PHOTOSYNTHESIS AND PHOSPHATE METABOLISM 1703 



pounds. Phosphoglyceric acid, phosphopyruvic acid, phosphoglycolic 

 acid, triose phosphates, pentose phosphate, hexose phosphates, heptose 

 phosphate and sucrose phosphate have all been found among the early 

 tagged products of photosynthesis. (For a review of phosphates identified 

 in tracer experiments with C(14) and P(32), see Buchanan et al. 1952.) 



Some of the observed phosphate esters contain one, some two phos- 

 phoric acid residues; none of them — except phosphoenolpyruvate, whose 

 function in photosynthesis is not clear^ — are true "high energy phos- 

 phates." If it is true, however, that the main reduction step in photosyn- 

 thesis is (as postulated in the "one-carboxylation, one-reduction" mecha- 

 nism in section A, 12) the reduction of PGacid to PGaldehyde by reduced 

 pyridine nucleotide, then the cooperation of a high-energy phosphate is 

 indispensable; i. e., PGA has to be phosphorylated to DPGA (with one 

 H2PO3 residue attached to the carboxyl group, forming a "high energy" 

 ester) before it can be reduced. It has been suggested that the needed 

 high-energy phosphate (ATP) is produced by partial reoxidation of an 

 intermediate, such as TPNII2. If this is correct (and as of this writing it 

 appears a plausible hypothesis), then the beginning of photosynthesis in a 

 cell should lead to an increase in the concentration of high energy phos- 

 phate (and a corresponding consumption of orthophosphate, or of a "low- 

 energy" phosphate ester). The steady progress of photosynthesis beyond 

 the reduction level of PGA will require a certain steady concentration, 

 [ATP], which must be higher the higher the light intensity (at least up to 

 saturation). In the dark, this concentration will decline again as the cell 

 uses up its energy reserves (c/. Lynen 1941, 1942). 



In chapter 9, section 5, we reported some experiments by Emerson, 

 Stauffer and Umbreit (1944) indicating an effect of photosynthesis on the 

 phosphate household of Chlorella. (A more direct, but quantitatively not 

 very convincing evidence of storage of energy in high-energy phosphates 

 was obtained by Vogler and Umbreit (1943) with chemosynthetic bacteria, 

 c/. p. 114.) More recently, more significant evidence has been supplied. 



Wassink, Tjia, and Wintermans (1949) observed shifts in the phosphate 

 content of a medium containing the purple bacterium Chromatium D upon 

 transfer from light to darkness. The uptake of inorganic phosphate oc- 

 curred in light in N2, H2, and — to a smaller extent — in N2 -f- CO2 ; shift to 

 darkness caused a small release of phosphate if the gas phase remained 

 unchanged, and a marked release if No + CO2 was substituted for H2. 

 These results were interpreted as supporting the hypothesis of Vogler and 

 Umbreit that, in the energy-storing period (oxidation of sulfur in chemo- 

 synthetic bacteria, illumination in Ho in photosynthetic bacteria), high- 

 energy phosphate bonds are built up, while in the energy-utilizing period 

 (CO2 supply in the absence of O2 in chemosynthetic bacteria, CO2 supply 



