Processes in iVIovement of Materials 87 



concentration but is restrained by the membrane from moving 

 outward. 



Differentially permeable membranes necessary for osmosis. 



When a membrane permits water or other substances to pass 

 through, it is said to be permeable to that substance. For example, 

 animal membranes are permeable to water and to various dyes. 

 A membrane that allows one substance to pass through it, but 

 retards the passage of another substance, is said to be differen- 

 tially permeable. The membrane on the thistle tube is differ- 

 entially permeable, because it allows the passage of water but 

 restrains the sugar that is dissolved in the water. The diffusion 

 of water through a differentially permeable membrane to the side 

 where it is less concentrated is called osmosis. 



Osmotic pressure. If we close the upper end of the thistle 

 tube, the water will continue to rise and compress the inclosed 

 air. If a large amount of sugar is put inside the tube, the water 

 will rise rapidly and exert great pressure. If only a small amount 

 of sugar is present inside the tube, the water will rise slowly and 

 exert but Httle pressure. The pressure which is developed by 

 diffusion under these conditions is called osmotic pressure.^ 



The plant cell as an osmotic apparatus. In hving plant cells 

 the cytoplasm hning the cell walls is the differentially permeable 

 membrane. The cell walls of some tissues are also differentially 

 permeable. 



The cells contain sugars, salts, acids, and other substances 

 dissolved in the water of the vacuole, just as the sugar is contained 

 within the thistle tube in the experiment described above. The 



1 Experiments to show osmosis and osmotic pressure are best performed with 

 paper, or collodion, diffusion shells, or with specially prepared porcelain cups. The 

 thistle tube and animal membrane are used in this discussion because of the sim- 

 plicity of the apparatus. 



There is no agreement among scientists as to the complete explanation of osmotic 

 pressure. The explanation given above leaves out of account some of the factors, 

 principally electrical, involved in the process. This simple explanation is intro- 

 duced merely to help the student to form a mental picture of the mechanics of 

 osmosis as it occurs in plant cells. 



