140 



MICROSOMAL PARTICLES 



different intervals of time by means of a small syringe and filtered through an 

 S and S filter. The filters, after being stored to dry, were counted in the 

 ordinary way. 



For sulfate the same procedure was adopted because it was thought worth 

 while to compare phosphate and sulfate uptake under similar conditions. For 

 the uptake of sulfides we encountered considerable difficulty, as was pointed 

 out by Cowie, in that the sulfide very readily formed sulfate and does not stay 

 in its original form. We overcame this to some extent, though not always con- 

 sistently, by having an excess of cold sulfide present and by working as rapidly 

 as was consistent with the experiment. When fresh radioactive sulfide and 

 fresh sodium sulfide were used the data indicated a very clear difference from 

 those obtained with sulfate and could also be made to give consistent results 

 though not quite to the same degree as sulfate uptake. 



EXPERIMENTAL RESULTS 



The form of an uptake curve for phosphate is shown in figure 3. The ex- 

 periments differ from those of the previous paper in that there is no limitation 

 in the amount of tracer and so the bacteria can continue to incorporate phos- 

 phorus throughout a whole period of 2 hours. It can be seen that for the con- 

 trol and the slightly irradiated points there is an exponential uptake of P 32 but 

 that at the higher doses the uptake becomes linear. By comparing the early 

 slopes of the lines a survival ratio can be obtained which can then be plotted 



2000-- 



I600-" 



i- 

 z 



O 



o 



1200 -- 



800- - 



400" " 



PHOSPHATE 



CONTROL 

 /2,900r 



20 40 60 80 100 



TIME OF UPTAKE 



I20MIN. 



Fig. 3. The results of one individual experiment on phosphate uptake. The control and 

 least-irradiated cultures show exponential uptake; the more heavily irradiated cultures show 

 reduced linear uptake. 



