Cell Potassium and Membrane Potential

Cell and Potassium Concentration

• Cells typically contain a high concentration of potassium (K+).
• Intracellular K+ concentration: ≈ 150 millimoles per liter.
• Extracellular K+ concentration: ≈ 5 millimoles per liter.
• Brackets denote concentration.

Potassium-Sodium Pump

• Mechanism: Uses energy to pump 2 K+ ions in and 3 Na+ ions out.
• Sets up the concentration gradient.

Anions and Neutral Charge

• K+ ions inside the cell pair with negatively charged anions (e.g., proteins, chloride, phosphate)
• Ensures the net charge remains neutral.

Potassium Leak Channels

• Specific channels allow K+ ions to leak out of the cell.
• Proteins and anions do not leak out.

Movement Due to Concentration Gradient

• K+ ions move out: Driven by the concentration gradient.
• This leaves anions behind, generating a negative charge inside the cell.

Membrane Potential

• K+ ions outside sense the negative charge inside and are attracted back in.
• Two forces at play:
  • Concentration Gradient: Pushes K+ out.
  • Membrane Potential: Negative charge inside pulls K+ back in.

Graphical Representation

• Concentration Gradient Curve: K+ moves out over time until it reaches equilibrium.
• Membrane Potential Curve: The membrane potential becomes more negative and also reaches equilibrium.
• Equilibrium Potential for K+: ≈ -92 millivolts, where the movement of K+ in and out is balanced.

Concentration Considerations

• Though K+ moves out, the overall concentration inside the cell doesn't change much because of the large total number (≈ 10^20 moles) of K+ ions.
• Only a small number of ions are needed to impact the membrane potential.

Key Takeaways

• The cell uses a lot of energy to maintain the K+ concentration gradient.
• Anions help neutralize the charge inside the cell.
• Potassium leak channels allow K+ to move out, creating a dynamic balance between the concentration gradient and membrane potential.
• Equilibrium potential is achieved when the forces driving K+ out and in are balanced.