A metabolite that is labeled by incorporation of an isotopic atom into its structure. The metabolic fate of the labeled metabolite can then be traced in intact organisms. That is, one is able to ascertain where (in what kind of structure) the metabolite ends up as well as the transformation products (intermediate molecules) which were involved in its formation. Certain atoms of a given metabolite are labeled. This is done by substituting radioactive isotopes for the atom in question. Because an atom is replaced by an isotope, the metabolite as a whole is chemically and biologically indistinguishable from its normal analog. The presence of the isotope allows the metabolite and its transformation products to be detected and measured. Without this technique, many aspects of metabolism could not have been studied. These include: the process of photosynthesis, metabolic turnover rates, and the biosynthesis of proteins and nucleic acids.