52 



LIFE: ITS BEGINNINGS AND NATURE 





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Fig. 2-22. Diffusion. A. Particles such as colored ions from a salt diffusing in water. B. Diffusion of particles through 



the cell membrane and within the cell itself. 



toplasm. Any excess flow one way or the 

 other can be fatal. 



Diffusion and osmosis 



The constant strivmg of particles of all 

 dimensions to reach an equilibrium or even 

 distribution in a mixture of two or more 

 different kinds is known as diffusion, a proc- 

 ess that is constantly at work in protoplasm. 

 Once the ions and other particles pass 

 through the membrane of a cell, they diffuse 

 throughout tlie protoplasmic mass until they 

 have become evenly distributed ( Fig. 2-22 ) . 

 However, when some of the particles sur- 

 rounding a cell cannot penetrate the mem- 

 brane, a different situation exists. Let us 

 suppose the membrane is permeable to 

 water, as most membranes are, but imper- 



meable to sucrose molecules. The sucrose 

 molecules will constantly strive to intermin- 

 gle with the water molecules on both sides 

 of the membrane, but since they cannot pass 

 through there will be an uneven distribu- 

 tion of the water molecules on the two 

 sides. The water and sucrose molecules will 

 bombard the membrane with about equal 

 hits on one side; corresponding hits will be 

 made by the water molecules on the other 

 side, but since there are only water mole- 

 cules present one will pass thi'ough for every 

 molecule of sucrose on the other side which 

 cannot get through. The water molecules 

 will be passing toward the sugar side more 

 rapidly than they pass in the other direc- 

 tion, thus building up a hydrostatic pressure 

 on the sugar side ( Fig. 2-23 ) . This unequal 



sami-parmaobla mambrona 7/ 



Fig. 2-23. Osmosis. A. Movement of water molecules through an artificial semi-permeable membrane. The larger 

 molecules cannot pass through. B. A similar situation in which the plasma membrane of the cell is the semi- 

 permeable membrane. 



