580 



Jerorae J. V7olken 



Isocyclic ring 



Lipoprotein 

 layer 



h CliJ-oroplast, sclienaatic 

 cular -loclel indicating; the 

 orientation and ca.lculat8d 

 croGC-Gection of the chloro- 

 lohvll vaolecule in the lauellae, 



centration (l x 10^) hy twice the number of dense layers; hence 

 the nuraher \Tould he 25 x lO'i" molecules per layer. Tlie number of 

 chlorophyll molecules per imit area in the interfacial monolayers 

 is just the reciprocal of the area available to each molecule, 

 i.e., approxims-tely h x 10-"L3 per cra^, 



Tliere are several possible irays in vhich the chlorophyll mole- 

 cules could be oriented in the chloroplast lamellae. If the por- 

 phio-in heads of the clilorophyll molecules lie at 0° as flat plates 

 as^lepicted in Fls. ha, their greatest cross-section vould be a- 

 vs-ilable. However, if they are oriented at inci-easins angles up 

 to 90°, the cross-sectional area aArailable would be decreased. 

 Sttidies of chlorophyll monola--ers at various liquid surfaces sug- 

 rest tixat the chlorophyll molecules probably lie at angles from 

 25 to 55° ATithin the lamellae, thus reducing ovir c|lcu_lated cross- 

 section of the chlorophyll molecule to about 100 A . Chlorophyll- 

 a has been shw-m to crystallize out in thin sheets of 50 A, cor- 

 Fesioonding to two molecularr layers. Since t^e crystallized chl- 

 orophyll molec\AJ-es occupied an area of lOu A ^-^^ ^ , the porphyrin 

 "head" of the chlorophyll molecule could be tilted at an angle^ 

 near ''"^ 



^1-5'^. However, the clilorophyll molecules in the living cellt 



