Question

1. why is the resting membrane potential different than the equibilirium potential?

2. what ion/compound counters the ion to establish the final resting potential? why does it not move down its concentration gradient due to diffusion?

Answer 1

1. Resting membrane potential differ than the equilibrium potential:-

The difference between the membrane potential and the equilibrium potential (-142 mV) represents the net electrochemical force driving Na+ into the cell at resting membrane potential.

At rest, however, the permeability of the membrane to Na+ is very low so that only a small amount Na+ leaks into the cell.

The ratio, r, is negative since sodium and potassium ions are pumped in opposite directions. ... Note that the value of the resting membrane potential is closer to the value of the potassium potential. Thus, a greater driving force is needed for the influx of sodium ions across the membrane.

2. Ion / compound counters the ion to establish the final resting potential:

potassium
The typical resting membrane potential of a cell arises from the separation of potassium ions from intracellular, relatively immobile anions across the membrane of the cell.

Why does it not move down its concentration gradient due to diffusion?

For example, sodium ions are present at 143 mM outside the cell and 14 mM inside the cell, yet sodium does not freely enter the cell because the positively charged ion cannot pass through the hydrophobic membrane interior. ... In this case, sodium must move, or be pumped, against a concentration gradient.

A concentration gradient is the difference in the concentration of a substance of one place/area compared to another area. To have a molecule diffuse down its concentration gradient, means that the molecules moves from hypotonic areas to the hypertonic areas of concentration.

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. Brownian motion will cause the molecules (or ions) to spread/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space.

The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport.