204 



Daniel I. Arnon 



by the use of an inhibitor (CMU) at 66'^ m\x or by the use of far- 

 red monochromatic light (708 m^) . A possible explanation of 

 these effects of oxygen will be given later. 



Roles of chlorophylls a and b in chloroplast electron trans - 

 port . The results obtained with monochromatic light support and 

 extend our earlier tentative conclusion (8) that the participa- 

 tion of chlorophyll b is essential for oxygen evolution but not 

 for TPN reduction and ATP formation. 



The diagram shown in Fig. 9 incorporates the new data into 

 our 1961 scheme of electron transport in chlorop lasts (Fig. 3 in 

 ref. 7). We envisage that noncyclic photophosphorylation by 

 chloroplasts is coupled with an "uphill" flow of electrons from 

 water (0H~) to TPN--an electron flow which is driven by two 

 photochemical reactions working in series (cf. 57). The first 

 photoreaction lifts electrons from the redox potential (at pH 7) 

 of water-oxygen (E'q = 0.82 V) to that of plastoquinone (E^'^'O V) 

 and requires the participation of chlorophyll b (henceforth re- 

 ferred to as photoreaction B) . The second photoreaction lifts 

 electrons from the level of cytochrome f (E'q = O.365 V) to that 

 of ferredoxin (E'^ = -0.^3 V) and is driven by the chlorophyll a 

 pigment system (henceforth referred to as photoreaction A). A 

 "primary" phosphorylation site, common to both the cyclic and 

 noncyclic photophosphorylation pathways, is considered to be 

 coupled with a "downhill," dark electron transfer--one favored 

 by the thermodynamic gradient--which joins the two photochemical 

 reactions and probably involves a transfer of electrons from 

 plastoquinone (Q) to the chloroplast cytochromes and thence to 

 chlorophyll a (long black arrow in Fig. 9). In addition, the 

 ferredoxin-catalyzed cyclic photophosphorylation is envisaged as 

 having a least one more phosphorylation site, coupled with the 

 electron transport sector that is marked in Fig. 9 by a broken 

 line. 



Role of oxygen in cyclic photophosphorylation . The differ- 

 ential oxygen effect on ferredoxin-catalyzed cyclic photophos- 

 phorylation at 663 and 7O8 mn, mentioned previously, is explained 

 by the following hypothesis. Noncyclic electron flow in chloro- 

 plasts is a unidirectional electron transfer from water to TPN 

 and is driven by both photoreactions B and A. The problem of 

 "overreduction" of intermediate electron carriers does not arise 

 as long as the terminal electron acceptor, TPN, is available. A 

 different situation, however, arises in the case of cyclic photo- 

 phosphorylation. To maintain a cyclic electron flow from reduced 

 ferredoxin back to the electron transport chain, the intermediates 



