Sodium-Potassium Ion Pump: Mechanism and Function

Membrane Transport: Sodium-Potassium Ion Pump

Concept: Sodium-Potassium Ion Pump

The sodium-potassium ion pump (Na+/K+ ATPase) is a vital membrane protein that maintains cellular electrochemical gradients by actively transporting sodium and potassium ions across the plasma membrane. This process is essential for nerve impulse transmission, muscle contraction, and maintaining cell volume.

• Inside of cells: More negative compared to the outside, establishing an electrical gradient.

• Ion gradients:

  • High [Na+] outside the cell

  • High [K+] inside the cell

• Function: Maintains these gradients by pumping 3 Na+ ions out and 2 K+ ions in per ATP hydrolyzed.

Sodium-Potassium Ion Pump as a P-Type ATPase

P-type ATPases are a class of ATP-driven pumps that transport ions against their concentration gradients. The Na+/K+ pump is a classic example, crucial for cellular homeostasis.

• Active transport: Moves ions against their gradients using energy from ATP hydrolysis.

• Antiport mechanism: Na+ and K+ are transported in opposite directions.

• ATP hydrolysis: Provides the energy for conformational changes in the pump.

Mechanism of Sodium-Potassium Pump

The pump operates through a series of steps involving binding, phosphorylation, conformational change, and release of ions.

1. Binding of 3 Na+ ions inside the cell.

2. ATP hydrolysis and phosphorylation of the pump.

3. Conformational change releases 3 Na+ ions outside the cell.

4. Binding of 2 K+ ions from outside.

5. Dephosphorylation and conformational change.

6. Release of 2 K+ ions inside the cell.

Equation for ATP hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

Physiological Importance

• Establishes a membrane potential (more negative inside, more positive outside).

• Drives secondary active transport of other molecules (e.g., glucose, amino acids).

• Regulates cell volume and osmotic balance.

Example: Sodium-Potassium Ion Pump Cycle

The cycle involves alternating binding and release of Na+ and K+ ions, powered by ATP hydrolysis, resulting in the net movement of positive charge out of the cell.

Practice Questions (from notes)

• Type of transport: Active transport through a symporter (correct answer from notes).

• False statement: "It helps to create a transmembrane potential that is more negative on the inside and more positive on the outside." (correct answer from notes).

• Order of steps:

  1. Na+ ions bind.

  2. ATP hydrolysis and phosphorylation.

  3. Conformational change releasing 3 Na+ ions outside.

  4. K+ ions bind.

  5. Dephosphorylation and conformational change.

  6. Release of 2 K+ ions inside.

Summary Table: Sodium-Potassium Pump Steps

Step	Event
1	3 Na+ bind inside
2	ATP hydrolysis and phosphorylation
3	Conformational change, 3 Na+ released outside
4	2 K+ bind outside
5	Dephosphorylation and conformational change
6	2 K+ released inside

Additional info: The sodium-potassium pump is a classic example of primary active transport and is essential for many physiological processes, including nerve impulse transmission and muscle contraction.