Further explanation on the Na/K pump?

Question

I was hoping someone here could help me reconcile some different things I am hearing about the Na/K pump.

My understanding was that the pump pumps out three Na for every 2 K that it pumps in and that THIS creates an electrochemical gradient, which creates membrane potential and allows the membrane to do work.

Now I am listening to a lecture where the lecturer is saying that the charge gradient is created because the K and the Na both have negative ions to balance them and then K that the Na/K pump pumps in diffuses across the membrane, leaving behind the coupled negative ions and that THAT is what causes the charge that gives the membrane potential.

Are these both correct?

Answer 1

They're both correct; each is only one part of the whole picture.  However, the second explanation sounds a bit misleading, so maybe this will help clear things up.

The first part of the explanation is accurate.  Because the number of positive Na ions being pumped out is less than the number of positive K ions being pumped in, this creates a slight surplus of positive charge outside the cell.

The cell also contains what are called ion leakage channels, which allow Na and K ions to flow into or out of the cell.  However, there are more K ion channels than Na ion channels, which means that more K ions can flow out than Na ions can flow in.  This inequality also helps create and maintain the gradient.

Most critically, however, is that the cytoplasm naturally contains a large concentration of negative charge in the form of ions, amino acids, proteins, and so on, all of which cannot penetrate the cell membrane.  It is not the mismatched amount of positive ions itself that creates the voltage.  What matters is that the mismatched amount of positive ions leaves more positive charge on the outside of the cell while all of the negatively charged proteins and other molecules stay inside.