Sodium-Potassium Pump: An Example of Exchange Transport
- To transmit nerve impulses, neurons rely on concentration gradients of sodium (Na⁺) and potassium (K⁺) ions across their membranes.
- These gradients are maintained by the sodium-potassium pump, an active transport protein that moves ions against their concentration gradients using ATP.
How the Sodium-Potassium Pump Works
- Na⁺ Binding
- The pump is open to the inside of the cell.
- Three Na⁺ ions bind to specific sites, reducing intracellular Na⁺ concentration.
- ATP Phosphorylation
- ATP transfers a phosphate group to the pump.
- This induces a conformational change, closing the pump.
- Na⁺ Release
- The pump opens to the outside, releasing Na⁺ ions into the extracellular space.
- This increases the Na⁺ concentration outside the neuron.
- K⁺ Binding
- Two K⁺ ions bind to the pump from the outside.
- The extracellular K⁺ concentration decreases.
- Phosphate Release
- Binding of K⁺ triggers the release of the phosphate group, resetting the pump's conformation.
- K⁺ Release
- The pump reopens to the inside, releasing K⁺ ions into the cell.
- This allows the process to repeat as new Na⁺ ions bind.
- Use NOKIA as a memory aid for the sodium-potassium pump:
- NOKIA: Na⁺ Out, K⁺ In, ATP
- Breakdown:
- N: Na⁺ (sodium)
- O: Out (3 sodium ions are pumped out of the cell)
- K: K⁺ (potassium)
- I: In (2 potassium ions are pumped into the cell)
- A: ATP (energy from ATP powers the pump)
Purpose and Importance
- Maintains Membrane Potential: The pump generates a charge imbalance (3 Na⁺ out, 2 K⁺ in), creating a membrane potential essential for nerve impulses.
- Active Transport: Each cycle uses one ATP molecule to ensure directional movement of ions.
- Neuron Functionality: This exchange is crucial for resetting ion gradients after nerve signal transmission.
- Don’t confuse the sodium–potassium pump with ion channels.
- The pump uses energy (ATP) to move ions against their concentration gradients, whereas channels allow passive movement down concentration gradients.
The pump’s importance is underscored by the fact that mutations affecting its function can lead to severe neurological disorders, such as familial hemiplegic migraine and alternating hemiplegia of childhood.
Self review- What are the main steps in the sodium–potassium pump cycle?
- How does the pump contribute to the resting membrane potential?
- Why is the sodium–potassium pump considered an exchange transporter?
