134 



FUNDAMENTALS OF STRUCTURE AND FUNCTION 



are just as many molecules of the solute in one part of the solvent 

 as in another. In the case of the diffusion of red ink in water, 

 the eosin (which is the coloring material used) was more concen- 

 trated in the drop than in the water, so the molecules of eosin began 

 moving away from this place of high concentration until they were 

 equally dispersed throughout the water. As a general rule we may 

 say that, if other conditions are equal, the diffusion rate between 

 two points is proportional to the differences in concentration of the 

 substances at these two points. One thing which affects the diffusion 

 rate is the nature of the medium, w^hether it be a gel, emulsion, or 

 some sort of semisolid (porous). Gelatin, for example, which is a 

 gel, offers no effective resistance to the diffusion of molecules of a 

 crystalloid nature through its meshes, but, upon the other hand, 

 this network may serve to block effectively the passage of colloidal 

 substances. 



Suppose a membrane were stretched crosswise in a jar where 

 diffusion was taking place. Could the molecules of the diffusing 

 substance pass through the membrane? This depends on whether 

 the membrane is permeable to the diffusing substance. In some 

 membranes the ultramicroscopic "pores" are believed to be quite 

 large, thus letting through molecules of larger sizes, while in other 

 membranes the "pores" through which substances can diffuse are 



very small. Other substances 

 penetrate in proportion to their 

 lipoid solubility. Thus some 

 membranes allow certain sub- 

 stances to pass through, while 

 they keep out others. Such mem- 

 branes are said to be selectively 

 'permeable. An ordinary parch- 

 ment membrane will allow the 



'^l 



ff^ 



" . • - •'•. ^«.-- ■',■ ■' i'ly. l,v-i .'•:~-r 





'■f.^:.:^-. 



■■"iPV^i^- 



a 



W 



e p "'"-"-^f 



V 



Diagram of an imaginary section . i •. t-> - -u 



through the cell wall and protoplast to eosm to pass through it. But the 



show a, outer water ; iv, cell wall ; c, ecto- cell membrane does not act in the 



plast or cytoplasmic membrane next ^^^^ manner, as it is a vldsma 

 to the cell wall; p, general cytoplasm; i i j.- i 



/. tonoplast or inner cytoplasmic mem- membrane, and selectively per- 



brane next to the water, thus forming meable. 



a continuous pathway which carries ^j^^ plasma membranes sur- 

 solutes irom (a) to (?)) ; i\ vacuole. ,. ,. . , ,. , 



rounding living cells are believed 



to be colloidal in nature, made of a combination of fatty and protein 



substances. Careful experiments have demonstrated them to be 



