OSMOSIS 57 



to sugar solution," the molecules of the solute being 

 unable to pass through it, while the molecules of water 

 can. If common salt solution, instead of sugar, is 

 within the bag, the molecules of salt can pass out 

 (though at first the liquid rises in the bag because 

 the water passes in more quickly than the salt can 

 pass out), since the membrane is permeable to them. 

 The solution within the bag becomes less concentrated 

 owing to the entrance of water and the escape of salt, 

 while the liquid outside becomes a salt solution of 

 increasing strength till the two are of equal concentration 

 and equilibrium is attained. But in the case of the 

 sugar equilibrium cannot be attained in this way 

 because the sugar molecules cannot escape through 

 the membrane, and so the water continuously enters, 

 rising in the bag if it is open at the top, distending its 

 extensible wall if the bag is closed. This process of 

 the passage of a solvent through a semi-permeable 

 membrane which will not allow the solute to pass is 

 called osmosis, and the pressure developed on the wall 

 of the membrane is known as osmotic pressure. 



Various solutes, among which sugars are prominent, 

 to which the protoplasmic membranes bounding the 

 cells appear to be practically impermeable, are formed 

 within the living cells of plants and animals. These 

 are known as osmotic substances, and many of the pro- 

 cesses of absorption and movement of water within 

 the organism depend on the passage of water through 

 membranes on the other side of which such solutes 

 exist, in accordance with the universal tendency to 

 establish equilibrium. 



We now begin to see the importance of the fact that 

 the structure of organisms is so largely built up of 



