STRUCTURE 141 



rently thought by some to be associated with control of the size of the holes 

 through which penetration of ions and molecules occurs. 



Although the membrane may have a total thickness of hundreds of ang- 

 stroms, the hydrophobic lipid layer, probably continuous, (and certainly the 

 well-protected center layer), is estimated to be only 75 A thick. Figure 

 6-9 is an electron micrograph of two membranes touching each other, 

 from which the 75 A figure can be directly measured. This is a pattern which 

 has been found in practically all the living membranes so photographed. 

 The membrane is not perfectly symmetrical, as different staining methods 

 have shown; and in some cases — the erythrocyte wall, for example — there 

 is definitely an assymetry. 



'-■•■>-,,* ■ --.■-■■ - . 



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- 





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Figure 6-9. Electron Micrograph of the Double Membrane of a Nerve. Osmic acid 

 stains the outer protein layers (see also Figure 6-8), and scatters electrons (dark ridges), 

 but does not absorb into the (light) lipid layer in between. Total distance across one 

 membrane is about 75A. Magnification: 880,000 x. (Courtesy of J. D. Robertson, 

 Harvard Medical School.) 



When ones tries to penetrate deeper into the structure of the membrane, 

 one runs into singularly difficult problems. Although it must be made up of 

 macromolecules of protein, cellulose, and lipid, those molecules probably 

 are distorted and stretched, or cross-linked into a planar structure. Neither 

 the structure nor the properties of degraded or dissolved membrane mole- 

 cules would therefore be expected to reflect those of the living membrane 

 by conventional techniques of analysis. And yet not only are the complete 

 membrane structures too thin to be studied in bulk, but also they degenerate 

 when dried for X-ray or electron-microscopic study. In other words, good 

 techniques for studying living membranes in vivo are still needed. Certain 

 very specialized membranes, such as those enclosing nerve and muscle cells, 

 and the rod and cone cells of the retina, can be studied through examina- 

 tion of the details of their specialty. For instance, much progress has 

 recently been made in elucidating the structure of the mitochondrion mem- 



