The Nernst Equation
by Greg Crowther
Find the concentrations of ions out and in;
Figure out the quotient, and take the log (base 10).
Multiply by a constant like 58 mV;
Divide by ion valence to find potential E.
At the voltage you have found (says Walther Nernst),
There's no flux in or out (says Walther Nernst)!
This song reminds students of the Nernst equation for calculating an ion's equilibrium potential. In its simplest form, the Nernst equation says that, at 20 degrees Celsius (293 Kelvin), for a given ion, Eion = (58 mV/z)*log10([ion]out/[ion]in), where z is the ion's valence.
This song was originally written for Biology 220 at the University of Washington. The melody is loosely based on "I Want You Back" as recorded by the Jackson 5. (The words "says Walther Nernst" correspond to the words "I want you back" in the original.)
Questions: (1) Does the constant of 58 mV remain the same for all temperatures? (2) What is ion valence? (3) What units are carried by the equilibrium potential (E)? (4) What does the value of E mean? Answers: (1) No -- this "constant" varies with temperature. At an internal body temperature of 37 degrees Celsius, the value is 61 mV. (2) Ion valence is the charge carried by an ion, such as minus-1 or plus-2. (3) E, an electrical potential, generally is reported in units of millivolts. (4) E is the electrical gradient across the membrane needed to perfectly counterbalance any concentration gradient, such that there is no net driving force driving the given ion from one side of the membrane to the other.
• MP3 (demo)
• score (with melody play-back)