CARBOXYLATION AND THE {CO2} COMPLEX 207 



Figure 22 taken from McAlister's paper on wheat, illustrates the 

 pickup phenomenon. The ordinates represent the transmission of the 

 gas at 4.25 m^i: the higher the ordinate, the smaller the concentration, 

 [CO2]. The first, downward section of the curves represents the increase 

 in [CO2] in the dark caused by respiration; the upward section shows the 

 decrease in [CO2] in light through photosynthesis. At the beginning of 

 the upward section, an induction -period of the order of 1 to 2 minutes is 

 visible on all curves; at the end of the upward section, when illumination 

 stops, respiration usually immediately succeeds photosynthesis. How- 

 ever, on the curves which correspond to the strongest illumination, the 

 absorption of carbon dioxide continues for about 20 seconds in the dark. 



The maximum amount of pickup after very intense photosynthesis 

 was about one-half molecule of carbon dioxide per molecule of chlorophyll, 

 i. e., of the same order of magnitude as found by Ruben and Smith for 

 the binding of radioactive carbon dioxide in Chlorella and sunflower leaves. 



The pickup was observed also by McAlister and Myers (1940) with 

 a different experimental setup (cf. Vol. II, Chapter 33). This time, no 

 pickup was found at the high (saturating) concentrations of carbon di- 

 oxide, even in strong light, but positive results were obtained at low 

 (limiting) CO2 concentrations. The duration of the pickup was found to 

 increase with decreasing concentration, reaching a full minute at 0.006% 

 carbon dioxide. 



Similar observations were made by Aufdemgarten (1939) with Sti- 

 chococcus hacillaris. He noticed that the pickup is slowed down by the 

 presence of cyanide, e. g., from 30 to 150 seconds in a 1 X 10~^ M potas- 

 sium cyanide. 



It seems that the pickup occurs whenever the concentration of the 

 complex, {CO2}, is depressed, either because of the high rate of its 

 utilization by photosynthesis (McAlister), or because of the low rate of 

 its replacement, the latter being caused either by a low concentration of 

 carbon dioxide (INIcAlister and IMyers), or by an inhibition of the cata- 

 lyst Ea by cyanide (Aufdemgarten). 



Another kinetic observation, which must be mentioned here is the 

 "carbon dioxide gush" observed by Emerson and Lewis (1941) in the 

 first few minutes of illumination of Chlorella. It was followed, in the 

 dark, by a slow absorption of a quantity of carbon dioxide roughly 

 equivalent to that lost in the gush. The details of this phenomenon will 

 be discussed in chapter 33 (Vol. II), dealing with the induction phenom- 

 ena. The total volume of the gush is about 0.2 ml. per ml. of cell volume; 

 it can thus be attributed to a photochemical decomposition of the 

 complex {CO2}. (Details of this explanation, given by Franck in 1942, 

 were discussed in chapter 7, page 167.) The reabsorption of carbon di- 

 oxide in the experiments of Emerson and Le^ds is very slow (it may 



