CHAPTER XVI 



PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 



Problems in science have their day, but there is one biological 

 problem that has held a foremost position since its inception 

 nearly a century ago. That problem is 'permeability. It has 

 held its position because of the mystery that surrounds it and 

 because it is a protoplasmic quality of prime significance to the 

 cell. 



Just as an osmotic system (page 183) owes its capacity to 

 develop pressure to the selective permeability of a membrane 

 which permits water but not dissolved substances to pass through 

 freely, so also does the living cell owe this and other properties 

 to the selective permeability of the protoplasmic membrane. 

 To this quality does protoplasm owe its extraordinary capacity 

 to determine the extrance and exit of substances into and out of 

 the cell. The capacity of protoplasm to select is known as 

 selective, or differential, permeability. The more commonly 

 used term semiperTneability originally referred to the full perme- 

 ability of the membrane for water and its apparent imperme- 

 ability for dissolved substances. The expression is now used in 

 the same sense as selective permeability. Cell permeability 

 control is presumed to lie primarily in the outer plasma membrane. 



The capacity of the cell to select substances is presumed to 

 lie primarily in the protoplasmic membrane; the surface layer 

 is certainly the first barrier that must be passed. Permeability 

 problems, therefore, assume two aspects — ^that dealing with 

 the question of what substances enter and that having to do 

 with the nature of the membrane which gives to it its capacity 

 to select. 



PERMEABILITY 



There are four major methods of ascertaining the means by 

 which substances enter a cell — the visual method, by direct 



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