Outline – 1, BIO 3360, Membrane Potential
I. ECF (Extracellular Fluid) and ICF (Intracellular Fluid) show no electrical potential (0 mV) when examining them away from cell membrane
II. An electrical potential exists across the plasma membrane
III. All cells are polarized
-The value of the potential varies from cell to cell but they are all polarized.
-Negative inside, positive outside.
-The sign of the potential is referred by convention to that of the inside (e.g. -70 mV)
-Positive and negative charges accumulate on either side of the plasma membrane because the lipid bilayers can maintain separation of oppositely charged ions
IV. Why are cells polarized?
–If membrane is permeable to a certain ion, equilibrium is reached . For example, if inside of the cell has a concentration of .1M KCl and outside of the cell has a concentration of .01M KCl, AND the membrane is permeable to K+, the K+ will move from inside of the cell to outside, creating a region of electronegativity along the inner face of the membrane as it leaves, and a region of electropositivity along the outside of the membrane where it enters. The negatives inside the membrane create an electrical magnetic force that draws positives back into the cell. Eventually inward and outward movement of K+ balance each other.
A. Concentration gradients contribute to membrane potential
B. Permeability characteristics contribute to the membrane potential
C. Normal cells have higher concentrations of K+ inside and Na+ outside of the cell
D. Electrical potential– charge difference on either side of cell membrane measured in millivolts
V. Equilibrium potential– the potential difference across the membrane under equilibrium conditions. If only one ion is involved, the equilibrium potential equals the resting membrane potential.
A. Nernst equation –used to calculate the equilibrium potential for a given ion.
– Given a chemical gradient of ion X , the equilibrium potential Ex , can be calculated as:
Ex = (RT/ Fz ) ln [ X]out / [ X]in
– z is the charge of the ion
– R and F are constants. T is temperature in K
NERNST EQUATION SIMPLIFIED: Ex in millivolts = +/- 60 log [X]in / [X]out
at normal body temperature for univalent ions such as K+. – if it is a positive ion and + if it is a negative ion
-The more permeable the membrane is to an ion, the closer the resting membrane potential is to the equilibrium potential
-Potassium plays the major role in establishing the membrane potential because the resting membrane is not permeable to sodium
-A modification of the Nernst equation is the Goldman equation used to calculate resting membrane potentials based on CONCENTRATIONS and PERMEABILITIES of the ions.
B. Role of sodium in establishing resting membrane potential
C. Role of Na-K pump (= Na/K ATPase) in establishing resting membrane potential