Primair actief transport

Unlike passive transport, primary active transport utilizes energy from ATP hydrolysis to transport ions against their electrochemical gradients.

The sodium-potassium pump is one such transmembrane protein with its extracellular side closed and its intracellular region open and associated with a molecule of ATP. In this conformation, the transporter has a high affinity for sodium ions present in the cytoplasm. So, three of these ions enter from the cytosolic side and bind to the pump.

Such binding allows ATP to transfer one of its phosphate groups to the transporter, providing the energy needed to close the pump's intracellular side and open towards the extracellular region.

This new conformation decreases the pump's affinity for sodium ions such that they are released into the extracellular space, but increases its affinity for potassium, allowing it to bind two potassium ions present in the environment.

The phosphate group on the transporter then detaches, enabling a new ATP molecule to associate with the pump's intracellular side. The pump opens and allows the potassium ions to exit into the cell, returning the transporter to its initial shape and thus beginning the cycle again.