STRUCTURE OF NERVE CELL MEMBRANES 165 



particular instance, that of the theory proposed by Boyles and Conway, the 

 actual size of the hole was supposed to be such that it determined which ion was 

 able to go through. Now, does this sieve idea apply here to the model that Dr. 

 Mullins has presented? I think this is where I would like to bring in our fixed 

 charge hypothesis. 



If the different-sized, hydrated ions go into the cells through these channels 

 or pores and the permeability depends upon how large they are and how large 

 the sizes of these holes are, then a large size, charged particle like Na— I mean 

 hydrated charge particle — would certainly have a tendency to block the holes 

 more than a smaller one. So if you study the rate of entry of one charged parti- 

 cle, let's say, K 42 , then the rate of entry of K 42 should be blocked more by the 

 larger sodium than, say, by a nonhydrated potassium. This is on the supposition 

 that the number of holes are limited and that they are the rate limiting factor 

 in the rate of entry of ions. 



On the other hand, if you draw out the hypothetical charged size on one side 



of these triangular holes, then this applies almost equally well as a model for 



our thesis. The idea is to say that these proteins are highly charged and you can 



get some average value of the charge density by taking into account the total 



quantity of charged particles in the total amount of protein in the cell. I think 



Dr. Morales and Dr. Botts have calculated such an average distance between 



each of the charged particles if they are uniformly distributed and we had also 



made similar calculations. I think probably they would agree that something 



like 20 angstrom units would be the proper calculation. If you take that into 



account, then you arrive at a very interesting conclusion, because, if the surface 



is like the interior, filled with charged particles, then an externally placed ion 



will be subjected not only to a trans-membrane electrical field strength but it 



would also be subjected to the electrical field strength due to these ionized and 



charged particles. Now, their close proximity to each other makes it so that the 



electrical field strength, due to these charged particles, is extremely strong, so 



that an ion that would otherwise have entered the inside phase cannot do so 



directly but has to be attracted to these charges and then go in by disassociation. 



What I am saying now is that the charged particles cannot just go in blindly 



depending on their size, but their entrance has to be preceded by a phase of 



association with the fixed charges on the surface followed by disassociation. 



This is, of course, based on the simple model which is in agreement with Dr. 



Mullins' model except that I added these charges. 



A simple conclusion can be drawn from this because, if the fixed charges are 

 fixed in number, then the number of ions going in will be determined by the 

 number of fixed charges present. In this case you can write down an equation 

 which formally is analogous to the Michaelis-Menton equation for enzyme ac- 

 tion. More importantly, if you study two species of ions going in simultaneously, 

 then you will find that the one will affect the other in the rate of going in and 



