640 



J. A. Bassham 



also declined, suggesting that some of the rise in PGA concentration might be 

 due to a momentary reoxidation of sugar phosphates in addition to that result- 

 ing from continuing carboxylation of ribulose diphosphate coupled with a cess- 

 ation of the reduction of PGA to sugar phosphate. 



To investigate this point further, we undertook a kinetic study of the car- 

 bon labeling of cycle intermediates in preilluminated Chlorella . Following a 

 period of steady state photosynthesis the light was turned off and 1^002 was 

 immediately added without any alteration in the total level of CO2 which was 

 maintained at about 2%, 



Some of the results of this experiment are shown in Fig. 2. There was a rapid 



5 10 15 20 30 40 50 60 70 80 90 100 

 Time after Addition of '"^002 (sec) 



Fig. 2. Fixation of ^'^C into compounds in Chlorella following prelllumination . 

 Sugar monophosphates include monophosphates of fructose, glucose, sedoheptulose, 

 ribose and ribulose. "Total l^C fixed" includes all radioactive compounds, 

 including; t hose on the origin, found on the chraaatcgram. 



initial uptake of I'^COn during the first 15 seconds. This was followed by a 

 tenporary decrease in l^C-labeling of the canpounds. Presumably sane of the 

 newly incorporated I'^C was respired. After this drop, a slight loss of newly 

 incorporated l^C in such products as sugar mcnonhosphate occurs during the per- 

 iod from 15-30 seconds. Thereafter, l^C in these compounds remains essentially 

 constant . 



