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Why is the inside of a cell more negative then the outside?

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    ..I know there is more potassium inside the cell then the outside and more sodium outside the cell then the inside.. but why is the inside of the cell more negative relative to the outside if potassium and sodium are positive ions?
    I understand it's got something to do with membrane potential and voltage difference and something about equilibrium potential (which I have read in my textbook a million times).. but I don't get it and do not understand it still! Can someone kindly explain it to me please?

    (this module I'm taking is biology based and I'm not so good at biology!)
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    (Original post by snowflake23)
    ..I know there is more potassium inside the cell then the outside and more sodium outside the cell then the inside.. but why is the inside of the cell more negative relative to the outside if potassium and sodium are positive ions?
    I understand it's got something to do with membrane potential and voltage difference and something about equilibrium potential (which I have read in my textbook a million times).. but I don't get it and do not understand it still! Can someone kindly explain it to me please?

    (this module I'm taking is biology based and I'm not so good at biology!)
    Don't forget about the chloride (Cl-) ions and the carboxylate ions (COO-) Inside the cell (I believe).

    Also, we're talking about potential difference. Both are positive ions, yes, but there are more positive sodium ions outside than positive potassium ions inside, so the difference makes the outside relatively more positive.
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    There are potassium leak channels which are always open (different from the voltage gated potassium channels which only open with changes in voltage); potassium can move freely out of the membrane. You are right in stating that the external environment of the neuron is positive because of the high Na conc. and so thinking in terms of diffusion, a lot of K+ will diffuse out via these leak channels, down the concentration gradient. But the negativity of the inside of the neuron will cause some K+ to diffuse back making it slightly positive, but still negative compared to the outside. The equilibrium potential: membrane potential at which the net diffusion of K+ (for example) out of the cell ceases (ie: the point at which K+ diffuses out due to concentration gradient is balanced by its movement due to the negative electric potential). Also, I think that there is some Cl- inside the cell too, but you may need to double check that.
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    Has no one mentioned A-, proteins inside of cells, which are negatively charged?

    I suppose you could argue that there is quite a bit of negatively charged DNA (phosphate groups), as well as Pi, from the hydrolysis of ATP --> ADP + Pi, catalysed by the enzyme, ATP synthase.
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    Keyword is RELATIVE. You've got more sodium outside than inside and generally more potassium inside than outside. Plus, lots of anions inside compared to outside. (Though more chlorides are outside than inside)

    At rest, only potassium channels are open hence, they diffuse out and down their concentration gradient. This makes it relatively more negative inside (hence, potential difference is approx -70mV in nerve cells). Then there is an electrochemical gradient pulling potassium in because it is more negative inside. You get a dynamic equilibrium between the inside and outside= resting membrane potential.

    Basically: Its negative because of the diffusion of potassium out (NOT na+/k+-ATPse btw.)

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Updated: May 5, 2012
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