Among EK, ENa, and ECa, which is the most negative value?

Among EK, ENa, and ECa, which is the most negative value?

• A. EK
• B. ENa
• C. ECa
• D. All are positive

Answer: (A)

This tests the idea of ion equilibrium potentials—the voltage at which there is no net flow of a specific ion across the membrane, set by its concentration gradient and charge. Potassium has a high inside concentration and low outside, and it’s a monovalent cation, so the potassium equilibrium potential (EK) is negative, typically around -90 mV. Sodium has a large outside-to-inside gradient and is also monovalent, yielding a positive equilibrium potential around +60 to +70 mV. Calcium is divalent and experiences a very large gradient, giving an even more positive equilibrium potential, about +120 mV. Since -90 mV is more negative than +60 to +70 mV and +120 mV, EK is the most negative value among them. The resting membrane potential in many cells tends toward EK because the membrane is more permeable to K+ at rest, but the key point is the relative signs and magnitudes from these equilibrium potentials.

This tests the idea of ion equilibrium potentials—the voltage at which there is no net flow of a specific ion across the membrane, set by its concentration gradient and charge. Potassium has a high inside concentration and low outside, and it’s a monovalent cation, so the potassium equilibrium potential (EK) is negative, typically around -90 mV. Sodium has a large outside-to-inside gradient and is also monovalent, yielding a positive equilibrium potential around +60 to +70 mV. Calcium is divalent and experiences a very large gradient, giving an even more positive equilibrium potential, about +120 mV. Since -90 mV is more negative than +60 to +70 mV and +120 mV, EK is the most negative value among them. The resting membrane potential in many cells tends toward EK because the membrane is more permeable to K+ at rest, but the key point is the relative signs and magnitudes from these equilibrium potentials.