General Concept of Photosynthesis by Isolated 



Chloroplasts* 



DANIEL I. ARNON, M. B. ALLEN, and F. R. WHATLEY, Laboratory 

 of Plant Physiology, Department of Soils and Plant Nutrition, University of 



California, Berkeley, California 



The old concept, once dominant in plant physiologj'-, that the entire 

 photosynthetic apparatus is localized in chloroplasts was discredited 

 about twenty years ago when Hill showed that isolated chloroplasts 

 could not fix COo although they retained a capacity to evolve oxygen 

 when illuminated in the presence of an artificial electron acceptor in 

 accordance with equation L 



Light + A + H,0 > H2A + V2O2 (1) 



in which A represents an electron or hydrogen acceptor other than 

 carbon dioxide. The failure of isolated chloroplasts to fix CO2 was 

 later confirmed with isotopic carbon [see review, 4]. The old concept 

 was never based on direct evidence for CO2 fixation by chloroplasts 

 but on the view that the oxygen of photosjaithesis came from the 

 sphtting of CO2; hence oxygen evolution by chloroplasts, first ob- 

 served by Engelmann in the 1880's, was accepted as evidence of 

 simultaneous COo fixation. 



In the light of modern knowledge eciuation 1 has, until recently, 

 represented the only known photochemical acti\aty of isolated chloro- 

 plasts, and has, for this reason, sometimes been called "the chloro- 

 plast reaction" [Hill (1), Whittingham (2)]. 



However, as was discussed by Allen and by ^Miatley in this sym- 

 posium isolated chloroplasts have now been found capable of carry- 

 ing out two additional photochemical reactions: photosjiithetic phos- 

 phorylation, represented by equation 2,t and also, as shown by equa- 

 tion 3, the reduction of CO2 to the level of carbohydrate with an 

 accompanying evolution of O2 (5). 



* Aided by grants from the National Institutes of Health and the Office of 

 Naval Research. 



t P,- represents orthophosphate; ADP, adenosine diphosphate; ATP, adenosine 

 triphosphate. 



296 



