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So my teacher asked: "$[K^+]_i$ (k+ on the inside) is about forty times higher than $[K^+]_o$ (k- on the outside). With so many more positively charged potassium ions inside versus outside, why isn’t the interior of the cell positively charged compared to the exterior?"

My Thoughts: I am assuming that there must be enough negative charges inside the cell from other things like chlorine or ATP and proteins that the net coulombs on the inside is lower than the net coulombs on the outside. Does this seem like the correct answer? I am hesitant to use this because so far my teacher has only talked about potentials and not charges. I completely understand that the potential is negative, but potential and charge are two different things.

Edit: posted exact question from teacher as opposed to original summary of question

Abid Rizvi
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  • Since we're just talking about a potential differences, it doesn't have to be additional negative charges inside; it can also be additional positive charges outside. The Na/K ATPase also pumps out a lot of Na+ ions. See this page: https://en.wikipedia.org/wiki/Na%2B/K%2B-ATPase – Roland Sep 14 '16 at 22:06
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    this is a fair point. So Na+ is pumped out and K+ in at a ratio of 3:2 respectively. Therefore is it safe to assume that there are a greater number of Na+ outside than there are K+ inside, hence the greater positive charge on the outside? – Abid Rizvi Sep 14 '16 at 22:18
  • That's right. Now you could answer your own question! :-) – Roland Sep 15 '16 at 20:30
  • There are anions too, inside the cell. For say in a electrical-chargeless 0.1 (M) NaCl, there is 6.023 * (10^22) particles of Na+ and exact same amount of Cl- particles; and a slight increase of Cl- concentration at the same-time at certain place, could give rise to overall negative charge. Also, not only mineral ions, large-biomolecules contain carboxyl residues (COO-). – Always Confused Sep 19 '16 at 12:06

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Net Potential on a cell surface is always measured in comparison to the extracellular media...so it is a relative parameter. So inside of the cell membrane is negative implies that there is more negative charges inside or more positive changes outside compared to the respective opposite side of the membrane. As we do not deal with the "absolute potential" we need not calculate the values of them.

K+ concentration is more inside the cell compared to outside...but there are other ions in and outside cells...e.g. Na+, Ca+, proteins, Cl- etc...all of them together determine the potential difference across cell membrane.

Arnb
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