Question

can someone explain how action potential works using the sodium potassium pump? the steps and order of the process? when Na and K go in and out of the cell? thank you

Answer 1

The sodium-potassium pump transports sodium out of and potassium into the cell in a repeating cycle of conformational (shape) changes. In each cycle, three sodium ions exit the cell, while two potassium ions enter. This process takes place in the following steps:

1.To begin, the pump is open to the inside of the cell. In this form, the pump really likes to bind (has a high affinity for) sodium ions, and will take up three of them.

2.When the sodium ions bind, they trigger the pump to hydrolyze (break down) ATP. One phosphate group from ATP is attached to the pump, which is then said to be phosphorylated. ADP is released as a by-product.

3.Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space. In this conformation, the pump no longer likes to bind to sodium ions (has a low affinity for them), so the three sodium ions are released outside the cell.

4.In its outward-facing form, the pump switches allegiances and now really likes to bind to (has a high affinity for) potassium ions. It will bind two of them, and this triggers removal of the phosphate group attached to the pump in step 2.

5.With the phosphate group gone, the pump will change back to its original form, opening towards the interior of the cell.

6.In its inward-facing shape, the pump loses its interest in (has a low affinity for) potassium ions, so the two potassium ions will be released into the cytoplasm. The pump is now back to where it was in step 1, and the cycle can begin again.

This may seem like a complicated cycle, but it just involves the protein going back and forth between two forms: an inward-facing form with high affinity for sodium (and low affinity for potassium) and an outward-facing form with high affinity for potassium (and low affinity for sodium). The protein can be toggled back and forth between these forms by the addition or removal of a phosphate group, which is in turn controlled by the binding of the ions to be transported.

For every three ions of sodium that move out, only two ions of potassium move in, resulting in a more negative cell interior. While this charge ratio does make the cell’s interior slightly more negative, it actually accounts for only a tiny fraction of the sodium-potassium pump’s effect on membrane potential.

Instead, the sodium-potassium pump acts primarily by building up a high concentration of potassium ions inside the cell, which makes potassium’s concentration gradient very steep. The gradient is steep enough that potassium ions will move out of the cell (via channels), despite a growing negative charge on the interior. This process continues until the voltage across the membrane is large enough to counterbalance potassium’s concentration gradient. At this balance point, the inside of the membrane is negative relative to the outside. This voltage will be maintained as long as long as K+ concentration in the cell stays high, but will disappear if K+ stops being imported