WEEK 5: THE MEMBRANE POTENTIAL Flashcards
What is a membrane potential?
What units is it measured in?
The membrane potential is the difference in electric charge between the interior and the exterior of a cell.
Separation of opposite charges across the membrane
Difference in the relative number of cations and anions in the ICF and ECF’
Measured in mV.
Between the interior and exterior of the cell, which one is more negative?
In any cell the potential will be negative inside with respect to the outside
Describe the membrane permeability to sodium and potassium.
What is the concentration difference of potassium across the membrane?
The membrane is much more permeable to potassium than sodium
There is a considerable concentration difference of potassium across the membrane (150:5)
Describe the effect of K+ alone on the membrane potential.
*Potassium will move out of the cell down its concentration gradient (=chemical gradient)
*This will result in a negative charge inside cell
*This (-ve) charge will tend to prevent the potassium leaving the cell (=electrical gradient)
*Concentration difference and electrical potential difference will come to an equilibrium= equilibrium potential
State the Nernst equation for calculating equilibrium potential for an ion.
E= 61/ z log Co/C1
Where ;
z= ion’s valence is 1for sodium and potassium.
Co= concentration of the ion outside the cell
Ci= concentration of the ion inside the cell.
Describe the effect of sodium ions alone on the membrane potential.
1.The concentration gradient of sodium ions move it to the inside of the cell
2.The inside of the cell becomes more positive as sodium ions move in.
3..There is negative charge left on the outside of the cell
4.The resulting electrical gradient tends to move Sodium ions out of the cell.
5.The electrical gradient will counterbalance the inward concentration gradient. ( Equilibrium concentration for Sodium ion, +60mV)
6.The membrane potential is at equilibrium and no ion movement.
What is the equilibrium potential for Potassium ion and sodium ion?
Potassium ion= -90mV
Sodium=+60mV
What is the value of the resting membrane potential?
-70mV
Describe the effect of concurrent Potassium and Sodium ion movement on establishing membrane potential.( How the membrane potential is established)
- The sodium -potassium pump actively transports 3Na+ out of the cell and 2K+ into the cell.
2.The potassium ion tries to drive the The membrane potential towards the equilibrium potential for K+ while the Na+ ion tries to drive the membrane potential towards the equilibrium concentration for Na+.
3.The K+ will exert a dominant effect as the membrane is more permeable to K+. As a result, the resting ,membrane potential will be -70mV which is closer to the equilibrium concentration for K+.
State the Goldman-Hodgkin equation for calculating membrane potential.
Vm= 61 log Permeability for Na+ and K+ outside the cell/ Permeabilities for K+ and Na+ inside the cell.
Where:
Vm = membrane potential
Describe the role of Na+ Pump in RMP.
How much voltage contribution does it make to the RMP?
The Na pump makes very small direct contribution to the Resting Membrane Potential (1 to 3mV)
This is through its UNEQUAL transport of ions
3 Na ions out & 2 K ions in
State the role of Cl- in RMP.
In summary, all other ions including Cl- do not contribute to the RMP
Summary
Plasma membrane of all living cells has a membrane potential (polarized electrically)
There is a separation of opposite charges across plasma membrane (= membrane potential)
Due to differences in concentration and permeability of key ions (= origin of RMP)
The Na pump plays a small role in the origin of the resting membrane potential
