ALBERT W. IRENKEL 



589 



CHLOROPLASTS 



XH 



nhv 

 HjOH 



CHROMATOPHORES 



n'hY 



YOH- 



ADP+P; 



r 



^ATP 



"cyclic" e TS 



vPMS 



CMU 

 -^11- 



>0, 



Fe(CN): 



Fe(CN): 



^-^-ATP 



ADP + P, 

 TCPIP 



ox 



"non-cyclic" e TS 



TCPIPred 

 "oxidotive" e TS 



non-cyclic 

 e TS 



TPN+ TPNH °P^' °^^" ^^^^^ ^^^ 



Fig 1. Comparative representation of electron transport systems and associated 

 phosphorylation sites in chloroplasts and bacterial chromatophores. 



Chloroplast scheme after Jagendorf (11). XH, photochemically produced re- 

 ductant; YOH, photochemically produced oxidant (precursor of Oo); CMU, 

 p-chlorophenyl dimethyl urea; TCPIP, 2,3',6-trichlorophenol indophenol; PMS, 

 phenazine methosulfate: e TS, electron transport system; "oxidative" e TS, cf. Krog- 

 man and Vennesland (13). 



Chromatophore scheme (for Rhodospirillum rubrum). X'H, Y'OH, photochemi- 

 cally produced reductant and oxidant; cyt c.,. Rhodospirillum cytochrome c (1916) Q, 

 possiijle site of action of 3-iiydroxy, 1-heptyl quinoline-N-oxide and of Antimycin 

 .\ in blocking electron transport and light-induced phosphorylation (16); site of 

 action of these inhibitors bypassed by PMS (4). 



reason for as.suming that the manner in which electron flow is initiated 

 in illuminated bacterial systems should be basically different from 

 the mechanism which prevails in chloroplasts. The strongest argu- 

 ment in favor of an identical basic photochemical mechanism is the 

 work of Larsen, Yocum, and van Niel (14) who demonstrated that 

 the quantuin efficiency for bacterial photosynthesis was close to that 

 for photosynthesis of oxygen-producing plants, and that the quantum 

 yield of bacterial photosynthesis was independent of the chemical 

 potential of the exogenous hydrogen donor. It therefore appears 

 reasonable to assume that the quantum efficiency for the production 

 of an electron pair must thus be the same for chloroplasts as for 

 bacterial chromatophores, but one is then faced with the well-known 

 fact that the total light energy available to the bacterial system, per 

 unit number of effective quanta absorbed, will be only about 75 per 

 cent of that of the chloroplast system (based on the energy levels 

 of the first excited state of bacteriochlorophyll versus that of chloro- 

 phyll a) (Fig. 2) . Whether this difference in available energy [dis- 

 regarding possible two-quantum mechanisms and energy losses which 

 may or may not be the same for the two systems (15) ] is sufficient 



