THE ENERGETICS OF PHOTOSYNTHESIS 103 



the order of magnitude of 10 mm HoO for the O2 pressure in the cells, when 

 the O2 pressure outside the cells is zero. 



Brown (7) tried to prove that no additional respiration (induced respiration) 

 occurs in photosynthesis. In his experiments Chlorella respires Oo enriched 

 with '"'O. As the alga is suspended in normal water, normal O2 should be 

 developed, according to the concept of water photolysis. However, under 

 normal conditions of photosynthesis when the light reaction and the back 

 reaction overlap, -/^ of the Oo produced in light is reabsorbed so rapidly that 

 the Oo can be imagined to be oscillating between the free state and the C- 

 bound state. If the molecular O2 at the disposal of the cells is labeled and the 

 CO2 provided is not labeled, we cannot expect that more labeled O2 will be 

 used during illumination than in the dark. As a matter of fact, on spatial 

 grounds, the normal O2 produced by the cells upon illumination will be used 

 much more rapidly than the labeled Oo introduced from outside that must 

 diffuse into the cells. Brown found, in fact, that labeled respiration was not 

 increased but was often decreased during illumination. This result shows that 

 not only induced respiration but also dark respiration favors the unlabeled O2 

 produced in the cells from the unlabeled photolyte. Warburg (43, 64) 

 regards this as a beautiful example of isotopic discrimination. Further 

 weighty arguments against the logic of Brown's method have been put forward 

 by Decker (17). 



The necessity of O2 in photosynthesis is also demonstrated by experiments 

 carried out by Krall and Burris (31) who found that C02-fixation by barley 

 leaves is inhibited to about 97.5% by keeping the leaves anaerobic in an 

 atmosphere of nitrogen in the light or in the dark for 30 min or more before 

 exposure to ^^CO?. Due to O2 evolution, this inhibition decreases with in- 

 creasing time of exposure to CO2 in the light. Though these experiments 

 are quite convincing, Krall tries to explain his results in a different way. 

 Because of his further research on the influence of CO on photosynthesis of 

 barley leaves, he considers cytochrome oxidase participates in the photo- 

 synthetic fixation of CO2 (29, 30). 



Calvin el al . (66) tried to measure induced respiration by illuminating the 

 cells in the presence of labeled CO2, expecting that more unlabeled COo 

 would be given off" in the light. However, they found no increase in un- 

 labeled CO2. This finding is quite in agreement with Warburg's equations 

 of photosynthesis, from which it follows that the induced respiration must 

 be labeled when the CO2 is labeled. The induced respiration can, of course, 

 only be measured as it was discovered by Burk and Warburg: when it is 

 separated in time from O2 development (43). 



The close connection existing between photosynthesis and aerobic re- 

 synthesis of glutamic acid, to be discussed in Part D of Chapter 4, gives further 

 irrefutable evidence of the necessity of O2 in photosynthesis. 



In § 68 and § 70 the necessity of respiration in photosynthesis will be 

 further discussed. 



