1688 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



The tagging of triose and hexose phosphates was inhibited much more 

 strongly. It thus seems that cyanide establishes blocks not, or not only, 

 in the carboxylation reaction, but also, or mainly, in the subsequent trans- 

 formation of the carboxylated compounds. (This, of course, does not affect 

 the conclusion that cyanide affects the participation of carbon dioxide in 

 photosynthesis, and has little influence on the parts of the photosynthetic 

 process which do not involve carbon dioxide — such as the primary photo- 

 chemical process, and the reactions which lead to the liberation of oxygen 

 from water.) 



5 X 10-^ M hydroxylamine, added under similar conditions, produced 

 a 75% inhibition of total C* uptake. The tagging of malic acid was 

 least affected by this poison; the tagging of glutamic, succinic, fumaric 

 and citric acids was even enhanced. Calvin interpreted this as an indica- 

 tion that hydroxylamine removes a block that normally prevents, in light, 

 the utilization of photosynthetic intermediates as material for respiration 

 (c/. section 12), 



Gaffron, Fager and Rosenberg (1951) found cijanide to inhibit the post- 

 illumination C*02 fixation strongly, whether it was added during the illumi- 

 nation or after the light had been turned off. The inhibition affected par- 

 ticularly the water-soluble fraction, indicating (in some contrast to the 

 data of Calvin et al.) an inhibition of the PGA formation. Hydroxylamine, 

 on the other hand, inhibited the postillumination fixation of C*02 only if 

 given during the illumination period, but had no effect if given together 

 with the tracer after the return to darkness. 



One-minute photosynthetic C* fixation in Scenedesmus was constant, 

 according to Calvin et al. (1951), between pR 4 and 9; the rate dechned 

 by about 50% at pR 2 and 10, and dropped abruptly to zero at pH 10.5. 

 The rate-depressing effects of excess acidity (pH 2) or alkalinity (pH 10) 

 did not increase with time; cells could be kept at these extreme pH 

 values for 30 minutes, and the full rate of C* fixation restored upon return 

 to pH 7. 



The main changes in C* distribution caused by variations in pH were 

 an increase in the proportion of tracer found in malic acid (e. g., from 5% 

 at pH 1.6 to 25% at pH 11.4) and a drop in its proportion in sucrose, from 

 7 to 0%. The absolute amounts of tagged malic acid and phosphopyruvic 

 acid showed sharp maxima at pH 9. 



12. Evolution of the CO2 Reduction Mechanism 



Between 1946, when the C(14) tracer was first used systematically for 

 the study of photosynthesis, and mid-1954 (the time of the final revision of 

 this chapter), our knowledge of the chemical mechanism of carbon dioxide 



