364 DANIEL I. ARNON 



Table IV shows that, at the high Hght intensity at which cycHc photo- 

 phosphorylation by purified grana was measured, the highest rates were 

 obtained in the system catalyzed by phenazine methosulphate. Photo- 

 phosphorylation in this system was not increased by the addition of an 

 aqueous chloroplast extract. In contrast, photophosphorylation catalyzed 



Fig. II. Electron micrograph of isolated spinach grana prepared for electron 

 microscopy by a freeze-drying technique, i cm.- of leaf surface is estimated to have 

 about 50 million chloroplasts. Whole chloroplasts were disrupted by a sonic vibra- 

 tion treatment for 10 sec. The grana were isolated in sucrose by a density gradient 

 centrifugation technique and the sucrose removed by washing with a",, NaCl. 

 The grana were used in electron microscopy involving a modified freeze-drying 

 technique [119] that avoids possible artifacts resulting from chemical fixation and 

 retains the natural shape of particles. Magnification : 59 000 x (Miiller, Steere, 

 and Arnon, [118]). 



by vitamin K or FMN proceeded at a lower rate and was markedly 

 increased by the addition of chloroplast extract ; however, even with this 

 increase, it failed to reach the rate of photophosphorylation in the phen- 

 azine methosulphate system. 



These results support the conclusion that grana are the site of the 

 "primary" photophosphorylation reaction (equation (3)), the one that is 



