Membrane Potentials

Question 1

Which channel is responsible for establishing the electrochemical gradient across cell membranes?

Answer 1

The Na/K ATPase pump

Question 2

What does the Na/K ATPase pump do to sodium and potassium with regards to their concentration gradients?

Where is the concentration of each of these larger, intracellularly or extracellularly?

Answer 2

It moves them against their concentration gradients.

[Na] = highest extracellularly

[K] = highest intracellularly (think burns patients)

Question 3

What ratio of Na molecule efflux to K molecule influx does the Na/K ATPase pump achieve?

Answer 3

X3 Na out for every X2 potassium in

Question 4

What is the difference between the membrane potential or membrane voltage and the electrical status of the cell/system as a whole?

Answer 4

As the cell membrane is selectively permeable to K, it leaves the cell due to the concentration gradient (more K inside than outside the cell) formed by the Na/K ATPase pump. As it does so, it leaves uncompensated negative change on the inside of the cell in the form of chloride ions. These opposing charges, however, line up on either side of the cell membrane and therefore are not free to interact with other ions or ‘float’ free on their respective sides. There is a charge difference across the membrane (RMP) but due to the ions lining up across the membrane the intracellular and extracellular solutions remain ELECTRICALLY NEUTRAL yet the resting membrane potential DECREASES.

This process happens in reverse with sodium ions however the membrane is&raquo_space; more permeable to potassium which results in a overall negative RMP.

Question 5

What is the formula for calculating out the concentration gradient of a positive ion?

Answer 5

Gradient = [Conc (outside of cell)] / [Conc (inside of cell)]

Question 6

What is the formula for calculating out the concentration gradient of a negative ion?

Answer 6

Gradient = [Conc (inside of cell)] / [Conc (outside of cell)]

Question 7

Why does ion movement eventually reach equilibrium?

What are the two forces involved in this?

NB: use K over Na for this example.

Answer 7

Because the force causing K to leave due to the concentration gradient is equal to the force pulling K back into the cell due to negatively charged ions inside the cell.

The CHEMICAL FORCE = ELECTRICAL FORCE

Question 8

What is the equilibrium potential of an ion and what is its symbol?

Answer 8

The equilibrium potential (E) for an ion is the membrane voltage a cell needs to have in order to stop the ion moving down its concentration gradient.

Question 9

What is the Nernst equation and what does it allow us to work out and from what.

Answer 9

The Nernst equation allows you to calculate the equilibrium potential from the concentration gradient of an ion.

The formula (for a positive ion) is:

E = [(R x T) / (Z x F)] x ln [Conc (outside of cell)] / [Conc (inside of cell)]

R = gas constant
T = temp
Z = ion charge
F = faraday’s constant

Question 10

How does the cell membrane act as a capacitor?

Answer 10

By storing the charge created by the increased distance between the positive and negative ions across the cell membrane.

Question 11

How can the Nernst equation be reduced for a monovalant ion?

Answer 11

R.T/Z.F becomes 58(mv)

Therefore:

E = 58 x ln [Conc (outside of cell)] / [Conc (inside of cell)]

Question 12

What are the intracellularly and extracellular concentrations of sodium?

Answer 12

Plasma = 140mol/L

Intracellularly = 10mmol/L

Question 13

What are the intracellularly and extracellular concentrations of potassium?

Answer 13

Plasma = 4mmol/L

Intracellularly = 120/140mmol/L

Question 14

What is shorthand for resting membrane potential?

Answer 14

Vm

Question 15

What do ENa and EK mean?

Answer 15

The equilibrium potential for sodium and potassium respectively.

Question 16

What is ENa?

Answer 16

+50mv

Question 17

What is EK?

Answer 17

-90mV

Question 18

Why is the Vm closer to EK than ENa?

Answer 18

As the membrane is X50 more permeable to K than Na.

Question 19

At a constant Vm in a quiescent state cell, what is the net flow of ions?

Answer 19

Zero

Passive Na leak = passive K leak

Question 20

What happens to the Vm if a cell becomes permeable to an ion?

Answer 20

The ion moves down its electrochemical gradient and will drive the Vm to that ions own E (equilibrium potential)

Question 21

What is the formula for the driving force of an ion?

Answer 21

Vm - E(eq)

NB: E(eq) means E for that ion

Question 22

What is the driving force for K?

Answer 22

Driving force = Vm - Ek

= -70 - (-90)
= +20mv driving K out of the cell

Question 23

What is the driving force of Na?

Answer 23

Driving force = Vm - Ek

= -70 - (+50)
= -120mv driving Na into of the cell

Question 24

The driving forces may be unequal, but why is the net flow of ions still?

Answer 24

Due to the membranes increased permeability to K, therefore less of a driving force is needed to move the ion.

Question 25

Permeability vs conductance.

What are they and what determines both?

Answer 25

Permeability
= the ease at which you can move ions across a membrane
= determined by the number of open ion channels

Conductance
= the amount of charge (no. of ions) which cross the membrane
= is a function of permeability (no. of open ion channels) AND the concentration gradient of the ion

Question 26

What is the name of the equation which builds on the Nernst equation to provide a more accurate estimation of the resting membrane potential (Vm) by taking into account the relative permeability of the membrane when combining ions?

Answer 26

The Goldman-Hodgkin-Katz equation

Question 27

What is the Goldman-Hodgkin-Katz equation?

Answer 27

See FBS1 PPP1 folder - could not be typed easily.

Basic principles
= both K and Na ions are combing (e.g. the extracellular ion concentrations for both are on the top and the intracellular for both on the bottom of the fraction section of the equation)
= each concentration is multiplied by its pk (permeability of the membrane to that ion) to account for the fact this is unequal for the two ions.