Membrane Potential Help?

[jamesac]

Hi there, I am currently learning about membrane potentials in a physiology class, but I'm having a little trouble. I understand that differences in concentration of an ion creates a chemical gradient, and that as these ions move across the membrane, an electric potential is created. The electric potential that exactly counterbalances the chemical force is the equilibrium potential. That much I'm okay with.

 

We went over the Goldman Equation (as seen in the attachment I have provided), and the key takeaway here is that membrane potential is mostly affected by those ions which are most permeable. This makes sense when thinking about the fact that if ions have the ability to move they can change the electric potential of a membrane. What I'm confused about though, is why, according to this equation, non-penetrating particles have zero effect on membrane potential. If there was a hypothetical cell with 1 million negatively charged non-penetrating ions inside of it and 1 million positively charged non-pentrating ions outside of it, this would clearly create an electric force pulling positively charged particles inside the cell. But according to this equation and what I've been told in class, only the non-penetrating ions have an effect on membrane potential. I was hoping someone here might be able to point out the fault in my logic and help me understand this better. Any help is greatly appreciated, thanks!

 

[Function]

Hmm this has been quite some time ago, and I'm not sure if my explanation is correct, but if I had to give it a shot, I'd blame "potential" for it, which can only be evoked by particles able of creating a gradient and moving from one side to the other. Since non-permeable particles (most proteins are negatively charged yet cannot pass through the membrane) can't 'move', they can't create a gradient and are quite static, thus not contributing to a dynamic potential.

 

Implementing particles that don't contribute to a change in potential would be quite useless. The contribution of other molecules and particles other than your charged ions to which the membrane is permeable is so small that it doesn't even matter.

Edited January 19, 2017 by Function

[jamesac]

^Thank you for the reply! I understand that permeable ions would be the only ones capable of creating a change in membrane potential, since they are the charged particles moving across a membrane and therefore changing the charge on either side. I guess I'm just having a hard time understanding how non-permeable ions don't at least contribute to the membrane potential since they would affect the electrical gradient. It could be that the contribution of the other particles is small enough like you noted, that it doesn't really matter. It's easy enough for me to memorize what happens, but actually understanding how this works will help me to better recall it in the long run.