BENTLEY GLASS 887 



in ihe primitive oxygenless atmosphere. Just as oxidative phos- 

 pliorylation during respiration is superimposed on tlie primitive 

 anaerobic fermentation processes of glycolysis, so the primitive anae- 

 robic cyclic form of photophosphorylation is retained in the photo- 

 synthesis of green plants, the later elaborations which are super- 

 imposed upon the primitive process being concerned with the utili- 

 zation of ATP and the sul)stitution of different electron donors for 

 the most primitive one, molecular hydrogen itself. The assimilation 

 of CO2 gained in importance as the supply of organic compounds 

 in the environment was depleted. Electron donors at a lower re- 

 ducing potential than hydrogen — donors such as succinate or thio- 

 sidfate — became utilizable through the use of light not only to supply 

 ATP but also to provide electrons with high enough energy to re- 

 duce pyridine nucleotides. Finally, photosynthesis advanced in green 

 plants to the stage wliere CO^ could serve as sole source of carbon 

 and where water became the electron donor. With this achieve- 

 ment there appeared the evolution of oxygen, a byproduct of the 

 removal of electrons from hydroxyl ions and reaction of the result- 

 ing [OH] radicals to form water and release molecular oxygen. 

 The scheme outlined is reminiscent of Norman Horowitz' theory 

 of the gradual lengthening of biochemical chain reactions during 

 evolution, when exhaustion of one after another of the primeval 

 organic components of the environment forced living organisms to 

 add steps to existing chains until they had finally attained some 

 inexhaustible substrate. In the case of photosynthesis, the inex- 

 haustible supplier of electrons was water, and of carbon it was CO2. 

 Oxygen, the byproduct, altered the composition of the atmosphere 

 until aerobic metabolism became possible. This made oxidative 

 phosphorylation possible. Hence mitochondria, now believed to have 

 a common phylogenetic origin with proplastids, arose in evolution 

 after the chloroplasts. The oxygen furthermore made possible the 

 final accretion to the balanced photosynthetic system, the addition 

 of oxygen-catalyzed cyclic photophosphorylation. 



Certain aspects of Arnon's views have been challenged. Albert 

 W. Frenkel, for example, believes it premature to state definitely 

 that there are basically different photochemical mechanisms in green 

 plants and in bacteria, that in the latter the non-cyclic stage of photo- 

 phosphorylation is lacking, and also that no photooxidant and photo- 

 reductant are formed as primary products of the photochemical split- 

 ting of water. In his paper, Frenkel describes a bacterial chromato- 

 pliore system from Rhodospir ilium rubrum that in fact does carry 



