Introduction to NAS

Question 1

what makes the bilayer impermeable to charged molecules?

Answer 1

it is hydrophobic

Question 2

what do ion pumps in the membrane do to membrane potential?

Answer 2

maintain concentration gradients
They don’t set the membrane potential of the cell – they providing the starting point for the ability of the cell to generate electrical signals.

Question 3

what 2 ions are in the PM and what are their concentrations?

Answer 3

The 2 important ions are K+ and Na+
K+ concentration is high inside the cell, low outside
Na+ concentration is low inside the cell, high outside

Question 4

what are the basic electrical processes of the PM?

Answer 4

The inside of all cells contains an excess of anions leading to a negative voltage within the cell.
The size of the negative voltage inside different cells differs due to a difference of anions.
This negative voltage is known as the Membrane Potential (Em)
In neurones, this may be −65 mV
Balance of charges determines value of Em

Question 5

why is the term “chemically heterogenous” used when referring to ion channels?

Answer 5

the exposed ends of a channel proteins are hydrophilic and their middle surfaces (embedded in the membrane) are hydrophobic.

Question 6

what are the 3 different classifications for ion channels?

Answer 6

non-gated (leak)
voltage- gated
ligand

Question 7

what is the gating mechanism for ion channels?

Answer 7

A stimulus will cause this gate to open and cause a conformational change to allow an ion through

Question 8

what are non-gated (leak) channels?

Answer 8

set resting membrane potential, they don’t have gates and are open all the time

Question 9

what are voltage gated channels?

Answer 9

generate Action Potentials

Question 10

what are ligand channels?

Answer 10

generate Em changes at synapse, where the ligand is a neurotransmitter which binds to the channel, acting as a stimulus to open the gate.

Question 11

what is the resting membrane potential and what does it mean?

Answer 11

Resting Membrane Potential (Em) = -60 to -70 mV
This means:
There is an excess of negative charge inside the cell
The voltage is stable – so there is an equal movement of ions inside and outside the cell

Question 12

how is resting membrane potential achieved?

Answer 12

The plasma membrane contains Non-gated (leak): K+ channels and Na+ channels
Therefore, the membrane has a permeability to each ion (PK , PNa)
Chemical gradient: There is an unequal ion distribution so ions will flow down their concentration gradient
K+ concentration is higher inside 🡪 driven to leave cell through ion channels (efflux)
Na+ concentration is higher outside 🡪 driven to enter cell through ion channels (influx)
Electrical force: Ions are charged (Na+/K+) thus are attracted by voltage inside cell (Em)
At negative Em, drive for K+ and Na+ to move into the cell (influx)

Question 13

what determines the permeability of a ion in a PM?

Answer 13

The number of the leak channels for each different ion is the factor which sets the permeability for that ion.

Question 14

what does driving an ion across the membrane electrically require?

Answer 14

the membrane possesses channels permeable to that ion to provide conductance
There is an electrical potential difference across the membrane

Question 15

how is a chemical gradient generated in a PM?

Answer 15

There is a higher concentration of K+ inside the cell than outside so K+ efflux
This results in a loss of positive charge from inside cell
This creates negative Em which sets up an Electrical Force

Question 16

why is the chemical gradient referred to as “constant” ?

Answer 16

Over short time scales, changes in concentration are extremely small - (i.e. chemical gradient is “constant”
Concentrations are not changing in any discernible way -the changes are very small (i.e. chemical gradient is “constant” in terms of the influence. But the electrical impact of this is substantial as the membrane potential changes dramatically and becomes negative.

Question 17

how is the electrical force generated in the PM?

Answer 17

K+ begin to influx when Em becomes negative as they are attracted to the negative Em inside the cell
Initially, K+ efflux is greater than K+ influx – so there is a net movement of K+ outside the cell
As more and more potassium ions leave the chemical gradient is not changed, but the electrical force is getting greater (Em more negative) and the more potassium comes in to the cell (attracted by the negative charge).
At a sufficiently negative Em, chemical and electrical influences are balanced so there is no net K+ movement as the electrical force causing K+ to enter is equal to the chemical gradient causing K+ to leave.
K efflux = K influx

Question 18

when does Em become positive?

