THE PHOTOTROPHIC ASSIMILATION OF CARBON 37 



the alga is supplied with radioactive carbon dioxide for a 

 short period in the light then killed and extracted immedi- 

 ately, it is found that not only is the rate of fixation from 

 ten to a hundred times greater than that in the dark, but 

 that different compounds become labelled, phosphoglyceric 



SO - 



o ^o 



Q 



o 



O 



k 



ki 



20 



ORGANIC PHOSPHATES 



O PH0SPH06LYCER/C 

 ACID 



O MALIC AND 



AS PART I C AC I DS^ 



MINUTES 



FIG. 8. Distribution of radioactive carbon among products ex- 

 tracted with 80 per cent ethanol from Scenedesrmis sp. after 

 varying periods of photosynthesis in the presence of carbon 

 dioxide labelled with radioactive carbon (after ref. 52). 



acid, pyruvic acid, malic acid, alanine, triose and hexose 

 phosphates, and sucrose now being the principal substances 

 concerned. ^^' ^^"^ As the period of exposure of the actively 

 photosynthesizing alga to radioactive carbon dioxide is 

 shortened a greater proportion of the radioactive carbon 

 appears in phosphoglyceric acid (Fig. 8). This and other 



