Membrane Potential I

Question 1

The electric force is much, much stronger than the ______,

Answer 1

osmotic force

1018 times greater

Question 2

Ions crossing a membrane are driven by both _________.

Answer 2

an electrical force (strong) and a concentration gradient (weak).

Question 3

Like charges repel and opposite charges attract, so a negatively charged Cl- ion diffusing across the cell membrane will be repelled by _______ but will also be attracted by __________.

Answer 3

the internal negativity of the cell

the lower concentration of Cl within the cell

Eventually these forces will reach an equilibrium whereby for every Cl entering the cell, another leaves. Together these two forces make up an electrochemical gradient.

Question 4

Equilibrium potential is the

Answer 4

electrical potential difference across the membrane that must exist if the ion is to be a equilibrium. It relates a concentration gradient to an electrical force.

Question 5

Equilibrium potentials are:

Answer 5

what the voltage would be for a particular ion to be at equilibrium across a membrane

Question 6

Membrane potential (Vm) is a

Answer 6

real voltage that exists across a membrane. It is a reflection of the equilibrium potentials of all the ions and their given equilibrium state across a membrane.

Question 7

If Vm is not the same as a given ion’s equilibrium potential, then

Answer 7

the membrane is either impermeable to that ion, or that ion must be pumped across the membrane.

Question 8

Equilibrium potential is ultimately dependent on

Answer 8

concentration (see the Nernst equation) while Vm is dependent on the electric force.

Question 9

Every ion in solution has its own equilibrium potential based solely on

Answer 9

its concentration inside and outside the membrane.

Question 10

nernest equation

Answer 10

E= 60 log10/Z (Concentration outside)/ Concentration inside

Question 11

If the membrane potential equals the equilibrium potential, then

Answer 11

you are done! There is no pump and the ion is at equilibrium.

Question 12

If the membrane potential does not equal the equilibrium potential then _______. The sign is determined by _________.

Answer 12

the ion must be pumped across the membrane because it is not at equilibrium

figuring out which way the ion wants to flow to make Vm equal equilibrium potential, and thus what way the ion leaks

Question 13

The ions must be pumped out the other direction from

Answer 13

the direction of leak (i.e. if they leak into the cell, the must be pumped into the ECF).

Question 14

If the membrane is impermeable to both Na+ and Cl-,

Answer 14

nothing will happen and the cell will survive.

Question 15

If the membrane is permeable to both Na+ and Cl-,

Answer 15

they both will diffuse into the cell down their respective concentration gradients. This will increase internal osmolarity, water will enter the cell, and the cell will swell and burst.

Question 16

This electric force is about ___times more powerful than the osmotic force!

Answer 16

10^18

Question 17

1. The passive movement of an ion across the membrane is governed by two forces:

Answer 17

its concentration difference and the electrical potential difference across the membrane (the membrane potential). The two forces, combined, make an electrochemical gradient.

Question 18

2. Membrane potentials are produced by only one thing,

Answer 18

namely an imbalance in the number of cations and anions inside a cell.

Question 19

3. The electric force is very much more powerful than the diffusional force produced by a concentration difference, which means

Answer 19

that relatively few excess ions are needed to counter large concentration differences. it is very safe and appropriate to consider that the concentration of chloride in the cell does not change, or more generally, bulk solutions are always electrically neutral.

Question 20

equilibrium potential for the ion

Answer 20

it relates a concentration gradient to an electrical force

Specifically, it is the electrical potential difference across the membrane that must exist if the ion is to be at equilibrium at the given concentrations.

Question 21

membrane potential, Vm, and can be measured by

Answer 21

impaling the cell with a microelectrode and measuring the voltage difference between the tip of microelectrode inside the cell and a reference electrode outside

Question 22

What if Vm is not the same as ECl?

Answer 22

a) the membrane is impermeable to Cl-,

b) Cl- must be pumped across the membrane, because it is not distributed at equilibrium. In this case, Cl- ions will perpetually diffuse in one direction, ‘down’ their electrochemical energy gradient, and an exactly equal number of Cl- ions will be pumped in the opposite direction.
So the Cl- concentration in the cell won’t change so long as the pump works.