Lecture on Cellular Potentials

Overview

• Continuation of lectures on cellular potentials.
• Focus on resting membrane potential and equilibrium potential.

Resting Membrane Potential

• Inside of cell is negative compared to outside.
• Cell maintains this negativity as its resting membrane potential.
• The actual voltage varies depending on solute concentration.

Equilibrium Potential

• Calculated using the Nernst equation.
• Determines voltage necessary to maintain concentration of a single ion.
• Assumes the ion is the only one moving across the membrane.

Sodium (Na) and Potassium (K) Equilibrium Potentials

• Sodium: High concentration outside; requires positive inside cell to maintain.
• Potassium: High concentration inside; requires negative inside cell to maintain.

Real Life vs. Experimental

• Multiple ions and forces in real life, unlike the single-ion assumption in the Nernst equation.
• Use the Goldman Hodgkin Katz (GHK) equation to calculate real resting membrane potential.

Goldman Hodgkin Katz (GHK) Equation

• Includes permeability (P) for each ion: P for Na, P for K.
• Incorporates concentration differences and permeability to find average potential.
• Resting membrane potential is closer to K equilibrium potential, indicating easier K permeability.

Example Calculation

• Na: 142 mM outside, 14 mM inside.
• K: 4 mM outside, 140 mM inside.
• Permeability ratio: K is 100 times more permeable than Na.
• Result: Approx. -86.2 mV, close to actual resting potential.

Handling Negative Ions

• Chloride (Cl-) requires modification in GHK: inside/outside ratio.
• Graduate student suggested reversing the ratio for negative ions to fix calculation errors.

Summary

• Nernst Equation: For single solute equilibrium potential experimentally.
• GHK Equation: For real resting membrane potential considering multiple ions.
• Resting membrane potential is influenced by all solutes and their permeabilities.

Membrane Properties

• Membrane is not perfect; it is leaky.
• Sodium-potassium pump maintains resting potential by moving ions against their gradients.

Sodium-Potassium Pump

• Moves Na out and K in, maintaining negative inside.
• Sodium has strong pressure to enter the cell due to concentration and charge differences.

Cellular Work

• Sodium's pressure used to bring in glucose through co-transporters.
• Sodium-glucose co-transporter leverages sodium's drive to enter the cell.

Conclusion

• Understanding voltage and concentration differences is key to understanding cellular functions.
• Encouragement to review content and ask questions in discussions and labs.