120 



PHOTOSYNTHESIS 



Chlorella cells there was a small initial production of 

 oxygen immediately upon illumination attaining a pressure 

 of the order of 5/100,000 mm. Hg but after this initial 

 evolution the production of oxygen fell to a lower value to 

 be followed in the course of some minutes by a gradual rise 

 to a higher steady rate (see Fig. 7.1). The immediate response 

 of the cells to light may be compared with the chloroplast 

 reaction using natural hydrogen acceptors such as are in 

 acetone leaf extract, and present in the cell in limited 



O 



/ 



3^56 

 T/me (minutes) 



7 



FIG. 7.1. Time course of oxygen evolution from Chlorella cells 

 under anaerobic conditions illuminated at 0° C. (After Franck, 

 Pringsheim, and Lad.) 



amount, the subsequent rise representing the slow develop- 

 ment of the normal photosynthetic mechanism (Hill and 

 Whittingham, 1953). Further work by Allen in Franck's 

 laboratory has shown that in Scenedesmus, unlike Chlorella, 

 the development of the photosynthetic mechanisna is inde- 

 pendent of the presence of free oxygen. Hitherto induction 

 phenomena have been interpreted by Franck in terms of the 

 formation and removal of natural inhibitors of the photo- 

 chemical system and by Wassink as due to changes in the 

 concentration of a suitable hydrogen acceptor for the photo- 

 chemical system. More precise details are necessary before 



