Sodium-Potassium Pump

Question

From my understanding, in the sodium-potassium pump we have Na⁺ inside the cell and K⁺ outside the cell, thus forming a so called "salted banana." After reading my textbook I found many statements saying that "potassium ions tend to diffuse out of the cell...sodium ions tend to flow into the cell." I initially thought that when the action potential hits Na⁺ leaves the cell and K⁺ enters the cell, which to me contradicts the previous statement.

Can someone please clarify this and differentiate between the two? I am trying to understand the action potential process as a whole.

Thank you !

Answer 1

Your confusion is caused by the assumption that Na⁺ always leaves the cell and K⁺ always enters. The Na⁺/K⁺ pump is there to maintain membrane potential and relative Na⁺ and K⁺ ion concentrations stable inside. When an action potential (AP) is generated, sodium channels open and sodium rushes inside to depolarize the cell( 1st phase of AP). Next, the sodium channels close and K⁺ begins to leave the cell (since the inside of the cell contains too many positive charges and the cell would remain depolarized if nothing was changed). Potassium leaves through leak channels. So in summary, Na⁺/K⁺ pump and the channels that are part of AP generation result in different movement of Na⁺ and K⁺ across the cell membrane.

Here is a nice review of Na⁺/K⁺ pump and action potential generation. https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/v/sodium-potassium-pump