Can someone tell me the fundamental reason why K+ has low concentration outside of the cell and more inside of the cell?
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asked Oct 4, 2014 at 15:55
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$\begingroup$ Is this a homework question? $\endgroup$– MattDMoCommented Oct 5, 2014 at 2:07
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1$\begingroup$ By "why" are you looking for the purpose (physiological/evolutionary) or are you looking for the mechanism (membrane transporters/ion channels)? $\endgroup$– SusanCommented Oct 5, 2014 at 14:43
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$\begingroup$ Just why. Why is it not Sodium that is dominant on the outside of the cell? $\endgroup$– Project BacklogCommented Oct 5, 2014 at 16:03
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$\begingroup$ @ProjectBacklog it is sodium that is dominant on the outside of the cell. $\endgroup$– stochastic13Commented Oct 12, 2014 at 7:13
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1$\begingroup$ I maintain that there is no "just why?" here. It's either "for what purpose?" or "how is this accomplished?" To try to cover both seems too broad to me. $\endgroup$– SusanCommented Dec 11, 2014 at 0:04
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2 Answers
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Why nature has done it this way around is difficult to explain. What is certain, however, is that the membrane potential which results from the imbalance of these ions across the membrane is used for a variety of purposes, such as transport of other ions and molecules, action potential generation in neurons among many other things.
How it is accomplished is mainly through the Na+,K+-Atpase pump and secondarily through Cl- channels that exchange Na+ for Cl- to lift the amount of Cl- outside the cell.
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$\begingroup$ The question is kind of broad and once narrowed down I am happy to include citations if and where needed. $\endgroup$– AliceD ♦Commented Dec 11, 2014 at 3:12
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Your question isn't all that precise but I think looking at individual ionic equilibrium potentials begins to give us some understanding.
Looking at the equilibrium potentials(EK for potassium and ENA for sodium) for these two ions and their channels you have that EK = -90mV and ENA = +60mV in a typical neuron. This means that at these voltages you will have no NET flow of that ion across the membrane. Inside a cell you will have a certain amount of negatively charged ions called anions which will not diffuse, or move across the membrane regardless of the concentration gradient. To compensate for this (because the inside and outside of the cells like to have equal charges when there are not action potentials) you need a certain amount of a positively charged ion on the inside. NOW, there is no NET flow of EK at a negative voltage, this is around the same voltage of the entire membrane resting potential (this varies). If you look at the numbers and consider the voltage at which an action potential occurs, then you cannot have sodium be the cation inside the membrane since ENA = +60 since an action potential propagates when the membrane is driven towards a positive voltage.
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$\begingroup$ A few remarks here (1) "..you will have no NET flow of that ion across the membrane" - There is a leak current yes, but when the channels are closed you can assume there is no flow. (2) "called anions which are impermeable" - ions are never (im)permeable, the membrane is (not). (3) "they cannot and will not move across the membrane regardless of the concentration gradient" - you just claimed there is a net flow of both ions? (4) By calculating the Nernst potential, you already use the pre-existing concentration gradients, so of course you cannot swap the ions to meet the Nernst equation. $\endgroup$– AliceD ♦Commented Dec 11, 2014 at 12:08
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$\begingroup$ @ChrisStronks When I say there is no net flow of ions I mean this for sodium and potassium. At resting membrane potential the exchange of these ions across the membrane is essentially equal, sodium and potassium are cations. The anion I'm talking about which will not move across the membrane unless you break the membrane and remove them are proteins and other molecules essential to the cell. Also, equilibrium potential for a specific ion is different from the resting membrane potential. The resting membrane potential will however tend towards the potassium equilibrium potential. $\endgroup$– NolohiceCommented Dec 11, 2014 at 15:21
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$\begingroup$ If anyone wants a reference, specifically, chapter 6 in "From Neuron to Brain" fifth edition by Nicholls and others. However to get a overall picture of what I'm talking about I suggest going over chapters 4 through 6 in that book. $\endgroup$– NolohiceCommented Dec 11, 2014 at 15:41
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$\begingroup$ agreed, but your answer still does not address the question I think, as the question deals with any type of cell, while you focus on action potentials. $\endgroup$– AliceD ♦Commented Dec 11, 2014 at 22:36
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$\begingroup$ yes, because action potentials don't just occur in neuronal cells. My answer may not be the "fundamental reason" but it does provide a reason for the difference in distribution of potassium and sodium in a variety of cells termed excitable cells(those which produce action potentials). Aside from this all cells, not just excitable cells have the static - electric properties of resting membrane potential, ionic potentials and concentration gradients. Differences in distribution here can be blamed on pumps and leak channels. $\endgroup$– NolohiceCommented Dec 11, 2014 at 23:57