Crafts et al. — 30 — Water in Plants 



of the free energy : water content curve and hence separate a region where 

 energy change with change of moisture is low from one where energy 

 change is high. The changes through these regions however are smooth 

 and continuous ; they are quantitative rather than quaHtative. Evidences 

 for breaks or plateaus on such curves usually involve changes of state 

 (sol ^ gel etc.) or very low water contents. 



Early attempts to explain deviation of observed osmotic pressure meas- 

 urements from ideality involved association of the solvent and hydration 

 of the solute. The graph for observed osmotic pressure curved rapidly 

 upward at high sugar concentrations whereas the law of ideal solutions 

 followed a straight line. Differences between the curves represent losses 

 of water activity that could as well have been attributed to water binding. 

 These losses result from intermolecular forces characteristic of the solute 

 and solvent. Of the various methods for measuring hydration the vapor 

 pressure method should excel for it integrates these forces and gives a 

 true picture of water activity in the system ; it also avoids pitfalls such as 

 super-cooling in the freezing methods and molecular interaction where 

 a reference solute is used. Chandler (1941) has obtained accurate re- 

 sults by this method. 



Greenberg (in Schmidt, 1938) states " — hydration does not appear 

 to be an important factor in determining the properties of protein solu- 

 tions." Bull (1943), on the other hand says "bound water may or may 

 not contribute greatly to the understanding of physiology and pathology, 

 but its importance for the understanding of protein reactions is extreme." 

 Greenberg along with many others has pointed out the shortcomings of 

 the various methods for determining bound water. If these methods are 

 inadequate for measuring the amount of bound water in a protein how 

 much less adequate are they for indicating its importance. 



In conclusion it should be emphasized that the differences in degree of 

 hydration that have been measured and that surely exist between different 

 biological systems reflect differences in the intensity of binding forces and 

 in the packing and configuration of the molecules of solvent and solute that 

 these forces control. The nature of the forces and of the water molecules 

 doesn't vary ; the differences in state between the substrates however are 

 real and important. They typify the differences that exist between water 

 in the beaker, solutions as they occur in cells, and protoplasm, seat of the 

 complex and baffling activities of the living organism. 



Mobility of Ions: — The migration of ions and molecules in aqueous solution is of 

 great importance to biology. Much research has been directed toward problems of 

 permeability and solute uptake by living organisms. 



A perplexing problem in ion mobility is the high migration velocity of hydrogen 

 and hydroxyl ions. Velocities of K'^ and NH4"^ are quoted as 6.7 X 10'^ F cm. per sec. 

 where F =: the potential gradient in volts per cm. Those of H* and OH" are 32.5 X 

 10-1 p cm. per sec. and 17.8 X 10"* F cm. per sec. respectively. Bernal and Fowler 

 explain these high velocities by a mechanism involving a proton jump from one water 

 molecule to another when favorable configurations are presented. Such a mechanism 

 presupposed an appreciable degree of organization of the structure of water. Much 

 of modern physical chemistry involves the types and strengths of chemical bonds. The 

 structure of water as related to such bonds, and the forces between ions and water are 

 all concerned in ion mobility. Also involved are the membranes encountered and the 

 interfaces between them. It is evident from this discussion that the problems of the 

 mobility and uptake of solutes by plants are exceedingly complex when viewed from 

 the standpoint of molecular kinetics as exemplified by the process of diflfusion. When 

 one adds the metabolic activities of the growing cell to this picture it is easy to under- 

 stand the difficulties encountered in the study of plant nutrition. The material presented 

 in this chapter indicates the progress that is being made in the study of solutions. It 



