BIOSYNTHESIS OF PENTOSES 255 



Fig. 4. The cyclic conversion of phosphate esters in photosynthesis (after Calvin 

 aTid co-workers^°). 



tion, phosphogly eerie acid, sedoheptulose monophosphate, fructose mono- 

 phosphate, and pentose mono- and diphosphates contained, respectively, 

 32%, 24%, 19%, and 9% of the total radioactivity (C^";. More recent 

 kinetic studies on steady-state photosynthesis'''' show that the steady-state 

 concentration of ribulose diphosphate is relatively high, the concentrations 

 of phosphoglyceric acid, ribulose diphosphate, sedoheptulose phosphate, 

 fructose phosphate, and glucose phosphate being, respectively, 1.4, 0.5, 

 0.18, 0.12, and 0.4 )uM. per ml. Scenedesmus cells. Although degradation 

 data are not yet available, ribulose diphosphate is believed to arise from 

 sedoheptulose phosphate, and the latter to be formed by aldolase conden- 

 sation of triose phosphate and a tetrose (presumably erythrose). It has 

 been suggested^* '^^ that sedoheptulose phosphate and ribulose diphosphate 

 are involved in the regeneration of the Co fragments utilized in CO 2 fixation 

 rather than in the synthesis of hexose. This is supported by Calvin and 

 Massini's findings^^ that the sudden rise in phosphoglyceric acid during a 

 period of darkness, following a preliminary period of illumination, is ac- 

 companied by a decrease in both ribulose diphosphate and sedoheptulose 

 phosphate. Since the rate of production of phosphoglyceric acid was higer 

 during the first minute of darkness than in photosynthesis, it was tenta- 

 tively suggested that the C3 cleavage product of ribulose diphosphate is 

 triose phosphate during photosynthesis and phosphoglyceric acid in the 

 dark, this hypothesis being supported by the fact that triose phosphate 

 also decreases in the dark. Experiments with iodoacetamide-poisoned 



^^ M. Calvin and P. Massini, Experientia 8, 445 (1952). 



