858 LIGHT AND LIFE 



demonstrated by the instantaneous rise of luminescence to the steady- 

 state level. Formation of the intermediate requires both ADP (or 

 AMP) and luciferin, as well as the enzyme. The intermediate might 

 be luciferyl phosphate. In that case, the AMP formed in the reaction 

 might in turn react, after the luminous reaction, with the oxyluciferyl 

 phosphate to form oxyluciferin and regenerate the ADP in a cyclic 

 type of mechanism. For the present, this remains merely a working 

 hypothesis. 



Photosynthesis 



Current studies of photosynthesis involve a number of aspects of 

 the over-all process: the nature of the primary activation of chloro- 

 phyll and the transfer of energy from chlorophyll to other parts of 

 the system; the transfer of absorbed light energy from accessory pig- 

 ments to chlorophyll a; the flow of electrons in the primary reduction 

 of pyridine nucleotides, in the Hill reaction, and in the reduction of 

 cytochromes; the relation of photophosphorylation to photosynthesis; 

 and the catalytic participation of oxygen in the process. The paper by 

 Daniel Arnon on the nature of Cell-Free Photosynthesis and the 

 Energy Conversion Process presents a number of schemes which en- 

 deavor to unify thinking about these aspects into a coherent scheme 

 of photosynthesis as a device that evolved for producing ATP, rather 

 than as one concerned primarily with carbon dioxide fixation. It is, 

 however, evident alike from the other papers and from the vigorous 

 discussion that followed these sessions that a number of key points 

 in Arnon's scheme represent concepts either controversial or not yet 

 fully tested. It seems best, in view of the tentative nature of many 

 parts of this scheme, to abstain from organizing the entire considera- 

 tion of photosynthesis in the Symposium around this single theory. 

 Nevertheless, a very large portion of the general picture presented 

 by Arnon seems likely to stand the test of further experimentation, 

 and is certain to take a prominent place in this and futine discussions 

 of photosynthetic mechanisms. 



Chlorophyll and Organic Free Radicals 



When a quantum of light is absorbed by chlorophyll in solution, 

 an excited state of the molecule arises, and two things may happen. 

 Excited chlorophyll in some way promotes the oxidation of substrates 

 by oxygen (e. g., oxidation of allylthiourea, studied by Gaffron). Also, 

 chlorophyll in the excited state will sensitize a transfer of hydrogen 



