Isotopic labeling of a specific substrate carbon with 13C, which has an extra neutron, is particularly useful for investigating biochemical pathways because:

a. the radioisotope blocks each pathway at a specific step.
b. radiolabels generally are easier to track in cells than fluorescent labels.
c. the radioisotope readily exchanges neutrons so that all neighboring carbons become labeled and easier to detect with NMR.
d. the position of the labeled carbon can be traced through products of the pathway to characterize the pathway reactions.
e. the presence of the radioisotope greatly accelerates the pathway, which makes it easier to study.