Online Lab - Virtual Membrane Potentials

Reducing the concentration of KCl in the ECF created a concentration gradient for K+ and Cl- to move across the membrane

This caused K+ ions to flow out of the cell resulting in a more negative charge inside the cell

Ions move by diffusion from high to low concentrations

Only a small amount of ions need to move to generate a membrane potential of ± 100 mV.

The concentrations of ions don’t significantly change under most conditions.

the combination of a membrane permeability for K+ and a transmembrane KCl concentration gradient was all that was needed to generate a negative membrane potential

as the concentration gradient was larger, the membrane potential got larger

Throughout the experiment, ICF concentration did not change much

Ions will flow across the membrane until they are at electrochemical equilibrium

Electrical force always acts according to the law that opposite charges attract while like charges repel

If the membrane is only selective for one ion, the membrane potential reaches an equilibrium (at the Nernst potential) where the chemical and electrical forces are equal.

Permeant ions will continue to flow across the membrane until they are at electrochemical equilibrium

when there is an ion concentration gradient and a membrane selectively permeable for an ion, a membrane potential is generated

By changing the membrane selectivity, cells can change the membrane potential.

Changing membrane selectivity is done by opening and closing specific ion-selective channels in the membrane (ion channels)

Goldman-Hodgkin-Katz (GHK) equation to quantify relative membrane permeability (as done for the membranes below) or to predict a membrane potential when more than one ion passes across the membrane

Membrane potential = the equilibrium or Nernst potential when only one ion permeates

Note that at rest, the membrane potential does not equal the K+ equilibrium potential. So K+ is not at equilibrium, and a small force exits that moves K+ out of the cell. But the membrane potential doesn’t change – it is at a “steady state”.

The K+ efflux is exactly balanced by Na+ influx.  There is a high membrane permeability for K+ but a small electrochemical force for efflux, combined with a low permeability for Na+ but a high electrochemical driving force for Na+ influx.