1736 CHLOROPLAST8, CHROMOPLAStS AND CHROMATOPLASM CHAP. 37A 



Euglena, and one of 246 A- in Poteriochromonas. This, he suggested, is 

 just enough for a monomolecular layer of chlorophyll molecules, assuming 

 that their porphyrin ring systems lie flat on the surface. 



The smaller surface area of the lamellae in flagellates, compared to that of the 

 discs in granular chloroplasts, caused by the greater thickness of the laminae, is more 

 than compensated, in the calculation of the area per molecule, by the much smaller 

 numl)er of chloropli\il molecules to be accommodated jjer unit chloroplast volume. 



These estimates can be compared with the observations of chlorophyll 

 monolayers on water (Chapter 16, p. 449), and Chapter 32, section 5). 

 As noted in Chapter 32, chlorophyll layers of "crystalline" type cannot be 

 present in the living cell, because their absorption band lies at or beyond 

 725 m/i. Monomolecular layers of the "compressed gas" type, on the other 

 hand, have — similarly to chlorophyll in vivo — an absorption peak at about 

 670 mfi. The surface requirements of chlorophyll molecules in such layers 

 (about 1.1 m/i'-) is not incompatible with the above calculations for the 

 grana discs, and more than ample for Wolken's calculations for layered 

 chloroplasts. (In these monolayers, chlorophyll molecules do not lie 

 flat, but stand on edge like books on a half-filled shelf.) The possible sig- 

 nificance of a dense two-dimensional arrangement of chlorophyll molecules 

 for the excitation energy migration in vivo was discussed in Chapter 32. 



(c) Location of Chlorophyll 



In section 1, we described light-microscopic and fluorescence-micro- 

 scopic evidence indicating the concentration of chlorophyll in the grana 

 and its absence in the stroma. Electron microscopy cannot answer the 

 question of chlorophyll location directly, but an ingenious indirect method 

 can be used. It has been described in Chapter 14 that silver nitrate is re- 

 duced by chloroplasts to metallic silver even in strongly acid solution 

 {"Molisch reaction"), and that this reaction is accelerated by light. If the 

 silver precipitation is a photochemical process sensitized by chlorophyll, 

 silver should be deposited in the vicinity of the pigment, and could be used 

 as its "tracer" on electron micrographs. The "silver grana" observed in 

 some experiments (c/. fig. 40) would then be silver-coated chloroplast 

 grana. 



More recently, the silver precipitation by chloroplasts was studied by several 

 authors using histochemical methods. Some of them addressed themselves to the old 

 question of the probable nature of the reductant, not always clearly distinguishing 

 between the reduction in the dark and the additional (and perhaps chemically dif- 

 ferent) photochemical reduction. 



Nagai (1950) found the silver precipitates formed in light to be clearly associated 

 with the chloroplasts, but those formed in darkness to be distributed irregularly in the 

 cell. The selective chlorojjlast staining with silver nitrate could be obseiwed only at 



