PHOTOSYNTHESIS AND RESPIRATION 



1931 



C( 14)02, because no isotopic discrimination is observed, and no isotopic 

 exchange (e. g., of oxygen in O2 for that in H2O) seems to occur (fig. 37D.14). 

 In continuation of Van Norman and Brown's preUminary studies, more 

 detailed measurements of respiration in hght were carried out by Brown 

 (1953). A representative experiment, in which respiration (consumption of 



Fig. 37D.17. Tracer oxygen consumption bj^ a Chlorella suspension (Brown 1953). 

 Cells harvested after 4 days growth. 160 (A. cells suspended in 4 ml. Burk and War- 

 burg's medium at 22.9° C. Red light; intensity near compensation. O.xygen, 8.4%. 

 Carbon dioxide, 0.8%. Background gas, helium. Data recorded at 20 sec. intervals. 



0(18)0(16)) continues unchanged through two dark and two hght periods, 

 is shown in fig. 37D.15. The results were not always so clearcut. Fig. 

 37D.16 summarizes seventeen dark-light-dark cycles (all on Chlorella 

 pyrenoidosa) . It shows variations in behavior ranging from practically no 

 change (last experiment in the second row) to a 70% drop of respiration in 

 light (middle of last row) ; occasionally, a steady respiration increase, con- 

 tinuing through both light and dark periods (typified by the fifth cycle), 

 was observed. This trend could be, however, prevented by avoiding pro- 

 longed periods of anaerobiosis in the preparation of the suspensions. If, in 



