Permeability of the Cell Plasma Membrane. 57 



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The significance of changes in the permeability of the plasma 

 membrane of the living cell in the processes of 

 stimulation and contraction. 



By RALPH S. LILLIE. 



[Frofn the Physiological Laboratory , Zoological Department, Uni- 

 versity of Pennsylvania^ 



The general facts indicating that stimulation is dependent on 

 a temporary increase in the permeability of the surface layer or 

 plasma membrane of the irritable element are as follows : 



A. The nature of the motile process in such plants as Mimosa, 

 Dio7icea and the Cynarese, where the movement depends on a sud- 

 den loss of turgor. Such a change indicates either (i) a sudden 

 decrease in the concentration of the osmotically active substances 

 within the cell due to chemical action, or (2) a sudden loss of 

 impermeability relatively to the osmotically active substances. 

 The latter explanation is almost certainly the correct one. 



B. The identity of the electrical change accompanying stimu- 

 lation in motile plant cells with that observed in irritable animal 

 tissues (Burdon-Sanderson), indicating a fundamental similarity in 

 the conditions of stimulation in the two classes of organisms. 



C. The fact that the post-mortem increase in permeability is 

 accompanied by contraction in muscle cells ; the same is, of 

 course, true of motile plant organs where the movement depends 

 on loss of turgor. 



D. The nature of the electrical change accompanying stimu- 

 lation. If the irritable element represents a concentration-cell in 

 which a semi-permeable membrane (the plasma membrane) greatly 

 diminishes the velocity of the anion, while leaving that of the 

 cation practically unaltered (Ostwald-Bernstein membrane theory), 

 any marked increase in permeability relatively to the anion must 

 result in a fall of the potential difference between exterior and 

 interior of the irritable element. Such an electrical change actu- 

 ally occurs on death or injury of the element; also momentarily 

 during stimulation. A demonstrable increase in permeability 

 occurs at death ; inferentially, therefore, the same change occurs 

 during stimulation. 



