Radioisotopes are those forms of an element which exhibit rzdioactivity and so can be used as markers or tracers. For this purpose, we first introduce the radioisotope in a molecule in the cell and then detect the radioactivity with the help of certain device. For example, if we replace a suitable hydrogen atom of thymidine by 3H, we may trace the course of thymidine in the cell. Since thymidine is a component of DNA but not of RNA, we can be sure that we are following only DNA metabolism. Similarly, to know the course of protein synthesis we may use radioactively labelled amino acids.Radioisotopes are used to:
(a) measure the rates of metabolic turnover of a material within a cell or tissue,
(b) locate the site of synthesis of molecules,
(c) measure the rate of exchange of materials across cellular membranes,
(d) locate a molecule in a cell. Radioisotopes are widely applied to study also the precursor-product relathship.
Many times when a radioactively labelled mokcule (pncursor) is introduced into a system. The molecule is chemically converted into another form (product). For example, when radioactive iron is introduced in blood, radioactivity can be detected in liver femtin and haemoglobin of red blood cells. Similarly, radioactivity of 32P- labelled phosphate can be detected in many phospholipids.