CARBON-14 PREILLUMINATION EXPERIMENTS 



210 



dark, the tangents of the curves in the above-mentioned semilogarith- 

 mic plot represent rates of decay of R in the darkness. The effect of 

 cj^anide on the decay of R can be determined by comparing the 

 tangents of these curves. Once the magnitude of the effect of cyanide 

 upon the decay of R is kno\Mi, the suppressing effect of cyanide on 

 the steady-state R level can be estimated, since this level is de- 

 termined by the relative rates of formation and decay. The results of 

 this computation are sho^^^l in Fig. 9. For the sake of comparison, 

 corresponding data are included showing the effect of cyanide on the 



10"* KT* 10"' I0-' 



CONCENTRATION CM) OF CYANIDE 



Fig. 9. Concentration-inhibition curves of cyanide acting upon (i) the R-CO2- 

 reaction, (ii) stationary i2-level in the Hght, (iii) normal photosynthesis, measured 

 manometrically under the condition of hght- and C02-saturation, (iv) catalytic 

 H2O2 decomposition by intact Chlorella cells, measured titrimetrically with KMn04 

 after bringing the cells into contact with 0.015 M H2O2 for 30 seconds. All meas- 

 urements were made at 25°C. and pH 7.0. The degree of inhibition represents the 

 value 1 — vg/v, where Vg and v are the reaction rates or the i2-levels in the presence 

 and absence, respectively, of cyanide. 



reaction of R with C^^02, on the catalytic decomposition of H2O2 by 

 intact Chlorella cells, and on normal photosynthesis under conditions 

 of light and CO2 saturation. From these data w-e conclude that: 

 (a) cyanide has a dual effect upon R; first an acceleration of its decay 

 tending to lower the steady-state level of R in the hght, and second, 

 a hindrance of the reaction between R and C^''02, and (h) the sensi- 

 tivity of the R level toward cyanide is greater than that of the R-CO2 

 reaction, and indeed it is approximately the same as those of photo- 

 synthesis and of the catalytic decomposition of H2O2. 



We have seen that there are three kinds of reagents whose presence 

 entails more or less marked decrease of the concentration of R in the 



