118 - The Cell 



Water Equilibrium in the Cell: Osmosis. 



The bulk of all solutes present in protoplasm 

 — and this includes proteins, most sugars, 

 and inorganic salts — are substances that 

 penetrate the plasma membrane very slowly, 

 if at all (see Table 6-1). The solvent, water, 

 on the other hand, can enter or leave the 

 cell quite quickly. Moreover, water is ex- 

 tremely abundant in all the media (fresh 

 water, sea water, blood, lymph, sap, etc.) 

 that normally surround die cell, as well as 

 in the protoplasm itself. These facts are of 

 great importance. They mean that the bur- 

 den of establishing equilibrium between the 

 cell and a surrounding solution devolves 

 mainly upon the water. 11 a cell is placed in 

 an unbalanced solution in which the con- 

 centration of water is drastically different 

 compared with the protoplasm, so great a 

 quantity of water will enter or leave the cell 

 that the living structure will be destroyed. 



Isotonic Solutions. In an isotonic solution, 

 the concentration oi water is the same as in 

 the protoplasm. This condition is realized 

 only when the total concentration of solute 1 

 (taken collective!) ) in the solution and the 

 protoplasm is likewise equal. In an isotonic 

 solution, the water exchange between the 

 cell and the solution is exactly balanced. 

 Owing to the random movement of mole- 

 cules, water continues to escape across the 

 membrane, but the escaping water is exactly 

 balanced by an equivalent amount of water 

 that enters the cell simultaneously. As a gen- 

 eral rule the blood and lymph of animals are 

 approximately isotonic to the cells. In the 

 blood stream, for example, the red blood cells 

 are in osmotic balance with the surrounding 

 plasma. Under these conditions no net gain 

 or loss of water occurs, and consequently the 

 cells are preserved from abnormal changes of 

 volume. But if red cells are studied in an 

 artificial medium such as a salt solution, ab- 

 normal changes are bound to occur unless 

 the artificial solution is made isotonic to the 

 protoplasm (for example, 0.9 percent NaCl). 



1 Disregarding the small amounts ot rapidly pene- 

 trating solutes ("for example, oxygen). 



An isotonic solution contains a concentra- 

 tion of nonpenetrating (or very slowly pene- 

 trating) solute that approximates the total 

 concentration of nonpenetrating solutes (pro- 

 teins, phospholipids, salts) in the protoplasm. 

 Otherwise an equality of the water concen- 

 trations inside and outside the cell is not pos- 

 sible. Various nonpenetrating solutes may be 

 used in the preparation of an isotonic solu- 

 tion, although the inorganic salts, especially 

 NaCl, are most frequently employed. Best 

 results are obtained from salt mixtures in 

 which Na + , K + , Ca 2 + , and Mg 2 + ions are 

 represented in the proper proportions, since 

 otherwise the normal semipermeability of 

 the plasma membrane gradually deteriorates. 

 In the various kinds of "Ringer's solution," 

 as such balanced solutions are called, the 

 proper quantity and the proportions of inor- 

 ganic salts vary considerably from one kind 

 of animal to another. In transfusion fluids, 

 which are used in emergencies to replace the 

 blood plasma, a considerable proportion of 

 protein is included with the salts. Various 

 foreign proteins have been tried as substi- 

 tutes for the normal plasma proteins, but not 

 with much success. The main difficulty has 

 not been to obtain the proper osmotic con- 

 ditions, but to find foreign proteins that will 

 remain in the blood stream and that are not 

 toxic to the organism. 



Hypotonic Solutions. A hypotonic solu- 

 tion contains a relatively low concentration 

 of nonpenetrating solute (compared to the 

 protoplasm of the cell that it surrounds). 

 Because of the low concentration of solute, 

 the water concentration in a hypotonic solu- 

 tion is relatively high. Therefore, cells placed 

 in hypotonic solution tend to take in water 

 and to swell. The passage of water from the 

 external solution where the concentration is 

 higher into the cell where the concentration 

 is lower conforms, of course, with the funda- 

 mental law of osmosis and diffusion. If the 

 protoplasm can accommodate the extra water 

 and come into equilibrium with the sur- 

 rounding solution, the swelling will stop. 

 Otherwise, the swelling continues until the 



