28 



BIOLOGICAL CHEMISTRY 



DIFFUSION 



The first process to which we must refer is that of diffu- 

 sion ; that is the tendency for a dissolved substance (solute) 

 to become equally distributed throughout the solvent. The 

 molecules pass from the parts of the solution where they 

 are more concentrated towards those parts where they are 

 less concentrated. 



Diffusion is a slow process and is effective in living organ- 

 isms as a factor in the exchange of materials only when the 

 distances are short such as those of cellular dimensions.* 



A B 



FIG. 4. Diagram to represent : 



A. Diffusion ; the grading in concentration due to diffusion is represented by the shading. 



B. Dialysis, osmosis and osmotic pressure ; a substance which can pass through the wall 

 of the inner vessel will dialyse out until the concentrations throughout both inner and outer 

 vessels are equal. A substance which cannot pass through the wall of the inner vessel will 

 cause water to pass in by osmosis until a certain pressure is produced which is known as the 

 osmotic pressure. 



OSMOSIS 



If a semi-permeable membrane, that is one which allows 

 the solvent to pass through but not the solute, separates the 

 solvent from the solution the result is that the solvent passes 

 through the membrane to dilute the solution. This obviously 



* T. Graham, Phil. Trans. t 1854, p. 227. 



