140 PHOTOSYNTHESIS 



But this would necessitate the oxygen being removed 

 efficiently down to the level of io~^ mm. Hg pressure of Og', 

 that is, if the chemical energy available from the chloroplast 

 reaction is taken as for the reaction: — • 



R+H20=RH2+i02+38,ooo cals. 



Arnon (1951), in his experiment with the formation of 

 malate from pyruvate, used a malic enzyme preparation 

 from leaves and was able to measure the oxygen produced 

 manometrically. The amount of malic produced was in 

 about the same proportion to the pyruvate as in the experi- 

 ments of Vishniac and Ochoa. The final pressure of Og 

 would have been 0-75 mm. Hg pressure. This would require 

 the chemical energy from the chloroplast reaction to be 

 41,000 cals. and represents a discrepancy of 3,000 cals. on 

 the other methods of measuring the chloroplast reaction. 

 We could dismiss the other measurements as unreliable, 

 but even so the question of how much energy is directly 

 made available in the chloroplast reaction would be far 

 from solved. The green plant (whilst maintaining a pressure 

 of O2 above even that in the atmosphere) can carry out reduc- 

 tions which involve a much greater total amount of energy 

 than is represented by the formation of malate from pyruvate 

 to the extent observed in vitro. 



Thus the present position is that the chloroplast reaction 

 in vitro may correspond to the oxygen-producing mechanism 

 of photosynthesis while it does not correspond directly to 

 the reducing mechanism concerned in the assimilation of 

 carbon dioxide. This latter statement would imply either 

 that the substances of the type R cannot in vitro become 

 appreciably reduced because of the back reaction 



RH2+02=R+H202 



or that the substances of the type RHg are not strong enough 

 reducing agents, as their standard redox potentials are too 

 positive. Both these considerations limit the conclusions 

 about the mechanism of photosynthesis that may be drawn 

 at present from in vitro experiments. 



