PHOTOSYNTHESIS OF CARBON COMPOUNDS 



459 



unknown route), if it were a precursor to glutamic acid, and would have to be ap- 

 preciably labeled by the time glutamic acid reaches its maximum labeling rate. Yet 

 ije can detect no radiocarbon in y-aminobutyric acid in this experiment or in others 

 fy this series, even after the algae have been exposed to ^^COj for lo min. Clearly, 

 wottle if any of the labeled glutamic acid formed in our experiments is made b 



^ — i—i — i — * — > — * — ir-io 



Tims in Minuttt 



Fig. 8. Appearance of ^*C in PGA, glutamic acid and citric acid in Chlorella pyrenoidosa vs. time of 



photosynthesis with "COj. 



carboxylation of y-aminobutyric acid. Rather, it must arise from other intermediate 

 substances such as those formed by the two carboxylation mechanisms already 

 discussed. 



Note, however, that the rate of labeling of citric acid is by far too small to permit 

 it to be the precursor of the labeled glutamic acid in any sequence such as : 

 oxaloacetic acid + acetyl coenzyme A — >■ citric acid ->■->■->■ 

 a-oxoglutaric acid -|- COj — > glutamic acid 



Steady state Expt. 28 



All the results described thus far were obtained in an experiment (steady state 18) 

 in which the nutrient solution, though not automatically replenished, was sufficient 

 to maintain the rate of photosynthesis at a nearly constant level during the course 

 of the experiment. The results of steady state Expt. 28, in which the nutrient solution 

 was replenished during the course of the experiment led to the same conclusions. 



TABLE VI 



COMPARISON OF STEADY STATE EXPERIMENTS 1 8 AND 28 



Experiment 



COi uptake 



fimolesfminlml 



algae 



Rale of appearance 



cf "C in compounds on 



chromatograms 



(20-40 sec) 



RuDP 



saturation 

 at 40 sec 



PGA residual' carbon 

 saturalion according to 



Reaction D Reaction L 



I» 



28 



15-5 

 195 



13 

 17-if 



0-53 

 0.38 



0-57 

 0-43 



0.46 

 0.28 



* See subsequent discussion for explanation of the term "residual". The degree of saturation 

 at 40 sec is obtained by dividing the measured value of '*C in the compound at 40 sec by the 

 saturation level of **C in the compound (or residual atoms) after 10 min exposure of the algae 

 to "CO,. 



115 



