THE CHEMISTRY OF PHOTOSYNTHESIS 129 



rich in chloroplasts are generally poor in mitochondria. Wassink (68, 69), 

 Wintermans (76) and Kandler (31, 32) also observed the uptake of inorganic 

 phosphate by illuminated chloroplasts. According to Wassink, it is possible 

 that not ATP but another polyphosphate with a sulfhydryl group is produced. 

 The existence of such a still unidentified organic phosphate ester is also 

 claimed by Krall (35). 



Allen et al. (1) showed that the rate of ATP production by chloroplasts, 

 anaerobically, upon illumination is far higher than observed with mito- 

 chondria in oxidative phosphorylation. In § 57 it will be shown that, ac- 

 cording to Kandler (33), the rate of phosphorylation in photosynthesis of 

 whole cells is of the same magnitude as that of oxidative phosphorylation. 



It must be pointed out however that Ohmura (45) observed oxidative 

 phosphorylation with chloroplast fragments in the dark: these fragments 

 showed the Hill reaction in the light when intermediate products of the tri- 

 carboxylic acid cycle were added. Thus, it is possible to prepare chloroplast 

 fragments which show oxidative phosphorylation and photosynthetic phos- 

 phorylation (see § 51). Ohmura concludes that the latter is nothing but a 

 combination of water photolysis and oxidative phosphorylation. This as- 

 sumption would accord with the views of Ochoa and Vishniac. 



§ 51 Light Phosphorylation 



Arnon (4-14, 56, 71, 72, 73) called ATP formation by illuminated intact 

 chloroplasts photosynthetic phosphorylation. With the aid of light energy and 

 under appropriate conditions, chloroplasts can synthesize ATP 



light 

 ADP + ph ^ ATP (a) 



According to Anderson and Fuller (2) and Newton and Kamen (41), iso- 

 lated chromatophores of purple sulfur bacteria are also capable of light- 

 dependent ATP formation. 



The most important difference between photosynthetic phosphorylation 

 and oxidative phosphorylation is that the former does not need molecular 

 O2. In earlier work, Arnon (4) was not able to establish the role of DPN + 

 or TPN+ in light phosphorylation. Later, he found that TPN+ — but not 

 DPN+ — acts as a catalyst, a further fact demonstrating the difference be- 

 tween the two types of phosphorylation, DPN+ being indispensable to oxida- 

 tive phosphorylation. Arnon's photoreduction of TPN+ is expressed as 

 follows 



light 

 2 TPN+ + 2 H2O > 2 TPNH + 2 H+ + O2 (b) 



This reaction needs a TPN+-reducing factor which occurs in chloroplast 

 extracts It was found that when 1 mole O2 is produced 2 mole TPN + 

 are reduced and 2 mole inorganic phosphate are esterified, as the following 

 equation shows 



