284 RADIATION BIOLOGY 



tion of the indirect action theory so repeatedly presented before by Fricke 

 (1934). The great advances made in the field of free-radical reactions 

 and the production of mutations by those free radicals found on irradia- 

 tion of water have demonstrated the importance of this mode of action. 



In the study of the biochemical aspects of radiation — radiation bio- 

 chemistry — the dominant concept must be that the reactions take place 

 in aqueous systems and in the presence of oxygen, since most living cells 

 contain from 85 to 90 per cent water and require oxygen for the perform- 

 ance of their energetic activities. Knowledge gained from the study of 

 radiation chemistry in gaseous or solid systems is, of course, necessary and 

 may aid in the interpretation of biochemical reactions. Application of 

 this knowledge to biological systems must, however, be made with due 

 consideration of the biological environment. 



Water ^^ith oxygen dissolved at various tensions is the solvent of bio- 

 logical systems. A large number of ions are distributed in the intracel- 

 lular as well as in the extracellular spaces. Organic substances of varied 

 composition provide the materials for the framework, as well as for the 

 performance of energy. The effect of ionizing radiations on passing 

 through these systems will be the subject of this chapter. Great difficul- 

 ties, sometimes insuperable, will be found in the interpretation of findings, 

 most of them due to lack of knowledge of the components of the system 

 and to the difficulty of differentiating between the indirect effects of the 

 radiation products of water and the direct effects due to collision between 

 the ionizing track and the substance of biological importance. 



THE TWO THEORIES OF BIOLOGICAL ACTION OF RADIATIONS 



Biologists, w^ho studied the effect of ionizing radiations upon the proc- 

 esses of cell multipHcation or mutation, claimed that these effects could 

 be explained on the basis of the production of ionization in or in the imme- 

 diate vicinity of some partic\ilar molecule or structure vital to the life of 

 the organism, the so-called "target." They went so far as to calculate 

 the size of this "sensitive spot" or "point sensible," and to give the num- 

 ber of "hits." Chemists, who studied the effect of ionizing radiations on 

 substances in aqueous solutions, attributed the effects to the action of the 

 radiation products of water, the "activated water" of Risse (1929) and of 

 Fricke (1934). This controversy between the partisans of the "hit" the- 

 ory and the partisans of the "indirect action" is reminiscent of the con- 

 troversy which twenty years ago arose between the advocates of oxygen 

 activation and those of hydrogen activation to explain biological oxida- 

 tions. Just as time showed that the two theories of biological oxidations 

 are not incompatible but complementary to each other, time is also show- 

 ing that these two theories of biological action of ionizing radiations are 

 not contradictory and may take place within the same system. How- 



