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Nernst potential toy model

last edited by 8 years, 4 months ago

8.3.P9

Consider the inside and outside of a cell, where the 8 nanometer thick cell membrane has a large number of open Na+ Ion channels. The major contributor to these ions is dissolved NaCl. Let’s analyze a “toy model” of the cell’s resting potential in which the Na+ ion channels are the only ion channels, they are permanently open, and NaCl is the only contributor of Na+ ions. Assume the outside of the cell has a concentration of 150 mM (10-3 Molar) NaCl and a significantly lower concentration inside the cell.

A. Explain why in this situation a potential difference (Nernst potential for Na+) would develop across the membrane and specify which side of the membrane would have the higher potential.

B. If the Nernst potential for the sodium ions in our toy model is 60 mV, what would the concentration of NaCl be on the inside of the cell? Show your work.

C. Now let us refine our model by noting that there is a second source of Na+ ions in the cell: NaI. Suppose the outside of the cell has a concentration of NaI of 0.04 mM and the inside has a NaI concentration of 4 mM. Will the presence of these ions increase the Na+ Nernst potential across the membrane, decrease it, or leave it the same? Explain your reasoning. (Assume the NaI is fully ionized in solution.)

D. In the refined toy model can you calculate the Nernst potential for Cl- ions? If yes, calculate it. If not, explain why not.

Dave Buerhle 11/6/15