Cell Membrane Potential

Question 1

Describe in your own words what you understand by the term “Equilibrium Potential”.

Answer 1

Equillibrium potential is the charge of a membrane when the concentration of a specific solute is zero.

Question 2

What contribution to membrane potential is made by the K+ ion?

Answer 2

K+ ions contribute the greatest charge to a membrane due to high permeability affinity. K+ will travel down conc.grad until the electrochemical gradient reaches equillibrium.. As intracellulaar conc. of cell proteins (-vely charged) is higher in the intracellular environment, the movement of the K+ ion out of the cell creates a negative charge inside the cell. The K+ equillibrium potential is reached at a memebrane potential of -90mV with 150mMol K_ in ICF and 5mMol ECF.

Question 3

What contribution to membrane potential is made by the Na+ ion?

Answer 3

Na+ moves into the cell down conc.grad bringing +ve charge into ICF and -ve charge in ECF. Cell charge becomes more posistive creating greater force to slow and eventually cease Na+ movement. At equillibrium, the membrane potential = +60mV w/ 150 ECF and 15mMol ICF

Question 4

What contribution to membrane potential is made by the Na+-K+ pump?

Answer 4

NaK+ Pump takes 3Na+ out of the cell and puts 2K+ into the cell against their concentration gradients with energy released from the hydrolysis of ATP. This movement of molecules establishes the gradients necessary for K+ and Na+ to establish membrane potential, It pumps more cations out of the cell than bringing in therefore it causes a negative cherge inside the cell, This generates 20% of the memebrane potential.

Question 5

Why is the membrane potential closer to EK+ rather than the ENa+?

Answer 5

The resting membrane potential is closer to EK+ as the membrane is 50-75x more permeable to K+ than it is to Na+

Question 6

How is the Nernst equation useful for experimental purposes when investigating the distribution of ions across the cell membrane?

Answer 6

The nerst equation allows the calculation to determine resting membrane potential of a balanced equillibrium state of molecule at specific concentration.

• If measures and predicted values are same, equillibrium exists.
• If marked difference, equillibrium state not in effect and energy is being expendd to maintain non-equillibrium state.