J. A. BASSHAM AND M. CALVIN 



that there is a continuous flow of energy from one cycle to another 

 resulting in the conversion of light energy into the potential 

 energy of new chemical bonds. These cyclic processes, although 

 similar to better known cyclic processes occurring during 

 respiration in plants and animals, do not seem to be in any 

 instance simple reversals of respiratory cycles but do appear to 

 have many points of contact with respiratory reactions, including 

 common intermediates and enzymes. These points of contact 

 may well lead to interaction between the two processes which 

 could alter the course of each (18). It appears that the organiza- 

 tion of the green plant, both structural and enzymatic, may 

 provide mechanisms for partially preventing such interactions 

 where they would prove deleterious to the efficiency of the 

 photosynthetic reaction, and possibly for permitting or even 

 inducinsT them where interaction is beneficial. 



^& 



Function of the Chloroplast 



One such device for isolating photosynthesis from respiration 

 is the chloroplast itself. It appears likely that the entire reaction 

 of photosynthesis, from the absorption of light, carbon dioxide, 

 and water to the formation of various end products, may occur 

 in the chloroplasts. This fact was indicated long ago by the 

 observed formation of starch granules in chloroplasts. (However, 

 starch and other high molecular weight compounds are also 

 formed outside the chloroplasts from simple sugars, amino acids, 

 and other low molecular weight compounds which diffuse out 

 of the chloroplast.) It has long been known that isolated chloro- 

 plasts can, under suitable conditions, retain the ability to evolve 

 oxygen at rates comparable to those of photosynthesis, and at the 

 same time, transfer reducing power to a suitable oxidizing agent. 

 Efforts to demonstrate CO2 reduction with isolated chloroplasts 

 were generally unsuccessful in the past. However, Gerretsen 

 (32) found a decrease in the oxidation-reduction potential in 

 illuminated chloroplast suspensions when they were supplied 

 with carbon dioxide and concluded that there might be an uptake 



30 



