PHOTOSYXTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 35 I 



to /)-chloromercuribenzoate [cf. 89]. However, unlike chloroplasts [50, 

 88, 89], photophosphorylation by Chromatium particles was, under 

 our experimental conditions, resistant to inhibition bv methvlene blue 

 [cf. 66]. 



6. The electron flow mechanism of photosynthetic 

 phosphorylation 



Photosynthetic phosphorylation has provided direct experimental 

 evidence for the view that the key event in photosynthesis, the conversion 

 of light into chemical energy, is independent of the classical manifestations 

 of this process in green plants; oxygen evolution and COo assimilation. If 

 it is accepted that photosynthetic phosphorylation represents the simplest 

 common denominator of photosynthesis in green plants and bacteria, then 

 a mechanism for this process would be expected to provide a basic pattern 

 for the conversion of light into chemical energy. The salient facts which 

 must be explained are that a " high energy" pyrophosphate bond is formed 

 at the expense of absorbed light energy. There is no need, a priori, to 

 connect this reaction either with photolysis of water or with reduction of 

 COg. Photosynthetic phosphorylation catalyzed, for example, by phenazine 

 methosulphate or vitamin K, produces neither a reductant for COo 

 assimilation nor molecular oxygen ; the sole product is ATP. 



The simplest hypothesis to account for the formation of ATP in photo- 

 synthetic phosphorylation is to assume that, as in the dark phosphorvla- 

 tions of glycolysis and respiration, the formation of a pvrophosphate bond 

 is also coupled with a release of free energy which occurs during electron 

 transport, i.e. when an electron drops from the higher energv level (that 

 it has when it resides in the electron donor molecule) to the lower energy 

 level that it assumes on joining the electron acceptor molecule. But a 

 mechanism for photosynthetic phosphorylation must also account for its 

 unique features : ATP is formed without the consumption of an exogenous 

 electron donor and electron acceptor. Unlike oxidative phosphorylation, 

 photosynthetic phosphorylation consumes neither exogenous substrate 

 nor molecular oxygen, only light energy. 



A mechanism for photosynthetic phosphorylation must, therefore, 

 provide for the generation of both an electron donor and an electron 

 acceptor in the primary photochemical act when radiant energv is 

 absorbed by chlorophyll. Investigations of photosynthesis at the cellular 

 level, in which the main preoccupation has usually been with CO2 assimila- 

 tion and oxygen evolution, led to no cogent theory of the primarv act of 

 photosynthesis that would fit the experimental facts of photosynthetic 

 phosphorylation. As summed up recently by Livingston "physiologists 

 and biochemists appear to believe that this question (the primary act of 



