142 



MICROSOMAL PARTICLES 



CONTR< 



1500- 



SULFATE 



1000- - 



<n 



i- 

 z 

 z> 

 o 

 o 



500" 



39,000 r 



x„— — 78,000 r 



_o Ill.OOOr 



60 80 100 



TIME OF UPTAKE 



120 



rt 



140 MIN. 



Fig. 5. The initial uptake of sulfate at higher doses. For 10 minutes, even at a dose of 

 111,000 r, the uptake is normal. Thereafter, the rate of uptake breaks away from normal, 

 at earlier times for higher doses. The implication is that some synthesis can go on with 

 the original synthetic apparatus but that adequate provision for future synthesis is not made 

 in irradiated cells. 



The initial processes in the cell seem to be capable of continuing, but they run 

 out and the damage to the cell then becomes apparent. 



The incorporation of sulfide was studied in much the same way. The results 

 again look very similar to those found for sulfate except that still higher doses 

 were necessary to produce an effect, and also there was some indication that 

 the inactivation instead of being linear was curved. This might mean that 

 more than one process is involved in sulfide uptake. 



By way of check experiments, the number of colonies produced after irradi- 

 ation was also studied and the effect of radiation on the optical density was 

 measured. The optical density is that after a standard period of growth, usu- 

 ally about 80 minutes. 



The results of all these experiments can be seen in figure 6, where curves for 

 per cent remaining versus radiation dose in roentgens are plotted. It can be 

 seen that by far the most sensitive factor is the formation of colonies and that 

 this is followed, in order, by phosphate and sulfate uptake, optical density, 

 and sulfide uptake. Lastly, the effect on the methionine uptake is very small 

 indeed and can hardly be plotted on the graph. Analyzed in terms of the 

 probability of escape as mentioned earlier, the data are presented in table 1. 



