PHOTOSYNTHESIS AND RESPIRATION 



1929 



discrimination factor is in approximate agreement with theoretical expecta- 

 tion, but the high value for the C(14)-C(12) discrimination — first obtained 

 by Weigl et al. and confirmed by Brown — is about twice that expected theo- 

 retically even for a totally irreversible (not to speak of a reversible) reaction ; 

 growth experiments of the same authors with Scenedesmus ohliquus gave re- 

 sults in agreement with theoretical expectation for a partially reversible 

 CO2 uptake. 



(6) 0{18) Studies 



The assumption that respiration is essentially unchanged in light 

 was confirmed by measurements with isotopic oxygen tracer, 0(18)0(16). 



20 30 



MINUTES 



Fig. 37D.15. Metabolism of oxygen isotopes by a suspension of Chlorella pyrenoidosa 

 in phosphate buffer in dark and in Hght (Brown 1953). Mass 32 (ordinary oxygen) de- 

 creased in the dark due to respiration and increased in light due to respiration and photo- 

 synthesis. Mass 34 (tracer oxygen) decreased in the dark and in light, due to respira- 

 tion. The illumination intensities for the two light periods were different. 



Plants containing only ordinary oxygen, 0(16), exposed to air enriched in 

 0(18)0(16) were found by Van Norman and Brown (1952) to consume 

 0(18)0(16) uniformly through the dark and the light periods, while 

 0(16)0(16) was consumed by respiration in the dark, and evolved by photo- 

 synthesis in hght. These experiments give clearer results than those with 



