PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 353 



present in the photosynthetic particle [94]. This "terminal" cytochrome 

 component, i.e. a cytochrome that is adjacent to, and interacts with, the 

 excited chlorophyll molecule, becomes oxidized after donating an electron 

 to chlorophyll. We have visualized [94] that phosphorylation is coupled 

 with the oxidation of the terminal cytochrome, in a manner analogous to 

 the phosphorylations which accompany the oxidation of cytochromes by 

 oxygen in mitochondria [44]. Thus, chlorophyll (with the aid of light) is the 

 ultimate oxidant in photosynthetic phosphorylation and plays a part which 

 corresponds to that of molecular oxygen in oxidative phosphorylation 

 [cf. 97]. The terminal phosphorylation reaction is represented by 

 equation (3). 



2[Chl] + + zFe' -cyt + ADP + H3PO4 -> ATP + zChl + 2Fe'^ +cyt (3) 



(^-) 



i^PMS 



Chi ff ^*) t ^ M ^ Cyt 



LIGHT -f-2DP "Xz) 



Cyclic photophosphorylation (PMS type) 



Fig. 4. Scheme for anaerobic cyclic photophosphorylation catalyzed by 

 phenazine methosulphate (PMS). Details in the text. 



The photophosphorylation reaction would leave the cytochrome in the 

 oxidized state. Since cvtochromes are present in catalytic amounts, cyclic 

 photophosphorylation would soon cease unless the cytochrome could 

 become reduced again. Our theory provides that in cyclic photophos- 

 phorylation the reduction of cytochrome occurs by the return of the elec- 

 tron originally "expelled" from chlorophyll in the primary photochemical 

 reaction (equations (2) and (2a)). 



In isolated chloroplasts, the reduction of cytochrome by the electron 

 expelled from chlorophvU 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 methosulphate. Phenazine methosulphate is known to be a very 



