1282 THE PIGMENT FACTOR CHAP. 32 



needed reaction centers is n (or n/2) per oxygen molecule liberated in the flash. We 

 therefore conclude that, in the "substrate Umitation" theory of flash saturation, the 

 required number of "reaction centers" can be T[Chl]o, or 4T[Chl]o or 8r[Chl]o depending 

 on whether the postulated reaction mechanism permits each center to utihze 1, 4 or 8 

 quanta in each flash. 



Gaffron and Wohl (1936) sought an explanation of how the quanta ab- 

 sorbed anywhere 'in the "unit" can be utilized, without loss, for photochemi- 

 cal action in a single "center." They suggested that the "unit" may be a 

 closely packed system in which the individual pigment molecules are so 

 intimately associated that a light quantum absorbed by one of them can be 

 exchanged, from neighbor to neighbor, until it reaches the reduction center. 

 In other words, instead of the energy quanta being collected (as sug- 

 gested above) by chemical agents diffusing through the system, the quanta 

 themselves were supposed to move around until they find the substrate for 

 photochemical action. 



Weiss (1937) suggested that the "units" may be identical with the 

 chloroplast grana (Vol. I, p. 357). Frey-Wyssling (1937) pointed out that 

 each granum contains about 10^ chlorophyll molecules, while "units" were 

 supposed to consist of only about 10^ molecules each. However, the unit 

 may also be interpreted statistically as a structure containing about one 

 molecule of an enzyme per about 10^ molecules of chlorophyll. It could 

 be suggested, for example, that each protein disc in the grana (c/. chapter 

 37 A), carrying about 10^-10'^ chlorophyll molecules, is provided with about 

 10^-10* "reaction centers," e. g., molecules of an enzyme. Each of the 

 latter can be conceived of as "servicing," preferentially an area, or being 

 associated exclusively with an "island" of about a thousand chlorophyll 

 molecules (c/. Rabinowitch 1951). The "servicing" may be accomplished 

 either by the diffusion of material particles, or by energy migration be- 

 tween pigment molecules, or by a combination of both mechanisms. 



Aside from any special assumption concerning the nature an ddistribu- 

 tion of the "reduction centers" in the cell, we may ask: Is the postulated 

 efficient exchange of excitation energy between a large number of chloro- 

 phyll molecules physically possible? Can evidence be adduced for (or 

 against) the occurrence of such an exchange in living chloroplasts? 



The study of energy transfer in liquids or solids is a rather new de- 

 velopment. It was known for a long time that in simple gases a trans- 

 fer of excitation energy from molecule to molecule does occur in colli- 

 sions, and that its efficiency is a function of the "resonance" between the 

 collision partners. The closer the resonance, the stronger the interac- 

 tion, the higher the probability of energy transfer, and the greater the dis- 

 tance over which it can occur. For example, when an excited helium atom 

 approaches a normal atom of the same gas, the perfect resonance between 

 the two states, (He* -}- He) and (He -1- He*), causes the energy early to 



