THE CHEMISTRY OF PHOTOSYNTHESIS 



147 



Chlorophyll 



Fig. 54. The chlorophyll aggregate as a semiconductor. 



According to a further hypothesis, chlorophyll acts photochemically not 

 as a single molecule but as an organized, orientated aggregate. The lamellae 

 structure of the grana discussed in § 3 support the view that the chlorophyll 

 aggregate can be considered to be a kind of photoelectric battery and the 

 absorbed light energy its driving force. On one side of the aggregate a fat 

 or lipoproteid layer and on the other side a hydrophilic layer act as "elec- 

 trodes." The former would be composed of the lipoid soluble lipoic acid and 

 the latter would be water (Fig. 54). 



Commoner's experiments discussed in § 9 lead to the assumption that the 

 chloroplast acts as a semiconductor. Katz (35) and Calvin (9) compare the 

 chlorophyll aggregate to a semiconductor. As in the transistor (Fig. 6), the 

 electrons set free in the chlorophyll layer migrate to the lipoic acid layer 

 where they are accepted by lipoic acid. The "holes" produced are occupied 

 by electrons originating from water (Fig. 54). As sufficient water is present, 

 no back reaction can take place. The lipoic acid molecules are continuously 

 regenerated so that the electrons produced are continuously accepted. 



According to Levitt (36), the chlorophyll excited by one quantum would give 

 only one electron to lipoic acid and, after absorption of a second quantum, a 

 second electron. Bearing this in mind, we can imagine what would happen 

 at the "electrodes" of the chlorophyll aggregate. By the absorption of one 

 quantum about 42000 cal/mole would be taken up. Assuming a loss of 5000 

 cal/mole, only 37000 cal/mole of the light energy would be available. Con- 

 sidering the relationship between potential and change of free energy 



AG = -23074 nE (44) 



we have for n = 1 a potential difference between the aforementioned layers 

 of the chlorophyll aggregate ("electrodes") of 1.6 volt. It is now possible to 

 determine the potential of each of the "electrodes." According to reaction 

 b, the increase of free energy during the transfer of one electron to lipoic acid 

 is about 7000 cal/mole, corresponding to a potential of 0.3 volt. The remain- 

 ing energy is 37000 — 7000 = 30000 cal/mole, corresponding to a potential 

 of 1.3 volt. Thus, the potential difference of 1.6 volt is distributed over the 

 photoelectric battery in such a way that the potential is 0.3 volt on the nega- 

 tive side (lipoic acid side) and 1.3 volt on the positive side (water side). 



