DANIEL I. ARM ON 509 



a "high-energy" pyrophosphate bond is formed at the expense of 

 absorbed light energy. There is no need, a priori, to connect this re- 

 action either \\ith photolysis of water or with reduction of CO^. 

 Photosynthctic phosphorylation catalyzed by phenazine methosulfate 

 or vitamin K produces neither a reductant for CO2 assimilation nor 

 molecular oxygen; the sole product is ATP. 



The simplest hypothesis to account for the formation of ATP in 

 photosynthctic phosphorylation is to assume that, as in the dark 

 phosphorylations of glycolysis and respiration, the formation of a 

 pyrophosphate bond is also coupled with a release of free energy 

 which occurs during electron transport, i.e., when an electron drops 

 from the higher energy level (that it has when it resides in the elec- 

 tron donor molecule) to the lower energy level that it assumes on 

 joining the electron acceptor molecule. But a mechanism for photo- 

 synthetic 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, 

 photosynthctic phosphorylation consumes neither exogenous substrate 

 nor molecular oxygen, only light energy. 



A mechanism for photosynthctic phosphorylation must, therefore, 

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

 acceptor in the primary photochemical act when radiant energy is 

 absorbed by chlorophyll. Investigations of photosynthesis at the 

 cellular level, in which the main preoccupation has usually been with 

 COo assimilation and oxygen evolution, led to no cogent theory of 

 the primary act of photosynthesis that would fit the experimental 

 facts of photosynthctic phosphorylation. As summed up recently by 

 Livingston, "physiologists and biochemists appear to believe that this 

 question (the primary act of photosynthesis) was answered long ago 

 by physicists while physicists find the problem distressingly com- 

 plicated and therefore uninteresting" (93) . 



The mechanism of photosynthctic phosphorylation that we have 

 proposed (9) regards the photosynthctic particle, chloroplast or bac- 

 terial chromatophore, as a "closed" catalytic system. We have sug- 

 gested that during the primary photochemical act one component of 

 the "closed" system, chlorophyll (bound to protein) , becomes ex- 

 cited on absorbing a photon and "expels" one of its electrons that 

 has been raised to a higher energy level. The excited chlorophyll 

 thus -becomes the electron donor. On losing an electron, chlorophyll 

 assumes an oxidized state, and in this way also becomes the electron 

 acceptor in photosynthctic phosphorylation. 



