FREQUENTLY ASKED QUESTIONS

• “I have a quick question about how the equilibrium potential is described. The definition says that it’s the voltage that would have to exist across a membrane to prevent the net movement of an ion down its concentration gradient. The word ‘down’ is confusing me.

ANSWER: Let's take sodium (Na), for example. Simply, if the Na concentrations are 150 mM in the extracellular fluid and 15 mM in the intracellular fluid, the concentration or chemical gradient will tend to move Na into the cell. Under these conditions:

If Na is moving into the cell, scientists say: The net movement of Na is into the cell, with or down its concentration gradient.If Na is moving out of the cell, scientists say: The net movement of Na is out of the cell, against its concentration gradient.

• “How can I figure out which way the ions are moving?”

1. First ask yourself: Where is the actual membrane voltage (Vm or RMP) relative to the calculated equilibrium potential, which is your reference. Is the Vm more positive (above the equilibrium potential line) or more negative (below the equilibrium potential line) compared to your calculated equilibrium potential (Es)?

2. Draw a cell and put in the middle of that cell "more +" or "more -,” and only use the word "more,” not “less”! If you use the word “less,” it will mess you up. Also, write in the middle of your drawn cell the ion of interest, (e.g., Na+ or K+). Don’t worry about its concentration gradient. It’s built into the Nernst equation.

3. Ask yourself: Is that ion going to be repelled out of the cell, or be attracted into the cell? (Remember: Like charges repel and unlike charges attract.)

You’re done!

NOTE: The key is to compare the RMP to the Es, and NOT the other way around.