PHOTOSYNTHETIC PHOSPHORYLATION AND THE ENERGY CONVERSION PROCESS 387 



TABLE XIV 



Non-Cyclic Photophosphorylation by Chloroplasts with Ascorbate as 



THE Electron Donor 



(Whatlev, Dieterle, and Arnon [161]) 



Treatment i contained: washed chloroplast fragments (P1S2), prepared in 

 the absence of chloride, containing o -5 mg. chlorophyll; 0-05 ml. purified spinach 

 phosphopvridine nucleotide reductase, and the following in micromoles: tris/ 

 acetate buffer, pH 80, 40; MgSOj, 5; K0H3-PO4, 10; ADP, 10; TPN, 4; and 

 KCl, 10. In Treatment 2 KCl was omitted and 2 x 10^ m p-chlorophenyl- 

 dimethylurea (CMU) was added. Treatment 3 was the same as Treatment 2 

 except that 20 /^tmoles ascorbate and 0-2 /^mole 2,6-dichlorophenol indophenol 

 were added (cf. [160]). The experiment was run for 20 min. at 15" (at a light 

 intensity of 2000 foot candles). 



The omission of chloride (Fig. 26) and the addition of CMU, a powerful 

 inhibitor of oxygen evolution, prevented the use of water as an electron donor in the 

 chloroplast system. Catalytic amounts of dichlorophenol indophenol served as 

 an electron carrier [160] between ascorbate and the chloroplast system. 



events, i.e. ATP formation and TPN reduction. Oxygen evolution occurs 

 when water (OH "), on donating an electron to the photosynthetic particle, 

 becomes oxidized to oxygen. Under special experimental conditions, when 

 ascorbate displaces water as the electron donor, no oxidation of OH - 

 occurs, only the oxidation of ascorbate [i6o]. This concept of the non- 

 cyclic photophosphorylation in chloroplasts is represented in Fig. 27. 



The proposed mechanism assigns to cytochromes a role in transporting 

 electrons from the electron donor system to chlorophyll. Cytochromes are 

 known to accept electrons from ascorbate but the suggestion that a photo- 

 synthetic cytochrome svstem mediates the transfer of electrons from OH " 

 to chlorophyll is put forward only as a working hypothesis [i]. This 

 hypothesis implies that the chlorophyll-cytochrome complex must 

 generate a sufficient oxidizing potential to drive the reaction 2H.2O— *0.2 + 

 4H+ + 4^^, which at 25- and at pH 7, has E'^= +0-815 V. It might be 

 argued that cytochrome /', the most oxidizing cytochrome now known to 

 occur in chloroplasts, has a redox potential lower than oxygen [162], i.e. 

 Eq— +0-365 V. However, it would be premature to conclude that our 

 knowledge of redox potentials of cytochromes in chloroplasts is now 

 complete. 



