MECHANISM OF PHOTOSYNTHESIS I45 



It seems probable that under most conditions the photo- 

 synthetic system would be so well integrated that the 

 opposite phases of activity could not be separated, which 

 would account for these transitory gas exchange effects not 

 being apparent in previous experiments with Chlorella or 

 other plants. 



Further, Brown (1953) and colleagues have found that 

 with Chlorella cells the consumption of oxygen from the gas 

 phase is not generally affected by light and dark, i.e. the 

 total uptake of O2 from the gas phase is the same in light and 

 dark. This would indicate — if n>2 in equations a and ^ — 

 that the extra oxygen does not appear as such. In other 

 words, zn will represent the number of electron transfers 

 involved per equivalent of COg converted to the level of 

 carbohydrate. 



Cytochrome components in chloroplasis 



The chloroplasts have been shown to possess cytochrome 

 components (Chapter 3). The component cytochrome / is 

 different from c}lochrome c in its physical properties; the 

 haematin group and the properties of oxidation and reduc- 

 tion are similar in the two pigments. The chloroplasts also 

 contain a haem protein which is similar to component cyto- 

 chrome h. In respiration there is a stepwise transfer of H or 

 of electrons to the level of molecular oxygen. Davenport and 

 Hill have suggested that in the chloroplast there could also 

 be a stepwise transfer of H or electrons from the level of 

 oxygen to that of a reducing agent. They found that the 

 redox potential of cytochrome / was mid-way between the 

 potential of molecular oxygen and the measured reducing 

 potential of the illuminated chloroplast. Thus the chloroplast 

 reaction might consist of two steps, each step being depen- 

 dent on a light reaction. The oxygen molecule would be 

 supposed to arise by the reduction of cytochrome / which 

 itself becomes oxidized when the substance characteristic 

 of the second step becomes reduced. It happens that the 

 energy for the reaction (pH 7-0) 



4 cytochrome/ (Fe^+)+2H20=4 cytochrome/ (Fe2+)+4H++02 



