PROTOPLASMIC STRUCTURE 109 



The normal semi-permeability of the plasma mem- 

 brane is a function of the living state of the cell, i.e., is 

 dependent upon the continuance of the normal construc- 

 tive metabolism. When metabolism ceases, as at death, 

 the membrane soon loses its insulating or semi-permeable 

 properties, and free interchange of diffusible substances 

 then occurs between the cell and the medium. The loss 

 of turgor in plant cells after death is the most familiar 

 example of this type of effect; leaves and other paren- 

 chymatous parts then wilt because of the diffusion of the 

 osmotically active substances through the now permeable 

 plasma membranes. The osmotic tension which during 

 life keeps the cell walls in their normal stretched and rigid 

 condition disappears, the tissue becomes soft and flaccid, 

 and the protoplasm shows other evidences of increased 

 permeability (increased electrical conductivity, loss of 

 diffusible materials to the surroundings, ready entrance 

 of dyes and other substances). Similarly in animal cells 

 various substances, such as pigments and other com- 

 pounds, normally confined within the protoplasm, 

 diffuse rapidly into the surrounding medium on death, 

 and the plasma membrane admits substances such as 

 alkalis and salts, to which previously it was impermeable. 



It is a remarkable and apparently paradoxical fact 

 that the living plasma membranes usually show them- 

 selves highly impermeable to many dissolved substances 

 which are essential to the cell as foods or otherwise 

 (sugars, amino-acids, and neutral salts), and some 

 general explanation of this peculiarity seems required. 

 If we were to express the matter teleologically we might 

 say that the advantage to the cell consists in the insula- 

 tion of the living protoplasm from its environment, 



