Osmosis and Other Mechanisms - 115 



(p. 116), and the more viscous the medium 

 the slower the diffusion. Equilibrium is 

 reached very slowly when the distances in- 

 volved are macroscopic; but within micro- 

 scopic and ultramicroscopic limits, the 

 equalization of concentrations may be almost 

 instantaneous. 



Osmosis. If a semipermeable membrane is 

 placed between two different solutions, an 

 osmotic system is established. In biology, 

 however, only aqueous solutions are impor- 

 tant. Therefore, from a biological viewpoint, 

 osmosis may be defined as the exchange of 

 water between the protoplasm and any solu- 

 tion surrounding the cell. Various other 

 substances, when able to penetrate the mem- 

 brane, may also enter or leave the cell, but 

 these are diffusional exchanges that are super- 

 imposed upon the true osmotic exchanges. 



In a simple osmotic system, such as is 

 shown in Figure 6-3, both the solvent (water) 

 and the solute (sugar) are under the same 

 compulsion to diffuse, each from the region 

 of its own higher concentration. But in such 



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Fig. 6-3. Model of a perfect osmotic system. The 

 membrane, which is free to shift position, is permeable 

 to water (the dots) but not permeable to sugar mole- 

 cules (crosses). Note that equilibrium is attained solely 

 by the passage of water from the upper solution, where 

 the original concentration of H 2 is higher, to the 

 lower solution, where the concentration is lower. Finally 

 both sugar and water are equally distributed. 



a perfect system only the water is able to 

 penetrate the intervening membrane. Conse- 

 quently the two solutions can reach equi- 

 librium only by the transfer of water from 

 one solution to the other. In Figure 6-3, for 

 example, since sugar molecules are unable to 

 penetrate the membrane, equilibrium will 

 be reached solely by the passage of water 

 from the upper solution (where the water 

 concentration is greater) to the lower solu- 

 tion (where the water concentration is lesser). 

 Eventually, however, an equal distribution 

 of both water and sugar may be reached. As 

 water is lost from the upper solution, the 

 sugar concentration increases, and simul- 

 taneously, as water is gained by the lower 

 solution, the sugar concentration decreases — 

 until finally both solutions reach equality. 



The Cell as an Imperfect Osmotic Unit. The 

 cell, quite obviously, does not represent a 

 perfect osmotic system. Not only water, but 

 also many dissolved substances commonly 

 present in and around the protoplasm are 

 able to penetrate the plasma membrane in 

 significant amounts. Therefore, the exchange 

 of water between a cell and its surroundings 

 (true osmosis) occurs simultaneously with the 

 exchange (purely by diffusion) of other sub- 

 stances, such as oxygen and carbon dioxide, 

 which experience no difficulty in passing into 

 or out of the cell. 



One set of purely diffusional exchanges be- 

 tween the cell and its surroundings is shown 

 in Figure 6-4, which illustrates diagrammati- 

 cally the process of respiration as it occurs in 

 all typical animal cells. Both oxygen and 

 carbon dioxide are freely soluble in the pro- 

 toplasm as well as in the surrounding water; 

 and both can pass very freely through the 

 plasma membrane. Thus a steady supply of 

 2 molecules streams into the living cell so 

 long as the concentration of oxygen remains 

 relatively high in the outside medium. Inside 

 the cell the concentration of O a is low be- 

 cause free oxygen is used for oxidative me- 

 tabolism as soon as it enters the protoplasm. 

 The 2 concentration in the surrounding 

 water tends to remain relatively high, since 



