Question

Solutions A and B are separated by a membrane that is permeable to Ca₂⁺ and impermeable to Cl⁻. Solution A contains 10mM CaCl₂, and solution B contains 1 mM CaCl₂. Assuming that 2.3 RT/F = 60 mV, Ca₂⁺ will be at electrochemical equilibrium when

1) solution A is +60 mV
2) solution A is +30 mV
3) solution A is -60 mV
4) solution A is -30 mV
5) solution A is +120 mV
6) solution A is -120 mV
7) the Ca₂⁺ concentrations of the two solutions are equal
8) the Cl⁻ concentrations of the two solutions are equal

Answer 1

Ca2+ will be at electrochemical equilibrium across a permeable membrane when the driving forces of the concentration gradient and membrane potential balance each other out. The equilibrium position for Ca2+ is not determined by Cl− concentrations since Cl− cannot cross the membrane.

Step-by-step explanation:

To understand when Ca2+ will be at electrochemical equilibrium across a permeable membrane between two solutions with different CaCl2 concentrations, one must consider both the chemical gradient (concentration gradient) and the electric gradient (voltage) affecting ion movement. In this scenario, the membrane is permeable to Ca2+ ions but not to Cl− ions, meaning only Ca2+ can pass through. Since Cl− cannot cross the membrane, its concentration will not influence the equilibrium position for Ca2+.

The electrochemical equilibrium for Ca2+ is reached when the chemical driving force, due to the concentration gradient, is equaled and opposed by the electrical driving force, due to the membrane potential. The Nernst equation can be used to calculate the voltage at which electrochemical equilibrium for Ca2+ is established between two solutions of differing concentrations.