Equilibrium potential for an ion is calculated using-
**Core Concept**
The equilibrium potential for an ion is a measure of the electrical potential that must be applied across a cell membrane to prevent the net movement of that ion. It is calculated based on the ion's concentration gradient and its charge, as per the Nernst equation.
**Why the Correct Answer is Right**
The Nernst equation calculates the equilibrium potential (E) for an ion as follows: E = (RT/F) * ln([ion]_out / [ion]_in), where R is the gas constant, T is the temperature in Kelvin, F is the Faraday constant, [ion]_out is the concentration of the ion outside the cell, and [ion]_in is the concentration of the ion inside the cell. This equation takes into account the ion's charge, as well as its concentration gradient across the membrane.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the Nernst equation, which is the correct formula for calculating the equilibrium potential.
**Option B:** This option is incorrect because it mentions the Goldman-Hodgkin-Katz equation, which is used to calculate the resting membrane potential, not the equilibrium potential for a single ion.
**Option C:** This option is incorrect because it mentions the diffusion potential, which is a related concept, but not the correct formula for calculating the equilibrium potential.
**Clinical Pearl / High-Yield Fact**
The Nernst equation is a fundamental concept in understanding how ions move across cell membranes, and is essential for understanding the mechanisms of various physiological and pathological processes, including nerve conduction and muscle contraction.
**Correct Answer: C. Nernst equation. The Nernst equation calculates the equilibrium potential for an ion.**