[Vsavsa]

I think I understand that cells are at an electrochemical equilibrium when the negative charge of the intracellular space pulls ions (which exit the cell because of the higher concentration gradient) back at the same rate at which they exit, making the inflow and outflow of a specific ion equal and resulting in the formation of a voltage in the membrane (around -70mV for Potassium equilibrium). I would like to know first of all if that understanding is more or less adequate and secondly and most important. If equilibrium is reached when enough Potassium ions leave the cell on their own, why is a pump needed!?
And third, and I know it might be a dumb question, but, why does the cell need to be at equilibrium in the first place?
I'm sorry if I seem to be lost but I'm just beginning my education in cell biology. Thanks!

[Babcock_Hall]

If a cell were at equilibrium, it would be dead.  The value of -70 mV is the membrane potential. and it affects all ions, not just potassium ions.  Maybe a place to start is to ask what is true about sodium ions versus potassium ions and the sodium potassium ATPase.

[Vsavsa]

I see, so the cell is not at equilibrium, so what state is it in? Does the Na⁺/K⁺ ATPase help maintain that state? Is it correct to say that the ATPase is constantly increasing the intracellular electronegativity? I think I found why I was misled into thinking the cell was at equilibrium thanks to Section 15.4 of Molecular Biology by Harvey Lodish et al. which explains that when a membrane separates solutions containing different concentrations of ions, and the membrane becomes permeable to one of the ions, then the ions will flow down their concentration gradient until the point where the electronegativity and concentration gradient balance and cancel each other out and maintain an equal flow of the ion back and forth.
However, my mistake was saying that the cell is at equilibrium, because the cell is not a simple selectively permeable membrane, but rather a system which requires the action potential provided by the resting potential of the electric difference between intracellular and extracellular ions for some (which, though?) of its biological processes.
The cell uses the Na⁺/K⁺ ATPase to maintain this action potential, "as it would fall if it were to be regulated only by ion channels" says the book.http://www.ncbi.nlm.nih.gov/books/NBK21627/
However, now I would like to know why would the membrane voltage fall of it was not for the Sodium Potassium Pump? Also, is my little explanation more or less right? Thanks again!!!

[Babcock_Hall]

It might make sense to focus on two issues.

1.  Consider the stoichiometry of the pump, which is two potassium ions in and three sodium ions out for each ATP that is hydrolyzed.  What effect does each catalytic cycle have on the membrane potential?

2.  Electronegativity doesn't enter into this idea.  However, the thermodynamic favorability or unfavorability of moving an ion depends upon two things, its concentration difference across the membrane and the membrane potential.  For sodium ions to move out of the cell, are they moving against their concentration gradient?  Are sodium ions moving with or against the membrane potential?  If we answer these two questions correctly, we will know whether it is thermodynamically favorable or unfavorable to move sodium ions from inside to outside the cell.

[Babcock_Hall]

Perhaps I can add one more thing.  Whether an ion has a thermodynamic tendency to move across a membrane depends on the difference in concentrations on each side.  But the tendency also depends on the membrane potential.  The value of the membrane potential is typically in the vicinity of -30 to -60 millivolts under many conditions, treating the outside of the cell as the initial and the inside of the cell as the final state.  The membrane potential affects all ions.

[Babcock_Hall]

I am not sure of the best way to describe the state of a cell.  One way to describe the state of the cell much of the time is homeostasis.  It is probably true that many chemical species are in steady-state.