180 



MACROMOLECULAR COMPLEXES 



which Hght energy is used to create the energy-rich bond of the 

 terminal phosphate of ATP is receiving considerable attention. 



In higher plants, all of the many reactions of photosynthesis are 

 integrated in a complex organelle, the chloroplast. The modern 

 evidence in support of this old concept has been summarized by 

 Arnon ( 1958 ) . In order to study the energy-supplying step directly, 

 it is desirable to ehminate all of the non-pertinent variables and use 



CHLOROPLAST 



MITOCHONDRION 



0.5M 



CHROMATOPHORE 

 • 0.03 m 



Fig. 1. Relative size of organelles. There are great differences in the level 

 of organization of the organelles which supply chemical energy. The sub- 

 microscopic chromatophore converts light energy into the readily available 

 chemical energy of the pyrophosphate bond of ATP. The mitochondrion, which 

 lies just within the microscopic range, performs successive oxidations to generate 

 ATP from substrates which act as energy reservoirs. The microscopic chloroplast 

 converts light energy into pyrophosphate bond energy and uses the ATP to 

 forge carbon dioxide and water into stable energy reserves. 



the simplest possible system. The principal subject of this paper, the 

 chromatophore of the purple sulfur bacterium 'Chromatium,' meets 

 these conditions remarkably well since it is smaller than the chloro- 

 plast by two orders of magnitude ( Fig. 1 ) and appears to have only 

 one photosynthetic role, namely, the light-dependent formation of 

 adenosine triphosphate (ATP) from adenosine diphosphate (ADP) 

 and inorganic phosphate. 



