Which molecule acts as the selective carrier for potassium ions in some ion-selective membranes?

Which molecule acts as the selective carrier for potassium ions in some ion-selective membranes?

• A. Monactin
• B. Nonactin
• C. Streptomycin
• D. Valinomycin

Answer: (D)

The key idea is how ionophore-based membranes achieve selectivity by binding a specific ion and carrying it across the membrane. Valinomycin is the classic potassium-selective ionophore. It is a cyclic, neutral molecule that folds to form a hydrophobic cavity lined with carbonyl oxygens. This arrangement fits a potassium ion snugly, coordinating it efficiently while excluding the smaller sodium ion. Because the valinomycin–potassium complex is neutral, it can dissolve in the lipid membrane and shuttle the ion from one side to the other, enabling selective transport. This strong size and coordination fit for K+ over Na+ is what gives valinomycin its high selectivity in potassium-selective membranes. The other options aren’t the standard potassium-selective carrier: one is an antibiotic, and the remaining ionophores don’t provide the same potassium-focused selectivity as valinomycin.

The key idea is how ionophore-based membranes achieve selectivity by binding a specific ion and carrying it across the membrane. Valinomycin is the classic potassium-selective ionophore. It is a cyclic, neutral molecule that folds to form a hydrophobic cavity lined with carbonyl oxygens. This arrangement fits a potassium ion snugly, coordinating it efficiently while excluding the smaller sodium ion. Because the valinomycin–potassium complex is neutral, it can dissolve in the lipid membrane and shuttle the ion from one side to the other, enabling selective transport. This strong size and coordination fit for K+ over Na+ is what gives valinomycin its high selectivity in potassium-selective membranes. The other options aren’t the standard potassium-selective carrier: one is an antibiotic, and the remaining ionophores don’t provide the same potassium-focused selectivity as valinomycin.