Biochemical and Biophysical Phenomena 753 



plasma membrane of each individual cell plays an important role in 

 regulating the activities of the protoplasm within that cell. 



The force exerted by the pressure of moving molecules in a solution 

 against a membrane is known as osmotic pressure. The passage of a sub- 

 stance through a semipermeable membrane is known as osmosis. The 

 measurable force within living cells is considerable and usually keeps the 

 cell membrane distended. 



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Fig. 367. — Demonstration of osmosis in which the test-tube shaped semipermea- 

 ble membrane separates sugar solution and water. The pores of the membrane 

 are of a size that permit the passage of water molecules but not sugar molecules. 

 Hence, the passage of water is sufficient to cause it to rise in the upright tube. 

 Water molecules pass in either direction, but they pass faster into the tube than out 

 of it because of the greater concentration of water molecules on the outside. The 

 liquid will rise in the upright tube until it reaches a level at which its hydrostatic 

 pressure, due to its weight, is equal to the osmotic pressure produced by the sugar 

 solution. (From Roe: Principles of Chemistry, The C. V. Mosby Co.) 



A solution with greater concentration (less water) than the proto- 

 plasm, and which draws water from the protoplasm of the cell, is known 

 as a hypertonic solution. In this case, water will pass out of the cell 

 in an attempt to equalize the pressure. Under such circumstances (loss 

 of water), animal cells will tend to shrink as a whole because of their 

 delicate cell membrane, while the protoplasm of plant cells shrinks away 



