432 



LIGHT AND LIFE 



— o 

 UJ 



YOH 



XH 



Fig. 7. Oxidation-iediictioii potentials of some chloroplast and mitochondrial 



cytochromes. 



(2) Mitochondria oxidize hydrogen donors. Illuminated chloro- 



plasts reduce hydrogen acceptors. 

 (.^) Mitochondria oxidize DPNH. Illuminated chloroplasts reduce 



TPN. 

 (4) Mitochondria carry out oxidative phosphorylation. Illumi- 

 nated chloroplasts carry out "reductive" phosphorylation (10) .^ 

 In the mitochondria the cytochromes a -{- a^ catalyze the reduction 

 of oxygen. In the chloroplast the illuminated chlorophyll could be 

 regarded as supplying both the oxidase, as FOH, and the (H) ac- 

 ceptors, as XH. Hence, using such assumptions, one would expect 

 that the chloroplast cytochromes might function in a manner similar 

 to those of mitochondria. This suggestion is illustrated in Equation 5. 



YOH 



f 



be 



XH 



(^) 



In other words, in the illuminated chloroplasts YOH and XH might 

 react through the (H) transport system, /, and /;,; to give H._,0 and 

 the concomitant phosphorylation of ADP. In such a hypothesis one 

 woidd expect that addition of inorganic phosphate and ADP to 



*The use of the term "reductive" here is controversial, but it describes the 

 process as observed experimentally. 



