OSMOSIS 183 



To define osmosis, and other terms associated with it, satis- 

 factorily is difficult without reference to an osmotic system. 

 This is due in part to an unfortunate use of terms and in part to 

 the fact that osmosis is simply another well-known physical 

 process, viz., diffusion, taking place under certain conditions. 

 The usual osmotic system as assembled in a laboratory consists 

 of a solution of sugar in water separated from pure water by a 

 parchment paper or other membrane which is permeable to the 

 solvent water but not to the solute sugar. Water passes through 

 the membrane in both directions but in excess from the pure- 

 water side and therefore produces a pressure on the solution side. 

 A membrane permeable to one kind of molecule (water) and not 

 to another (sugar) is said to be semipermeable. Selectively or 

 differentially permeable would be a more precise designation. 

 Actually, parchment paper is permeable to sugar in solution but 

 sufficiently poorly so, in contrast to the much more rapid diffu- 

 sion of water through it, as to cause it to function as an osmotic 

 or selective membrane. Few membranes are wholly impermeable 

 to any molecularly dispersed substance. Even protein molecules 

 get through collodion membranes but so slowly that the mem- 

 brane acts as if it were impermeable to the solute, protein, and 

 permeable to the solvent, water. Practically every skin from 

 any source in nature is a selectively permeable membrane which 

 may function in an osmotic system. The importance of mem- 

 branes in vital processes rests in a large measure upon their 

 selective permeability. The most delicate of living membranes 

 is that at the surface of protoplasm. It determines in part the 

 substances that enter and leave the cell. The pig's bladder is a 

 more substantial osmotic membrane often used in the past for 

 laboratory experimental purposes. The solvent in natural 

 osmotic systems is water. The solute may be any water-soluble 

 substance for which the membrane is not (readily) permeable. 

 Salts and sugars are the most common. Organic substances of 

 high molecular weight {e.g., proteins) are not satisfactory because 

 of the large size (colloidal nature) of their molecules. 



Types of Osmotic Systems. — A common type of laboratory 

 osmotic system is a glass thistle tube over the mouth of which a 

 membrane of parchment paper is tightly fastened (Fig. 100). 

 An animal membrane may also be used. Still another form is 

 made from a sack or thimble of gelatin or collodion into which a 



