134 PHOTOSYNTHESIS 



acceptor. The maximum free energy change that has been 

 found possible under conditions where oxygen is being 

 produced corresponds to a reducing potential of about zero 

 at pH 7, referred to the normal hydrogen electrode, i.e. to 

 a value of +12 on the rH scale and to 76,000 cals. per Og 

 molecule. 2A+2H2O ^ O2+2AH2. The mean energy for the 

 quantum of light corresponding to the fluorescence of 

 chlorophyll a in the plant is about 41,000/Ar cals. 

 (N = Avogadro number). In the over-all reaction, 



CO2+H2O - (CH0H)+02 



the mean free energy change per H transferred (4H are 

 involved for each (CHOH) produced) is about 28,800 cals., 

 i.e. 115,000 cals. per O2 molecule. This corresponds to a 

 reduction potential at pH 7 of -0-5 volt or to a value of 

 about 5 units on the rH scale. Hence no direct reduction of 

 carbon dioxide can result in the chloroplast reaction. Thus 

 as Rabinowitch has stated, the chloroplast reaction can only 

 represent one half of photosynthesis. 



Physically the active system represents from some plants, 

 minute fragments of the chloroplasts, whereas in others 

 whole chloroplasts alone preserve activity. The particles 

 derived from the fragmented chloroplasts may be obtained 

 many times smaller than the individual grana of the chloro- 

 plasts and still show a measurable activity. Even at this level 

 the particles are very complex chemically. They contain all 

 the known pigment components of the chloroplasts, the 

 chlorophylls, the carotenoids and the characteristic cyto- 

 chrome components. Even if the chloroplast preparations 

 are amply supplied with a hydrogen acceptor, the oxygen 

 production still shows the phenomenon of light saturation, 

 thus indicating that at least one dark process is concerned 

 in addition to the light reaction. 



If this subcellular system is accepted as being a part of 

 the intracellular mechanism in vivo then the property of 

 evolution of oxygen together with the simultaneous transfer 

 of hydrogen possessed by it may be considered to be part of 

 the mechanism of photosynthesis. If so, it would appear 

 that CO2 is not directly concerned in oxygen production as 



