26o FINE-STRUCTURE OF PROTOPLASM II 



microscopic thickness of 0.08 fx may be due to desiccation during the 

 preparation for the electron microscope, and we may estimate the 

 thickness of the fresh grana to be about o.i 5 [x. The whole chloroplast 

 has a diameter of 5 fx and its thickness in the fresh state is about half 

 of this. If we calculate the volume of the chloroplast as an ellipsoid, 

 4/3 X 2.5^ X 1.25 X n, and that of the 50 grana as cylindrical discs, 

 50 X 0.3^ X 0.15 X n, wt obtain a volume ratio of 15/1. Thus, the 

 total volume of the grana would be only 1/ 1 5 or 7 % of that of the 

 whole chloroplast. Since there is 7.7% of chlorophyll in the chloro- 

 plast (Menke, 1940b), this would mean that the grana consist entirely 

 of chlorophyll. This is obviously impossible, for the grana are still 

 visible in the electron microscope when the pigments are extracted. 



We must conclude, therefore, that the discs visible in Fig. 131b do 

 not represent individual grana, but piles of grana. The number of 

 piles in the chloroplast of tobacco leaves is about 50, thus the same 

 as in spinach leaves, and their diameter 0.4 fx. The chloroplast of about 

 2.5 // thickness can lodge not more than 12 layers of grana. With these 

 figures the volume ratio chloroplast/total number of grana is 3, i.e. 

 the grana occupy 1/3 and the stroma 2/3 of the plastid volume^. This 

 ratio enables us to calculate the chlorophyll content of the grana 

 protein. 



According to Table XXIV (p. 246), half the weight of the chloroplast 

 is protein and 7.7% chlorophyll (mol.wt. 893). This yields a molar 

 xatio of 3 chlorophyll to i Svedberg unit (mol.wt. 17600). Since the 

 chlorophyll is restricted to the grana and their volume being only one 

 third of the chloroplast, this ratio must be 9/1 in the grana, if the 

 protein concentration is the same as in the stroma. This result seems 

 to prove that chlorophyll cannot be a prosthetic group of an enzyme, 

 for, considered as a co-enzyme, its carrier would have a molecular 

 weight as low as 2000, which has never been found for apo-enzymes. 



Stoll's chloroplastin (1936) has a molecular weight of roughly 

 5 millions. If it is really the chromoprotein of the chloroplast, it must 

 come from the grana alone and cannot be contaminated by stroma 

 protein. It is doubtful whether these two proteins can be separated 

 quantitatively by fractionated precipitation. Supposing the chloro- 



1 Thus about 25 % by weight of the grana consists of chlorophyll; this is astonishingly 

 high, as compared with the haematin (mol. wt. 592) content of the erythrocytes (p. 265) 

 which is only ca. 3 % of the cell interior. 



