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GENERAL CONCEPTS 



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Figure 3.10. Diagram illustrating osmosis. When a solution of sugar in water is 

 separated from pure water by a semipermeable membrane which allows water but not 

 the larger sugar molecules to pass tlirough, there is a net movement of water molecules 

 through the membrane to the sugar solution. The water molecules are diffusing from 

 a region of higher concentration (pure water) to a region of lower concentration (the 

 sugar solution). 



occurs with equal speed in both directions through the semipermeable 

 membrane and there will be no net change in the amount of water in 

 the pouch. The pressure of the column of water is called the osmotic 

 pressure of the sugar solution. The osmotic pressure results from the 

 tendency of the water molecules to pass through the semipermeable 

 membrane and equalize the concentration of water molecules on its 

 two sides. A more concentrated sugar solution would have a greater 

 osmotic pressure and would "draw" water to a higher level in the tube. 

 A 10 per cent sugar solution would cause water to rise approximately 

 twice as high in the tube as a 5 per cent solution. 



It is evident from this discussion that dialysis and osmosis are 

 simply two special forms of diffusion. Diffusion is the general term for 

 the movement of molecules from a region of high concentration to a 

 region of lower concentration, brought about by the inherent heat 

 energy of the molecides. Dialysis is the diffusion of dissolved molecules 

 through a semipermeable membrane and osmosis is the diffusion of 

 solvent molecules through a semipermeable membrane. In biologic sys- 

 tems the solvent molecules are almost universally water. 



The salts, sugars and other substances dissolved in the fluid within 

 each cell give the intracellular fluid a certain osmotic pressure. When the 

 cell is placed in a fluid with the same osmotic pressure as that of its 

 intracellular fluid, there is no net entrance or exit of water, and the cell 

 neither swells nor shrinks. Such a fluid is said to be isotonic or isosmotic 

 with the intracellular fluid of the cell. Normally, the blood plasma and 

 body fluids are isosmotic with the intracellular fluids of the body cells. 

 If the environmental fluid contains more dissolved substances than the 

 fluid within the cell, water Avill tend to pass out of the cell and the cell 

 shrinks. Such a fluid is said to be hypertonic to the cell. If the environ- 

 mental fluid has a lower concentration of dissolved substances than the 

 fluid in the cell, water tends to pass into the cell and the cell swells. This 



