CHAPTER 



12 



Isotopic Tracers 



Probably the single most valuable technique to become available to 

 the biologist in recent years is the isotopic tracer method. Increased un- 

 derstandings in a number of important areas can be attributed directly 

 to these materials, which came into general use shortly after the end of 

 World War II. 



The use of tracer isotopes is such a broad and highly technical subject 

 that no attempt at complete coverage will be made here. Because the use 

 of radioactive tracers, particularly, has potential dangers, I recommend 

 that no one should amuse himself by simply playing with radioactive 

 materials but instead should undertake tracer experiments only after 

 competent instruction in the laboratory. Fortunately, although it is im- 

 possible to give more than a summary here, a number of excellent pub- 

 lications are available for the reader who is interested in pursuing the 

 subject. 



Several kinds of problems, otherwise insolvable, are experimentally 

 easy if tracers are used. If a material moves from one place to another 

 within an organism but several different pathways are possible, the tracer 

 can identify the pathway taken. For example, mineral ions move from 

 the roots where they are absorbed upward to the leaves of plants. They 

 might move through either xylem or phloem; the proper application of 

 tracer experimentation tells which tissue is the actual path. In an animal, 

 a certain material might move from place to place through blood or 

 lymph, and a tracer could be used here also. 



The other major kind of problem is the chemical problem. We know 

 that A is converted to Z, but paper chemistry tells us that any of several 



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