MECHANISM OF PHOTOSYNTHESIS I43 



of Arnon with the reducing properties of the chloroplast 

 observed by other methods. It would be in accord with the 

 experiment of Tolmach (mentioned previously on p. 125), 

 who showed that at very low O2 pressures the addition of 

 TPN to illuminated chloroplast preparation produced much 

 more O2 than would correspond to the reduction of the co- 

 enzyme. This, however, might lead back to the extreme 

 view that the energy fixed by light is only equivalent to a 

 reaction of free energy approximately equal to the oxidation 

 of succinate to fumarate by Og. (-£"0' = o-o volt.) The oxida- 

 tion of succinate by mitochondria can lead to phosphoryla- 

 tion with a quite high efficiency. An elucidation of the 

 mechanism of the process would be of interest in the present 

 connexion; though there is no evidence to show that the 

 chloroplasts react directly with succinate or fumarate. 



Experiments of Warburg and Burk 



Some of the results obtained from physiological measure- 

 ments on living cells have been quoted in support of 

 the chemosynthetic hypothesis. Warburg's discovery that 

 oxygen is required for the uptake of CO2 by illuminated 

 Chlorella cells is most easily interpreted in this way. The 

 influence of cyanide as an inhibitor of the uptake of CO2 in 

 light can be interpreted as due to the blocking of an oxidation 

 reaction. In this connexion the discovery of the adaptation 

 of the alga Scenedesmus to hydrogen made by Gaffron is 

 relevant. This organism can be adapted to oxidize hydrogen 

 by molecular oxygen and assimilate CO2 in the dark. Gaffron 

 measured the n referring to the equation (j (p. 142) and found 

 a value of 6. In the absence of COg it was found that the 

 oxidation of Hg was slower and there was evidence that 

 peroxide accumulated. (Compare the Mehler reaction with 

 chloroplast preparations (Chapter 7).) In the absence of 

 oxygen and at low light intensities the H2 is oxidized to 

 water during the photoreduction of the CO2. If the oxygen- 

 producing mechanism of the cells is inhibited (for example 

 by o-phenanthroline) photoreduction may proceed in strong 

 light; but normally if the light intensity is increased photo- 

 reduction is at once replaced by photosynthesis with the 



