PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 389 



From the standpoint of cellular physiology it was interesting to 

 contrast the role of oxygen in ATP formation in photosynthesis with that 

 in respiration. The participation of oxygen as the terminal electron 

 acceptor in oxidative phosphorylation has conferred a marked superiority 

 on respiration over fermentation, in the efficiency of converting the free 

 energy of substrate into the energy of the pyrophosphate bonds of ATP. 

 Was the efficiency of conversion of light energy into ATP also increased 

 by the presence of oxygen : 



To answer this question, photophosphorylation by chloroplasts was 

 investigated in air and in nitrogen, at different concentrations of FAIX or 

 vitamin K, and particularly, at a limiting light intensity, when the efficiency 



LIMITING LIGHT orx) HIGH CHLOROPHYLL 



0003 001 005 01 ,0-3 

 //moles vit, K3 added 



0003 001 0-05 0-1 3 10 

 //moles FMN added 



Fig. 28. Effect of FMX and vitamin K^ concentration on cyclic photophos- 

 phorylation by spinach chloroplasts in nitrogen and air, at a low light intensity. 

 The reaction mixture included, in a final volume of 3 o ml., chloroplast fragments 

 (Cij) containing 1-5 mg. chlorophyll; and in micromoles : tris buffer, pH 8-3, 

 80; MgS04, 5; K.H3-PO4, 15; ADP, 15; TPX, 03 (only in the FMN series). 

 FMN or vitamin K^ was added as indicated. The reaction was run for 30 min. at 

 an illumination of 2 000 Lux (Tsujimoto, Hall, and Arnon, [92]). 



of the energy conversion process could be best observed (compare Sec- 

 tion 8). The results are shown in Fig. 2S. 



In limiting light, the highest rate of photophosphorylation was obtained 

 in nitrogen at a concentration of approximately 10^ m of either FMN or 

 vitamin K. No photophosphorylation occurred in nitrogen without added 

 cofactors but when these were added at an optimal concentration, the 

 anaerobic system was about twice as efficient in con\erting light energy 

 into ATP as the aerobic system. 



The experiments represented by Fig. 28 were carried out with relatively 

 high concentrations of chloroplast material. Under these conditions the 

 aerobic system showed little increase in photophosphorylation from adding 

 FAIN or vitamin K. However, high concentrations of chloroplast material 

 were found to be necessary to insure the effective operation of the anaerobic 

 FMN system. The anaerobic vitamin K system functioned optimally at 



