Membrane Potential II

Question 1

The Principle of Electrical Neutrality

Answer 1

bulk solutions (inside and out) have to be electrically neutral: 
the total cation concentration in the external solution must equal the total anion concentration in the external solution. 
The same holds for the internal solution: [cations]i = [anions]i.

Question 2

Donnan Rule:

Answer 2

[K+]o[Cl-]o = [K+]i.[Cl-]i

Question 3

Osmotic balance:

Answer 3

Osmotic balance: [K]i + [Na]i + [Cl]i + [A] = [K]o + [Cl]o + [Na]o

Question 4

Because the concentration of Na+ outside is higher than inside, the concentration gradient is directed

Answer 4

into the cell.

Question 5

Because the inside of the cell is electrically negative, the electrical gradient is

Answer 5

directed into the cell.

Question 6

given the opportunity, Na+ is going to

Answer 6

leak into the cell.

Question 7

If Na pump is blocked,

Answer 7

Na+ enters the cell, water follows, and the cell swells.

Question 8

The Na pump can be blocked by

Answer 8

interfering with ATP production or by specific drugs

Question 9

examples of specific drug that blocks Na pump works by:

Answer 9

that selectively block the Na+ pump by binding to a site on the outside surface of the membrane.

Question 10

block Na pump by interfering with ATP production examples

Answer 10

low temperature, cyanide, hypoxia

Question 11

The Na+ pump also pumps

Answer 11

potassium into cells

Question 12

the Na pump can’t extrude Na+ from the cell very well if the _______.

Answer 12

potassium concentration in the ECF is low

Question 13

For the Na pump to work,

Answer 13

both Na+ and K+ ions must be present simultaneously, or the pump won’t work.

Question 14

Cells with a poisoned pump lose _____ as they gain_____

Answer 14

K+

Na+

(they still swell, because they also gain Cl- as Na+ leaks in).

Question 15

The sodium pump is really an ________

Answer 15

obligatorily coupled sodium-potassium exchange pump.

Question 16

The Na/K pump is “_______”, that is, it has an ____________ cycles per second.

Answer 16

“saturable”

an easily demonstrable maximum rate of activity of only about 100

Question 17

“saturable” is characteristic of ________,

Answer 17

carrier mediated transport

Question 18

the Na/K ratio is _______

Answer 18

3 Na+ for 2 K+

Question 19

Thus, the pump is __________. This has the effect of making Vm __________.

Answer 19

electrogenic, and not electro-neutral

a little negative

The effect in most cases is very small, so that for our purposes we can ignore its electrogenicity

Question 20

the ion binding affinity of the channel switches between

Answer 20

Na and K, depending on which gate is open.

Question 21

______ provides the energy for the transitions (gates swinging and affinity changing).

Answer 21

ATP

Question 22

The pump contains a large _____ subunit, and a smaller _____ subunit

Answer 22

alpha

beta

Question 23

The Na/K pump cycle.

Answer 23

A. Both gates are closed, 2 K+ ions are inside.

B. ATP binds, the inner gate opens, and affinity changes from K+ to Na+. So K+ leaves and Na+ enters.

C. ATP is split, leaving the pump
phosphorylated. The inner gate closes.

D. Spontaneously, the outer gate
opens and affinity changes from Na+ to K+.

D to A. The pump loses its phosphate group, and the outer gate closes, completing the cycle.

Question 24

Na+ is constantly _____ into the cell, trying to get to equilibrium

Answer 24

leaking

Question 25

The real cell is in a steady state, which means that,

Answer 25

the ion concentrations aren’t changing over time,

constant input of energy is needed

Question 26

What is equilibrium for Na+?

Answer 26

Vm to equal ENa.

Question 27

For potassium, equilibrium is achieved when________. So a struggle ensues between the two ions

Answer 27

Vm is equal to EK

Na+ leaking in, trying to pull Vm up to ENa, and K+ leaking out, trying to pull Vm down to EK.

Question 28

What are the differences in resting membrane potentials due to?

Answer 28

relative permeability.

Question 29

A cell with many more K+ channels than Na+ channels will have a membrane potential close to _____.

Answer 29

EK

Question 30

Conversely, a cell with relatively more Na+ channels will have a membrane potential closer to ____.

