THE CHLOROPLAST 91 



study the chloroplast by xarious microscope techniques and have 

 attempted to correlate structural changes of the chloroplast with 

 pigment synthesis and with photosynthesis in light ^ dark adapta- 

 tion (Wol'ken axid Palade, 1953; Wolken and Sciwertz, 1953; Wol- 

 ken et al, 1955; Wolken and Mellon, 1956; Wolken, 1956a). 



E. gracilis when light-grown contains manv green elongated 

 cylindrical chloroplasts 5 to 10/>t in length and 0.5 to 2/>i in diameter. 

 When the chloroplasts are viewed by interference-phase, by absorp- 

 tion, and by polarizing microscopv, an ordered structure is indicated. 

 Previously, it was proposed from studies of negative birefringence 

 that the chloroplast is a lamellar structure of 20 to 30 lipid layers of 

 the order of 50 A, each separated from layers of aqueous protein of 

 the order of 250 A by monomolecular films of chlorophyll molecules 

 ( Frey-Wyssling, 1957). There is now excellent proof of this lamel- 

 lar structure from electron microscope studies, and x-ray diffraction 

 techniques have corroborated the electron microscope observations, 

 showing a repeating unit of the order of 250 A. 



In order to determine more precisely the lamellar structure and 

 to test the pigment monolayer hypothesis, the diameter, length, 

 number, and thickness of the lamellae of the chloroplasts were meas- 

 ured and statistically evaluated (Wolken and Schwertz, 1953). For 

 example, the chloroplast of E. gracilis consists of 21 dense lavers of 

 the order of 250 A in thickness, with less dense interspaces of 300 to 

 500 A in thickness. Each dense layer appears to be covered on both 

 sides by a thinner and denser layer (lamellae) of the order of 50 

 to 100 A in thickness. The average thickness measured for a variety 

 of plant chloroplasts ranges from 20 to 100 A for the electron-dense 

 lamellae (Leyon, 1956; Sager and Palade, 1957; Sager, 1958). Frey- 

 Wyssling ( 1957 ) had noted in previous studies that the thin lamellae 

 consist of spherical particles 65 A in diameter; these were considered 

 to be the protein or lipoprotein macromolecules (Fig. 6). 



There is experimental evidence to indicate that the chlorophyll 

 molecules are preferentially oriented parallel to the lamellae and are 

 within the very dense (lipoprotein) lamellae (Thomas, 1955). The 

 electron-dense layers are believed to be lipoprotein and lipid mate- 

 rial. The areas of least density are considered to be aqueous pro- 

 teins, enzymes, and dissolved salts. From the geometry of the 

 chloroplast, the number of dense layers, and the chlorophyll con- 

 centration per chloroplast (Table 1), we have calculated the cross- 

 sectional area occupied by each chlorophyll molecule to be 222 A" 



