ESTABLISHMENT OF CONCENTRATION DISTRIBUTION & MEMBRANE POTENTIAL

A. Electrogenic 3:2 Na-K-ATPase Pump

1. Powered by the energy stored in cellular ATP

2. Every cycle of the active transport pump moves 3 Na+ out of the cell and 2 K+ ions into the cell; this results in

a) intracellular K+ increase
b) intracellular Na+ decrease
c) net removal of 1 positive charge from the cell

3. Although this contributes to the negative intracellular potential, it is responsible for only a few mV of the resting potential

B. Donnan (or Gibbs-Donnan) Equilibrium

1. Intracellular fluid contains negative ions (e.g., proteins) to which the cell membrane is relatively impermeable, contributing to the excess of negative charges within the cell

2. Also contributes to the negative resting potential, but is not the major cause

C. P-K >> P-Na (combined with the concentration gradients)

1. Active transport of the Na-K-ATPase pump is followed by passive back diffusion; since the membrane is much more permeable to K+ than to Na+, potassium back diffusion predominates, causing the intracellular potential to become negative -- major cause

2. Eventually, the concentration and electrical gradients so developed lead to a steady state for K+, Na+, and Cl-