Maintaining Cell Ion Gradients

The concentration difference of ions across the cell membrane, specifically low intracellular $Na^+$ and high intracellular $K^+$, is actively maintained by the $Na^+ K^+$ ATPase.

Mechanism of Action

• The $Na^+ K^+$ ATPase is an enzyme and a transmembrane protein.
• It functions as a pump, using energy from ATP hydrolysis to move ions against their concentration gradients.
• Specifically, it pumps 3 $Na^+$ ions out of the cell for every 2 $K^+$ ions it pumps into the cell.
• This continuous action depletes intracellular $Na^+$ and accumulates intracellular $K^+$, establishing and maintaining the characteristic gradients crucial for cell function, particularly nerve and muscle cells.

Role of Ion Channels

• $Na^+$ and $K^+$ channels facilitate the passive movement of these ions across the membrane, typically down their electrochemical gradients.
• While channels are essential for establishing the resting membrane potential and enabling ion flux, they do not actively *maintain* the concentration gradients against the natural tendency of ions to equalize. The active transport performed by the $Na^+ K^+$ ATPase is required for this maintenance.

Therefore, the $Na^+ K^+$ ATPase is directly responsible for maintaining the low $Na^+$ and high $K^+$ concentrations inside the cell relative to the extracellular fluid.