41 6 BIRGIT VENNESLAND 



coupled with the reduction of quinone, ferricyanide and TPN is shown in 

 Equation 4, 5, and 6, respectively. These equations have been balanced 



Q + ADP2- + Pf- + H+ >HoQ + ATP3 +1O0 (4) 



2Fe(CN)3- + ADP2- + Pf- > 2Fe(CN)^- + ATP3- + H+ + i02 (5) 



TPN + + ADP2 + Pf- > TPNH + ATP^ + iQ, (6) 



completely with respect to hydrogen ions. Note that water is not required 

 to satisfy the stoicheiometry (since the oxygen may be regarded as coming 

 from the phosphate), but that hydrogen ions are consumed during the 

 reaction with quinone and formed during the reaction with ferricyanide, 

 whereas there is no net formation or consumption of acid during the 

 reaction with TPN.* 



Equations 4-6 show that the P/ze ratio for all three reactions is one. 

 The photoreduction of TPN requires addition of a soluble protein to the 

 grana [32-34], and so may be assumed to be somewhat indirect, but 

 reactions 4 and 5 do not require such a soluble activator. This and other 

 evidence has shown that TPN is not a natural mediator in the reactions 

 with most other Hill reagents [30]. The photoreduction of TPN is dis- 

 cussed in the paper by Dr. Davenport, and so will not be further mentioned 

 here. It is generally agreed that the Hill reactions per se can readily be 

 dissociated or uncoupled from the generation of high-energy phosphate 

 bonds. Ageing of almost any preparation of chloroplasts or grana usually 

 (but not always) results in a loss of their capacity for catalyzing photo- 

 phosphorylation before the loss of their capacity for catalyzing the various 

 Hill reactions. Here again there is an obvious analogy to the behaviour of 

 mitochondria. 



The role of oxygen in cyclic photophosphorylation 



After the occurrence of stoicheiometric photophosphorylation had been 

 demonstrated, a re-examination of the process of cyclic photophos- 

 phorylation has led to the conclusion that it generally occurs in a manner 

 somewhat more complicated than that depicted in Scheme 3. If the chloro- 

 plasts have an unimpaired oxygen-evolving system, cyclic phosphorylation 



* The equations are written for one ionization state of each of the three forms 

 of phosphate (Pf~, ADP^", and ATP^~). Since there will be two ionization states 

 of phosphate present at the pH's generally employed for carrying out these reac- 

 tions, a complete description of the reaction would be more complicated than that 

 indicated. The overall change in acid-base balance indicated by the equation is, 

 however, largely correct. The equations have been balanced in detail to show what 

 is meant by the statement that the chemical nature of X and Y must be known 

 before we can say how the elements of water participate in their oxidation-reduc- 

 tion reactions. 



