| MadSci Network: Biophysics |
hi ritta,
Hmm, this looks awfully like an assignment question to me and we're not encouraged to answer those here on MadSci. Perhaps I can just point you in the right direction and you can figure the rest out for yourself.
In some ways, muscle (and many other cells) are a lot like batteries. They store charged ions in their interior at a different concentration than that found outside of the cell. This leads to a potential difference across the cell membrane - electrical energy which is available to do work.
There's a quite simple equation that describes the relationship between the ion concentrations and the potential difference. It's called the Nernst equation, after Walther Nernst who first formulated it. Try "Nernst equation" at Google or Wikipedia and you'll find lots of information about it.
The equation itself looks like this:
VK = -RT/zF * ln([Kin]/[Kout])VK is the membrane potential: -70 mV in your example. [Kin] is the internal ion concentration, which was 120 mM potassium in your question. The other values are R (the universal gas constant, 8.314510 J K-1 mol-1), T (the temperature in Kelvin), z (the valency of the ion involved, +1 for potassium) and F (the Faraday constant, 9.6485309 x 10e4 C mol-1).
So - as you did not specify a temperature in your question, I would not have been able to answer it anyway! But let's assume 37 °C, which is 310 K. You now have all the numbers that you need - you'll have to rearrange the Nernst equation given above so as [Kout] (the answer that you want) is on the left hand side, then plug in all of your numbers and see if the answer makes sense to you.
Good luck and remember - biologists need math and physics skills too,
Try the links in the MadSci Library for more information on Biophysics.