414 BIRGIT VENNESLAND 



oxidation-reduction dismutations, and that these are associated with the 

 photophosphorylation process [10, 11]. The apparent association of photo- 

 phosphorylation with electron transport or flow suggested an obvious 

 analogy to mitochondrial oxidative phosphorylation. The simplest way of 

 picturing this analogy is to insert the ATP-generating step into an oxida- 

 tion-reduction back reaction between the oxidant and reductant generated 

 in the light, as shown in Scheme 3. 



OX > +0.. 



R+OX + Pi + ADP >OR + X+ATP 



Scheme 3 



This is the interpretation which was adopted initially and which has been 

 generally accepted [8, 11-14]. ^^ ^^ consistent with the fact that the O2 

 evolving step is not a required part of the photophosphorylation reaction. 

 It explains why chromatophores and green grana have this step in common, 

 that is — because they both cause an appropriate oxidation-reduction 

 dismutation — and it explains why the overall Hill reaction may be inhibited 

 without necessarily inhibiting photophosphorylation. 



With a mechanism such as that depicted in Scheme 3, the function of 

 the cofactor is to serve as a bridge between the reductant, R, and the 

 oxidant, OX, which are regarded as built-in chloroplast components. The 

 need for such a bridge highlights the separation of R and OX in the 

 chloroplast. Whatever the cause of this separation, whether it be geometry 

 or the absence of an adequate catalyst, R and OX must not back-react freely 

 or the system would not be able to store any chemical energy. The cofactor 

 is pictured as closing a gap and thus permitting electron flow along an 

 electron transport chain from R to OX. The generation of high-energy 

 phosphate bonds is considered to occur in association with the electron 

 flow, in analogy to mitochondrial oxidative phosphorylation. 



Though it is a reasonable picture, Scheme 3 cannot be regarded as a 

 statement of known fact. We do not actually know the site of high-energy 

 phosphate bond generation. All that can be said with reasonable certainty 

 is that reaction [3] occurs and that the cofactor is alternately reduced and 

 oxidized while this reaction takes place. It has been suggested more 

 recently [15-17] that the generation of high-energy phosphate bonds may 



