4. Reactions in Chloroplasts and Cell Extracts 



Some Features of the Chloroplast Reaction 



C. p. WHITTINGHAM, Botany Department, Cambridge University, 



Cambridge, England 



HYDROGEN ACCEPTORS FOR THE CHLOROPLAST REACTION; 

 SOME PROPERTIES OF A NATURAL HYDROGEN ACCEPTOR 



Chloroplast preparations liberate oxj^gen from water in the light in 

 the presence of a suitable hj^drogen acceptor ; the general equation is : 



2H2O + 2A — ^^ 2H,A + O2 



This now well-known reaction has been shown both with "whole" 

 chloroplasts and chloroplast fragments and with quinone, ferric salts, 

 and such dj^estuffs as dichlorophenolindophenol as hydrogen acceptor. 

 Numerous other compounds have been added to this list including 

 Janus green, cytochrome c, coenzymes 1 and 2, and flavin derivatives. 

 As reported some years ago Davenport, Hill, and Whatley (1) have 

 prepared from leaves a protein fraction which acts as a natural hydro- 

 gen acceptor catalyzing the reduction of methemoglobin by illu- 

 minated chloroplasts. Activity has now been shown to be confined to 

 a component in the protein fraction which runs electrophoretically as 

 a single band. This substance also catalyzes the reduction of cj^to- 

 chrome c. In the latter case whole chloroplasts from chard were used 

 which had little cytochrome c oxidase activity (2). Both compounds, 

 methemoglobin and cytochrome c, whose reduction is accelerated by 

 the factor are heme compounds. 



WTiole Chlorella cells give in presence of ferricyanide a reaction ap- 

 parently similar to the chloroplast reaction, but here, as shown by 

 Warburg and Krippahl (3) and others previously, carbon dioxide is 

 necessary. With Chlorella and quinone this is not so and the system 

 essentially shows the chloroplast reaction. There is no evidence to 

 show that for the chloroplast reaction carbon dioxide is essential. 

 Many workers have shown photolysis in absence of any incorporation 

 of radioactive carbon dioxide (4,5). There remains only the pos- 



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