1926 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



in a side arm. This was taken as proof that, with photosynthesis inhibited 

 by lack of carbon dioxide, respiration continues at the same rate in hght as 

 in darkness; and that, consequently, it cannot be postulated — as Franck 

 did — that respiration intermediates are drawn into the reaction cycle of 

 photosynthesis when the carbon dioxide supply fails (for Franck's answer 

 to this argument, see section 4(d) below). 



Kok (1951) was unable to reproduce the experiment of Warburg e/ al. In a mano- 

 metric vessel with a center-well containing KOH-soaked filter paper, and a very thin 

 layer of algal suspension in the main compartment, the rate of oxygen consumption was 

 found by him to decrease even upon very weak illumination, indicating a beginning com- 

 pensation of respiration (presumabl}^ by photochemical "antirespiration"). 



While these experiments indicated that, in the absence of photosyn- 

 thesis, respiration was unaffected by light, subsequent experiments by Burk, 

 Warburg, Geleick and Briese (c/. sect. 4(a) below) lead them to the conclu- 

 sion that, when carbon dioxide is supplied, and photosynthesis thus allowed 

 to proceed, an "extra respiration" occurs in light, involving the consump- 

 tion of at least 2.8 volumes of oxygen (and the liberation of an equal volume 

 of carbon dioxide) for each volume of these gases liberated and consumed, 

 respectively, in "net" photosynthesis (meaning by this now the rate of gas 

 exchange in light, corrected for steady respiration, as observed in an ex- 

 tended dark period). 



Because of the significance of these observations for Burk and War- 

 burg's work on the quantum yield, they will be described in more detail in 

 section 4. 



(a) C{14) Studies 



Weigl and Calvin (1949) and Weigl, Warrington and Calvin (1951) 

 first used the isotopic tracer technique to find out whether the rate of respi- 

 ration underwent a change in light. For this purpose, plants (barley seed- 

 lings) were exposed to a circulating gas mixture, (N2 -h CO2), containing 

 tracer carbon, C*(14). The total carbon dioxide content was monitored 

 by an infrared photometer (p. 852), the oxygen content by a Pauling mag- 

 netic oxygen meter, and the content of C*02 by an ionization chamber. 

 The curves show^ing the change in specific activity, [C*02]/[C02] of the 

 circulating gas as function of time, obtained in this way, appeared com- 

 plex (fig. 37D.13). If the nonphotochemical uptake of C*02 by isotopic 

 exchange {cf. chapter 36, section Al, 2) was neglected (compared to the up- 

 take by reduction in light), these curves could be explained by taking into ac- 

 count, in addition to the known rates of net carbon dioxide evolution in dark 

 and consumption in light, two more factors : rate of respii'ation in light, and 

 the discrimination between C(12) and C*(14) in photosynthesis. Addi- 



