DIFFUSION MODEL 371 



Recently much emphasis was placed on the findings that some 

 externally applied chemical agents have biological effects qualitatively 

 similar to those of radiation, particularly as far as inhibition of cell 

 division and production of genetic effects are concerned. Such situations 

 may be also handled similarly to the above treatment, using the diffusion 

 model. The chief difference is in the boundary values of the diffusion 



Cell — ' ' — Poison 

 Nucleus 



Fig. 7. Diffusion model of the action of a poison on the cell. When the cell is placed 

 in a medium containing a uniform concentration of poison, migration of the poison 

 through the cell wall and cytoplasm begins toward the nucleus. This figure is a 

 graphic representation of the concentration gradient of the poison in the cell. 

 Whether or not the effect is genie, dose-effect relationships may be calculated using 

 the diffusion model wdth the proper boundary conditions. 



problem. Figure 7 shows a crude representation of this problem. When 

 a cell is placed in a medium containing the external agent, diffusion 

 through the cell membrane into the interior of the cell begins; the 

 density of the blackening in Fig. 7 shows a plausible concentration dis- 

 tribution of the poison in the process of diffusing to the nucleus of the 

 cell. Consideration of the figure shows that, because of the larger 

 volume of the cytoplasm to be crossed by the poison, the shape of the 

 "survival" curve as a function of the amount and duration of poisoning 

 might be different from the radiation survivals, and also that in some 

 cases the cell wall and cytoplasmic constituents might effectively protect 

 the nucleus even at the expense of their own destruction. Some German 



