ENERGY MIGRATION AND THE PHOTOSYNTHETIC UNIT 1281 



in order to complete the reduction of the carbon dioxide molecule associated 

 with it, how can the uptake of carbon dioxide start immediately upon illu- 

 mination, even in light which is so weak that individual chlorophyll mole- 

 cules absorb quanta at an average rate of only one everj^ 30 minutes? 



The second difficulty could be eliminated by the assumption of a single 

 photochemical reduction step, followed by nonphotochemical dismutations, 

 while the first one could be resolved by the assumption that the reaction 

 substrate (or an intermediate catalyst) moves freely through the photo- 

 synthetic apparatus, and can thus collect the required energy quanta 

 from several excited pigment molecules. The quantity of the substrate, 

 or catalyst, required for efficient energy collection may well be only 0.1 

 mole per cent of the amount of the pigment itself, particularly if the activa- 

 tion of chlorophyll is prolonged by transfer into a long-lived active state 

 (c/. Vol. I, chapter 18). 



An alternative, more exciting and more controversial interpretation 

 has been suggested. It was submitted that the absence of an induction 

 period in weak light, as well as the high quantum yield, can be explained 

 by postulating the existence of a "photosynthetic unit" of 300-2400 

 chlorophyll molecules. This concept was developed by Gaffron and Wohl 

 (1936) from initial suggestions by Emerson and Arnold (1932\ 1932^). 



In discussing, on the basis of substrate limitation, the figures in table 

 32.1, Emerson and Arnold (1932^, 1932^) and Arnold and Kohn (1934) sug- 

 gested that a "chlorophyll unit" of 2500 molecules may be associated with 

 one "reduction center" (for example, a molecule of the carbon dioxide- 

 acceptor complex, ACO2) in such a way that all these pigment molecules 

 can co-operate in bringing about the reduction of the molecule of carbon 

 dioxide attached to the one "center." 



In discussing the possible attribution of flash yield limitation to a finishing cata- 

 lyst, we suggested that to calculate the available amount of this catalyst the factor r 

 should be multiplied by n (or n/2) (n being the number of quanta required for the reduc- 

 tion of one molecule of carbon dioxide), because the catalyst might have to operate n 

 (or n/2) times in the reduction of one carbon dioxide molecule to the carbohydrate level 

 and the liberation of one oxygen molecule. (For example, in the Franck-Herzfeld 

 scheme, 7.VA, eight unstable intermediates must be stabilized by catalyst Eb.) In the 

 hypothesis of flash yield limitation by the available substrate now under discussion, the 

 necessity of dividing r by n (or n/2) is less certain, because a reaction center can con- 

 ceivably take up, during a single flash, all the n (or n/2) quanta that might be required 

 to complete the reduction of the associated carbon dioxide molecule (and the oxidation of 

 the associated water molecule). 



It is, however, equally possible that each reaction center can utilize only 1 (or 2) 

 quanta per flash, and that the intermediate photochemical products must complete 

 their conversion into the final products, carbohydrate and oxygen, by nonphotochemical 

 dismutations (or coupled reactions, discussed in chapter 9 under the name "energy dis- 

 mutations"). If one of these mechanisms is used in photosynthesis, the number of 



