PHYSIOLOGICAL STUDIES 73 



of carbon dioxide. All photochemical intermediates are 

 postulated as back reacting unless 'stabilized' by a catalyst, 

 catalyst B, whose amount limits the rate of photosynthesis 

 at high light intensities and therefore determines the maxi- 

 mal yield in brief light flashes. After effecting 'stabilization' 

 this catalyst is postulated as becoming inactive and only be- 

 comes active again as a result of a spontaneous unimolecular 

 reaction. The velocity constant of the latter determines the 

 dark time necessary between flashes for the attainment of 

 maximum yield. 



catalyst A ^ ^ ^^^ 



1. RH+CO2 ^R.COOH 



light 



2. H.Chl.R.COOH^ V Chl.R.COOH„+i 



catalyst B 



R'OH+Chl.R.COOH„+i — !^fl^l^ H.Chl.R.COOHn+i+R'O 



catalyst B 

 n = 1, 2, 2, 



3. 4R'0+2H20 - 2R'OH+2R'OOH 



2R'OOH - 2R'OH+02 



These three theories all agree in the division of the photo- 

 synthetic process into three distinct steps. All postulate a 

 role for other substances in addition to chlorophyll each of 

 which may limit the rate under certain conditions. 



The further development of the subject and the more 

 specific interpretation of these general ideas as to the 

 mechanism of photosynthesis came in the first place from 

 studies in the comparative biochemistry of photosynthesis 

 and later from studies with radioactive tracers and with 

 chloroplasts isolated from the cell. 



