L2 - Bioelectricity

Question 1

How to carry out a Vm measurement?

Answer 1

Glass electrode placed through cell membrane and sits with its tip in intermembrane space (-70 mV)
Reference electrode placed into solution of 0 mV

Question 2

Determination of Vm

Answer 2

Unequal distribution and selective movement of a few ions

Na+, K+ and A- (large anions)

Question 3

Distribution of Na ions

Answer 3

Extracellular - 150

Intracellular - 15

Question 4

Distribution of K ions

Answer 4

Extracellular - 5

Intracellular - 150

Question 5

Distribution of anions

Answer 5

Extracellular - 0

Intracellular - 65

Question 6

Contribution to the distribution of ions by Na/K ATPase

Answer 6

Direct - 20%
- Electrogenic – 3Na out, 2K in
- This leads to a loss of positive charge
Indirect
- Regulates intracellular concentrations of Na and K

Question 7

Contribution to the distribution of ions by K channels

Answer 7

K+ moved to extracellular down concentration gradient
- Leaves a negative intracellular charge
- A- cannot move from intracellular to extracellular
- V gradient draws K+ from extracellular to intracellular
Negative potential
Movement out = movement in
No net or current flow

Question 8

Contribution to the distribution of ions by Na channels

Answer 8

Na+ moved from extracellular to intracellular down the concentration gradient
- Leaves a positive intracellular charge
- A- cannot move from intracellular to extracellular
- V gradient draws Na+ from intracellular to extracellular
Positive potential
Movement out = movement in
No net or current flow

Question 9

Nernst equation

Answer 9

Eion = RT X log [ion]out

…………. z ……… [ion]in

Question 10

Explanation of the Nernst equation

Answer 10

-70mV is what we actually measure.
The closer the Nernst potential is to the actual measurement the more ions are moving across the membrane.
High K permeability at rest so closer to what we actually measure.

Question 11

Passive K movement only

Answer 11

Vm = Ek

-70 mV = -90 mV

Question 12

Passive Na movement only

Answer 12

Vm = ENa

-70 mV = +61 mV

Question 13

Goldman equation

Answer 13

RT log PNa[Na]o + PK[K]o

..Z ……. PNa[Na]i + PK[K]i

Question 14

Relative permeability of ions

Answer 14

Na - 1
K - 50-75
A - 0

Question 15

Electrogenic transport - voltage gated Na channels

Answer 15

At rest lots of K channels open, few Na channels open
Resting membrane potential - closer to the Nernst potential of K
Once you reach threshold opening of voltage gated Na channels
- Permeability of Na goes up
- Depolarises the membrane potential
- Closer to the Nernst potential of Na

Question 16

Electrogenic transport - Na/amino acid cotransporter

Answer 16

When cotransporter activated Na and amino acid enter the cell
Depolarises the membrane potential