186 



PROTOPLASM 



ferrocyanide) but permeable to others (potassium chloride). 



It is also impermeable to barium chloride, calcium chloride, and 



potassium sulphate. 



The simplest interpretation of the selective permeability of 



membranes is that in which the membrane is regarded as a sieve 



with pores that let small molecules pass but not large ones. 



There are some difficulties with this 



theory, in that big molecules occasionally 

 get through while little ones do not. 

 Wilhelm Ostwald suggested that the 

 electrical properties (sign of charge) of 

 membranes may be such as to permit one 

 type of ion (positive or negative) to 

 pass but not the other type. Sugar, 

 however, is not an ion. Possibly both 

 mechanical (sieve) and electrical (charge) 

 properties are at work (pages 281, 284). 



Terminology. — The osmotic pressure of 

 a solution is an evaluation. Actual 

 pressure exists only when the solution is 

 part of an osmotic system, i.e., when it is 

 confined in a sack made of a semiper- 

 meable membrane. In order to dis- 

 tinguish between actual pressure (turgor) 

 and osmotic pressure in the sense of a 

 rating of the solution, botanists have 



Whether 



W 



•:■■ :■: 



m 



n 



'^^mf~" 



Fig. 102. — Osmotic 

 system with a copper 

 ferrocyanide membrane 



precipitated within the substituted value for pressure. 



walls of a porous cup. ^^^^^ ^^ pressure, what we are dealing 



with is a type of (osmotic) activity, or tension. Osmotic pressure 

 is that pressure which must be applied to a solution confined by 

 a semipermeable membrane in order to prevent the entrance of 

 water. This pressure is the osmotic pressure of the solution, but 

 no pressure is developed until water has entered, and this pressure 

 is not the osmotic pressure but turgor. 



Turgor is a distending force; it is the pressure developed within 

 an osmotic sack and rarely equals the potential osmotic pressure 

 of the confined solution. The movement of the water is termed 

 osmosis; it is the excess diffusion of water from the pure-water 

 side to the solution side (under one atmosphere of pressure and 

 at the same temperature as the solution). 



