Cellular Membrane Potential and Ion Concentration

Membrane Structure and Ion Distribution

• Cells are enveloped by membranes separating inner and outer environments.
• Ions (positively and negatively charged) have unequal distribution across the membrane, creating concentration and charge differences.

Resting Membrane Potential

• Established by concentration differences and membrane permeability.
• Outside the Cell: Higher concentration of Na+ (sodium), Cl- (chloride), Ca2+ (calcium).
• Inside the Cell: Higher concentration of K+ (potassium) and anions (A-).
  • Anions include amino acids and proteins produced by the cell.

Sodium-Potassium Pump

• Uses ATP to exchange 3 Na+ ions out of the cell for 2 K+ ions into the cell.
• Helps establish the concentration gradient for potassium and sodium.

Potassium Dynamics

• Concentration: 150 mMol/L inside, 5 mMol/L outside.
• Potassium moves out due to a concentration gradient using potassium leak and inward rectifier channels.
• Loss of K+ leaves negative anions inside, creating an electrostatic gradient.

Equilibrium and Electrostatic Gradient

• Equilibrium potential (Nernst potential) for potassium: -92 mV.
• Equilibrium potential is where concentration gradient equals electrostatic gradient.

Nernst Equation

• Used to calculate equilibrium potential for each ion:
  • Single charge ion: Vm = 61.5 * log([ion outside]/[ion inside])
  • Double charge ion: Vm = 30.75 * log([ion outside]/[ion inside])

Ion Concentrations and Potentials

• Potassium: 150 mMol/L inside, 5 mMol/L outside, Equilibrium: -92 mV.
• Sodium: Equilibrium: +67 mV.
• Chloride: Equilibrium: -86 mV (flipped due to negative charge).
• Calcium: Equilibrium: +123 mV.
• Concentration gradients move ions from high to low concentration.
  • Potassium moves out, calcium, sodium, and chloride move in.

Resting Membrane Potential

• Sum of individual ion potentials influenced by their movement across the membrane.
• Potassium is the major contributor to resting membrane potential (~90% of ions moving).
• Calculation:
  • 90% of K+ (-81 mV) + 1% of Ca2+ (1.23 mV) + 1% of Na+ (0.67 mV) + 8% of Cl- (-6.88 mV) = -86 mV.

Adjusting Membrane Potential

• Cell can alter resting potential by changing permeability to different ions (adding/removing ion channels).

Recap

• Ion equilibrium potential: where concentration gradient equals electrostatic gradient.
• Calculated using the Nernst equation.
• Resting membrane potential is a summation of these potentials, governed by ion permeability.

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End of Lecture Summary

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