DANIEL I. ARNON 



511 



mitochoiulria (DO) . Thus, chlorophyll \\\i\\ tjic aid of light is the 

 ultimate oxidaiu in i)liotosynthetic phosphorylation and plays a part 

 which corresponds to that of molecular oxygen in oxidative phos- 

 phorylation (( f . 71) . The terminal phosphorylation reaction is repre- 

 sented by Equation 7. 



2[Chl] t^ + 2Fe-^ + cyt + ADP + Hj^PO^ -> 



ATP + 2Chl + 2Fe3 + cyt {4) 



The photophosphorylation reaction would leave the cytochrome in 

 the oxidi/ed state. Since cytochromes are present in catalytic amounts, 

 cyclic photophosphorylation would soon cease unless the cytochrome 

 could become reduced again. Our theory provides that in cyclic 

 photophosphorylation the reduction oi cytochrome occurs by the 

 return of the electron originally "expelled" from chlorophyll in the 

 primary photochemical reaction (Equations 3 and 3a) . 



In isolated chloroplasts, the reduction of cytochrome by the elec- 

 tron expelled from chlorophyll requires an added catalyst, i.e., an 

 intermediary electron carrier. In the simplest case, as shown in the 

 scheme in Fig. 4, the part of the electron carrier is played by a non- 

 physiological catalyst, phenazine methosulfate. Phenazine methosul- 

 fate is known to be a very effective electron cairier in reactions in- 

 volving cytochromes. For example, Massey (105) has found that 



lej 



i^PMS 



LIGHT ~P-^DP K^TP 



Cyclic photophosphorylation (PMS type) 



Fig. 4. Scheme for anaerobic cyclic photophosphorylation catalyzed by 

 phenazine methosulfate (PMS). Details in the text. 



