322 



CATALYST POISONS AND NARCOTICS 



CHAP. 12 



surface activity increases regularly with the increase in size of the radical 

 R; and the narcotization efficiency increases parallel with it. According 

 to Warburg, the quantities of urethan required to reduce respiration in 

 Chlorella are about three times as large as those which cause a similar 

 reduction of photosynthesis (c/. Table 12. VIII). 



Table 12.VIII 

 Inhibition of Respiration and Photosynthesis in Chlorella by Urethans 



Figure 30 shows the effect of increasing quantities of phenylurethan 

 on photosynthesis. The respiration of Chlorella is stimulated by phenyl- 

 urethan in quantities from 0.001 to 0.025%; it reverts to its normal rate 

 at 0.05%, and is inhibited by higher concentrations. Noack (1925) 

 found that Fontinalis, pretreated with urethan, becomes susceptible to 

 photoxidation even in the presence of carbon dioxide (c/. page 528). 



According to Warburg (1920) and Wassink, Vermeulen, Reman, and 

 Katz (1938), urethans inhibit photosynthesis equally efficiently at all 

 intensities of illumination. This is illustrated by figure 3 1 . The influence 

 of urethan is, according to Warburg, also independent of the concen- 

 tration of carbon dioxide. 



The inhibition of photosynthesis in weak light shows that urethan 

 prevents the transfer of excitation energy from chlorophyll to the reaction 

 substrate, thus interfering with the primary photochemical process. It 

 can do this either by enveloping chlorophyll molecules (or catalytic 

 complexes of which chlorophyll forms a part), or by protecting in a 

 similar way the "acceptors" on which the substrates are fixed while 

 awaiting photochemical transformation. The enhancing effect of urethan 

 on chlorophyll fluorescence in living Chlorella cells (which will be described 

 in Vol. II, Chapter 24) shows that urethan does come into contact with 

 chlorophyll during the excitation period of the latter. We may assume 

 either that urethan associates itself with chlorophyll, or that chlorophyll 

 molecules encounter, during their excitation period, photoactive sub- 

 strates with which they can react if they are not protected by urethan 



