XVI. STABLE ISOTOPES AS TRACERS 5G7 



moded by the wide range of application of isotopic tracers to trans- 

 port problems. The factor primarily responsible for this ready ac- 

 ceptance of a new method lies in the incontrovertibility of the stable 

 isotope evidence when properly obtained and evaluated. Nothing 

 foreign to the biological system is being introduced, as has almost 

 always been the case with labels of other descriptions. Nimierous 

 metabolic precursors and products that formerly were even impossible 

 to label may now be followed satisfactorily in living systems. 



It is to be expected that exploratory investigations by means of 

 tracer isotopes will confirm many of the conclusions reached earlier 

 from the use of other techniques. Contributions to date have been 

 largely of this character. The superior potentialities inherent in the 

 isotope tracer method should lead soon, however, to more extensive 

 and detailed studies on the translocation of a multitude of substances, 

 including important biochemical intermediates. A beginning has 

 been made, especially with metallic radioactive ions (3, 3b, 17), but an 

 immense field for research involving carbon compounds has barely 

 been touched. 



Transportation of the products of photosynthesis through plant 

 tissues has been studied by several workers and an introduction to the 

 literature on the subject may be found in an article by Rabideau and 

 Burr (26). An initial study on sulfur transport in plants has also 

 appeared (25). Significant future advances in our knowledge re- 

 garding the permeability of cells of all types seem inevitable now that 

 steady state conditions can be studied so readily by virtue of isotopic 

 labels. Here again, a beginning has been made (17, p. 118). 



2. Mechanisms of Biochemical Reactions 



Perhaps the most significant contributions resulting from the use 

 of stable isotopes in biology have been concerned with the intermedi- 

 ary steps in metabolism. Hitherto proposed mechanisms for bio- 

 chemical reactions were based primarily on circumstantial evidence 

 obtained in devious ways. Direct evidence available from reactants 

 labeled with stable isotopes has not only clarified and confirmed many 

 of the older conceptions of intermediary metabolism but also has re- 

 sulted in the introduction of a number of new ideas. The speed with 

 which hypotheses in this field can be tested has been increased almost 

 immeasurably and consequently progress can be expected to l)e rapid 

 in the future. 



