Membrane Physiology

Question 1

how are electrical signals generated?

Answer 1

generated by the transfer of ions across the membrane – via ion channels

Question 2

what carries charge inside a cell?

Answer 2

ions
organelles
anions/ cations
proteins

Question 3

how does the existence of a cell membrane generate a resting potential?

Answer 3

concentration difference of charge- carrying ions between intra and extracellular compartments
ions not allowed to flow freely through membrane (impermeable to charged molecules)

Question 4

what is the concentration of ions inside vs outside of the cell?

Answer 4

K+
inside- 150 mM
outside- 5.5 mM
Na+
inside- 15 mM
outside- 150 mM
Ca2+
inside- <10-7 mM
outside- 1.8 mM
Cl-
inside- 9 mM
outside- 125 mM

Question 5

what is the resting membrane potential in nerve cells?

Answer 5

inside is relatively negatively charged with respect to the outside of the cell
RMP around -65 to -70 mV (INSIDE)

Question 6

what are electrically excitable cells?

Answer 6

to the ability of some cells to be electrically excited resulting in the generation of action potentials i.e discharging of RMP

Question 7

what is the difference between electrically & non- electrically excitable cells?

Answer 7

electrically excitable cells are able to momentarily discharge the standing electrical potential between intracellular and extracellular compartments

Question 8

what happens after a cell discharges the resting potential?

Answer 8

an action potential
then the cell immediately works to restore RMP
within 1 millisecond

Question 9

what does permanent discharge of RMP do?

Answer 9

permanent discharge results in death of electrically excitable tissues- can’t generate AP
including:
- nerve cells
- muscle cells (cardiac)

Question 10

what characteristics of the membrane allow the generation of RMP?

Answer 10

The permeability of the membrane to the ion- what can cross?
presence of ion channels
ATPase- for K+ and Na+

Question 11

what factors can define whether or not a particle can cross the membrane?

Answer 11

size
electrical charge
whether they are recognised by specialised transport systems (ion pumps)
solubility in water- won’t allow it to go through if soluble

Question 12

what type of protein are ion channels made of and what do they facilitate?

Answer 12

transmembrane spanning protein
facilitate PASSIVE movement

Question 13

what is the structure of ion channels?

Answer 13

the exposed surface of a protein can be chemically heterogeneous. e.g. the exposed ends of a channel proteins are hydrophilic and their middle surfaces (embedded in the membrane) are hydrophobic.
- have ion selectivity
- a gating mechanism - essential for controlling Em (ion movement is passive)

Question 14

what is the difference between selective and non- selective ion channels?

Answer 14

selective- allow particular ions to pass through
non- selective- i.e non-gated (leak channels):
allow any molecules to pass through

Question 15

what are examples of ion channels?

Answer 15

Non-gated (leak): set resting membrane potential, they don’t have gates and are open all the time
Voltage-gated: generate AP Ligand: 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 16

define gating mechanism of an ion channel

Answer 16

• different stimuli control opening
• stimulus will cause this gate to open and cause a conformational change to allow an ion through

Question 17

to generate a membrane potential you need to consider?

Answer 17

chemical gradient
electrical force

Question 18

how does chemical gradient contribute to the generation of a membrane potential?

Answer 18

there is a higher concentration of K+ inside the cell than outside so K+ efflux
results in a loss of positive charge from inside cell- creates negative Em which sets up an electrical force

Question 19

why is the chemical gradient considered to be “constant” ?

Answer 19

the changes in conc of ions 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 20

how does electrical force contribute to the generation of a membrane potential?

Answer 20

• 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 21

what do ion pumps do to membrane potential?

Answer 21

Ion pumps maintain concentration gradients.
They DONT SET the membrane potential of the cell – they providing the starting point for the ability of the cell to generate electrical signals.

Question 22

how do ion pumps maintain the ionic gradient?

Answer 22

• sodium potassium pump is an enzyme – catalyses ATP breakdown
• it exchanges internal Na+ for external K+ against their concentration gradients
• has a very small 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