PROTOPLASMIC STRUCTURE 103 



concentration, a process necessarily involving osmotic 

 work; in this work O2 is consumed and energy set free. 

 The problem of the nature of this physiological trans- 

 porting mechanism is often distinguished as the problem 

 of "physiological permeability" from the simpler prob- 

 lem of the conditions of simple diffusion across the cell 

 surface, or '^ physical permeability." The majority of 

 recent studies on protoplasmic permeability have had 

 reference to this latter problem. 



It will be apparent that the problem of the nature 

 and conditions of cell-permeability is not a special or 

 limited one but underlies the whole problem of the 

 essential nature of protoplasmic structure and activities. 

 Apparently the most fundamental property of the plasma 

 membrane is semi-permeability; water can pass, although 

 with considerable resistance in many cases; but the 

 normal water-soluble constituents of the protoplasm 

 and its surroundings do not diffuse across the cell bound- 

 ary, or only under special conditions. This property 

 of "semi -permeability" is associated with a high degree 

 of electrical resistance (signifying impermeability to 

 crystalloidal ions) and water-insolubility. The physico- 

 chemical conditions of these properties of the plasma 

 membrane have been much investigated of recent years, 

 and since the results of this work have an important 

 bearing on our general problem, a somewhat detailed 

 review will be given. 



The general importance of membranes in organic 

 structure and activities has long been recognized, and 

 much study has been devoted by physiologists to various 

 artificial types of membrane, such as precipitation 

 membranes, membranes of parchment and collodion, 



