CHLOROPLAST STRUCTURE 461 



one-fourth of the vokime of this intradisc region. The high concen- 

 tration of chlorophyll that may be estimated to exist in specific 

 regions of the grana would be compatible with the hypothesis of a 

 photos>Tithetic unit in which light energy could be transmitted 

 through se\'eral hundred chlorophyll molecuiles before it was trapped 

 in a "sink." 



Is there any evidence for the specific orientation of chlorophyll 

 molecules with respect to each other? Such orientations, if they oc- 

 curred, might help to explain the great efficiency with which the 

 energy of a photon could be transmitted through a photosynthetic 

 unit. Goedheer (6) has concluded from studies of the birefringence of 

 chloroplasts in the neighborhood of the chlorophyll band that there 

 is a small amount of orientation, the porphyrin planes of the chloro- 

 phyll being oriented parallel to the disc membranes. However, 

 Menke and Menke (7) question this interpretation. The very low 

 yields of polarized fluorescent light, emitted by Chlorella when 

 illuminated with incident polarized light, suggest that there is very 

 little orientation of chlorophylls in a photosynthetic unit (8). How- 

 ever, on the basis of calculations of Arnold and Oppenheimer (9) it 

 does not appear to be necessary to assume a rigid orientation of 

 porphyrin planes stacked parallel to each other in order to account for 

 energy transfer in the photosjoithetic unit. 



Sometimes when we know what a structure is we can explain what 

 it does. Here, the knowledge of the structure of the chloroplast makes 

 us aware of how very much we don't know of the mechanisms of 

 photosynthesis. 



Discussion 



Latimer : There was mention of Goedheer's paper in connection with dichroism 

 of chloroplasts. Goedheer also observed double refraction as did Dr. Menke later. 

 Menke did this work in 1944 and just got around to publishing it. Double refrac- 

 tion and dichroism are essentially due to the same phenomenon, namely, to orien- 

 tation of the molecules. If they were not oriented, a double refraction would not 

 have been observed, but it was very clearly observed with the very interesting 

 wavelength dependence which you saw, indicating some very definite organization 

 (Latimer, this book, page 100). The degree of organization is hard to measure 

 quantitatively but it showed up very nicely. 



Granick: I believe Menke had a recent paper in Z. Naturforsch., 10b, 416 (1955) 

 in which he revised and criticized Goedheer's work. 



Rabinowitch: One word about double refraction. Of course, double refraction 

 indicates that some molecules are arranged in a parallel way, and the suggestion 



