460 S. GRANICK 



The chloroplast of an alga like Euglena would appear to represent 

 a single granum. Such a granum contains about 20 dense parallel 

 membranes which extend the length and width of the chloroplast. 

 These membranes are thicker and the interspaces are greater than 

 those observed in the grana of higher plants. 



When a Euglena organism is placed in the dark, it loses its chloro- 

 phyll; at the same time the membranes disappear from the chloro- 

 plast. On placing such a colorless Euglena in the light it gradually be- 

 comes green. At the same time the dense membranes appear (3). 

 The greening thus appears to consist not only of a process of chloro- 

 phyll synthesis but concomitantly there must occur carotenoid and 

 protein synthesis and the structural organization of these molecules. 



The fine details that have been revealed by the electron microscope 

 depend on a technique of tissue fixation in buffered OSO4, subsequent 

 dehydration through alcohols, embedding in a methacrylate polymer, 

 and fine sectioning (4). Our studies of this procedure applied to chlo- 

 roplasts suggest that the chlorophylls are leached out during the de- 

 hydration stages so that the final sections as viewed in the electron 

 microscope may be assumed to be devoid of chlorophyll. We have 

 also found that by this procedure carrot chromoplasts lose their 

 carotenes. 



The fixed insoluble membranes that are observed in thin sectioning 

 of chloroplasts probably represent protein material. A photograph b}^ 

 Von Wettstein (5) suggests that a disc membrane may be made up of 

 spherical protein molecules of about 35 A diameter. This would mean 

 that the membrane would represent a monomolecular layer of pro- 

 tein. 



Where might chlorophyll be localized in the chloroplast? There is 

 sufficient evidence to indicate that chlorophyll is probably localized 

 in the grana and not in the stroma. There is also evidence from 

 studies with polarized light that chlorophyll is localized in layers within 

 the grana. Let us assume that chlorophyll is localized within the 

 disc, i.e., within the 65 A space between the upper and lower disc 

 membrane. If the disc contained a monolayer of chlorophyll molecules 

 with the plane of the porphyrin heads all lying flat and adjacent to 

 each other in the monolayer, then there would be room for about 

 90,000 molecules per disc, and the concentration of chlorophyll inside 

 the disc would be about 0.15 M. Other estimates suggest that if 

 chlorophyll were localized here, then chlorophyll would take up about 



