ISOLATED CHLOROPLASTS I23 



plant or in the bacteria oxidizes some hydrogen donor (HgS) 

 supplied externally. According to this view photosynthesis 

 was represented by the following equations: — 



4H2O + 2A + 4X -- 2AH2 + 4(XOH) 

 CO.3 + 2AH2 - (CH2O) + H2O + 2A 



4(XOH) + 2H2O - 4H2O + 4X + O2 

 or 4(XOH) + 2H2S - 4H2O + 4X + 2S 



Thus the chloroplast reaction would then be represented 

 essentially as 



2A+4H2O - 2AH2+O2+2H2O 



the reaction only occurring in light and in the presence of 

 a suitable hydrogen-accepting reagent which would react 

 with the reduced compound AH2. 



In order that the hydrogen donor produced can reduce 

 carbon dioxide at the concentration in which it is present in 

 air the oxidation reduction potential of the hydrogen donor 

 AH2 must be even more negative than the standard hydrogen 

 electrode. It is therefore important to consider the reducing 

 properties of the chloroplast system, conveniently summar- 

 ized in Table 7.1 and in Fig. 7.2, which shows the relation 

 between the electrode potentials and the percentage reduc- 

 tion of the effective reagents. The reduction of ferric oxalate 

 represents the greatest reducing potential that has yet been 

 obtained in the chloroplast reaction, and in this case com- 

 plete reduction would only be approached at an oxygen 

 pressure far below atmospheric. This would suggest that at 

 pH 7 and with the highest light intensities yet used the 

 chloroplast reaction has a limiting potential of about zero. 

 If reagents of more negative potential are used with the 

 same high light intensity they will be reduced in appreciable 

 amount only at infinitesimally small oxygen pressures. 



Hence chloroplast preparations do not 'split water' to the 

 extent required by the simple formulation of photosynthesis 

 proposed by van Niel. This may be due to the present 

 methods of preparing chloroplast preparations. Alterna- 

 tively two other interpretations are possible, namely that 

 either (i) the chloroplast reaction in vitro is an artefact and 

 has no direct relation to processes of reduction occurring in 



