THE GENERAL BIOLOGICAL SIGNIFICANCE OF 

 CHANGES IN THE PERMEABILITY OF THE 

 SURFACE LAYER OR PLASMA-MEM- 

 BRANE OF LIVING CELLS. 1 



RALPH S. LILLIE. 



The osmotic properties of living cells have been the subject of 

 repeated investigation since the fundamental experiments of Pfeffer 

 and de Vries, and especially since the rise of the theory of 

 osmotic pressure and the extension of the gas laws to dissolved 

 substances by van't Hoff in iSS/. 2 One remarkable result, of 

 the highest significance for general physiology, has been perhaps 

 the main outcome of these researches ; living cells of the most 

 diverse kinds have been found to possess in common a high degree 

 of physical impermeability to most of the diffusible non-colloid 

 substances normally occurring in the cells and in their surround- 

 ings. The surface film or so called plasma-membrane of cells, 

 while readily permeable to water, offers a highly efficient barrier 

 at least during the resting or unstimulated state to the 

 entrance or exit of many dissolved substances of relatively low 

 molecular weight, even of those normally present in protoplasm, 

 as sugars, amino-acids, and neutral salts. This impermeability is 

 a property of the normal living cell, unmodified by experimental 

 conditions. A direct proof of this is the characteristic turgor of 

 plant cells, which is due to the osmotic pressure of crystalloid 

 substances dissolved in the cell-sap ; it is clear that the osmot- 

 ically active substances sugars, organic acids, and other rela- 

 tively simple compounds are ordinarily unable to traverse the 

 limiting surfaces of the protoplasm ; were they able to do so the 

 maintenance of the high internal pressures known to exist would 

 be impossible. Such impermeability is a peculiarity of the living 



1 From the Physiological Laboratory, Department of Zoology, University of 

 Pennsylvania. 



2 A full account of the earlier researches will be found in Hamburger's " Osmo- 

 tischer Druck und lonenlehre in den medizinischen Wissenschaften," Wiesbaden, 

 I. F. Bergmann, 1902-4. See also Hober's " Physikalische Chemie der Zelle und 

 der Gewebe, ad ed., Leipzig, Engelmann, 1906. 



188 



