The resting membrane potential (RMP) of a cell is primarily determined by the distribution of ions across the cell membrane and the permeability of the membrane to those ions. The concepts of active and passive determinants relate to how this potential is established and maintained.
Passive Determinants:
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Ion Concentration Gradients: The primary passive determinants of the resting membrane potential are the concentration gradients of ions (such as sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and others) across the membrane. These gradients result from the differential distribution of ions maintained by various cellular processes.
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Membrane Permeability: The resting membrane potential is largely influenced by how permeable the membrane is to different ions. At rest, the membrane is significantly more permeable to K⁺ ions than to Na⁺ ions. This is due to the presence of specific ion channels, particularly potassium leak channels, that allow K⁺ to flow out of the cell more freely than Na⁺ can flow in.
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Nernst Equation: The passive contribution to the resting membrane potential can be described using the Nernst equation, which calculates the equilibrium potential for a specific ion based on its concentration gradient across the cell membrane.
Active Determinants:
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Ion Pumps: Active determinants include the action of ion pumps, such as the sodium-potassium pump (Na⁺/K⁺ ATPase). This pump actively transports three Na⁺ ions out of the cell and two K⁺ ions into the cell, which helps maintain the concentration gradients necessary for resting membrane potential. This action requires energy in the form of ATP.
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Restoration of Gradients: The active transport mechanisms help restore and maintain the ion concentration gradients that might be altered during cellular activity or following depolarization events, ensuring that the conditions for the resting membrane potential are preserved.
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Overall Contribution to RMP: While passive determinants establish a baseline resting membrane potential based on equilibrium potentials for different ions, active determinants continually adjust the ion concentrations and maintain the gradients that allow the resting potential to remain stable over time.
Summary
- Passive determinants influence the resting membrane potential through ion concentration gradients and membrane permeability to ions, driven by physical processes.
- Active determinants involve the use of metabolic energy to maintain those ion gradients and restore them after disturbances, primarily through pumps like the Na⁺/K⁺ ATPase.
Both types of determinants work together to establish and maintain the resting membrane potential necessary for the functioning of excitable cells such as neurons and muscle cells.