152 PROBLEMS OF PHOTOSYNTHESIS 



ley (3) have shown, CI ions exert a protective action upon chloroplasts in 

 vitro. The fact that the molecules of vitamin K, and chlorophyll possess a 

 phytol chain (Fig. 56) may also be of interest. Both substances probably 

 occur in the monomolecular layers between the lipoid and the protein-water 

 lamellae of the grana. In spinach leaves the vitamin Ki content amounts to 

 about 0.04 mg per g dry weight, so that the ratio chlorophyll/vitamin Ki is 

 of the magnitude 200. The ratio chlorophyll lipoic acid is about 1000. 



According to Marre and Servettaz (38), the reduction of TPN+ does not 

 follow that of a compound with a more positive redox potential, such as vita- 

 min K or FMN {E = -0.06). They found a distinct effect of TPN+ on 

 the reduction of cytochrome c (or f) in illuminated chloroplasts and on the 

 rate of photosynthetic phosphoryladon, indicating that reduction of TPN + 

 precedes and conditions the production of ATP. This is in agreement with 

 the findings of Marre and Forti (37) mentioned in § 51. Thus, it seems that 

 the role of TPNH in photosynthesis would be not only that of a reducing agent 

 of some product of CO2 fixation, but also that of an electron carrier in photo- 

 synthetic phosphorylation, as Arnon (4) postulated. 



§ 57 Some Other Investigations 



According to Calvin's hypothesis, the essential reduction process in photo- 

 synthesis is the reduction of 3-phosphoglyceric acid 



TPNH A TP 

 3-phosphoglyceric acid 7 ^ triose phosphate 



This reaction is the reversal of the oxidation of triose phosphate discussed in 

 § 44. According to Bassham et al. (11), the concentration of 3-phosphogly- 

 ceric acid in the light is about 30% lower and that of triose phosphate about 

 70% higher than in the dark. Nevertheless, it is assumed that in the light 

 the reaction distinctly proceeds from left to right. Kandler (30) points out 

 that this can only be possible when the concentrations of the necessary cofac- 

 tors are changed accordingly and when ATP is present in particularly high 

 concentrations. We learnt in § 53 that 3 mole ATP would be used per mole 

 CO2 in photosynthesis. Thus, Calvin postulates a stoichiometric relation- 

 ship between CO2 reduction and ATP turnover. In respiration, 6 mole ATP 

 are produced per mole CO2 (3 mol ATP per V2 mole O. used). When res- 

 piration of Chlorella is overcompensated 40-fold, 40 mole COo are reduced in 

 photosynthesis against one mole CO2 produced in respiration. This means 

 that the ATP turnover is increased 20-fold. However, the investigations of 

 Kandler (30), Strehler (50) and Bradley (14) clearly show that the ATP 

 content of the cells does not increase upon illumination. A stoichiometric 

 relationship between CO2 reduction and ATP turnover is therefore improb- 

 able. These results make the conversion of 3-phosphogIyceric acid to triose 

 phosphate questionable. 



