524 



LIGHT AND LIFE 



Fig. 12. Electron microgiaph of isolated spinach grana prepared for electron 

 microscopy by a freeze-drying technique. [One spinach chloroplast contains about 

 1000 grana (147a). 1 cm- of leaf surface is estimated to have .50 milhon chloro- 

 plasts.] Wliolc chloroplasts were disrupted by a sonic \ibration treatment for 10 

 sec. The grana were isolated in sucrose by a density gradient centrifugation tech- 

 nicjue, and the sucrose was removed by washing with 2% NaCl. The grana were 

 used in electron microscopy in\()l\ing a modified freeze-drying lcchnic]ue (142) tiiat 

 avoids possible artifacts resulting from chemical fixation and letains the natural 

 shape of particles. Magnification: .'>9,000 X. (Midler, Steere, and Arnon, 110). 



cytochrome and is .shown in Fig. 4 as being catalyzed by phenazine 

 niethosulfate. [Significant in this connection are the recent findings 

 of James and Leach that, like the bacterial cytochrome in chromato- 

 phores, the chlorojjlast cytochromes, / and b, are "entirely confined 

 to the grana" (80)]. At high light intensity, the "bypass" pathway 

 catalyzed by j>hcnazine methosidfatc gives high rates of photojihos 

 phorylation because it is not limited by the absence of chloroplast 

 constituents that lie otitside the grana. The beneficial effect of added 

 chloroplast extract on the FMN and vitamin K systems (Table 5) 

 suggests that the extract contains some chloroplast constituents that 

 are involved in these pathways but not in the pathway catalyzed by 

 phenazine methosidfate. 



