Lecture 13

Question 1

Understand that the difference in free energy of a solute or solvent between two components can
have chemical, electrical and/or hydrostatic pressure components. At equilibrium, for a given component,
the free energy difference between the two compartments is zero.

Answer 1

Know

Question 2

Based on the principle of ionic attraction, explain how a potential difference across a membrane will influence the distribution of a cation and an anion.

Answer 2

Cation: + ion

Anion: - ion

Membrane is more negative inside cell so attracts cation inside and anion outside

Question 3

Write the Nernst equation and indicate how this equation accounts for both the chemical and electrical driving forces that act on an ion. Explain the effects of altering the intracellular or extracellular Na+, K+, Cl-, or Ca2+ concentration on the equilibrium potential for that ion.

Answer 3

Electrochemical force = when electrical and chemical gradients are equal

• electrical is charge
• chemical is amount of ions
• net flux is zero when electrical and chemical are equal

Nernst equation
- calculate what electrical energy (Em) is when next flux is zero

Note: bigger ion gradient = bigger voltage needed to get net flux be zero

Multiple species
More permeability for ion = closer membrane potential is to the equilibrium potential for that ion alone

Question 4

Based on the Nernst equilibrium potential, predict the direction that an ion will take (follow) when the membrane potential a) is at its equilibrium potential, b) is higher than the equilibrium potential, or c) is less than the equilibrium potential. List values in a typical non-excitable cell for the membrane potential, for ENa, EK, ECl, and ECa.

Answer 4

Equilibrium potential
Na in
K out
Cl out

Higher than equilibrium potential
- want to make it more negative 
Na out
K out
Cl in 

Lower than equilibrium potential
- want to make it more positive 
Na in
K in
Cl out

Question 5

Describe the normal distribution of Na+, K+, and Cl- across the cell membrane, and using either the Goldman-Hodgkin-Katz chord conductance equation explain how the relative permeabilities of these ions create a resting membrane potential. Given an increase or decrease in the permeability of K+, Na+, or Cl-, predict how the membrane potential would change. (Not required to calculate on exams)

Answer 5

Extracellular K influences the resting potential more because more channels for k = more conductance = more permeability

Extracellular Na at equilibrium is zero at rest so does not affect cell membrane potential at rest = no permeability