no SUBCELLULAR PARTICLES 



difference found between bacterial chromatophores and green plant chloroplasts. 

 It is likely that both systems function identically, or very similarly, in effecting 

 photophosphorylation. 



We may discuss briefly the mechanisms which may be operative on the basis of 

 the facts presented and the comparative biochemistry of photosynthesis. The vari- 

 ous observations recorded can be fitted into a scheme in which it is supposed that 

 the phosphorylating particles contain a system of electron transport carriers com- 

 prising a 'chain' of interacting oxidation-reduction (redox) systems. It seems rea- 

 sonable to suppose that an optimal steady-state relation exists between oxidized 

 and reduced forms of the interacting electron carriers (8). Upon illumination of 

 the chromatophores, both electron donors and acceptors are made available to 

 this system because it is coupled so closely to the photo-active pigments (19). The 

 resultant recombination reaction, or back-oxidation, is mediated by this electron 

 transport chain, resulting in accumulation of ATP. In green plants, only a 

 small fraction of the electron transport possible is required for the ATP needed 

 to activate the biosyntheses ascribed to the photochemical apparatus; accordingly, 

 most of the photo-oxidant can be dispensed with as oxygen, while the unreacted 

 photo-reductant is utilized for reductive assimilation of carbon dioxide. In the 

 bacteria, no oxygen-evolving system is present. It seems possible that one of the 

 several important functions of the added accessory hydrogen donor is to prevent 

 destructive peroxidation reactions, which, as a corollary, may inhibit metabolism 

 by over-oxidizing components of the photo-activated electron transport chain. 



It is not certain that the electrochemical potential difference generated by the 

 light reaction between photo-reductant and oxidant is the same in green plants 

 and photosynthetic bacteria. That is, there may be oxidants of different electro- 

 chemical potentials produced. In plants the terminal oxidant certainly has a po- 

 tential close to that of the oxygen electrode at physiological pH (Eo= +0.8 v) 

 so that the full potential difference between the hydrogen and oxygen electrode 

 potentials (approximately 1.2 v) may be available. The electron transport chain 

 coupled to the chloroplast system may span this whole range so that photophos- 

 phorylation efficiency is maximal for each electron transferred. On the other hand, 

 the bacterial photo-oxidant may be generated at a considerably lower electro- 

 chemical potential, particularly in the strict anaerobes, so that a much smaller 

 potential span is available in bacterial photosynthesis. Hence, the photophosphory- 

 lation efficiency in the bacteria may be significantly less. This may be correlated 

 with the fact that although green plants dissipate most of their photo-oxidant as 

 molecular oxygen, they still make enough ATP by photophosphorylation to 

 satisfy all requirements for carbon dioxide assimilation. On the other hand, the 

 bacteria with a less efficient system may require all their photo-oxidant to be 

 reduced through the electron transport system. 



We have remarked that the electron transport chain couples maximally when 



