756 



SUMMARY 



tional question is how does each photosystem obtain its 

 appropriate share of the absorbed quanta. Of two possible 

 mechanisms the first one, which is supported by much of the 

 data, assumes that the respective pigment systems and conver- 

 sion centers are independent of each other. The second 

 assumes that energy can "spill over" from system II to system 

 I, but not vice-versa (Myers, Govindjee, Bannister and 

 Hommersand). In the extreme case, this mechanism could oper- 

 ate with a single assembly of absorbing pigments if only 

 given a switching device at the end point of energy transfer. 

 One thing is certain — deciding about these possibilities is 

 made much more difficult by the fact that it is never possible 

 to irradiate system II by itself. 



Further evidence as to the nature of the cooperation 

 between the two photosystems comes from sensitive measure- 

 ments of oxygen evolution (French, Fork, de Kouchkovsky; 

 VVhittingham). These experiments show that oxygen evolu- 

 tion due to irradiation of photosystem II is not initiated 

 unless the chloroplasts or algae have been previously 'J^rimed't- 

 either quickly by exposure to light activating system I or 

 much more slowly by a dark incubation. Thus system I, accord- 

 ing to this evidence, produces a precursor for system II to 

 work on. Data of Govindjee and Whittingham, indicating that 

 one sensitization band can operate in either photosystem, seem 

 unexplainable as yet. 



The biochemical details of the oxygen evolution process 

 remain an area of almost complete ignorance. Earlier work 

 showed that Mn is involved in the oxygen evolution system. 

 Recent studies (Gaffron) suggest that flavin enzymes might be 

 playing a part here, primarily from the discovery that the 

 oxygen evolution inhibitor DCMU interacts very strongly with 

 model photoreactions of soluble flavins. Some of the experi- 

 ments of Trebst suggest that photo-oxidation of some of the 

 quinones, or of ascorbic acid, may proceed by a reversal of 

 the oxygen evolution pathway; however, all alternative ex- 

 planations have probably not yet been ruled out. 



The pathway of carbon continues to be a fruitful field of 

 inquiry for a few groups. Increasing doubt seems to have 

 developed as to whether the Calvin cycle is true in every 

 respect in vivo. Most essentially, recent critical experi- 

 ments seem to suggest that phosphoglyceric acid is not the 

 only first product in photosynthesis; it appears that fixa- 

 tion of CO2 onto ribulose diphosphate may, in part, produce 



