Table of Contents

• Prequiz...
• Part I Two Opposing Gradients
• 1: Getting through membranes
• 2: Going with the flow
• 3: Permeability is key!
• 4: Meet the neighborhood
• 5: Getting groovy with gradients
• 6: Some real numbers...
• Part II Two Useful Equations
• 7: The Goldman Equation
• 8: Playing with the Goldman Equation
• 9: The Nernst Equation
• 10: Taking Nernst for a test drive
• 11: The world according to Nernst
• 12: How bout them logs?
• 13: Pumps and channels redux
• Part III The Action Potential
• 14: At equilibrium? Or far from it?
• 15: Opening the gate
• 16: The action potential and the Goldman equation
• 17: Action potential and the Nernst equation

 

The Nernst Equation

 

Remember what happened when we specified that the membrane should only be permeable to ONE ion (in this case K+): (rollover with your mouse for the simplified equation)

Here is an even simpler way of stating the same thing, called the "Nernst Equation", and as an added bonus I'll even let you in on the value of the mysterious "k":

   

This equation was actually discovered half a century before the Goldman equation, by a Nobel prize winner named Walther Nernst -- hence, the Nernst equation. "C" in the standard version stands for "concentration". I included the second version just to make the relationship between the Nernst and the Goldman equations more apparent.

By the way, the value of K = 62 is valid only at body temperature (37 degrees Celsius). A deep sea fish would have a different value, because of the lower effect  of temperature on the potential energy of the ions !