11th Class Biology The Living World / जीव जगत Radioisotope Or Tracer Technique

They are unstable isotopes which function like normal elements but emit positive or negative particles, e.g., \[^{3}H\](Tritium), \[^{14}C\](Carbon), \[^{32}P\](Phosphorus), \[^{35}S\](Sulphur), \[^{42}K\](Potassium), \[^{131}I\](Iodine). Radioactivity is recorded in different parts by Geiger counter or scintillation counter or autoradiography to know regions of use and transport. The tracers have been used for knowing pathway of mineral transport (Stout and Hoagland, 1939), organic solute transport (Vernon and Aronoff, 1952), carbon assimilation (Calvin, 1955).

Where radioactive elements are not available, heavy isotopes are used, e.g., \[^{15}N{{,}^{18}}O.\] Their fate is recorded by mass spectroscopy and density gradient centrifugation. Meselson and Stahl (1958) studied DNA replication and Ruben et al (1941) evolution of oxygen (photolysis of water) in photosynthesis by using heavy isotopes.