PHOTOSYNTHESIS AND PHOSPHATE METABOLISM 



1707 



phosphate bond energy storage in photosynthesis. The "extreme" variant 

 (suggested, e.g., in the paper by Emerson, Stauffer and Umbreit 1944) is 

 that all light energy utiHzed in photosynthesis is first converted to phos- 

 phate bond energy; our objections, based on the undesirabiHty of sphtting 

 43 kcal energy quanta into 10 kcal portions before accumulating them again 

 (chapter 9, p. 228), were directed against this extreme theory. An "in- 

 termediate" suggestion is that one (luantum of light is used to lift hydrogen 

 from water (Eq ^ 0.8 volt) to an intermediate redox catalyst with a poten- 

 tial around 0.0 \olt, and that high energy phosphates (created by partial 



90^ 



60 120 leo 



TIME OF ILLUMINATION (SEC) 



Fig. 36.30. Transient changes in ATP eoncentration (E) at the begin- 

 ning of illumination of Chlorella, compared with those of fluorescence (A), 

 chemiluminescence (B), and C02-uptake (D) (after Strehler 1952). 



reoxidation of this intermediate) take over from there (Kandler 1950, 

 Strehler 1952). Finally, there is the "modest" suggestion (Ruben 1943, 

 Calvin et al. 1954), which assigns to phosphate energy only the bridging 

 of the final gap between Eo — 0.3 volt (pyridine nucleotides) and the energy 

 needed to reduce a carboxyl to a carbonyl {Eo ^ 0.5 volt). 



All the above-discussed hypotheses suggested partial reoxidation of 

 intermediates as source of ATP energy. Burk and Warburg postulated in- 

 stead (c/. chapter 37D, section 3), partial reoxidation of the final products 

 of photosynthesis (carbohydrates) . To complete the review of alternatives, 

 the oxidant in the back reaction need not l^e molecular oxygen— as postu- 

 lated by most of the above-enumerated authors— but could be an inter- 

 mediate oxidation product ("oxygen precursor") formed by the photo- 

 chemical process. (This assumption becomes particularly plausible if one 

 wants to apply the same general concept to photosynthesizing bacteria and 



