540 LIGHT AND LIFE 



TABLE 10 



Effect of Chloride on Non-cyclic Photophosphorylation 



BY Isolated Chloroplasts 



(BovE, BovE, Whatley, and Arnon, 30) 



mary phosphorylation reaction at the site of the cytochrome adjacent 

 to the excited chlorophyll molecule (Equation 4) . Chloroplasts have 

 no hydrogenase and hence can give no hydrogen evolution. The elec- 

 tron "expelled" in the primary photochemical act is accepted by TPN 

 and used (along with an accompanying proton) for the production 

 of TPNH2. TPNH2 can either accumulate under special conditions, 

 as in the experiments with isolated chloroplasts (Table 10) , or be 

 used jointly with ATP for the formation of carbohydrates during 

 normal photosynthesis. 



In a non-physiological variant of reaction 5, TPN can be replaced 

 by ferricyanide. In that case the electrons from chlorophyll are ac- 

 cepted by ferricyanide which becomes reduced and accumulates as 

 ferrocyanide. The protons also accumulate, causing the acidity to 

 increase, as has been found to be the case with potassium ferric 

 oxalate by Holt and French (71a) . 



To keep the chlorophyll system functional the electrons removed 

 by non-cyclic transport must be continuously replenished. We sug- 

 gested that this is accomplished in a reaction between hydroxyl ions 

 (or water) and a cytochrome peculiar to the photosynthetic apparatus 

 of green plants but absent in photosynthetic bacteria (9) . This reac- 

 tion is visualized as being possibly an "anode"-type, dark reaction, in 

 which OH- ions yield molecular oxygen and donate electrons, via a 

 cytochrome chain, to chlorophyll, where they are raised at the ex- 

 pense of absorbed light energy to the reducing potential required for 

 TPN reduction. A corollary of the scheme is that water is still the 

 source of the oxygen evolved in photosynthesis by green plants (9) . 



In the electron transfer sequences visualized in non-cyclic photo- 

 phosphorylation (Fig. 19), we have ruled out the participation of 

 cytochromes in the transfer of electrons from chlorophyll to TPN 

 because the redox jKjtential of TPN (£'„ =: —0.32 v) is altogether 



