High School

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Equilibrium Potentials

Use the simplified Nernst Equation from the lecture to calculate the equilibrium potentials for Ca²⁺ and Cl⁻ based on the concentrations provided.

Which of the following is the equilibrium potential for Ca²⁺ considering the extracellular Ca²⁺ concentration is 2.5 mM and the cytosolic concentration is 100 nM?

A. 270.5 mV
B. -98.5 mV
C. -49.3 mV
D. 135.2 mV

Answer :

Tthe equilibrium potential for Ca²⁺ with extracellular Ca²⁺ concentration of 2.5 mM and cytosolic concentration of 100 nM is D. 135.2 mV.

The Nernst Equation is used to calculate the equilibrium potential for a specific ion based on the concentration of that ion inside and outside the cell. The simplified form of the Nernst Equation for an ion with charge z is:

E_ion = (61/z) * log([ion]_out/[ion]_in)

For Ca²⁺ (which has a charge of +2), considering the given extracellular concentration ([Ca²⁺]_out) of 2.5 mM and the cytosolic concentration ([Ca²⁺]_in) of 100 nM (which is equal to 0.0001 mM), we can substitute these numbers into the equation:

E_Ca²⁺ = (61/2) * log(2.5/0.0001)

Solve the logarithm and the calculation as follows:

  • log(2.5/0.0001) = log(25000) ≈ 4.3979
  • E_Ca²⁺ = (61/2) * 4.3979 ≈ 135.2 mV

Therefore, the correct option is d. 135.2 mV.

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