Nernst potential of Na+ is
**Core Concept**
The Nernst potential, also known as the equilibrium potential, is a measure of the electrical potential difference that would exist across a cell membrane if all the ions on one side of the membrane were in equilibrium with those on the other side. For a given ion, the Nernst potential is determined by the ion's concentration gradient and its charge.
**Why the Correct Answer is Right**
The Nernst potential for sodium (Na+) is calculated using the Nernst equation, which takes into account the ion's concentration gradient and its charge. In a typical mammalian cell, the extracellular concentration of sodium is around 140 mM, while the intracellular concentration is around 10-15 mM. The Nernst potential for Na+ is given by the equation: E_Na = (RT/F) \* ln([Na+]_out / [Na+]_in), where R is the gas constant, T is the temperature in Kelvin, F is Faraday's constant, and ln is the natural logarithm. At room temperature, this works out to approximately -61 mV, which is the value given in the correct answer.
**Why Each Wrong Option is Incorrect**
**Option A:** The value of 94 mV is incorrect because it is too positive. This would indicate a Nernst potential that is far from the actual value for Na+ in a typical mammalian cell.
**Option C:** The value of -64 mV is incorrect because it is close to the actual value, but not quite correct. This value is often cited as the Nernst potential for Na+, but it is a simplification that does not take into account the actual concentrations of Na+ in a typical mammalian cell.
**Option D:** The value of -94 mV is incorrect because it is too negative. This would indicate a Nernst potential that is much more negative than the actual value for Na+ in a typical mammalian cell.
**Clinical Pearl / High-Yield Fact**
The Nernst potential is an important concept in understanding how ions move across cell membranes and how they contribute to the resting membrane potential. A key point to remember is that the Nernst potential is a equilibrium potential, meaning that it is the potential at which the net flow of ions across the membrane is zero.
✓ Correct Answer: B. 61