128 PROBLEMS OF PHOTOSYNTHESIS 



actions were confirmed by Arnon (3), Tolmach (55) and San Pietro and Lang 

 (51), although Jagendorf (29) using highly purified chloroplasts observed the 

 Hill reaction with TPN+ only. The reactions proceed as follows 



light 

 2 DPN+(TPN+) + 2 H2O ^ 2 DPNH(TPNH) + 2 H+ + O. 



With the DPNH(TPNH) produced, reductive carboxylations may occur. 

 In § 46 some examples of reductive carboxylations were discussed. As these 

 are reacdons of CO2 fixation, it was reasonable to examine whether such re- 

 actions were responsible for CO2 fixation in photosynthesis. Ochoa showed 

 that illuminated chloroplasts are able in the presence of malic enzyme, TPN+, 

 Mn ions and pyruvic acid, to produce appreciable amounts of malic acid. 

 When a-ketoglutaric acid and iso-citric dehydrogenase were used, much 

 z^o-citric acid was obtained. Malic enzyme occurs in the cytoplasm of plant 

 cells but not in the chloroplasts; this may explain why isolated chloroplasts, 

 which cannot catalyze reductive carboxylations, are not capable of exhibidng 

 true photosynthesis. However, the question arises whether reductive car- 

 boxylations are really the main reactions of COo fixation in photosynthesis. 

 In sections C and D of this chapter other important reactions of COo fixation 

 will be discussed. 



It is doubtful if the reduction of DPN+ (TPN+) must be considered to be a 

 primary reaction. Although this has been postulated by Arnon in the case 

 of TPN+, other authors, for reasons to be discussed later, assume that a pri- 

 marily acting hydrogen acceptor has to be inserted. Calvin (18) considers 

 this primary hydrogen acceptor to be lipoic acid but Wessels (70) supposes 

 it to be vitamin K. If the primary hydrogen acceptor is indicated by X, the 

 reduction of TPN+ is 



X + H.>0 -^ XH2 + V2 O2 



XH2 + TPN+ -> X + TPNH + H + 



The reduction of phosphopyridine nucleotides in the Hill reaction links the 

 latter with enzymatic processes and with the reaction of oxidative phosphoryl- 

 ation found by Lehninger (37) 



O2 

 2 DPNH + 2 H+ + 6 ADP + 6 ph — > 2 DPN+ + 2 HoO + 6 ATP 



The enzymes responsible for oxidative phosphorylation are localized in the 

 mitochondria so that a relationship between chloroplasts and mitochondria 

 should exist. Vishniac and Ochoa (58) proved in their experiments with 

 chloroplast-mitochondria systems that formation of ATP takes place with 

 simultaneous oxidation of reduced phosphopyridine nucleotides by molecular 

 O2. Investigations by Arnon (4, 6, 7, 10, 73), to be discussed in § 51, showed, 

 however, that both intact chloroplasts and chloroplast fragments without 

 any external enzyme systems — i.e., without mitochondria — are able to pro- 

 duce ATP without being able to fix CO2. It is also to be noted that cells 



