一次能動輸送

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

The sodium–potassium pump is one such transmembrane protein. Its extracellular side is closed, while its intracellular side is open and associated with an ATP molecule.

In this conformation, the transporter has a high affinity for sodium ions present in the cytoplasm. Three of these ions enter the pump from the cytosolic side and bind to it.

This binding allows ATP to transfer one of its phosphate groups to the transporter. This step provides the energy needed to close the pump’s intracellular side and open the extracellular side.

This new conformation lowers the pump’s affinity for sodium ions, causing it to export three sodium ions into the extracellular space. At the same time, its affinity for potassium increases, allowing it to bind two potassium ions.

The phosphate group on the transporter then detaches, allowing a new ATP molecule to bind to the pump’s intracellular side. The pump opens and imports two potassium ions into the cell, returning the transporter to its initial shape and beginning the cycle again.

This process maintains the ion gradient necessary for cellular functions, such as nerve signaling, muscle contraction, and secondary active transport.