A. A. BENSON 395 



Location of Pigments in the Lamellae 



Since the galactosyl monoglycerides^ are situated within the lamellar 

 structure, their interfacial orientation would place the galactose 

 moieties in contact with the stroma and the fatty acids in a lipid 

 layer adjacent to the protein-rich phase visualized by Hodge, McLean, 

 and Mercer (7) . There must be a vigorous competition between the 

 chlorophyll and the surfactant lipids for adsorption at the interface. 

 Then, either the chlorin groups are situated in the same layer as the 

 galactose or in an "inner" interface of the granum which is not in 

 contact with the stroma. The close packing of the chlorophyll, as 

 estimated by Rabinowitch (9) and others (H) , would hardly allow 

 appreciable space for interfacial adsorption of surfactant lipids which 

 far exceed its concentration. It would, therefore, appear reasonable 

 to anticipate two types of lipid laminae, an outer one dominated by 

 galactolipids in the region of carbohydrate synthesis and an inner 

 one dominated by chlorophyll and electron transport systems in the 

 region of oxygen production. Gas diffusion from the inner region of 

 the granum would be more plausible than would be transport of 

 carbohydrate. Such a picture would be in accord with the "semi- 

 conductor" concept of granum function. 



Recent observations by French (5) indicate the existence of several 

 forms of chlorophyll a in plants. Franck, earlier, suggested that the 

 differences in observed fluorescence properties of chlorophyll a were 

 due to variations in the medium surrounding the pigment. It now 

 appears reasonable to consider that heterogeneous distribution of 

 surfactants may provide the pigment molecules with media of widely 

 differing polarity. 



Summary 



The high lipid content of the chloroplast is associated with the 

 lamellar pigmented structures. The major chloroplast lipids are the 

 galactosyl monoglycerides and the phosphatidyl glycerols, both of 

 which are actively involved in the carbohydrate metabolism of photo- 

 synthesis. The rapid turnover of the galactose and glycerophosphate 

 moieties suggests that these surfactant lipids are oriented in the 



'New evidence indicates that the glycolipids are, in fact, glycosyl diglycerides 

 rather than glycosyl monoglycerides as reported. C'-labeling of both fatty acids 

 is slower than anticipated and only about 18 units of C" are readily incorporated 

 into these moieties. "Lyso"-glycolipids result from controlled enzymatic degrada- 

 tion, thereby indicating that each glycerol bears two fatty acid ester groups as 

 in the glycerol phosphatides. 



