92 MACROMOLECULAR COMPLEXES 



for the Euglena chloroplast and 246 A- for the Poteriochromonas 

 chloroplast (Wolken and Schwertz, 1953). Elbers et al. (1957) have 

 since collected data on chlorophyll concentrations and geometry of 

 chloroplasts in a variety of plant species, and by similar analogy 

 have calculated the mean area available for the chlorophyll molecule 

 in the monolayer to be of the order of 200 A-. Studies of the di- 

 chroism, birefringence, and polarization of fluorescence in Motigeo- 

 tia chloroplasts also indicate that chlorophyll resides as a monolayer 

 on the lamellar surface, and the area available per chlorophyll 

 molecule was calculated to be ~250 A" (Goedheer, 1955, 1957). 



where n is the average number of dense layers (double lamellae), providing 2n sur- 

 faces for the pigment molecules, d is the average observed length of the chloroplast, 

 and N is the average number of chlorophyll molecules per chloroplast (Wolken and 

 Schwertz, 1953). 



Since the cross-sectional area of the porphyrin head of the chloro- 

 phyll molecule is known from x-ray studies to be about 225 A" to 

 242 A", these results indicate that all the available chlorophyll mole- 

 cules could be packed into the interfacial area and cover all of the 

 dense surface of the lamellae as a monolayer. 



On the basis of these calculations, a simplified schematic molec- 

 ular model was proposed (Fig. 5). The suggestion of Bass-Becking 

 and Hanson (1937), that four chlorophyll molecules are united to 

 form tetrads in which the reactive isocylic rings turn toward each 

 other, was employed (Wolken and Schwertz, 1953). Interaction be- 

 tween the phytol tails was eliminated in the model by arranging the 

 tetrads in such a way that one, and only one, of the phytol tails is 

 located at each virtual intersection in the rectangular network. If the 

 chlorophyll were packed as a monolayer as shown in the schematic 

 molecular network in Fig. 5(b), there would still be space available 



