1932 



KINETICS OF PHOTOSYNTHESIS 



CHAP. 37D 



addition to this precaution, the cells were conditioned to the intensity of 

 light to be used in the measuring period, practical constancy of 0(18)0(16) 

 consumption in light and in the dark became the general rule — provided the 

 partial pressure of oxygen was high enough (and light intensity low enough) 



< 



2 



Z) 



in 

 in 



UJ 

 IT 



a 



_i 

 < 



12.0 



1.0 



< 



a. 



UJ 



> 



10.0 



10 15 



MINUTES 



Fig. 37D.18. Tracer oxygen consumption by a Chlorella suspension (after Brown 

 1953). Cells harvested after 7 days growth. 300 lA. cells suspended in 5 ml. at 23.5° C. 

 Burk and Warburg's medium. Red light; intensity about 3 times that giving com- 

 pensation. Oxygen, 0.7% in helium. Data at 20 sec. intervals. 



to avoid marked dilution, during the light periods, of tracer oxygen in the 

 gas by the photosynthetically evolved 0(16)0(16). Figs. 37D.17 and 18 

 illustrate the latter point, by comparing a curve obtained in 8.4% O2, 

 close to compensation, with a curve measured at 0.7% O2, at three times 

 the compensating light intensity. (Correcting for isotope dilution would 

 restore the linearity in the second case.) Low oxygen tension, and high 

 light intensity also increase the danger of incomplete mixing of photo- 

 synthetic oxygen with the oxygen pool in the gas. Re-utilization of photo- 

 synthetic oxygen probably has been responsible for the apparent inhibition 

 of respiration by light in some of the experiments in fig. 37D.16. 



