Ion channels sit in the membrane of cells, and allow current to flow from outside the cell to inside, or the other way around.

This current is carried by charged ions, such as Na+

, K+ or Cl−

.

Let’s consider a Na+

channel, just to be specific (although other ion channels behave the same way). The current through a Na+

channel depends on the voltage difference, V, across the membrane of the cell, as well as on the Na +

concentrations inside and outside the cell, which we will call ci and co respectively.

According to the Nernst equation, the current through the Na+

channel is zero when V =

RT F

ln 

co ci



.

This value for V is called the Nernst potential of Na+
. R is the gas constant, and F is Faraday’s constant.

a. What are the units of F (look this up online)? Show that the units all work out correctly in the equation for the Nernst potential.

b. What is the equation for V if you use log instead of ln?

c. Write co as a function of the other variables.

d. Look up some typical values for ci and co (look up values for the Na+
concentration inside and outside of a typical mammalian cell) and compute the Na+ Nernst potential at body temperature.