188 MACROMOLECULAR COMPLEXES 



would be expected with progressive dilution. Since the pigment 

 content of the chromatophore is known, it can be calculated that, in 

 these experiments, each chromatophore was forming about 500 high- 

 energy phosphate bonds per minute ( cf . Bergeron, 1958 ) . 



The enzymes involved in the assimilation of carbon dioxide are 

 not integrated in the chromatophore and are found in the uncolored 

 supernatant fluid. If the chromatophores are recombined with the 

 supernatant fluid, the reconstituted system fixes COo when the sys- 

 tem is illuminated (Table 2). When the light: dark ratios of CO^. 

 incorporation are compared, the reconstituted system is 30 per cent 

 as effective as the intact cells. It should be noted in passing that, 

 unlike higher plants, Chromatium does not synthesize large quan- 

 tities of reserve carbohydrates, and the rate of incorporation of COo 

 into amino acids such as aspartic is proportionately more rapid. 



TABLE 2 



COo-Fixation by Various Cell Fractions 

 of Chromatium. 1 Hr * 



Dark 

 or Total CO. Fixed 



Cell Fraction Light as C* in Counts/Min 



Whole cells Light 4,500,000 



Whole cells Dark 50,000 



Chromatophores Light 750 



Supernatant Light 1,750 



Supernatant and chromatophores Dark 1,950 



Supernatant and chromatophores Light 69,000 



" The cell-fractionation procedure separates the energy-yielding and energy-requir- 

 ing steps of photosynthesis. The chromatophore fraction catalyzes photophosphoryla- 

 tion, but the enzymes which use ATP for the synthetic reactions are in the supernatant 

 fraction. When the two fractions are combined, their fixation of COj proceeds in the 

 light. 



The Chromatophore as the Photochemical Organelle 



The procedure used to isolate a given set of cellular properties 

 serves as an operational definition of the fraction; correlated data 

 from both direct and indirect methods of observation are needed to 

 characterize the fraction and relate it to the cells from which it is 

 obtained. In this investigation, the requirement for direct observa- 

 tion was met by the electron microscope study of intact cells, crude 

 pigmented fractions, and the purified chromatophore fraction (Ber- 



