ALTERATIONS IN PERMEABILITY 111 



pass through while retaining smaller particles. It may also 

 appear to " seleet " certain constituents of the surrounding fluid 

 to pass in, seemingly quite irrespective of their size compared 

 with their fellows. Again, the process of negative osmosis may 

 take place through an animal membrane. That is, water may 

 pass from the more concentrated side to the more dilute side. 

 This flow of water takes place against osmotic pressure. Of 

 course a cell is in close juxtaposition to several other cells, and 

 therefore the composition, structure, and permeability of any one 

 cell membrane may vary from place to place according to the 

 nature of the interface. One interface may be such as to allow 

 free passage of solutes to which other interfaces may be semi- 

 permeable. 



Alterations in permeability may be due to (1) alterations in 

 the membrane or (2) alterations in the material presented to it. 



(1) The membrane itself may undergo change in composition 

 and permeability as the cell contents or the environment change 

 in (a) composition, or in (b) physical state. The composition of 

 the surface layer depends on the substances present in solution 

 in the interior and on the nature of the interface. Any alteration 

 in the chemical state of either of these phases will produce such 

 an alteration at the surface as will alter permeability. The 

 electrical double layer on the surface plays a considerable part in 

 deciding the composition of the membrane. If a solute of opposite 

 electrical sign to the membrane come within the electrical sphere 

 of attraction it will be adsorbed and will either thicken the 

 membrane or may occlude, wholly or partially, some of the 

 interstices. In any case, adsorption will alter the permeability 

 of the adsorbing surface. It may have a further effect. The 

 adsorbed material may enter into combination, chemical or 

 physical, with the membrane, producing a second alteration in 

 permeability. It may even cause a third alteration, by ultimately 

 passing through the membrane and going into solution on the 

 other side. 



If the adsorbed material be an amphoteric colloid, then the 

 electrical charge on the membrane may be modified and so 

 produce apparently abnormal osmosis. Collodion membranes, 

 for instance, are practically indifferent as regards electrical charge. 

 Water passes through these membranes into solutions of non- 

 electrolytes and of electrolytes more concentrated than ~- at a 

 rate in accordance with the van t'Hoff theory of osmotic pressure, 

 i.e. a linear function of the concentration of the particles (colloidal 



