SITE ISOLATION? MEMBRANE SEPARATION 



can be explained by binding of potassium ion by hemoglobin or 

 other dissolved cytoplasmic constituents. It has also permitted intro- 

 duction of otherwise nonpenetrating solutes inside the cell; for ex- 

 ample, Gardos (1954) showed that ATP introduced during the 

 intermediate porous stage could actuate subsequent potassium up- 

 take for several hours. 



Glvnn (1957a) has provided an extensive review of the com- 

 position and properties of the red blood cell membrane, undoubtedlv 

 the most satisfactory plasma membrane preparation available. 



Model membranes, artificially produced from lipid or lipid and 

 protein components, have so far been more useful for understand- 

 ing the barrier action than the transport behavior of natural mem- 

 branes. Mueller and his associates (1962) indicate, in a preliminary 

 note, that they have prepared bimolecular membranes from the lipid 

 and proteolipid of the white matter of brain. These membranes have 

 very high electric resistance, but macromolecules adsorbed from the 

 aqueous solutions separated by the membranes may greatly lower 

 the resistance. An unidentified adsorbed macromolecule may cause 

 the membranes to respond to an applied potential difference by a 

 quickly reversible resistance change, a phenomenon seen with vari- 

 ous natural membranes and undoubtedly relevant to the passage of 

 charged particles. The authors suggest that they have reconstituted 

 a cell membrane structure. 



75 



