35 



PHOTOSYNTHETIC ELECTRON TRANSPORT 

 Bessel Kok 



I, The first photoreaction of photosynthesis mediated by P700 



It is well established that in aerobic photosynthesis two 

 photoreactions occur. Photosystem I is sensitized by "long 

 wave" chlorophyll which feeds absorbed quanta into a special 

 long wave pigment "P700" (13). Upon excitation P700 loses an 

 electron and remains in a bleached form which exerts the prop- 

 erties of a weak oxidant (normal potential P/P"'"= 0.^3 volt). 

 The electron acceptor ("X~") is a strong reductant of a po- 

 tential lower than -0.42 volt (one can show that it reduces 

 methyl viologen) . Figure 1 and Table 1 bring quantitative 

 evidence that photoact I is mediated by P700: Fig. 1 shows 

 an experiment made by Hoch and Martin (l) with fresh chloro- 

 plasts. The reduction of TPN was studied as a function of 



Equivalence Between Rates of P7Q0 Turnover and TPN Reductio 



Reaction mixture (pH 7.8) contained per ml(in u. moles) : 

 Chlorophyll ; 0.0 17 ; TPN : 0. 5 ; ADP : 1 ; PO4 : 10 ; 

 Mg** : 2.5 ; PPNR .- saturating 

 Illumination 7 10 ml/ 



/leq. /min P700 = AO. P. 700 (A-B signal) x 1800 (RPM) 

 80 000 (E mol. ) x mg Chlorophyll 



fL eq. /min TPNH2 



AO D 340(B-W signal) 



3100 (E mol/2) x I{min) x mg Chlorophyll 



660 670 6B0 690 700 7)0 



WAVELENGTH (m/i.) 



Figure 1 Table 1 



wavelength both in the absence and the presence of the poison 

 DCMU. Reduced indophenol was added in the latter case to re- 

 lieve the DCMU inhibition (2). In the unpoisoned chloro- 

 plasts, two quanta of 65O-68O mu light, are required to 

 transfer one electron to TPN, the quantum yield drops severly 

 in wavelengths beyond 69O mu. In the presence of DCMU and 

 reduced dye, however, the quantum yield is relatively low at 



