Na+ / K+ ATPase: How does it restore resting membrane potential?
The Na+/K+ ATPase plays a critical role in restoring the resting membrane potential of a cell. Resting membrane potential refers to the difference in electrical charge across the cell membrane when the cell is at rest.
The Na+/K+ ATPase is an enzyme that is embedded in the cell membrane and actively transports sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. This process requires the hydrolysis of ATP (adenosine triphosphate), which provides the energy needed for the pump to function.
The mechanism by which the Na+/K+ ATPase restores the resting membrane potential involves the following steps:
1. Binding of intracellular Na+ ions: The Na+/K+ ATPase has binding sites specific to sodium ions on the inside of the cell. Three Na+ ions bind to these sites, causing a conformational change in the pump.
2. ATP hydrolysis: The binding of the Na+ ions triggers the hydrolysis of ATP, leading to the release of a phosphate (P) group and ADP (adenosine diphosphate). This provides the energy necessary for the pump to perform mechanical work.
3. Translocation of Na+ ions: The conformational change in the pump allows it to release the bound Na+ ions to the outside of the cell. This translocation occurs against the concentration gradient, as the concentration of Na+ ions is higher outside the cell compared to inside.
4. Binding of extracellular K+ ions: After releasing the Na+ ions, the pump has empty binding sites on the extracellular side, specifically for potassium ions. Two K+ ions from the extracellular fluid enter these binding sites due to their electrochemical gradient.
5. Release of phosphate group: The binding of the K+ ions causes further conformational changes in the pump, leading to the release of the phosphate group from step 2.
6. Translocation of K+ ions: These conformational changes also allow the pump to translocate the bound K+ ions back into the cytoplasm, against their concentration gradient. This completes one cycle of the pump.
Overall, the Na+/K+ ATPase actively transports three Na+ ions out of the cell while bringing in two K+ ions, resulting in an overall net export of positive charges from the cell. This helps to restore the resting membrane potential by contributing to the negative charge on the inside of the membrane, making it more polarized.
The continuous operation of the Na+/K+ ATPase is essential for maintaining the resting membrane potential, which is critical for processes such as nerve conduction, muscle contraction, and other cellular activities.
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