to follow and detect molecules of interest. For example, photosynthetic intermediates produced during carbon dioxide conversion to sugars were detected by exposing algae to carbon dioxide containing a radioactive form of carbon. This radioactive carbon could be rapidly detected in molecules produced by the algae during carbon fixation and sugar production. Why can radioisotopes substitute for non-radioactive isotopes of elements in experiments