156 M. D. KAMEN 



there is now considerable experimental backing for the existence of 

 such entities. 



The notion of a back oxidation has intrigued many writers and has 

 appeared in one form or another too often to recount here. (See, for 

 example, 2,3,23,39.) Davenport and Hill (8) have elaborated the 

 concept of a back oxidation coupled to the photochemical process in 

 terms of the hematin compound, cytochrome /, and some other 

 oxidizing agent which is reduced in the photochemical act. They 

 point out that the potential for cytochrome / lies at a point more 

 negative than that of the oxygen electrode by an amount which, for 

 the movement of four electrons, is precisely equivalent to the energy 

 of one quantum in the characteristic red emission spectrum of 

 chlorophyll. The next drop in potential brings one to the region at the 

 limit of the Hill chloroplast reaction and to the potential of cyto- 

 chrome be- Here, again, the potential difference results in an energy 

 change for four electrons corresponding to the energy in one quantum 

 of red light. 



Gaffron and Rosenberg (12) have summarized the experimental 

 findings which indicate that no back oxidation occurs involving 

 molecular oxygen as such. However, preoccupation with oxygen as 

 the H acceptor in such a process is not requisite to the Hill postulate. 

 The oxidizing system generated in the light can be a substance other 

 than oxygen, such as a complex organic free radical which produces 

 oxygen irreversibly and partly back reacts in the manner suggested by 

 Hill. 



It is fortunate that techniques now exist for making a start in de- 

 termining the state of cell constituents in intact cells during metabo- 

 hsm. These are dynamic spectrophotometric methods, which have 

 been employed by Duysens (9), Lundegdrdh (24), and others, and 

 have reached a notable degree of development in the hands of Britton 

 Chance and his co-workers (4-7) . 



Duysens first observed that, in intact cell suspensions of R. ruhrum., 

 there were changes in optical density brought about by a transition 

 from a steady state in the dark to a steady state in the light. These 

 changes coincided with the difference spectrum between oxidized and 

 reduced forms of a cytochrome type compound or compounds. Vernon 

 and Kamen had showTi about the same time (37) that the R. ruhrum 

 cytochrome c could be photochemically oxidized by air in an en- 

 zymic reaction, and that compounds at potentials more negative than 



