PHOTOCHEMICAL PART OF PHOTOSYNTHESIS 145 



Discussion 



Gaflfron : We have expressed regret that Dr. Franck could not be here. This, 

 I feel, applies in particular to our discussions on the chemiluminescence data. 

 What has been missing is a more thorough debate in biochemical terms on what 

 Dr. Franck thinks is the most logical interpretation of Strehler's and Arnold's, 

 as well as Brugger's, observations. 



Dr. Franck's main contention is that there is no necessity to introduce a 

 hydrogen transferring enzyme for the photochemical reduction of PGA (or 

 whatever carboxylated substance becomes reduced). The task of an enzj^me is to 

 facilitate reactions which otherwise require too much activation energy. In the 

 photochemical reaction we have an excess of energy to take care of activation. 



Dr. Franck believes the majority of observations available to date indicate 

 that a photochemical hydrogen transfer happens, indeed without a transferring 

 agent, directly between the chlorophyll complex and an attached ultimate ac- 

 ceptor. Such a direct transfer could also be written in the form of an electron 

 movement though, he contends, this entails more conceptual difficulties than is 

 generally recognized. What the biochemist should consider carefully is whether 

 it is wrong, for experimental reasons, to assume that a reducible intermediate, say 

 of the Calvin-Benson cycle, is directly attached to the chlorophyll complex. 

 By means of the one or two quantum process (which one is here irrelevant) the 

 metabolic intermediate receives one hydrogen and becomes a radical. Then it has 

 to wait for the opportunity to get a second hydrogen. During this time it can react 

 back and produce luminescence. Franck belongs among those scientists who be- 

 lieve that a theory becomes respectable only after it has been worked out and re- 

 vised to a point where no experimental contradictions are left that are fairly 

 obvious. I remind you of Dr. Brugger's curve showing the chemiluminescence of 

 Chlorella in nitrogen during steady-state illumination. The luminescence is rela- 

 tively low. Then one puts in a little carbon dioxide (see Fig. 4 on page 137). 

 Since carbon dioxide causes photosjmthesis to start, energy is drained away and 

 one gets a depression in the luminescence immediately followed by an increase. 

 Upon another addition of a little carbon dioxide, the luminescence again goes 

 down and then up. It can be pushed up in steps by adding small amounts of car- 

 bon dioxide to the maximum the luminescence can reach under any steadj'-state 

 conditions. There is a staircase effect due to short-lasting additions of carbon 

 dioxide. This is typical for one of those observations which require an explanation 

 consistent with the biochemistry and the physics of the process before one can 

 say the matter is understood. Higher luminescence means a higher concentration 

 of photochemically produced radicals. Here they seem to increase in proportion 

 to the amounts of C02-dependent intermediates, whenever the latter are de- 

 prived of the opportunity to complete the cycle. 



Bassham : I think we can explain this easily in terms of intermediate-reduced 

 enzymes; and, secondly, I would point out that as far as photochemical reduction 

 of PGA is concerned, we can reduce PGA in the dark following preillumination. 



Gaffron: In this case PGA is formed by carboxylation and some of it is cer- 

 tainly reduced in the dark. Does this prove it is exactly the same reaction which 

 goes on in the light? 



