579 



Jerome J. Wolken 



able for the porphyrin of the chlorophyll molecule '^^^« This was 

 found to be 222 bt, vhich is about the right cross-section for a 

 porphyrin molecule spread on a water-air interface. Similar cal- 

 culations since made for the chlorophyll cross -sectional area in 

 the cliloroplast of a, variety of plants were also found to be of 

 the order of 200 Sr ^12,13]^ 



TABLE I 

 Chloroplast Euglena gracilis 



diameter 1.23 H (l.04 - l.Jj-2) 



length, D 6.5O |i (5.2 - 9.3) 

 number of dense layers, n 21 (18 - 2^1-) 



danse layer thickness, T 2if2 A (18O - 303 ) 



interspace layer thickness 37^ ° (300 - h'jG) 



chlorophyll molecules, N 1.02 x 10^ (O.88 - I.36) 



Mean and Extremes in the Measurements. 



On the basis of the chlorophyll concentration, structural data, 

 and calculations of the area and volume occupied by the pigment 

 molecules of the chloroplast, .the schematic molecular model illus- 

 trated in Fig. k was proposed^^^. The model (Fig. ka.) includes 

 the suggestion of Baas Becking and Hans on ^1^) that k chlorophyll 

 molecules are united to form tetrads in such a \ray tliat only one 

 of the phytol tails is located at each intersection in the rect- 

 angular network. This arrangement has the advantage of leaving 

 adequate space for the carotenoid pigments. If these spaces are 

 occupied as illustrated, there would be at least one carotenoid 

 molecule for every three chlorophyll molecules in the network. 

 Since the molecular weights of the carotenoid molecules are one- 

 half to two-thirds of the molecular weight of the chlorophyll 

 molecules, a weight ratio , chlorophyll to carotenoid, of approx- 

 imately k:l to 6:1 would be expected. On the other hand, the car- 

 otenoid molecules are slender linear molec\iles, probably 5 A in 

 diameter, and therefore more than one molecule could conveniently 

 fit into the I5 £ x I5 A holes formed by the clalorophyll tetrads. 

 From symmetry, one might expect as many as four molecules per 

 hole, but this would lead to very tight fitting, which would be 

 energetically improbable. One can therefore put a lo\-/-er limit on 

 the number of chlorophyll to carotenoid mo].ecules of roughly 1:1, 

 and a weight ratio of 2 :1. The close packing of the chlorophyll 

 and carotenoid molecules in the pigment monolayers of the chloro- 

 plast could permit energetic interaction between the pigment mole- 

 cules. The average number of chlorophyll molecules in the inter- 

 facial layers is obtained by s^.mply dividing the chlorophyll con- 



