1740 CHLOROPLASTS, CHROMOPLASTS AND CHROMATOPLASM CHAP. 37 A 



cause of the continuation of the reaction in killed cells. Addition of ascor- 

 bic acid to grana suspensions considerably increases the yield — perhaps by 

 reducing again the chlorophyll oxidized by Ag+ ions. 



Thomas et al. also measured the action spectrum of AgNOs reduction by 

 grana suspension, and found it to be quite similar to this absorption spec- 

 trum, and essentially determined by chlorophyll (fig. 37 A. 19). 



Electron diffraction study of the grana after the Molisch reaction showed 

 a strong scattering pattern, indicating the presence of crystalline silver 

 deposits (no diffraction rings were visible in the scattering pattern of grana 

 not covered with silver). 



Electron-microscopic observations of grana after the Molisch reaction 

 showed uncoated (fig. 37A.20 (top)) grana as well as grana in different stages 

 of silver coating (fig. 37 A. 20 (middle)) ; after prolonged reaction, silver de- 

 posits grew and hid the grana (fig. 37A.20 (bottom)). When grana were 

 disintegrated into single discs by ultrasonic treatment, and the latter exposed 

 to silver nitrate in light, they, too, were overgrown with silver deposits. 



3. Ultracentrifuge Study 



Pardee, Schachman and Stanier (1952) disintegrated purple bacteria by 

 grinding, sonic waves or sudden release of pressure, and subjected the 

 products to fractional ultracentrifugation. They found that all pigments — 

 the bacteriochlorophylls as well as the carotenoids — came down in a single 

 fraction, consisting of large particles, and called the latter "chromato- 

 phores." It seems that these particles, though smaller than the grana of 

 the higher plants, may be functionally similar to them ; we will therefore 

 refer to them as "grana." 



Particles of the same size could not be found in non-photosynthetic bac- 

 teria, or in photosynthetic bacteria grown in dark and containing no pig- 

 ment. Their presence thus seems to be associated with that of the photo- 

 synthetic pigments — conceivably, they are held together by the strong res- 

 onance attraction that exists between pigment molecules. 



From the sedimentation constant of the colored fraction of bacteria, 

 Schachman et al. calculated — assuming spherical shape — a particle diam- 

 eter of 0.04 n. Electron micrographs of the same fraction showed the pres- 

 ence of flattened ellipsoids, or low cylinders, with a diameter of 0.11 m- The 

 authors suggested that these particles originally had been spherical, but be- 

 came flattened in the preparation of samples for electron microscopy. On 

 this assumption, a value of 0.06 /x was calculated for the diameter of the 

 original spheres — in fair agreement with the value derived from sedimenta- 

 tion experiments. However, agreement can be achieved also by assuming 

 that the })acterial grana have the approximately cylindrical shape observed 



