194 SURFACES AND MEMBRANES 



than plant or animal cell contents when the cells are placed in such 

 solutions. 



If a cell is placed in a solution having a higher osmotic pressure (hyper- 

 tonic solution) than the cell contents, water tends to flow out of the cell, 

 thus reducing its internal osmotic pressure. The cell therefore shrinks 

 in size. If the solution in which a cell is introduced has a lower osmotic 

 pressure (hypotonic solution), then water tends to flow into the cell, 

 which expands, eventually bursting as the result of the increased internal 

 osmotic pressure. 



Hamburger, a Dutch physiologist, was probably the first to apply 

 physical laws to animal physiology. He showed, as early as 1886, that 

 erythrocytes from different species of animals have different limits of 

 fragibility and that in no case was the solution producing hemolysis 

 (stretching of red blood cells so that pigment escapes) isotonic with the 

 cell content. He found that an isotonic solution of sodium chloride 

 containing 0.951 gram of NaCl per 100 grams of water could be used to 

 prevent the rupture of the membrane of human red corpuscles. 



Osmotic Pressure of Electrolytes 



We find experimentally that an aqueous solution of sucrose 

 (M = 342.2) having a molar concentration of 0.2 M at 0° C develops an 

 osmotic pressure equal to 4.76 atmospheres. A similar concentration of 

 sodium chloride (M = 58.5) develops an osmotic pressure of 8.75 

 atmospheres. The sodium chloride behaves osmotically as if its concen- 

 tration were nearly twice as great as that of sucrose. 



This abnormal osmotic activity of salt solutions was first observed by 

 the botanist de Vries during his investigation of the previously men- 

 tioned experiments on the plasmolysis of plant cells. 



If " normal " means the values calculated in accordance with the 

 van't Hoff laws of osmotic pressure and molecular weights of non- 

 electrolytes, then electrolytes may be said to possess an abnormally high 

 osmotic pressure. This abnormality is found to become more pro- 

 nounced by diluting the solution. 



The explanation is found in the fact that salts, acids, and alkalies 

 dissociate when dissolved in water. The molecules in solution dissociate 

 into positively and negatively charged entities called ions. These ions 

 participate in creating the osmotic pressure just as if they were individ- 

 ual particles. Sodium chloride, in water, dissociates into one positively 

 charged entity, the sodium ion, and one negatively charged entity, the 

 chloride ion. If all the molecules which make up a solution of sodium 

 chloride in water were completely dissociated, then such a salt solution 



