42 DAVID R. BRIGGS 



3. Importance of Osmotic Pressure Determinations 



The determination of the osmotic pressure of solutions has two 

 realms of primary usefulness in biophysics. The first is in plant and 

 animal physiology, where the osmotic pressure is of chief interest and 

 importance because of its capacity to influence the distribution of 

 fluids and solutes, to which the membrane may be permeable, across 

 semipermeable membranes in the organism. Where isolated cells or 

 tissues are being subjected to studies while immersed in solutions, it is 

 usualh^ essential that the surrounding solution be of the same osmotic 

 characteristics as the fluids that bathe them in their normal habitat. 

 When fluids are to be injected into the blood stream, intercellular 

 spaces, or into the cells themselves, a mininuim of discomfort or shock 

 to the organism will occur when the osmotic properties of the solution 

 approximate those of the fluids in the locus of injection. It is neces- 

 sary for the biologist to know, with a fair degree of approximation, 

 the osmotic pressure of the extra- and intracellular fluids of the speci- 

 men with which he is working. Because the body fluids usually con- 

 tain a variety of osmotically active solutes for which a semipermeable 

 membrane would be difficult to obtain, osmotic pressures are seldom 

 measured directly in such cases, but are calculated from measurement 

 of the freezing point depressions, the vapor pressures, or a complete 

 chemical analysis of the solutions under consideration. 



The quantity osmotic pressure is commonly employed in physiology 

 to express numerically the difference in degree to which the activity- 

 of the solvent may in any particular case be maintained across cellu- 

 lar or tissue membranes in the body and as a measure of the concen- 

 tration work (5) required to bring about this difference. When solu- 

 tions of imequal composition exist in dynamic equilibrium on the two 

 sides of a cellular or tissue meml>rane, it can general!}^ be assumed 

 that the membrane has performed work in bringing about and main- 

 taining this difference. The minimum amount of work required can 

 be calculated as the summation of the concentration works or minimum 

 free energy changes for all the components of the two solutions, and 

 will be given by the expression : 



W,nin = ^nUT In (C'b/C'a) 



where Cb and ('a icfer to the concentrations (more accurately, the 

 activities) of any individual component in solution B (taken as the 

 final solution) and solution A, respectively, n is the number of moles 

 of the component transferred from solution A to B, R is the gas con- 



