573 



A. A. Benson 



Lipids of Isolated Chloroplasts 



Present estimates of concentrations of the lipid components of plant 

 cell organelles may be affected by enzyme -catalyzed degradation. The 

 lipids of runner bean leaf chloroplasts ^^ contained no galactolipids as a 

 result of activation of galactolipases by disruption of the cells. Water 

 homogenates of the leaves catalyzed rapid liberation of fatty acids from 

 pure galactolipids. In other instances we have observed specific degrad- 

 ation of the sulfolipid as a result of breaking Scene desmus cells^^. It is 

 possible that inactivation of chloroplasts and their failure to retain cer- 

 tain protein components is a result of lipase action and consequent alter- 

 ation of the stability of their lipid membranes. Such phenomena are 

 well understood in the case of erythrocyte lysis by phospholipase A. 



The unsaturation characteristic of chloroplast lipids leads to the 

 suggestion that the hydrophobic phase of an adsorbed layer will be a 

 liquid rather than possessing the highly oriented structure characteristic 

 of compressed monolayers of saturated fatty acids. This 'liquid' region 

 may be 'dissolved' in hydrophobic sections of adjacent protein molecules. 

 The cytochromes c for which amino acid sequences are known have such 

 hydrophobic regions. It may be found that 'liquid' fatty acid chains like 

 that of linolenic acid are particularly strongly adsorbed by certain 

 hydrophobic regions of proteins. 



Chlorophyll's Environment 



The phytyl ester moiety of chlorophyll may induce orientation of the 

 molecule to an extent which may approximate that of the glycolipids. 

 This surfactant pigment, then, must be surrounded, in the main, by the 

 predominant chloroplast lipid, digalactosyl diglyceride. The porphyrin, 

 therefore, would be compressed at an interface by hydrated galactose 

 molecules. Since these lipids vary in concentration with illumination 

 one should anticipate changes in association of propinquitous pigments 

 and thereby in their absorption spectra. James Franck suggested that 

 the fluorescent and non-fluorescent forms of chlorophyll are the result 

 of their be situated in aqueous and hydrocarbon phases of the chloroplast. 

 The chlorophyll compressed in a membrane of galactosyl, sulfoquinov- 

 osyl, and L-1-glycerophosphoryl groups of the oriented chloroplast lipids. 

 Chlorophyll, more deeply embedded in the hydrocarbon phase of the li- 

 pids or adjacent protein would correspond to the non-fluorescing form. 



The precise location of the lipid layers which the electron micro- 

 graphs suggest is still uncertain. The developing understanding of the 

 chemistry and metabolism of these substances promises to provide 

 more probable bases for interpreting present pictorial concepts of 

 chloroplast lamellar structure. 



