THE NERNST EQUATION RELATES THE MEMBRANE
POTENTIAL TO THE DISTRIBUTION OF AN ION AT
EQUILIBRIUM
Ej = 59mV log Cjo / Cji
(6.11)
1. A tenfold difference in concentration (outside > inside)
corresponds to a Nernst potential of +59mV (Co /Ci = 10/1; log
10 = 1).
2. A tenfold difference in concentration (inside > outside)
corresponds to a Nernst potential of -59 mV (Co /Ci = 1/10; log
1/10 = -1).
Using the Nernst Equation
Ej = 59mV log Cjo / Cji
1. Bathe tissue in a known external concentration of solute, or
measure the external concentration of the solute.
2. Allow the tissue to come to equilibrium, then measure the internal
concentration of the solute.
3. Measure the membrane potential using an electrode.
4. Compare either the measured vs. predicted membrane potentials or
the measured vs. predicted concentration differences.
THE NERNST EQUATION CAN BE USED TO DISTINGUISH
BETWEEN ACTIVE AND PASSIVE TRANSPORT
1. If, at equilibrium, the measured membrane potential equals the
calculated Nernst potential, transport is passive.
2. Also, if the predicted ion distribution based on the Nernst Equation
equals the observed ion distribution, transport is passive.
3. However, if the measured membrane potential differs from the
predicted membrane potential based on the Nernst equation,
transport is active.
4. Or, if the measured ion distribution differs from that predicted by the
Nernst equation, transport is active.
Sample Questions
Ej = 59mV log Cjo / Cji
1. What is the Nernst potential for K+ under the following conditions:
Co = 1 mM; Ci - 100 mM? If the measured membrane potential is 120 mV, is transport of K+ active or passive?
2. The measured membrane potential of a cell at equilibrium is -80
mV. Assuming K+ is transported passively, what is the predicted
internal concentration when the external concentration of K+ is 5
mM?
(1 mM = 0.001 M)
KINETIC ANALYSES CAN ELUCIDATE
TRANSPORT MECHANISMS:
CHANNELS VS. CARRIERS
PROTON PUMPS PLAY IMPORTANT
ROLES IN ACTIVE (ATP-DEPENDENT)
TRANSPORT
SOLUTES MOVE THROUGH BOTH
THE APOPLAST AND SYMPLAST
XYLEM PARENCHYMA CELLS PARTICIPATE IN
XYLEM LOADING