Chapter V 



45 



Mechanism of Osmosis 



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a tissue, an organ, or a whole plant, that relates to the hydrostatic pressure 

 of the free solution or liquid contained in it. Because of the tensile strength 

 and the elastic nature of plant cell walls, the plant cell usually exhibits a 

 certain degree of turgor. 



Turgor pressure at full turgor is the excess pressure exhibited by solu- 

 tion inside a turgid cell or osmometer above the external pressure. The 

 equal and opposite pressure exerted by the walls upon the cell contents is 

 termed wall pressure. For cells in contact with free water the external 

 pressure is approximately one atmosphere. Cells within a plant must ob- 

 tain their water from the xylem in which the diffusion pressure of water 

 may vary between wide limits. Such cells are seldom at full turgor; at 

 times the DPD of water in the xylem may equal or exceed the osmotic 

 pressure of cells in surrounding tissues : such a condition brings on wilting. 



The Kinetic Basis of Osmosis: — Although the kinetic view of osmotic pressure 

 has been criticised from many angles, it seems inevitable that any true explanation of 

 pressure must rest upon a kinetic foundation. Ultimately the manifestation of pres- 

 sure must be a result of bombardment by the molecules of the substance exerting the 

 pressure, be it gas, liquid, or solid. 



The mechanics of the process of osmosis may be clarified by use of a model to 

 illustrate the difference in the rates of diffusion of different gases and the effects upon 

 pressure. 



In Figure 11, A is a porous clay thimble closed 

 at its lower end with a rubber stopper through which 

 extends a glass tube B. The lower end of B is sub- 

 mersed in water in the beaker C. As the apparatus 

 stands the thimble and tube are filled with air — mostly 

 nitrogen and oxygen. If an inverted beaker filled with 

 hydrogen is lowered very slowly over A nothing hap- 

 pens, but if it is lowered quickly bubbles of gas pro- 

 ceed from the lower end of B. If the beaker of hydro- 

 gen is kept over A bubbling proceeds for a time, slows 

 down, stops, and then water enters B and is pulled up 

 to a considerable height above the level in the beaker. 

 If the hydrogen is removed from A quickly, bubbling 

 ceases at once and the water rises in B. In each ex- 

 periment water rises to a certain height in B and then 

 slowly lowers again until it comes to the same level as 

 it is in C. 



The explanation of these phenomena is as follows. 

 As the apparatus stands at rest, the thimble A and 

 tube B are full of air and the molecules are diffusing in 

 and out through the pores of the thimble at equal rates. 

 When hydrogen surrounds A its molecules, having 

 higher velocities and being smaller in size, diffuse in- 

 ward more rapidly than air diffuses out ; an increased 

 pressure is built up in A and B and air bubbles from 

 the tube. Rapidly, if the hydrogen is removed, or 

 slowly, if it is allowed to dissipate, hydrogen concen- 

 tration decreases around A and inward diffusion of 

 molecules is lowered to the same rate as outward. 

 Bubbling ceases. Then, if no more hydrogen is in- 

 troduced, outward diffusion exceeds inward because, 

 again, hydrogen moves through the porous thimble 

 more rapidly than air. The pressure inside the apparatus becomes less than atmos- 

 pheric and water rises in the tube. Finally, as hydrogen moves out and is slowly re- 

 placed by air again the apparatus attains its initial state. If hydrogen were main- 

 tained around A for a long time bubbling would continue as long as any air was left 

 in the apparatus ; when all air had left, either as bubbles from B or by diffusion out- 

 ward through A, the water in B would return to the initial level in C and the apparatus 

 would come to rest, being completely filled with hydrogen. Then if the hydrogen 

 around A were removed the pressure would be reversed, as described above, the water 



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Fig. 11. — Apparatus for 

 demonstrating differential diffu- 

 sion of gases. A is a porous 

 clay thimble, B a glass tube, and 

 C a beaker of water. For ex- 

 planation, see text. (Redrawn 

 from Meyer and Anderson, 

 1939). 



