2 PHYSICAL PROCESSES IN LIVING MATTER 



With ultraviolet a similar effect would be produced by a steady rain of 

 quanta, with a few million photons independently absorbed more or less 

 uniformly in the protoplasmic molecules, excluding the water but some- 

 what concentrated in the side chains of the nucleoproteins. It is no 

 wonder that these very different first steps in the distribution of the 

 available energy lead to differing mechanisms of the consequent effects; 

 it is perhaps more remarkable that the final effects can be so similar. 

 Doubtless this simplicity is in large part an artifact of our gross means 

 of observation. 



1000 rep 100 rep 



ofPoa's of Coy's 



1 micron 



Fig. 1. The distribution of disturbed atoms immediately after irradiation of an 

 E. coli cell with dosages indicated. Note the great difference in spatial distribution 

 and the various delta rays, scattered electron paths, etc. 



The history of radiobiology has led to the emphasis on ionization as 

 the measure of absorbed energy, since, in the x-ray domain especially, 

 the means of measurement depended on the easy collection of ions in 

 air. The roentgen and its definition in terms of ion pairs produced have 

 fixed this point of view; but we shall use the roentgen, or perhaps better 

 the so-called rep, as a measure of the density of energy absorbed. The 

 generality of the definition is evidently helpful; to say that when 93 

 ergs per gram of ordinary tissue is the energy density absorbed we have 

 1 rep is to connect this physical factor with all the biological experience. 

 But it must be evident that the description of the complex class of radia- 

 tion reagents by the absorbed energy density alone is inadequate, even 

 if we have already far extended the ion-pair definition of the radiologists. 

 We know that qualitative differences can exist among radiations and 

 their effects, even for identical gross energy transfer. The examples 

 above for E. coli colony growth corresponded in energy units to about 

 5000 rep for the x-rays, 20,000 rep for the alphas, and roughly half a 

 million rep if we stretch the concept to include the non-ionizing ultra- 

 violet. The last figure is equivalent to a temperature change of 

 about 1°. 



With this general picture in mind, we shall here attempt to outline the 

 principal mechanisms for the transfer of energy from the hot beam of 



