THE NERNST EQUATION RELATES THE MEMBRANE THE NERNST EQUATION RELATES THE MEMBRANE POTENTIAL TO THE DISTRIBUTION OF AN ION AT POTENTIAL TO THE DISTRIBUTION OF AN ION AT

EQUILIBRIUMEQUILIBRIUM

Ej = 59mV log Cjo / Cj

i (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 / Cj

i

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 THE NERNST EQUATION CAN BE USED TO DISTINGUISH BETWEEN ACTIVE AND PASSIVE TRANSPORTBETWEEN 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 QuestionsSample Questions

Ej = 59mV log Cjo / Cj

i

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