36 ELEMENTS OF BIOLOGY 



of sugar molecules the greater the number of impacts and the 

 greater the pressure developed. 



If we add to the water outside the cylinder sugar or any salt 

 that will not pass through the sausage casing, then the migration of 

 water into the cylinder will be reduced and will cease altogether 

 when the concentration of molecules that do not pass is exactly 

 equal outside and inside the cylinder. If we continue to add sugar 

 to the water outside the cylinder, then the migration of water mole- 

 cules is reversed and the contents of the cylinder shrink. Three 

 terms are employed to describe the osmotic pressure relations be- 

 tween the contents of the cylinder and the surroundings. When the 

 osmotic pressure of the sugar solution is greater than that of the 

 water outside, it is termed a hypertonic solution. When the con- 

 centration of molecules that do not pass the membrane is exactly 

 equal within and without the cylinder, the solution is called iso- 

 tonic. When the contents of the cylinder are more dilute than the 

 surrounding solution, it is spoken of as hypotonic. 



Function of the Cytoplasm. The relations between the cyto- 

 plasm of the cell, the cell boundary, and the surroundings are in 

 many respects similar to the relation between the contents of the 

 glass cylinder and the water. Colloids exhibit osmotic pressure, for 

 colloidal particles do not readily pass through a semi-permeable 

 membrane. Moreover, the cell contains large, osmotically active, 

 molecules. The colloidal cytoplasm of the cell is placed against the 

 semi-permeable plasma membrane; if the concentration of salts or 

 other substances that do not pass through this membrane is less 

 outside the cell than the concentration necessary to balance the 

 osmotic pressure of the cytoplasm, then water continually flows into 

 the cell. If this occurred in all living cells, then the cells would dis- 

 tend until the osmotic pressure within exactly balanced that with- 

 out. On the other hand, cells would shrink if the osmotic pressure 

 of the cytoplasm were materially less than that of the surround- 

 ings (Fig. 7). 



