THE PHYSICAL CHEMISTRY OF PROTOPLASM 229 



through them of both solvents and solutes. The first and last of 

 these types of membranes have no effect upon osmotic pressure; 

 but osmotic pressure is at once set up whenever a semi-permeable 

 membrane is interposed between solutions of different concentra- 

 tions. It is due to the molecular motion of both the liquid and the 

 dissolved solids, as a result of which a greater number of molecules 

 are " bombarding," or pressing upon the membrane from the side 

 of the more concentrated solution. This sets up an unequal 

 pressure upon the two sides of the membrane, and if the latter be 

 semi-permeable there will result a passage of the liquid through the 

 membrane toward the denser solution so as to equalize the pres- 

 sure. The resultant tendency is for the solutions on the two 

 sides of the membranes to become equal in concentration by 

 movement of the liquid from the less dense to the more dense 

 portion, instead of by movement of the dissolved materials toward 

 the less dense part of the solution as in the case of diffusion when 

 solutions of different concentrations are brought in contact with 

 no membrane to interfere with free diffusion. 



Osmotic pressure tends, therefore, to force the movement of 

 solvents through semi-permeable membranes from more dilute 

 toward more concentrated solutions. Protoplasm acts in general 

 as an approximately semi-permeable membrane or material. 

 For example, if the concentration of sugar in any given mass of 

 protoplasm becomes greater, by reason of the photosynthetic 

 activity, osmotic pressure is set up and water enters the mass, 

 thus preventing loss of turgidity due to increased concentration. 

 Similarly, any other increase in concentration of synthetic products 

 is compensated for by entrance of water because of increased 

 osmotic pressure, unless the products are insoluble and, therefore, 

 incapable of effecting the osmotic pressure. . 



Hence, osmotic pressure provides for the movement of water 

 into and out of protoplasm and so tends to keep the proportion of 

 water uniform throughout the entire tissue. It will at once occur 

 to the reader, however, that if the statements in the preceding 

 paragraph were unqualifiedly true, and if the protoplasmic mass 

 were absolutely semi-permeable in character, there would be no 

 possibility of the passage of dissolved solids into or out of the cell; 

 i.e., if the protoplasm acted as an ideally semi-permeable mem- 

 brane, only water could pass into or out of it. But we know that 

 mineral salts from the soil must pass into any cell before the syn- 



