136 



FUNDAMENTALS OF STRUCTURE AND FUNCTION 



sugar solution by a selectively permeable membrane, the water mole- 

 cules tend to pass through the membrane (since it is permeable to 

 water) from the water, to the sugar solution where the water is in 

 less concentration. Actually it is a question of the water molecules 

 of the solvent reaching an equilibrium. 



Osmotic pressure, in living cells, is one of the factors that accounts 

 for the rise of water in roots and up the stems of plants. Its effects 

 can easily be demonstrated experimentally in the laboratory by plac- 

 ing, for example, living cells of Spirogyra in a 10 per cent solution of 

 salt and water. The water from within the cell (where it is in greater 

 concentration) passes out through the cell membrane to enter the salt 

 solution (where water is in less concentration than in the cell). The 

 result is that the cell body shrinks away from the cell w^all and the 

 shrunken cell is said to be plasmolyzed. A solution which contains 



a greater number of mole- 

 cules of the substance in 

 solution (solute) per vol- 

 ume than the interior of 

 the cell is said to be hy- 

 perosmotic; if it has less 

 concentration than the in- 

 terior of the cell it is 

 hyposmotic; and if it has 

 the same number of solute 

 molecules per unit volume 

 as the interior of the cell 

 the solution is isosmotic to 

 the cell. 



When a cell is placed in 

 a hyposmotic solution it 

 will tend to swell up, be- 

 cause water is diffusing 

 more rapidly inward, and so, unless the cell is surrounded by 

 heavy walls as in the case of plants, the cell will tend to burst. 

 When this happens it is called cytolysis. This may be demon- 

 strated when human red blood corpuscles are placed in distilled 

 water. It is evident, therefore, that osmotic pressure differs greatly 

 in the cells of different organisms, possibly depending on whether 

 they live in fresh or salt water and the consequent concentration of 

 salts present. As a matter of fact, fresh- water organisms live in a 



pelliole — 



.cytoplasm. . 



YlllClsZJiS. 



..ceU\/cdl 



Plasmolysis in a plant and an animal cell. 

 Note how the cytoplasm has shrunk away from 

 the wall in the case of the plant cell and the 

 pellicle in the case of Paramecium. Why has 

 this occurred ? 



