256 RADIATION BIOLOGY 



characteristic of living matter. The peculiarity of biochemical reactions 

 is further caused by the intricate inhomogeneity of cell structure, with 

 membranes, colloids, and external and internal surfaces acting as barriers 

 of varying, though controlled, permeability. Physical and physico- 

 chemical phenomena are closely interwoven with purely chemical 

 reactions. 



This manifold integrated structure makes it necessary to investigate 

 biochemical radiation effects on the broadest basis possible and, in fact, 

 we can discern three different lines of approach: 



1. The apparently simple but in fact very difficult research on pure 

 water itself. 



2. The investigations on substances in aqueous solutions. 



3. The investigations on cells, tissues, and uni- and multicellular 

 organisms, e.g., yeast cells, bacteria, viruses, and plant roots. 



The results of experiments which attack the problem from such different 

 angles are sometimes contradictory and demonstrate that experience 

 with an isolated system does not always apply when the same material 

 forms part of a more complete whole. 



After a brief discussion of the chemical action of radiation on water, we 

 shall discuss some effects on aqueous solutions and then pass on to more 

 complex systems. The radiation chemistry of water is still in its infancy. 

 Water, one of the simplest constituents of living matter, occurs in living 

 matter in the highest proportion and is of great functional importance as 

 a solvent, as will be seen in the discussion of radiosensitivity. It is 

 sufficient for the purpose of this general article to state that the primary 

 radiochemical action in water irradiated with ionizing radiations is 

 believed to consist in splitting the water molecule into H atoms and OH 

 radicals via ionization and excitation of water molecules. These two 

 uncharged entities are chemically very reactive and can act as oxidizing 

 and reducing agents as well as being able to break C — C bonds. It is 

 worth stressing that oxidizing power is not confined to the OH radicals, 

 since H atoms can bring about oxidation by dehydrogenation, while 

 Evans and Uri (1950) and Haissinsky, Lefort, and Le Bail (1951) 

 have pointed out that OH radicals can also be the agent of reduction. 

 Although H atoms and OH radicals are believed to be the primary 

 products of the interaction of radiation with water, secondary products 

 such as hydrogen peroxide and HO2, resulting from the interaction 

 between radicals, may be of considerable importance. Further, the 

 interaction of dissolved oxygen with H atoms will lead to the reactive 

 radical HO2 which, by dismutation according to the equation 



2HO2 = H2O2 + O2 

 could increase hydrogen peroxide formation. The complete picture of all 



