114 - The Cell 



take at any particular moment is entirely un- 

 predictable. The particle may move in any 

 direction, depending upon its chance colli- 

 sions with other particles or with the wall of 

 the containing vessel. Nevertheless the mass 

 movements of each kind of particle present 

 in a solution can be predicted accurately on 

 a statistical basis. These mass movements 

 are governed by the fundamental law of 

 diffusion. The particles of each different sub- 

 stance present in a solution will diffuse from 

 a region where these particles are more con- 

 centrated toward a region where they are less 

 concentrated. Moreover, diffusion will con- 

 tinue until every component reaches equal 

 concentration throughout the whole solution. 

 In the sugar solution of Figure 6-2, for exam- 

 ple, the mass movement of the water in a 

 downward direction occurs simultaneously 

 with the upward movement of the sugar 



SSs 



SSs 



*xmym± 



:*i<iii:*}i*ff*e: 



Fig. 6-2. Diagram of free diffusion. A weak sugar 

 solution, layered on top of a stronger one. The attain- 

 ment of equilibrium (equal distribution of all the mole- 

 cules) depends upon two factors: (1) water molecules 

 (the dots) diffuse downward from the upper solution 

 where the water concentration is greater, and simul- 

 taneously, (2) sugar molecules (the crosses) diffuse up- 

 ward from the region where the sugar concentration is 

 greater. Each kind of molecule in a solution obeys the 

 same law. 



molecules, until complete equality in the 

 distribution of both kinds of particles is 

 finally reached. 



The concentration of each different sub- 

 stance present in a solution is of critical im- 

 portance in determining the direction of the 

 diffusion of that substance. The concentra- 

 tion of a substance specifies the number of 

 its particles present in a unit volume of the 

 solution. No two molecules or ions can oc- 

 cupy the same space at the same time, and 

 therefore it follows that the concentration of 

 any one substance cannot be increased with- 

 out displacing from each unit volume of the 

 solution an equivalent number of the par- 

 ticles of all other components. Accordingly, 

 whenever the total solute concentration is 

 high, the concentration of solvent must be 

 low; or whenever the concentration ol one 

 solute is increased, the concentration of the 

 other solute and of the solvent must undergo 

 a corresponding decrease. 



The foregoing relationships mav be seen 

 in the sugar solution diagramed in Figure 

 6-2. The sugar and the water molecules reach 

 equilibrium by diffusing in opposite direc- 

 tions. But each is obeying the same funda- 

 mental law: each is escaping from the region 

 of its greater concentration toward the region 

 of its lesser concentration. In the lop part of 

 the original solution, the higher concentra- 

 tion of water is determined by the lower 

 concentration of sugar, and consequently 

 the water diffuses downward. In the bottom 

 of the test tube, the higher concentration of 

 sugar corresponds to the lower concentration 

 of water, and consequently the mass move- 

 ment of the sugar molecules is in an upward 

 direction. 



The velocity of diffusion is determined b\ 

 a number of factors. The whole process de- 

 pends upon thermal energy, and conse- 

 quently equilibrium is attained more quickly 

 in warmer solutions. When the concentration 

 difference (concentration gradient) is greater, 

 the tendency of the particles to escape from 

 the concentrated region is greater. Large 

 particles diffuse more slowly than small ones 



