CARBON MONOXIDE INHIBITORS 328 



nant energy, transfer of energy from the chlorophyll to the cytochrome, but rather 

 perhaps a charge transfer or a hydrogen transfer or something else, and that this 

 is the reason he didn't insist on the band. 



Rosenberg : I am reminded of a paper by Emerson and Lewis. They pointed out 

 how the quantum efficiency of photosynthesis falls off very sharply beyond the 

 red peak of chlorophyll towards longer wavelengths. At a wavelength above 680 

 the quanta absorbed do practically nothing for photosynthesis. If there were such 

 cytochrome pigments in large amounts in the chloroplast, perhaps they would 

 absorb a big part of the energy in that region which turns out to be very useless 

 for photosynthetic activity. 



Duysens : May I reply to this? This decrease of quantum yield in the deep red 

 region parallels the decrease of fluorescence yield of chlorophyll in the same re- 

 gion, and the same phenomenon occurs in solution. So it is the property of the 

 pigment and not of other absorbing substances in the plant. We find this in all 

 kinds of fluorescent substances. 



Smith : Isn't it true that when j'ou go beyond that band j'ou cannot excite the 

 fluorescence any more? 



Duysens : You can excite fluorescence, but the fluorescence yield drops. 



Brown : Regardless of the anomalies in this region, I wonder if I could ask a 

 very naive question. Maybe some physical chemist can answer it from known 

 fact. If 5'ou have an exciton mechanism for energy transfer is it not by definition a 

 primary light absorber: The exciton mechanism that Dr. Kamen is suggesting 

 should extend the action spectrum of photosynthesis out into the infrared. 



Allen: Can anybody really say what an exciton is? Perhaps we can start a 

 little farther back. 



Rabinowitch: This is a matter of definition. The word "exciton" has been used 

 in two different senses. One of them is identical with resonance transfer of excita- 

 tion, a system in which energy can be transferred, by resonance, from one molecule 

 to another, staying with each molecule long enough for it to preserve its individu- 

 ality as an absorber. 



However, a more accepted usage now is to speak of exciton when we have such 

 strong interaction between the molecules that they are acting in absorption as a 

 single large super-molecule, in this case, the whole system is different from that of 

 the individual molecule. There is definitely no such exciton present in the cell. 



I would also like to mention, in connection with this drop-off in the far red, that 

 Vavilow has received the Stalin prize for the explanation of this phenomenon as a 

 general consequence of thermodynamics, but according to Pringsheim his explana- 

 tion was wrong. 



I think we still don't have a satisfactory explanation, although it obviously is 

 not a specific property of one pigment, but a general property. Of course, one could 

 explain it by saying if an anti-Stokes reaction occurred with a 100 per cent yield, 

 we would have a machine that does nothing but convert smaller quanta into larger 

 quanta, taking energy out of the heat vibrations, which seems to be against the 

 second law of thennodynamics. But this is not a satisfactory proof, since the law 

 could be satisfied by a lowering of the yield below 100 per cent. This is one case in 

 which the physical chemists should hang their heads in shame. 



Wassink: What I got from the Emerson-Lewis article is that the absorjition is 



