The question asks:
Calc. the membrane potential from Goldman equation using pNa/pK = 0.02 & the conc. of Na & K inside & outside the cell. Calc. the membrane potential using pNa/pK = 20. Compare these calculated values of membrane potential with the numbers from the simulation. Why does the membrane potential shift when pNa/pK moves from 0.02 to 20? Use [Na]i =12, [Na]o = 120, [K]i = 125, [K]o = 5.
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In a very simple sense, the cell is a container full of water and dissolved ions -- Na and K are the ones we're primarily concerned with.
There is a Sodium-Potassium pump that will pump potassium into the cell and sodium outside. This creates a concentration difference for the two ions - diffusion will attempt to cancel this concentration difference out, so you can think of a "diffusive force" acting on the ions to try to restore balance between the concentrations inside and outside the cell.
However, the ions also have a charge, so as they start to move across the cell membrane, they create a voltage that will repel more charges from coming in. There is a certain point at which the electric forces exactly cancel the diffusive forces, known as the Nernst potential. This is the point at which there will be an equilibrium in the flow of ions, and the membrane potential will be stable.
That description, though, is only for *one* ion. What happens if we need to worry about both potassium and sodium? Well, in that case we have to consider both the Nernst potentials for each ion, as well as the relative permeability to each ion ...
This job covers the Goldman equation. The membrane potentials from Goldman equations are given.