Answer 18

If the membrane is permeable to Na+ only (and cell is initially at zero mV)
Chemical gradient 🡪 Na+ influx
Em becomes positive
Electrical force causes Na+ efflux (Na+ are repelled by voltage inside the cell)
When Em reaches sufficiently positive value chemical and electrical influences are in balance so there is no net movement

Question 19

what is the equilibrium potential?

Answer 19

he voltage of the membrane potential necessary to perfectly oppose the net movement of an ion down its concentration gradient - i.e. the Ion is in equilibrium (no net flux).

Question 20

how does each ion have its own equilibrium potential?

Answer 20

Different ions have different concentrations on either side of the membrane and the Chemical force will be different for each ion. Therefore, the membrane potential that must be achieved to equalise the two forces will be different. So, each ion has its own equilibrium potential.

Question 21

why do skeletal muscle cells and glial cells have a membrane potential that is equal to the equilibrium potential for K+?

Answer 21

If the membrane is permeable to only one ion, then the resting membrane potential will be equal to the equilibrium potential for that ion. This is actually true for skeletal muscle cells and glial cells within the nervous system. Their membranes are permeable to K+ only and so their membrane potential is equal to the equilibrium potential for K+.

Question 22

what is the Nernst equation used for?

Answer 22

Works out the voltage of the equilibrium potential for each ion.
(Inside log Bracket) - Equilibrium potential depends on the ratio of the concentration of ions (ion/out) as permeability factor (Pion) cancels

Question 23

what is the ionic driving force?

Answer 23

The Net force resulting from chemical (conc. gradient) and electrical influences (attract to opposite charge)
Driving Force is present whenever Em is different from equilibrium potential for the ion (i.e. Em - Eion ≠. 0) (This equation is how you calculate driving force for an ion)

Question 24

why are ions not in equilibrium?

Answer 24

Em is approx. = -65mV
If Em ≠ EK then the influences on K+ movement are unequal
At -65 mV the chemical influence (efflux) is greater than the electrical influence (influx) so the ionic driving forces causes a net movement K+ out of the cell (efflux)
Em ≠ ENa At -65 mV so both chemical and electrical influences result in an ionic driving forces which causes Na+ to move into the cell (influx)

Question 25

how does K+ and Na+ influence Em in neurones?

Answer 25

Both Na+ and K+ ions are permeable (i.e. PK and PNa > 0)
Both ions are moving across membrane (ionic driving force)
K+ efflux (trying to bring Em to -80 mV 🡪 EK)
Na+ influx (trying to bring Em to +62 mV 🡪 ENa)
So, Em must rest between EK and ENa

Question 26

what does the Goldman equation allow you to calculate?

Answer 26

Em

Question 27

how would permeability affect resting membrane potential?

Answer 27

There are more K+ leak ion channels than Na+ leak channels (as Em is -65 which is close to Ek rather than ENa)
At rest the permeability of K+ is around 40 times that of the permeability of Na+ (PK = 40 x PNa)
As there are more non-gated K+ channels, it has more influence in setting membrane potential: Membrane potential (Em) controlled (mostly) by K+ movement).

Question 28

how would driving force affect resting membrane potential?

Answer 28

Na+ influx: due to both chemical gradient and electrical force
Na+: Large Driving force ( -65mv – 62mv = -127mv)
K+ efflux: Chemical gradient (causing efflux) is greater than electrical force (causing influx)
K+: Small Driving force (-65mv - -80mv = 15mv)

Question 29

how are ionic gradients maintained using ion pumps?

Answer 29

The Sodium potassium pump is an enzyme – catalyses ATP breakdown
It exchanges internal Na+ for external K+ against their concentration gradients
Sodium-potassium pump – very minute impact on membrane potential in comparison to ion channels. They just keep the concentrations at the correct levels:
It operates in background continuously:
Over long time periods, the constant efflux of K+ (for example) will eventually lead to significant change in concentration
Pumps maintain concentrations over long-term (by active transport) – NOT for controlling Em