FREE RADICALS IN PHOTOSYNTHESIS 



235 



by molecular oxygen so that, in these organisms, the HX radicals can 

 be formed without the help of light. 



It was pointed out once before that the interpretation of chemo- 

 synthesis leads to a problem of energy accumulation similar to that 

 arising in photosynthesis, because the energy liberated by the oxidation 



4C02 



4 {CO,} 



2HaO 



4X 



8Y 



u 



8hv 



r 



8HY 



[Q.IOo 



8HZ 



+ 42 

 (9.10 b) 



4Y 



(9.10c) 



4Y 



+ 4Z 



(9.10 g) 



4HZ 



{5.} 



(9.10 d) 



4HX 



_J 



4HZ 



(9.IOe) 



4. 

 4X 



4{hC02} + 

 EBJO-IOf) 

 ICHzOJ+aCO^+H^O 



Scheme 9.III. — Photosynthesis according to reaction system (9.10) (according to 

 chapter 19, page 552, HZ may stand for chlorophyll and Z for oxidized, e. g., dehy- 

 drogenated, chlorophyll). 



of several substrate molecules, has to be used for the reduction of one 

 molecule of carbon dioxide. The hypothesis of "energy dismutation," 

 achieved by a coupled oxidation of the intermediate H2X by oxygen and 

 carbon dioxide, is intended to provide an answer to this problem. 



The coupling between oxidative and reductive processes in autotrophic 

 organisms has often been oversimplified. The oxidation processes were 

 supposed to run to completion, and the liberated energy was assumed to 

 be available for the chemically independent reduction of carbon dioxide 

 by water. This picture may be used for calculating thermodynamic 

 efficiencies (c/. Table 5. VII), but it does not represent the real mechanism 

 of chemosj^nthesis. A pure "energy coupling" of two independent 

 chemical reactions is impossible in the living cell. The explosion of a 

 detonator may create a local temperature sufficient for the ignition of a 

 charge of explosives; but no "heating" by the enzymatic combustion of 



