Overview
This lecture provides a foundational overview of membrane potentials, including the differences between diffusion and equilibrium potentials, the roles of sodium and potassium ions, and the importance of the sodium-potassium pump.
Diffusion Potential vs. Equilibrium Potential
- Diffusion potential is created when ions move down their concentration gradient across a permeable membrane, generating an electric charge difference.
- No diffusion (and thus no diffusion potential) occurs if the membrane is not permeable to the ion.
- Equilibrium potential is the membrane potential measured once ion movement reaches equilibrium, with no net ion flow.
- The equilibrium potential for an ion can be calculated using the Nernst equation if membrane permeability and ion concentrations are known.
Ion Gradients and Membrane Potentials
- Potassium (K+) is concentrated inside the cell; sodium (Na+) is concentrated outside.
- K+ moving out makes the inside of the cell negative (diffusion potential), and at equilibrium, the potential is typically –94 mV for K+.
- Na+ moving into the cell makes the inside positive, with an equilibrium potential around +61 mV for Na+.
- Both chemical (concentration) and electrical gradients affect ion movement direction and net flow.
Generation of Membrane Potentials
- Membrane potential arises from electric charge differences across the membrane due to passive ion diffusion (through channels) and electrogenic pumping (e.g., sodium-potassium ATPase).
- Sodium-potassium ATPase pumps 3 Na+ out and 2 K+ in, creating a net negative charge inside the cell.
- Resting membranes are more permeable to K+ due to potassium leak channels, influencing the resting membrane potential.
Key Terms & Definitions
- Diffusion Potential — Voltage difference created by ion movement down its concentration gradient through a permeable membrane.
- Equilibrium Potential — Membrane potential where there is no net movement of a specific ion; calculated with the Nernst equation.
- Membrane Potential — Overall electrical potential difference across a cell membrane.
- Sodium-Potassium ATPase — Active pump moving 3 Na+ out and 2 K+ into the cell, contributing to membrane potential.
- Potassium Leak Channels — Channels allowing passive movement of K+ out of the cell, increasing K+ permeability at rest.
Action Items / Next Steps
- Review the differences between diffusion and equilibrium potentials.
- Study the impact of ion gradients and permeability on membrane potential.
- Prepare for the next lecture covering resting membrane potentials in more detail.