Bioelectricity

Question 1

What are the consequences of ion flow through plasma membrane ion channels?

Answer 1

Transmembrane potentials (ie. bioelectricity)
- where electrical and concentration gradients interact
- a build-up of ‘like’ charge tends to counteract the concentration gradient
-eventually reached an equilibrium (neither concentration or charge is perfectly balanced) - equilibrium potential

Question 2

What is bioelectricity

Answer 2

-electrical phenomena in biological systems
-include action potentials (nerve and muscle)
-but all cells use/display bioelectricity - na+ and k+ pump

Question 3

bioelectricity results from…

Answer 3

and interaction between membrane chemistry/ultrastructure solute concentration and charge and some basic physics

Question 4

Any cell with a permeating ion will tend to develop what

Answer 4

a transmembrane equilibrium potential
- only requires a concentration gradient in the permeating ion plus an impermeant counter ion

Question 5

What is the nernst equation and what does it calculate

Answer 5

size of the membrane potential (Vm) calculated by nernst
E= (RT/zF ) ln (Cout/Cin)

Question 6

What is E or Vm?

Answer 6

Potential
- difference in electrical charge between regions
- measured in volts (V)

Question 7

What is I?

Answer 7

Current
- flow of electrical charge between regions
- measured in amperes (A)

Question 8

What is R?

Answer 8

Resistance
- Forces that oppose the flow of charge
- measured in Ohms (Ω)

Question 9

What is g

Answer 9

Conductance
- inverse of resistance (ease of current flow)
- measured in siemens (S)

Question 10

What is C

Answer 10

Capacitance
- ability of a nonconductor to store charge
- measured in farads (F)
Without this bioelectricity wouldn’t happen
Bio membranes have to be tiny
opposite charges can react through thin membrane

Question 11

What is Ohm’s law?
and what does it relate?

Answer 11

relates potential, current and resistance
E=IR or I=E/R
current x resistance or potential divided by resistance

Question 12

What are the three sources of membrane potentials in cells?

Answer 12

1. Electrogenic ion pumps
   ie. 3:2 Na+K+ ATPase ~-10mV in all cells
2. Equilibrium Potentials
   - a special type of diffusion potential
   - due to [gradients] in permeating ions
   - found in some specialized cells
3. Donnan equilibrium
   - a special equilibrium potential
   -impermeable ions alter distribution of permeable ions
   ~ - 5mV in all cells

Question 13

in the donnan equilibrium, K+ and Cl- are …

Answer 13

freely permeable
- diffusion results in [equimolar] and there is no Vm
However, an impermeant anion (A-) will disrupt the concentration gradients of permeant ions

Question 14

in the donnan equilibrium, how does K+ move and how does Cl- move?

Answer 14

K+ moves due to its concentration gradient, Cl- moves due to charge gradient (likes repel)
opposites attract
At equilibrium, neither charge or concentration are balanced

Question 15

How do equilibrium and Donnan potentials differ?

Answer 15

They differ due to equilibrium potentials

Question 16

Donnan potential is found in all cells…
Equilibrium potential is only found in specialized cells…

Answer 16

depends on concentration gradient for non-permeating ion

Question 17

Frog example of transmembrane potentials in other biomembranes

Answer 17

• live in hypotonic environment, tend to lose ions in urine, by diffusion out of body
• is countered by a skin ion transport mechanism
  originally investigated by Hans Ussing
• The two sides of the skin cell act like separate membranes, each with their own Vm
  Thus Vskin = Ena + Ek
  2 separate membranes, 2 separate channels
  ATP found on basal side (blood side)

Question 18

Any membrane with a permeating & nonpermeating ions will develop a transmembrane potential

Answer 18

only requires a concentration gradient
- size of the membrane potential calculated by the nernst equation

Question 19

reminder: Goldman’s equation in neurons and muscle cells

Answer 19

• single membrane but MULTIPLE ions with differing permeabilties

Question 20

what is the z in the nernst equation

Answer 20

valence/charge of ion/particle (+1, -1, +2, etc.)

Question 21

What is the R in the nernst equation

Answer 21

R = universal constant

Question 22

what is the F in the nernst equation

Answer 22

Faraday’s constant

Question 23

what is Cout

Answer 23

concentration of ion on outside of membrane

Question 24

what is Cin

Answer 24

concentration of ion on inside of membrane

Question 25

what is ln

Answer 25

natural log of the concentration gradient Cout/Cin

Question 26

The size of the potential is directly proportional to the ion concentration gradient

Question 27

What are the two different simplified nernst equations:
@18 C - british room temp
@37 C - body temp

Answer 27

@18 = E (in mV)= 58 log10 (Cout/Cin)
@37 = E (in mV) = 61 log10 (Cout/Cin)

Question 28

Why are the nernst equation calculations important

Answer 28

• because they allow comparison of actual & theoretical potentials
• can generate hypotheses about ions involved & then test these experimentally