Answer 30

ENa

Question 31

Glial cells are nearly perfect :

Answer 31

‘potassium electrodes’ (permeable only to K)

Question 32

while red blood cells are

Answer 32

about equally permeable to Na and K.

Question 33

A second determinant of Vm is

Answer 33

ion concentration

Question 34

A small change in ECF potassium concentration has a big effect on

Answer 34

EK and (in nerve, muscle, and other cells highly permeable to potassium) a big effect on Vm.

Question 35

Ohm’s Law,

Answer 35

V=I.R
G=1/R

V is the ‘driving force’ on the ion, 
I is the
current carried by the ion, and 
R is the membrane resistance to the ion
G, which is the reciprocal of resistance

Question 36

Ohm’s Law for sodium is as follows:

for K+:

Answer 36

INa= GNa.(Vm-ENa).

IK= GK.(Vm-EK).

Question 37

more channels, more ______, more _______

Answer 37

current, and more driving force, more current.

Question 38

When these two currents are equal and opposite (INa = -IK) there will be:

Answer 38

no net movement of charge across the membrane (we are ignoring other ions), so Vm will be at its resting value.

Question 39

Gr is the

Answer 39

the relative conductance of the membrane to sodium and potassium.

Question 40

membrane potential is determined by

Answer 40

relative conductance.

Question 41

One (leaky membrane) has

Answer 41

100 K channels and 10 Na channels

Question 42

The other (tight membrane) has

Answer 42

only a tenth as many channels (10 K and 1 Na).

Question 43

the tight and leaky have membrane potential

Answer 43

the same!

The leaky cell will have ten times more sodium leaking in, and ten times more potassium leaking out, compared to the tight cell, and so will have to expend 10 times more energy to keep the Na/K pump working. Thus, the tight cell is ten times more efficient, energetically speaking.

Question 44

if the Na/K pump is completely blocked by drugs,

Answer 44

nothing much happens initially (in many cells).
Gradually, of course., the cell will fill up with Na, and lose its K.
As these changes occur, both ENa and EK move towards zero, and cells will depolarize.

Question 45

Ions crossing a membrane are driven by

Answer 45

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

Question 46

Like charges repel and opposite charges attract, so a negatively charged Cl- ion diffusing across the cell membrane will be repelled by _________. Eventually these forces will reach an equilibrium whereby for every Cl entering the cell, _______. Together these two forces make up _______

Answer 46

the internal negativity of the cell but will also be attracted by the lower concentration of Cl within the cell

another leaves

an electrochemical gradient.

Question 47

Equilibrium potential is the:

Answer 47

the electrical potential difference across the membrane that must exist if the ion is to be a equilibrium

Question 48

Equilibrium potentials are not real voltages. They are:

Answer 48

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

Question 49

Equilibrium potential relates a concentration gradient to

Answer 49

an electrical force.

Question 50

Membrane potential (Vm) is a:

Answer 50

real voltage that exists across a membrane

Question 51

Vm is a reflection of the:

Answer 51

equilibrium potentials of all the ions and their given equilibrium state across a membrane.

Question 52

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

Answer 52

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

Question 53

Equilibrium potential is ultimately dependent on _______ while Vm is dependent on the _______

Answer 53

concentration (see the Nernst equation)

electric force

Question 54

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

Answer 54

its concentration inside and outside the membrane.

Question 55

In acute hyperkalemia, the main danger concerns the .

Answer 55

the reliable conduction of electircal signals (action potentials) in the heart.

These synchronized electrical signals can become disrupted during acute hyperkalemia, causing cardiac arrhythmias as conduction blocks occur and maverick pacemakers arise in various locations of the conduction system.

Question 56

The causes of hyperkalemia mostly concern

Answer 56

loss of potassium from cells.

Crush injuries, burns, and other trauma that disrupt cell membranes can do it.
So can immunological attack of red blood cells (causing hemolysis).

Question 57

One of the most important determinants of the clinical course of the hyperkalemia is the

Answer 57

status of the kidney,

whose normal job it is to excrete excess potassium.

If kidney function is compromised, hyperkalemia can be much more serious than if the kidney is functioning normally.

Question 58

The diagnosis of hyperkalemia usually is via an

Answer 58

electrocardiogram (EKG) to detect cardiac arrhythmias, followed by measuring plasma potassium ion concentration.