R. rubrum-CYTOCKROME c reactions 



181 



(oxidation of cytochrome c); then the slow dark oxidation ceases 

 after a period of illumination. This gives additional evidence that the 

 slow dark oxidation does not function in respiration, since oxygen 

 uptake of the extract was found to be unaffected or slightly increased 

 by illumination. As the cytochrome c becomes more oxidized following 

 ilhimination, a slow reduction is observed on cessation of illumination. 



Effect of Illumination on Rhodospirillum rubrum + cytochrome c 



,3- 



E 

 O 



in 





o 

 u 



Q. 



O 



•ferricyanlde 



ferrlcyonJde 



n — \ — r 



2 3 4 



1^ 

 15 



Time (minutes) 



Fig. 1. Rhodosfirillum rubrum extract was added to reduced mammalian cyto- 

 chrome c at zero time. In curve (1) the cytochrome c was 95% reduced and in 

 curve (2) the cytochrome c was about 70% reduced at the beginning of the experi- 

 ment. Illumination was from the side, as described in the text. There was no 

 measurable change in optical density at 550 ran on illumination of the bacterial 

 extract in the absence of cytochrome c. The cytochrome c is completely oxidized 

 on addition of ferricyanide. 



Also the rate of reduction of cytochrome c on addition of succinate in 

 the dark is often increased after a period of illumination. Illumination 

 of the mixture results in the oxidation of the cytochrome c until a 

 certain ratio of reduced cytochrome c is reached; then no further 

 oxidation takes place. If a mixture of oxidized cytochrome c and 

 rubrum extract is illuminated, the cytochrome c becomes partly re- 

 duced. It was found that all the changes iji optical density observed 



