Biological Membranes - A. N. Ogurtsov 2012

Electrogenesis of Biomembranes
Electrical Potentials of Biomembranes
Thomas Equation

The Membrane Potential calculated using the Goldman Equation proved to be smaller in absolute value than the membrane potential calculated via the Nernst formula, and closer to its experimental values in large Cells.

Both the Nernst formula and the Goldman equation do not account for The Active Transport of ions across the membrane by ion pumps, which play a decisive role in maintaining ionic equilibrium in small cells. The cytoplasmic membrane contains Na+/K+-ATPases that pump potassium into The Cell and sodium out of the cell.

Taking into account the operation of the Na+/K+-ATPase, the Thomas equation was derived for the membrane potential

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where n is The ratio of the number of sodium ions to the number of potassium ions pumped by ion pumps across the membrane. Most commonly, the Na+/K+-ATPase operates in a mode where n = 3/2, and n is always greater than 1.

There are no ion pumps for Cl- ions, which is why the Thomas equation lacks the PCl[Cl-] terms.

The coefficient n > 1 enhances THE CONTRIBUTION OF the potassium concentration gradient to the Generation of the membrane potential; therefore, the membrane potential calculated using the Thomas equation is larger in absolute value than that calculated using the Goldman equation, and shows agreement with experimental values for small cells.

Disruption of bioenergetic processes within the cell and impairment of Na+/K+-ATPase function lead to a decrease in |φM|; in this case, the membrane potential is better described by the Goldman equation.

Damage to The cell membrane leads to an increase in membrane permeability for all ions: an increase in PK, PNa, and PCl. Due to the reduction in permeability differences, the absolute value of the membrane potential |φM| drops.

For severely damaged cells, |φM| is even smaller, yet a negative membrane potential φM is maintained due to intracellular polyanions—negatively charged Proteins, Nucleic Acids, and other large molecules that cannot penetrate the damaged membrane.

METABOLISM/35.html">Selection/41.html">Review Questions and Exercises

1. What Two Types of electrical potentials can arise in Cells and Tissues during metabolic activity?

2. What is meant by membrane potential?

3. Describe the setup for measuring intracellular potential using the microelectrode technique.

4. What is the Resting Potential?

5. What two factors determine the resting potential?

6. Write down the Nernst formula for the equilibrium membrane potential.

7. What is the reason for the significant discrepancy between experimental values of the membrane potential and those calculated using the Nernst formula in squid giant axon cells?

8. Write down the Goldman equation for the equilibrium membrane potential.

9. Under what conditions does the Nernst formula represent a special case of the Goldman equation?

10. Write down the Thomas equation for the membrane potential.



Last update: 13/08/2026

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