The sodium-potassium pump is a protein pump
found in the cell membrane of all animal cells. Its main function is to transport sodium ions
out of the cell and potassium ions into the cell. This serves a number of purposes, but is critically
important to the function of neurons as it helps to maintain a higher concentration of
sodium ions outside the cell, and a higher concentration of potassium ions inside the
cell. Preserving these differences in ion concentrations
helps to stabilize the cell’s membrane potential. This is critical for the neuron to be able
to fire an action potential, which is the basis of the electrical signaling of neurons. The sodium-potassium pump is a large membrane-associated
enzyme that uses the energy of adenosine triphosphate, or ATP, to transport sodium and potassium
ions across the cell membrane. All of the details of how this is achieved
are not yet fully clear, but the general process involves the binding of ATP to the pump, which
promotes the binding of 3 sodium ions and the release of 2 previously bound potassium
ions. ATP is broken down and transfers a chemical
group known as a phosphate group to the pump, which prompts the pump to undergo a conformational
change, or a change in its shape. This causes the bound sodium ions to be released
into the extracellular fluid. At the same time, the pump binds 2 potassium
ions, which also prompts another conformational change that returns the pump to its previous
configuration and the cycle begins again. Each cycle of the pump causes 3 sodium ions
to move out of the cell and 2 potassium ions to move into the cell. Because there is one more positive charge
leaving the cell than entering, there is a net loss of positive ions, which makes the
resting membrane potential of the cell slightly more negative.