370 ENERGY LOSS AND BIOLOGICAL EFFECTS 



Both the ionic efficiency jS of an intermediate P and its mean action 

 radius p are functions of the composition of the medium. In the limiting 

 case of dry organisms, or viruses, or crystalline enzymes, etc., very little 

 migration should take place; the mean action radius will then for all 

 practical purposes define the size of the target volume in the sense of 

 the classical hit theory. As water is added, the mean action radius maj^ 

 increase, so that the slope of the survival curve will not immediately 

 correspond to the true "sensitive volume." One expects that the same 

 organism would be more sensitive to radiation in its hydrated, wet form 

 than in its dry state. The sensitive volumes obtained in some earlier 

 work for various organisms should be re-examined in terms of the 

 diffusion model. There are some cells, among them yeasts, that may be 

 dried and irradiated in that form. Some work along this hne was done 

 by Failla (35), and suggestive data were given by Dale at this symposium 

 for enzymes with different water dilution. In the diffusion model most 

 ion pairs or their products, the intermediates, end their life by extragenic 

 interaction. As long as the intermediates originate reasonably close to 

 a sensitive site of the cell, they have some chance to reach this site in 

 the course of their migration. 



It seems possible that the diffusion model can be intelligently used for 

 evaluating changes in the radiosensitivity of cells due to their state of 

 division or to changes in the external medium. For example, the intro- 

 duction of dissolved oxygen into an oxygen-free medium may change 

 the ionic yield and mean action radius of H2O2, OH, and O2H. Using 

 radiations with low REL in the presence of oxygen increases the ionic 

 yield of OH and HO2 ; most tested organisms, indeed, have shown greater 

 sensitivity to radiation in oxygenated media than in oxygen-free media. 

 At high REL, however, the presence of oxygen does not alter the 

 tendency to form peroxides and the yield of OH and O2H will be small. 

 Presence of oxygen should have less effect on radiosensitivity at high 

 REL. The intelligent use of approximate knowledge of ionic yields 

 should be helpful, even in the absence of detailed chemical knowledge, 

 in predicting radiation protection of added substrates on living 

 organisms. 



There is a continuity of procedure between evaluating the magnitude 

 of radiation effects on isolated sensitive sites and extragenic effects on 

 cytoplasmic constituents, for example enzymes. In the latter case a 

 fairly large number of individual enzyme molecules may be present in 

 the cell, and a known or measurable fraction of these may be inactivated 

 by the radiation. 



Calculations based on the diffusion model can provide varying shapes 

 of survival curves, usually single- or multiple-hit curves. 



