Vz2- is a hypothetical ion with an equilibrium potential of -80mV. There are no leak channels for this ion in the cell membrane.
If membrane potential is -70mV, in which direction will this ion passively diffuse across the membrane?

Question

BlackTom AI · Accepted Answer

Vm - Eion = -70 mV - (-80 mV) = +10 mV, so the electrical driving force on Z2- is 10 mV. The equilibrium (Nernst) potential for an ion is the membrane voltage at which net passive flux of that ion is zero; any difference Vm - Eion gives the driving force that would cause passive movement if the membrane is permeable to that ion. Passive flux (or ionic current) is proportional to membrane conductance for that ion (I = g(Vm - Eion)).

Because Z2- is a negatively charged ion (charge -2), a Vm that is 10 mV more positive than Eion indicates the ion “wants” to move in whatever direction will drive Vm toward -80 mV. Bringing the membrane from -70 mV to -80 mV requires adding negative charge inside (or removing positive charge). Thus, if Z2- channels were present, the passive flux would be inward (Z2- would enter the cell), carrying negative charge in and hyperpolarizing toward -80 mV.

However, the problem explicitly states there are no leak channels for this ion in the membrane. Passive diffusion across a membrane requires a permeation pathway (channel or transporter) and cannot occur through an impermeable lipid bilayer. Therefore, regardless of the calculated driving force (+10 mV), there will be no passive movement of Z2- across the membrane.

Why other answers are wrong: “Into the cell” would be the direction of passive flux only if Z2- channels existed; “out of the cell” would be incorrect given both the sign of the driving force and the direction needed to reach Eion. Because the membrane is impermeable to Z2- (no leak channels), the correct outcome is no net passive diffusion.

Final answer: c. No net.

Vz2- is a hypothetical ion with an equilibrium potential of -80mV. There are no leak channels for this ion in the cell membrane. If membrane potential is -70mV, in which direction will this ion passively diffuse across the membrane?单项选择题

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