■ 3 6 



HANDBOOK OF PHYSIOLOGY 



( !IR( I'LATION II 



primarily a characteristic of the membrane but is a 

 measure of some real difference between pure solvent 

 and solution. The membrane merely allows this 

 difference to show itself. 



The osmotic pressure, IT, of any solution is propor- 

 tional to temperature and concentration: 



II*kCT 



Accordingly, in dealing with a cell, we must con- 

 sider the osmotic pressure not only of the solution 

 inside the cell but also that of the environment which 

 bathes it. The cell contains an amount of nondif- 

 fusible material in solution which is essential to its 

 metabolism. Clearly, the amount of this material 

 which can be retained without causing the cell to 

 swell will be sharply limited unless there is also a 

 counterbalancing nondiffusible material outside its 

 membrane. This is perhaps the major niche into 



o 7 



o 3 



I 



(No) +- 5-7 



(X 100) 



02 



06 



10 



I -A 



IB 



pig. I. The dependence of sartorius weight on [Na + ] of 

 the medium (24-hour immersion at 3 C). Closed circles : stepwise 

 NaCl reduction replaced by KC1. Crosses: stepwise NaCl reduc- 

 tion not replaced by K.C1. Open circles: stepwise NaCl reduction 

 in the presence of cyanide (2 X io -3 m). Volume control at 

 equilibrium depends on [Na + ] even in the presence of cyanide 

 and is independent of [K + ] above maintenance level (20 

 meq/liter). [From Conway (31). 



which evolution has fitted the sodium ion. Cell 

 membranes are almost impermeable to this ion and, 

 since the cellular nondiffusible material is almost 

 constant, the amount of cellular water is controlled 

 In variations in extracellular sodium, Na . Conway 

 (31) has demonstrated this point by showing, for 

 example, that K. concentration in the medium, K„, 

 can be varied over wide limits without influencing 

 the basic dependence of cell volume on Na<, after 

 equilibration (fig. 1 ). This essential point is often 

 overlooked in experiments dealing with alterations 

 of the medium. 



The development of osmotic pressure is one of the 

 colligative properties of solutions. Ideally it is de- 

 termined by the number of particles in solution : 



/7i/--/?/?r 



Its expression, however, depends on whether or not 

 the membrane is permeable to the particle in question. 

 At equilibrium, no osmotic pressure is exerted by 

 a particle to which a membrane is freely permeable. 



ion size. Ions may be defined as particles which have 

 gained or lost an electron on passing into solution. 

 The elements of Group 1 of the periodic table, the 

 alkali metals Li, Na, K, Rb, and Cs, are all charac- 

 terized by possessing a single electron in their outer 

 orbital shell. In solution this is lost so that the element 

 loses its electroneutralitv and remains positively 

 charged as Li + , Na + , etc. Other elements, like those 

 in Group 7, take up one or more electrons into their 

 outer shell and so become, in solution, negatively 

 weighted, e.g., Cl — , Br - , I , etc. The formation of 

 ions is not restricted to elements but also occurs with 

 more complex radicals which can collectively gain 

 or lose one or more electrons, e.g., OH - , XH 4 + , 

 etc. 



Ions share the ordinary colligative properties of 

 substances in solution as, for example, freezing point 

 depression, osmotic pressure, etc. Additionally, they 

 possess a number of special properties based on the 

 fact that they are electrically charged. 



We must distinguish here the size of the ion con- 

 sidered as a solid ball, so to speak, and defined by its 

 crystal radius, from its size when associated with 

 water molecules and defined by its hydrated radius. 

 The increase in size of the monoatomic crystals of 

 Group 1 falls naturally into the same order as their 

 periodic arrangement Li < Na < K < Rb < Cs. 



For many years it has been the accepted practice 

 to emphasize the hydrated ion in biological systems. 



