To biologists, then, the essential feature in the use of radioactive
isotopes is the possibility of preparing “labeled” samples of any
organic molecule involved in biological processes. With labeled samples
it is possible to distinguish the behavior and keep track of the course
of molecules involved in a particular biological function.
In this capacity the isotope may be likened to a dynamic and
revolutionary type of “atomic microscope”, which can actually be
incorporated into a living process or a specific cell. Just as a real
microscope permits examination of the structural details of cells,
isotopes permit examination of the chemical _activities_ of molecules,
atoms, and ions as they react within cells. (Neither optical nor
electron microscopes are powerful enough for us to see anything as small
as a molecule clearly.)