C*02 FIXATION IN DARKNESS WITH AND WITHOUT PREILLUMINATION 1643 



even for this species, table 36. IV indicates ten minutes as "preillumination 

 time needed to ensure maximum C*02 uptake in the dark." 



The illumination time required to accumulate the maximum capacity for C*02 

 uptake in the dark is significant for the interpretation of this uptake. If this time is 

 10-60 min., a "photosynthetic" origin of the CO2 gulp after preillummation is less 

 likely than if this time is of the order of 20 sec. (as suggested by Calvin and Benson). 

 The reasons are as follows: 



If the ma.ximum accumulation of the carbon dioxide acceptor in photosynthesis is 

 of the order of magnitude of that of chlorophyll (as indicated, e. g., by the volume of the 

 "pick-up") then we can expect this peak concentration to be reached in the absence of 



(/5 



o 



10 . 



E 

 E 



cr 



iij 

 a. 



Q 

 LlI 

 X 

 Li. 

 10 

 O 



O 



6 

 E 



0.5 10 



CARBON DIOXIDE PRESSURE, mm 



Fig. 36.4. Dependence of dark C* fixation on CO2 pressure: A, non-pre- 

 illuminated algae; B, preilluminated algae (after Calvin and Benson 1948^). 



carbon dioxide after a time of the order of magnitude of that required for each chloro- 

 phyll molecule to reduce one carbon dioxide molecule ("assimilation time" — which ac- 

 cording to chapter 28, is about 20 sec. in saturating light). If saturation of the cells 

 with the acceptor requires a much longer time, it is unlikely to be due to the main reac- 

 tion sequence of photosynthesis (since the rate of any partial process in this sequence 

 cannot be slower than that of photosynthesis as a whole). This consideration applies 

 also to the accumulation of "reducing power." It does not apply to side effects, such 

 as decarboxylation of respiratory intermediates (in consequence of CO2 withdrawal into 

 the photosynthetic system), since the yield of these processes could be much smaller 

 than that of photosynthesis itself. 



The origin of the "C*02-gulp" by cells which have been preilluminated in the ab- 

 sence of carbon dioxide and then exposed to C*02 in the dark, has been further discussed 

 by Calvin and Benson (1948). They argued that if the gulp were due to C*02 uptake 

 by the respiratory intermediates decarboxylated during the illumination period, the dif- 

 ference between the initial slopes of the curves showing C* fixation as function of [CO2] 

 (fig. 36.4) would indicate that the partial pressure of CO2 inside the cells has been re- 

 duced, by preillumination, from ^^0.1 mm. to as httle as 0.001 mm., and this, they said, 

 light cannot do. (In support of this assertion they recalled the known falling-off of 



