SITE ISOLATION? MEMBRANE SEPARATION 



by elements of the endoplasmic reticulum might perhaps contain 

 extracellular fluid, in which case the surface area available for trans- 

 port could be substantially increased. At the same time, the mem- 

 braneous or vesicular material obtained from cell homogenates might 

 then be functionally and structurally akin to the plasma membrane, 

 even if it originates largely from the reticulum. 



This interpretation, however, is by no means a secure one. Serial 

 sectioning has failed to bear out the proposed continuity of deep in- 

 foldings with the plasma membrane (cf. Andersson-Cedergren, 

 1959). The view that tubules filled with extracellular fluid pene- 

 trate deeply into the cell does not appear to have general support 

 among cytologists. Nevertheless, analogous structure and transport 

 behavior by these membranes is not ruled out. 



Bacterial cells, being smaller, yield much higher proportions of 

 membrane, for example, about 10 per cent. But such preparations 

 include a large fraction of the catalytic machinery of the cell, which 

 means that a much wider range of binding sites than those involved 

 in transport from the environment must be exposed when such 

 membranes are isolated. Apparently a very compact organization 

 is characteristic of bacteria, so that much of the cell machinery is 

 isolated with the membrane. Therefore, the natural accessibility 

 of transport sites on unmodified cells probably presents such sites 

 in higher purity than can be readily procured after tearing the 

 membrane from the cell. 



The isolation of red blood cell ghosts (Figure 25) may repre- 

 sent an interesting purification of membrane material. The erythro- 

 cyte can be lysed with water so that its hemoglobin (and other dis- 

 solved internal constituents) is distributed throughout the lysate. 

 If electrolyte solutions are then added to restore isotonicity, the 

 membrane gradually regains its integrity, selective permeability, and 

 transport behavior, leaving most of the hemoglobin and other cyto- 

 plasmic components outside (Ponder, 1948; Szekely et al., 1952; 

 Teorell, 1952; Stein, 1956). Glucose entry is again mediated (Le- 

 Fevre, 1961c), and in the presence of suitable energy sources, potas- 

 sium ion may again be concentrated (Straub, 1954; Gardos, 1954; 

 Hoffman, 1958). Perhaps the reversible hemolysis of the red blood 

 cells supports the presence of pores in the plasma membrane, which 

 may be enlarged by stretching that membrane. 



How valuable the isolation of cell ghosts may be to the isola- 

 tion and identification of the transport mediator remains to be seen. 

 The technique has disposed of the view that potassium accumulation 



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