MECHANISM OF PHOTOSYNTHESIS I35 



Studies of the direct response of the illuminated plant had 

 at one time suggested. Thus the experimental possibility of 

 producing oxygen by means of the chloroplast preparations 

 provided support for the generalization of the process of 

 photosynthesis first announced by van Niel. It follows from 

 this theory that in the green plant v^^ater itself is the ultimate 

 hydrogen donor, and evidence for this using isotopes has 

 been discussed (see Chapter 7). 



Comparison of the chloroplast reaction with an oxidase in 

 respiration 



In respiration oxygen is the ultimate hydrogen acceptor 

 and the water originally used in photosynthesis is formed 

 again. In actively respiring cells of both plants and animals 

 oxygen is reduced through the cytochrome system. This 

 system, discovered by D. Keilin, may be prepared as a very 

 fine particulate suspension and is derived from mito- 

 chondria. The component cytochrome c does not react 

 directly with oxygen but is immediately oxidized if added 

 to a mitochondrial preparation containing cytochromes a 

 and flg. If succinate is added in absence of oxygen the cyto- 

 chrome c is immediately reduced. The hydrogen transport 

 from succinate to oxygen occurs in steps. The complete 

 step from succinate-fumarate to cytochrome c would cor- 

 respond to 0-27 volt; the step from cytochrome c to oxygen 

 0-53 volt. 



The active chlorophyll-containing system derived from 

 the chloroplasts when illuminated will reverse the action 

 of cytochrome oxidase; water is oxidized to oxygen. The 

 'hydrogen' has then to reduce a hydrogen acceptor. A simple 

 experiment serves to illustrate the complementary nature of 

 the two systems. Cytochrome c is added to a crude chloro- 

 plast preparation from, say, the pea plant. Chloroplasts and 

 mitochondria are both present. The oxidation in the dark 

 and the reduction in the light of the added cytochrome c 

 is easily followed by the observation of the visible absorp- 

 tion spectrum. The sharp absorption band of reduced cyto- 

 chrome c (550 m^) appears during an interval of illumination 

 and temporarily disappears during a dark interval. 



