KINETIC STUDIES 1679 



tion of the relative distance from the various compounds to the final, 

 stable products of photosynthesis. 



Calvin and Massini (1952) described tagging experiments at 24° C, 

 in a large vessel (2 ml. wet algae in 200 ml. water), from which successive 

 20 ml. samples could be withdrawn, first as photosynthesis in C*02 pro- 

 ceeded, and then after the light had been switched off. The results were 

 rather erratic, but several compounds (or types of compounds) were 

 clearly shown to be saturated with C* after illumination periods of the order 

 of 5 min. ; these could be assumed to be intermediates in the direct path of 

 photosynthesis (rather than its by-products, or final products). By assum- 

 ing that in the saturated state the reservoirs of these intermediates are 

 fully equilibrated isotopically with the external C*02, the concentrations 

 of these intermediates in the photostationary state could be calculated 

 from the amount of radioactivity found in them in the chromatograms. 

 Table 36.VIII shows the results. 



Table 36. VIII 

 Photostationary Concentration of Photosynthetic Intermediates in Scenedes- 

 mus Illuminated by White Fluorescent Light (10 kerg./cm.^ sec.) at 24° C. 1% 

 CO2, 2 ML. Cells in 200 ml. Water (after Calvin and Massini 1952, and Benson 



1952) 



Concentration, millimole/1. 

 (wet cell volume) 



Compound 



Phosphoglyceric acid 



Triose (dihydroxyacetone) phosphate. 



Fructose phosphate 



Glucose phosphate 



Mannose phosphate 



Sedoheptulose phosphate 



Ribulose diphosphate 



Alanine 



However uncertain the numbers in table 36. VIII probably are, they 

 represent the first estimate of the concentration of intermediates in photo- 

 synthesizing cells, and are as such of the greatest interest. It will be noted 

 that the photostationary concentration of PGA (1.4 X 10~' mole/1.) is 

 about one order of magnitude lower than that of chlorophyll in algal cells 

 (c/. chapter 15, section B7). The amount of activity in most of the com- 

 pounds listed in table 36. VIII continued to increase slowly after "satura- 

 tion" has been reached, indicating that they were also "fed" from the 

 growing reservoir of labelled final products of photosynthesis (polysac- 

 charides). 



