142 METABOLISM AND PHYSIOLOGY 



Reasoning on the basis of redox potentials, the formation of H2 from 

 succinate by illuminated cells of purple bacteria would be expected 

 to require an even greater input of energy than the reduction of NAD. 

 Although we could calculate the probable magnitude of the energy re- 

 quirements for such reactions, or for formation of H2 from NADH2, it 

 seems wise to heed Mansfield Clark's admonition (55) in this regard. 

 He advises us to: "Beware when the attempt is made to apply such 

 [thermodynamic] data too casually to the dynamic affairs of living cells 

 and to such heterogeneous systems as are living cells." In any event, 

 the contribution of the photochemical system to light- stimulated net 

 electron flow in bacterial photosynthesis can be reasonably visualized 

 in terms of energy-dependent "reverse" electron transfer (see Fig. 2). 



Hydrogen (electron) 



donor; inorganic 



or organic 



Fig. 2. Scheme for hydrogen (electron) flow from donors to acceptors and 

 photoproduction of H2 in bacterial photosynthesis. The pyridine nucleotide may 

 be either NAD or NADP; for convenience, only the former is shown. 



This hypothesis assumes that electrons (or hydrogen) required for 

 net generation of reduced pyridine nucleotide are derived from an ac- 

 cessory inorganic electron donor or organic compounds. Depending 

 on the redox potential of the donor, or on the steady- state concentra- 

 tions of the reduced and oxidized forms of donor and pyridine nucleo- 

 tide, the formation of NADH2 may be promoted by energy- rich inter- 

 mediates created by the action of light on the photochemical apparatus. 

 Parenthetically, it may be noted that a similar promotion of "reverse" 

 electron transfer by intermediates of oxidative phosphorylation could 



