298 D. I. ARNON, M. B. ALLEN, F. R. WHATLEY 



physiological coiulitioiis when neither photosynthetic phosphoryla- 

 tion nor CO2 fixation takes place. 



This postulated increasing order of complexity is supported by the 

 experimental separation of the three chloroplast reactions (equations 

 1, 2, and 3) by means of differential inhibitors. Chloroplast prepara- 

 tions capable of carrying out complete photosynthesis were used in 

 three parallel series of experiments in which CO2 fixation, photosyn- 

 thetic phosphorylation, and photolysis (Hill reaction) were meas- 

 ured separately. It was possible to inhibit a more complex phase of 

 photosynthesis without affecting the simpler one which preceded it 

 and, conversely, inhibition of a simpler phase of photosynthesis was 

 invariably paralleled by an inhibition of the more complex phase 

 which followed it (Table I). Thus iodoacetamide and arsenite in- 

 hibited CO2 fixation but not photosynthetic phosphorylation or the 

 photolysis reaction. Dinitrophenol inhibited photosynthetic phos- 

 phorylation and CO2 fixation more than photolysis. Methylene blue 

 and p-chloromercuribenzoate inhibited both CO2 fixation and photo- 

 synthetic phosphorylation but not the photolysis reaction. On the 

 other hand o-phenanthroline, which inhibited photolysis, also in- 

 hibited photosynthetic phosphorylation and CO2 fixation. A more de- 

 tailed discussion of the effects of inhibitors on photosynthesis by 

 chloroplasts is given elsewhere (3). 



As shown in Fig. 1, it is envisaged that the recombination of the 

 products of decomposition of water in photosynthetic phosphoryla- 



TABLE I. Differential Inhibition of Photolysis, Photosynthetic Phosphorylation, 



and CO2 Fixation 



