Stephen G. Saupe - Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; (320) 363-2782;

Gink and Go Talk about Bank Accounts

SettingGink and Go are sitting at the kitchen table.  Go is quietly reading his biology textbook.  Gink is hunched over his checkbook, his calculator and broken pencils are scattered on the table in front of him.

Gink: Aaaaaaaaaagh! (scream and throw up your hands)
Go: Gink, what's wrong dude?
Gink: I can't balance my checkbook (scream again, and pull on your hair)
Go: That's too bad.  I'm sure you'll figure it out.  Speaking of balancing checkbooks, did you like the bank analogy Saupe used in biology class today to explain membrane potentials?
Gink: Duh, of course not!
Go: Oh, come on now - it helped me to understand action potentials by comparing them to deposits and withdrawals of money from a bank account.
Gink: Whatever.  Who cares that potassium and sodium ions are like money and that when they move across the membrane it’s analogous to making a deposit or withdrawal from the bank.
Go: The interesting thing is that just like you would expect the deposits and withdrawals from a bank to balance, you would expect the electrical charges to balance.    But they don’t.  The inside of the cell has a negative charge compared to the outside.
Gink: Hey I like these neurons.  It sounds like they maintain a negative account, just like my bank balance.  The bank would never allow that.  Why does it happen in cells?
Go: The membrane has passive leakage channels that allow potassium ions to diffuse outward, following a concentration gradient.  This means that there is a net outward flux of positive charges, or cash withdrawals, out of the neuron.  Plus, there is a sodium/potassium pump that pumps 3 sodium ions out for every 2 potassium that move in.  This also lowers the account.  
Gink: Wow.  Neurons have a major cash flow problem - their bank account is perpetually in the red. 
Go: Right.  In fact, at rest there is an electrical difference of 60 millivolts between the inside and outside of the cell.  We say that the resting potential is –60 millivolts because the inside is more negative that the outside.  The inside of the neuron is used as a reference point.
Gink: How do biologists know this?  I get a bank statement showing my account, but how do biologists know the neuron account is negative? 
Go: The electrical potential difference can be measured with tiny electrodes connected to an oscilloscope or voltmeter.
Gink: If my bank account were overdrawn by -60 millivolts I'll bet that would be a lot of money.
Go: You might think so but the electrical difference is caused by just a few anions - in fact the difference is only about 1 anion for every 100,000.
Gink: So how does the Nerd equation fit in?
Go: Nernst, not Nerd, equation (act disgusted)
Gink: Relax.  Talk to the hand (flip up your hand at Go)
Go: Yeah, yeah.  The Nernst equation is just a fancy way of relating membrane potential to the distribution of ions at equilibrium.  Remember that at equilibrium, the concentration of ions is counterbalanced by voltage on the different sides of the membrane.
Gink: Huh?  Are you trying to tell me that the negative charge inside the neuron actually help to pull the potassium ions back into the cell?
Go: Right, now you've got it.  And if you know the concentration of ions on either side of the membrane at equilibrium you can use the Nernst equation to calculate the electrical potential across the membrane.
Gink: Does Saupe really expect us to memorize that stupid equation?
Go: Nah...he just wants us to remember that it is important for understanding the relationship between membrane potential and ion concentration across the membrane.
Gink: That's a relief.  But it still seems like a nerd equation to me - just like Saupe!

Reference:  The bank account analogy was derived from Lovell JA (1999) A new approach to teaching membrane potentials. American Biology Teacher 61: 696 - 700. 


| SGS Home | Disclaimer

Last updated: March 26, 2007    � Copyright by SG Saupe