Fig. 3. Electron micrograph {65 000 x ) of a thin section of the purple sulphur 

 bacterium, Chromatium. The organism is smooth contoured and is bounded by a 

 dual membrane which can separate into two distinct structures, the cell wall (W) 

 and the plasma membrane. The cell is filled with the chromatophores (Ch) which 

 are minute vesicles. These appear as annular images with an outer diameter of 

 about 300 A and a cortical thickness of about 70 A. Large vesicles (V) about 1000 A 

 in diameter are also visible. The irregular areas of low density (N) are considered 

 to be parts of an irregularly shaped nuclear compartment. The closely packed 

 chromatophores conceal the small particles (Sp) which are observed in the fractions. 



