128 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



If cation transport across the red cell membrane requires a chemical reac- 

 tion between ion and some membrane reactant, the process will not, in general, 

 follow the first order kinetics which probably characterize unidirectional dif- 

 fusion in this system, but will rather show some type of 'saturation' kinetics. 

 At high concentrations of the ion in the medium, for example, the influx will 

 become limited by the concentration of some reactant in the membrane. The 

 kinetics of such a system will be described by equations similar to those formu- 

 lated by Michaelis (62), Van Slyke (115), Segal (91) and others. 



Temperature Coefficient. It is well known that the apparent activation 

 energy for free diffusion in aqueous solutions is about 4000 to 5000 calories/ 

 mole. Therefore, it is often assumed that demonstration of a high apparent 

 activation energy is evidence against the importance of diffusion as the mech- 

 anism of transport. While this sometimes may be valid, there are enough 

 exceptions to minimize the usefulness of temperature data in evaluating the 

 mechanism of transport (13). For example, the apparent activation energy for 

 diffusion of sodium into a 27% cross-linked cation exchanger can be as high 

 as 8560 cal/mole (98). Furthermore, transport of erythritol into human red 

 cells, presumably by diffusion, has a Qio of 2.5 (47), or an apparent activation 

 energy of about 12,500 cal/mole. 



Since the activation energy of most chemical reactions is rather high, ion 

 transport involving such a process will probably have a high temperature co- 

 efficient. However, even this deduction is equivocal. In a complex system in 

 which many chemical reactions of different activation energies occur, a reduc- 

 tion in temperature may increase the concentration of a reactant which is rate 

 limiting for ion transport, thus yielding a relatively low apparent activation 

 energy. 



Competition Between Ions for Transport. In the case of diffusion of an ion 

 through a very porous membrane, competition between different diffusing 

 ions will not play a prominent role in the transport process. However, cation 

 diffusion through the red cell membrane, if it occurs, is very slow as compared 

 with free diffusion in dilute aqueous solutions. This restriction on cation dif- 

 fusion may be due to several factors, e.g. the electrical potential distribution 

 in the membrane, variations in the activity coeflficient of the ion in the mem- 

 brane, or to steric factors. If the addition of some other ion to the outside 

 solution modifies any of these factors, some type of competitive kinetics may 

 result. Without further knowledge regarding the structure of the red cell 

 membrane, therefore, it is not possible to make sure deductions regarding 

 cation diffusion across the red cell membrane from competition experiments. 

 Nevertheless, in some cases the assumption that cation transport across the 

 red cell membrane by diffusion involves no competition yields results which 

 are consistent with other methods of estimating the role of diffusion in cation 

 transport in a given system (107, no). 



