OSMOSIS 117 



solution through the membrane to the stronger. Sometimes 

 some of the dissolved substances may pass through the membrane, 

 but often the membrane permits only the water to pass, in which 

 case it is known as a semi-permeable membrane. In case of the 

 pig's bladder not much sugar is allowed to pass through its wall, 

 which is, therefore, semi-permeable in reference to this particular 

 solution. When a membrane will allow a dissolved substance 

 to pass, it is said to be permeable to that substance. Most 

 membranes are permeable to some substances and impermeable 

 to others. 



The causes of the movement of the water or other solvents from 

 the less dense to the denser solution are not thoroughly under- 

 stood. Some think that it is due to the affinity of the dissolved 

 substances for the solvent, which is pulled to the substances with 

 a force increasing with the amount of the substances in solution. 

 Others think that it is due to the checking of the diffusive power 

 of the dissolved substances by the membrane, which permits the 

 two liquids to approach an equilibrium only through the passing 

 of more of the solvent to the denser solution. 



In comparing osmosis in the cell with the illustration, the cell 

 membrane corresponds to the wall of the pig's bladder, the cell 

 sap to the solution within the bladder, and the solutions around 

 the cell correspond to the water or solutions in the jar. If the 

 cell sap in denser than the solution on the outside of the cell mem- 

 brane, then water with those dissolved substances to which the 

 membrane is permeable will pass in; but, on the other hand, if the 

 cell sap is less dense than the solution without, water and prob- 

 ably some dissolved substances will pass out. Thus the passing 

 of liquids through the cell membrane from a less dense to a denser 

 liquid is also the chief feature of osmosis in cells. It should also 

 be noted in connection with osmosis in cells: (1) that the more 

 the two solutions separated by the cell membrane differ in con- 

 centration, the more rapid is the process of osmosis ; and (2) that 

 the solvent, which is water in case of cells, passes through the 

 membrane independently of its dissolved substances, which are 

 either carried along or left behind according to whether or not 

 the membrane is permeable to them. 



However, it is only in principle and not in practice that osmosis 

 as demonstrated with the pig's bladder is identical with that in 

 the cell. In the first place, instead of a solution containing only 



