DIFFERENT OXIDANTS 1583 



anism of photosynthesis. Tolmach was not certain whether the strong 

 effect of TPN on oxygen hberation by crude leaf juice was due to intermedi- 

 ate reduction of TPN (as in the pyruvate-mahc enzyme system), or to the 

 conversion of some sap component into an effective Hill oxidant by TPN- 

 mediated autoxidation (since he observed that addition of oxygen-saturated 

 water to TPN containing juice also caused a new oxygen burst in light). 

 The much greater volume of oxygen produced from crude leaf juice com- 

 pared to washed chloroplast-pyruvate-bicarbonate-malic enzyme system 

 indicated that only a small part of the former could be due to the reductive 

 carboxylation of pj^ruvatc. Tolmach also pointed out that the successful 

 competition of carbon dioxide with oxygen as hydrogen acceptor in photo- 

 synthesis would be difficult to understand if the reduction of carbon dioxide 

 were to be mediated by an intermediate (TPN) whose concentration in 

 chloroplasts is very low (a value of 10~^ mole/1, was mentioned in Tol- 

 mach's paper; however, subsequent determinations in Vennesland's labora- 

 tory gave higher values) . 



According to Franck, efficient reduction in the presence of air Avould 

 require TPN to displace the much more abundant O2 (about 3 X 10~^ 

 mole/1, in cell sap equilibrated with air) from contact with chlorophyll. In 

 fact, if one assumes, as Franck does, that a molecule of the oxidant must be 

 associated with each chlorophyll molecule, only a primary oxidant with a 

 concentration of the order of 0.05 mole/1, (in the grana) can function with a 

 high quantum yield. If one assumes that effective energy exchange be- 

 tween chlorophyll molecules reduces the number oi centers in which oxidant 

 molecules must stand ready to accept the H atoms, transferred in the pri- 

 mary photochemical process, by a factor of the order of 10- (or 10^), then 

 a concentration of '~5 X 10~^ (or ^-^5 X 10 ~^) mole/1, may be sufficient 

 to keep the reaction centers occupied; this could possibly — but not very 

 likely — make TPN a successful contender for these sites. On the other 

 hand, the lack of discrimination, characteristic of photochemical processes 

 (because of the excess activation energy usually available in a quantum), 

 should warn us from reading too much significance into the reduction — with 

 a small yield- — of any reductant offered to the chloroplast system, partic- 

 ularly of a reductant whose reduced form we happen to know how to trap 

 very efficiently. 



Arnon (1951) also made experiments on the reductive carboxylation 

 of pyruvic acid in the presence of malic enzyme and illuminated chloro- 

 plasts. He demonstrated that the malic enzyme is present in the cyto- 

 plasmic fluid left after the precipitation of chloroplast fragments by high- 

 speed centrifugation, so that the complete photocatalytic sj^stem (except 

 for TPN) can be obtained from one and the same plant. Figure 35.19E 

 shows the photochemical evolution of oxygen from this system, whose de- 



