Cell Biology: Membrane Potential

Electricity and Cells

• When ions move, there is electrical current
• Voltage: caused by ions actively transported across membrane to make charge gradient
    * Differences in charge across membrane

Electrical Properties

• All membranes have voltage across them
• Can measure using electrode
    * Voltage inside the cell - voltage outside the cell is the resting membrane potential
    * About - 70 mv

Membrane Potential

• -60 mv to -90 mv in most animal cells
• Three factors
    * Tendency of chemicals to move from high to low concentration
    * Tendency of ion to be repelled by like charges/attracted to opposite
      * Electroneutrality
    * Composition of cations and anions different in cytosol and extracellular fluid
      * Cytosol: high K, low Na
      * Extracellular: low K, high Na
• Takes work to separate charges
    * Much of the work done by sodium potassium pump
• Magnitude and direction (sign) of voltage 
    * Two factors
      * Relative ability of different ions to cross membrane
      * Electrochemical gradient across membrane
• Membrane potential happens when force pushing ions from high to low is balanced by electrical force pulling on ions to keep it in place
    * Chemical gradient: high to low
    * Electrical gradient: negative attracted to positive
• Maintaining Electrochemical gradients 
    * K+ move out of the cell
      * K+ leak channels
      * Some K+ can exit the cell
      * K+ doesn’t move out much because it is attracted to the negative fixed anions in the cell
        * Once a lot of + leaves the cell, there is a lot more negative inside which prevents more + from leaving
    * Na + move into the cell
      * Little or no Na + can enter through channels bc usually closed
• Nernst Equation
    * Vm = RT/zF * ln [Xo]/[Xi]
      * Vm = equilibrium potential because it is when delta G = 0
        * Can also be Ex 
          * Equilibrium potential for ion x
      * Will get volts
    * Only looking at movement of one ion
• Equilibrium Potential
    * Tells us what membrane potential will be for a particular ion moving
      * And which way ion will flow if membrane becomes permeable to it
    * Tells which ion is primarily responsible for determining RMP
      * Compare eq potential of ions to measured MP
        * One closest to Vm is most permeable ion
    * Why would calculated eq and measured be different?
      * In reality, multiple ions determine MP
      * Cells are not at equilibrium 
• Goldman Equation
    * Vm=RT/F*ln((Pk)[Ko] + (PNa)[Nao] + (Pcl)[Cli)(Pk)[Ki] + (Pna)[Nai] + (Pcl) [Cl o])
      * Takes into account multiple ions