PHOTOSYNTHESIS AND PHOSPHATE METABOLISM 



1705 



Elodea densa leaves instead of unicellular algae. They suggested that leaf 

 pieces can be freed from adsorbed radiophosphorus, by rapid rinsing before 

 killing, much more effectively than unicellular algae. After killing in 10% 

 TCA at o° C. the leaA'es were extracted three times with TCA, and the 

 P* activity determined in the total extract; the inorganic orthophosphate 

 (including perhaps some phosphate from very easily hydrolyzaV)le organic 

 esters) was precipitated by magnesium, with inactive orthophosphate as 

 carrier, and its activity measured; so was the P* activity of the remaining 

 TCA-soluble organic material and of the TCA-insoluble residue. 



T 

 20 



T 1 1 r 



40 60 80 100 



LIOMT INTENSITY (VOLTS) 



120 



-I 

 140 



Fig. 36.28. The effect of light on the steady ATP level in Chtorella (in 

 the presence of CO2, after 2-3 hours of anaerobic incubation). Measured 

 by the fiiefly extract luminescence method (after Strehler 1952). 



The experiments indicated a decrease in inorganic orthophosphate and 

 an increase in TCA-soluble organic phosphate in light; the effect was 

 stronger in the presence than in the absence of carbon dioxide (but was 

 definitely present also in the CO2 free system). The change could be ob- 

 served after 1-10 min. of illumination. An increase in TCA-insoluble^P* 

 (observed before by Kamen and Spiegelmann) was noted only after about 

 1 hour. The P* uptake in TCA-soluble fraction increased at first (and that 

 in an organic phosphate decreased) with light intensity, but reached 

 a constant level at about 2.5 klux. 



Strehler and Totter (1952) and Strehler (1953) (cf. also Strehler 1952) 

 applied to the problem of phosphate metabolism in photosynthesis the sen- 

 sitive techni(iue (discovered by McElroy) based on the stimulation of 

 chemiluminescence of firefly extracts by ATP. They estimated that in the 

 steady state of photosynthesis in Chlorella about one molecule ATP is 

 added to the cellular pool for every six molecules of liberated oxygen. If 

 one assumes that one (or two) ATP molecules are degraded to ADP to 

 make the liberation of one molecule of oxygen possible, the observed net 



