Sec. 24.3] CRITIQUE OF BIOLOGICAL APPLICATIONS OF ISOTOPES 509 



Universally accepted as essential to all life are the elements H, C, N, O, Mg, 

 F, S, and Fe. Although K and Ca are usually regarded as essential metabo- 

 lites, there is some evidence that certain bacteria and blue algae, for example, 

 may not require one or the other. In addition, various elements have been 

 shown to be unquestionably essential to certain plants or animals, and at 

 least some of these probably are universal micronutrient requirements of 

 living systems on our planet. These elements include B, Na, Al, Si, CI, V, 

 Mn, Co, Cu, Zn, Mo, and I. There is also rather good evidence that certain 

 plants require Ga, Cb, and W, and certain animals, Ni and Br. Other ele- 

 ments that have been postulated as essential micronutrients for certain forms 

 are Li, F, Cr, Rb, Sr, and Sn. It is quite possible, therefore, that some 30 or 

 more of the 96 known elements are essential to various forms of living 

 organisms. 



So far it has been the metabolism of the macronutrient elements and of two 

 micronutrients, Fe and I (in the higher vertebrates), that have been inten- 

 sively studied with tracers. Only certain of the micronutrients have been 

 subjected to the tracer approach, including, aside from Fe and I, the elements 

 Mn, Co, Cu, Zn, and Mo. The important micronutrient B in plants is still 

 to be investigated, also such probable micronutrients as Ca, Cb, and W in 

 plants and Ni in animals. 



The use of isotopic tracers has already divulged a tremendous store of data 

 on the metabolism of essential elements. In many cases, as with much of the 

 intermediary metabolism of organic compounds and the dynamics of elec- 

 trolytes in body fluids, this information would otherwise have been quite 

 unattainable. In the study of the role of micronutrients, isotopic tracer 

 techniques will very likely supply us with an understanding of mechanisms 

 now poorly or scarcely at all understood, and it may well be that these 

 techniques can, in addition, give us evidence for the essential role of other 

 elements not now definitely known to play such a role in any living process. 



Studies with trace amounts have been carried out for a number of elements 

 not known to be essential to life. Where these elements are normally present 

 in minute amounts in an organism, it is possible to elucidate their behavior 

 in the ;< normal" metabolism of that organism. 



The study of the metabolic fate of elements and compounds abnormal 

 to the body in form or amount has also been carried out for a large number 

 of substances. Such investigations have either a toxicological or a phar- 

 macological basis, or both. 



b. Nonmetabolic Tracing. Under nonmetabolic tracing may be included 

 study of the behavior and fate of normal body constituents above the purely 

 biochemical level, e.g., erythrocytes; and of foreign substances, e.g., bacterial 

 products and inert gases. 



In some cases, it is impossible to draw a clear distinction between metabolic 



