Understanding Membrane / Resting Potential from the perspective of ions?

Question

From wikipedia article RESTING potential: "there is no actual measurable charge excess in either side. That occurs because the effect of charge on electrochemical potential is hugely greater than the effect of concentration so an undetectable change in concentration creates a great change on electric potential."

the effect of charge on electrochemical potential significantly larger then the effect of charge on concentration - is this a general statement or true in all cases?

And if charge is more significant then concentration in influencing electrochemical potential why does a small change in concentration greatly effect electropotential? Or does it mean electrochemical potential? Either way how does it make any sense, is it correct or nonsense and what is the exact meaning behind the message?

other questions I have:

1) What is the reason Na+ stays at its location right across the barrier on the extracellular side of the membrane? Diffusion? Electrochemical potential? Electric Charge.. if so because of Attraction or Repulsion?

2) If the answer is the electrochemical gradient/potential, because it is trying to move to a "less" positive potential.. if its located extracellularlly isn't that the "great unknown" where any molecules/cells/protein etc. are able to float by and possibly influence it? What happens if one of these has a "LESS" positive potential then the membrane that Na+ is currently attracted?

3) Why can't Na+ be used for the entire process to establish the same voltage interactions leading from resting to action potential (as long as thier was less concentration intracellularly... if it is about LESS positivity ..Or vice versa for K+?)?

I am preparing for ACSM certification exam, so any help is appreciated.

Answer 1

Ah, what a classic biophysics problem.

One first needs to understand how a membrane gets a potential. The lipid bilayer is a large sea of hydrophobic interactions that essentially prevents any ion from crossing. As a result, Na⁺ and K⁺ concentrations remain constant and different on the cytoplasmic side and the extracellular side. However, ions can pass through ion channels like the K⁺ channel. It is important to understand that in K⁺ channels, only K⁺ can pass and these channels are actually selective against Na⁺ (answer to question 1).

There are two potentials at work here. First is a chemical potential created by the flux of K⁺ from high K⁺ to low K⁺. The second is a counteracting membrane potential created by a charge imbalance. Note, that the swapping of a few ions will a) result in a negligible change in the concentration ie. the chemical potential, b) result in a large change in the membrane potential. At some point, the flux out due to the chemical potential and the flux in due to the membrane potential will be equilvant and the cell will reach a resting potential otherwise known at the Nernst potential or equilibrium potential (technically a steady state).

When a cell depolarizes by closing these channels, the local charge will quickly go back to an equilibrium or a non-charged state.

So why K⁺ rather than Na⁺? For typical cells, the extracellular concentration of Na⁺ is 145 mM and cytoplasmic is 12 mM. For K⁺, it is 4 mM and 155 mM respectively. Doing the appropriate calculations of the Nerst potential, for Na⁺ it is +67 mV and for K⁺ it is -98 mV. Qualitatively we can see that this would result in vastly different things.

Most of this information can be found from Pollard and Earnshaw's Cell Biology