NITRATE METABOLISM AND PHOT(3SYXTHESIS 1709 



obtained to permit more detailed fractionation of the tagged compounds. 

 The P* distribution between glucose-()-phosphate, fructose-6-phosphate, 

 and mannose-6(sedoheptulose)-phosphate corresponded to the equilibrium 

 concentrations of these three esters, after exposures of from 1 to 25 min. in 

 dark or in light. Still longer experiments (1 hour exposure), permitted 

 a first estimation of the volumes of the ^•arious phosphate reservoirs in the 

 cell (on the assumption that after this time, a steady state has been reached 

 in respect to these volumes, and all reservoirs have been uniformly la- 

 belled). Table 36.IX shows the relative reservoir volumes computed on 

 this basis. 



It is interesting to note the smaller proportion of P contained in ATP 

 in light (compensated by a greater proportion in sugar phosphates). 



Frenkel (1954) described the formation of ATP from ADP + ortho- 

 phosphate in light by sonically disintegrated RhodospiriUum rubrum. 

 Arnon et al. made similar observations on whole chloroplasts. At first 

 (Arnon, Bell and What ley 1954) oxygen was reported to interfere with 

 "photosynthetic phosphorylation"; but later (Arnon, Whatley and Bell 

 1954) it was reported to occur also under aerolMc conditions. Mg + + ions, 

 the vitamins C (ascorbic acid) and K, and certain other compounds, were 

 found to act as "co-factors," stimulating the ATP formation. 



C. Nitrate Metabolism and Photosynthesis* 



This chapter would be the place to discuss also the relation of photo- 

 synthesis to the nitrate metabolism of plants. It, too, is affected by illu- 

 mination, and affects photosynthesis, e. g., by changing the photosynthetic 

 ratio ACO2/AO2 from about 1.0 in solutions containing no nitrogen or only 

 ammonia nitrogen, to much lower values in solutions containing nitrate, 

 in consequence of partial substitution of HN0,3 for CO2 as ultimate hydro- 

 gen acceptor. Early experiments on the photochemical reduction of 

 nitrate by ChloreUa by Warburg and Negelein have been described in 

 chapter 19, section Bl. It was stated there that "unfortunately, the sub- 

 ject has received no further attention since 1920." Since this was written, 

 some new studies have appeared in this field. Space limitations prevent 

 us from entering here into their description; we can only refer to the series 

 of papers by Burstrom (1942-1945), Myers and co-workers (1948, 1949), 

 Davis (1952) and Kessler (1953), and to the discussion by Kandler (1950). 



The radiocarbon tracer experiments, described in part A of this chapter, 

 have indicated how early the reduction of carbon dioxide in photosynthesis 

 can branch out into various side reactions, including transaminations 

 leading to simple aminoacids (and perhaps from there to proteins) before 

 the reduction stage of the carbohydrates has been reached. At some stage 



* Bibliography, page 1713. V 



