366 ENERGY LOSS AND BIOLOGICAL EFFECTS 



division is almost certainly a chain reaction initiated by changes in a 

 few molecules. How such chains of events develop in the cell we do not 

 know. All one may do is to trace out a few of the initial steps that occur 

 between irradiation and inactivation of essential molecules. In organisms 

 having high percentage of water one may draw an analogy between 

 radiation effects on water and on the cell. 



The physical basis of the diffusion model is very simple. In the proc- 

 ess of ionization, the lifetime of the ion pairs and excited atoms formed 

 is very short. Through mechanisms discussed by the radiation chemistry 

 section, the primary initial positive ion and negative electron and some 

 of the excited molecules may within a very short time interval give rise 

 to radicals and by chemical combination of these to more or less stable 

 molecules such as peroxides, which because of their ability to interact 

 further, act as "intermediates" in the production of the biological effects. 

 The exact chemical properties of these ionization products are of no 

 significance at the moment, except that it is important to find out how 

 many ion pairs are needed to form each intermediate ionization product. 

 This depends on the spacing of individual ion pairs of the positive and 

 negative ion columns, and on the chemical composition of the medium. 



The intermediate ionization products are usually longer lived than the 

 ion pairs themselves, and this is one reason why they are conceivably of 

 importance. Because of their thermal Bro^vnian motion, they will 

 migrate through the medium until they find suitable molecules for inter- 

 action, molecules from the cell medium or other ionization products. 

 Even if it is accepted that free radicals and other intermediates are the 

 most frequent immediate result of ionization, one immediately gets into 

 serious difficulties when one attempts to predict their physical and 

 chemical properties. The intermediates can obviously migrate, but 

 what are the laws of such migration? Even in pure water prediction of 

 the behavior of the radicals is a serious problem. Dissolved substances 

 are in a "solvent cage." When they migrate through the medium they 

 carry some water molecules along. When they find another solute 

 molecule in their migration, the two solute molecules may stay together 

 in the solvent cage for a reasonably long time, which they would be 

 unlikely to do if the approach of the molecules were in the gas phase. 

 In living protein, the laws of migration may be much more complex than 

 those for water. If one accepts some of the views of Delbruck (39), one 

 is forced to admit that protoplasmic structure and behavior may be very 

 different in live cells and in vitro. Not knowing the mechanism of migra- 

 tion and of interaction of solutes in cellular matter, one might proceed 

 by assuming the simplest admissible relationships and attempt to make 

 predictions based on these. Thus one might assume that in cells the 



