1280 THE PIGMENT FACTOR CHAP. 32 



photosynthetic apparatus can be kept stocked, in darkness, with a full as- 

 sortment of intermediates, so that, when illumination starts, carbon di- 

 oxide absorption can begin immediately, even in very weak light. 



If we assume only one photochemical step, followed by catalytic dis- 

 mutations, the co-operation of 4 or 8 light quanta in the reduction of one 

 molecule of carbon dioxide can be explained, without the assumption of 

 stable or continuously regenerated intermediates, by catalytic, nonphoto- 

 chemical reactions among several identical, primary photochemical prod- 

 ucts. The velocity of these reactions is in no way limited by the rate of 

 absorption of light quanta by a single chlorophyll molecule, since the inter- 

 mediate photoproducts, formed by several chlorophyll molecules, can dif- 

 fuse and react with each other. In this case, even if after a dark period 

 the cells were entirely devoid of reduction intermediates, oxygen evolution 

 and carbon dioxide consumption could begin immediately upon illumina- 

 tion. 



5. Energy Migration and the Hypothesis of the Photosynthetic Unit 



We consider the assumption of a "finishing" dark reaction of limited 

 maximum rate as the cause of flash saturation (as well as of saturation in 

 steady light) to be preferable to the assumption that this saturation is due 

 to the limited quantity of the reduction (or oxidation) substrate. The 

 arguments that influence our choice are {1) the observations of fluorescence, 

 which make it probable that usually all chlorophyll molecules are associated 

 with the photosensitive substrate, even after the light saturation has set in 

 (c/. page 1075), and (2) the observations which indicate that the primary 

 carbon dioxide absorber is present in the cells in a quantity roughly equiva- 

 lent to that of chlorophyll. As described in chapter 28, this theory, proposed 

 by Franck, can explain also the possibility of utilizing for photosynthesis 

 all the light quanta absorbed by chlorophyll in weak light and the absence, 

 under these conditions, of an extended induction period for the uptake of 

 carbo]! dioxide. 



Leaving aside for a while the arguments derived from fluorescence, and 

 from the apparent abundance oi the primary carbon dioxide absorber, we 

 ask whether the alternative theory of "substrate limitation" could explain 

 the two last-named phenomena: the high quantum yield and the absence 

 of induction losses in weak light. Offhand this appears difficult. If, at 

 best, only 0.1% of the chlorophyll molecules present in the cell can be as- 

 sociated with the reaction substrate (since this is the total amount of this 

 substrate supposed to be available in the cells), how can the quanta ab- 

 sorbed by all chlorophyll molecules be made useful for photosynthesis? 

 And if each chlorophyll molecule has to absorb 4, or, more likely, 8 quanta 



