755 



SUMMARY 



plained by cyclic photophosphorylation maintaining a high 

 ATP/ADP ratio inside the cell, thereby stopping electron flow 

 in tightly coupled mitochondria (Hoch). This control of res- 

 piration by a photosynthetic process is thus analogous to the 

 control over glycolysis by oxidative phosphorylation. 



Cyclic electron flow with isolated chloroplasts has been 

 observed with a physiological mediator, ferredoxin (Arnon). 

 Its significance for the in vivo mechanism is obscure, be- 

 cause of the low rates, sensitivity to oxygen, and unique 

 inhibition by low concentrations of antimycin A, 



The actual mechanism of ATP formation is still mysterious. 

 It was shown that a large pool of a non-phosphorylated, high 

 energy intermediate can be formed at pH 6 in the light, and 

 that the intermediate can be used for ATP formation subse- 

 quently, in the dark. The obligate role of this intermediate 

 in ATP formation, or possible alternative functions for it, 

 are still unknown (Jagendorf). Other aspects of the phos- 

 phorylation mechanism are exposed in the discovery of light 

 induced ATPases in chloroplasts, and in the finding that the 

 existence of high energy intermediates in the chloroplasts 

 causes an increase in light scattering, probably due to 

 structural changes (Packer). 



With respect to the site of the phosphorylation mechanism 

 in the electron transport chain, two possibilities are 

 actively considered. One (probably the favorite for most) is 

 in electron transfer from the primary reductant (E^ of 0.0 V) 

 made by photosystem II, to the P700 complex (Eq of + 0.^3 V). 

 Experimental proof does not yet rule out the alternative 

 possibility of phosphorylation associated with the re-oxida- 

 tion of the low potential primary reductant formed by photo- 

 system I which would have to have a potential of -0.6 V to 

 obtain sufficient energy in re-oxidation by ferredoxin or by 

 methyl viologen. 



Since clear evidence as to the number or position of phos- 

 phorylation sites is lacking, care should be taken to avoid 

 making definite commitments at the present time. Studies 

 with inhibitors or uncoupling agents such as HQNO or CCP 

 (Avron) as well as previous work with PCMB, do suggest that 

 there might be more than one phosphorylation site. 



The two photoreactions thus appec^r to be linked together 

 by reciox reactions and associated phosphorylation. An addi- 



