Imagine a scenario in which a negatively charged ion, Ion A-, is more concentrated on the outside of the cell membrane, yet is at electrochemical equilibrium (i. e., there is no net ion flux across the membrane, even though the membrane is permeable to Ion A-). Which of the following must be true in order for this scenario to be true?

a. The inside of the cell must be negatively charged relative to the outside of the cell (a negative Vm).

b. The inside of the cell must be positively charged relative to the outside of the cell (a positive Vm).

c. Ion A - must be only slightly negatively charged. Ion A - must be extremely negatively charged.

d. This scenario is impossible.

d. This scenario is impossible

Explanation:

This task is set to example the knowledge of how changes in the relative permeability values for ions (K + (pK), Na + (pNa), and Cl - (pCl)) in this case ion A - can lead to changes in the membrane potential which are electrochemical driving force acting on ions.

We need to examine the scenario below;

Imagine a scenario in which a negatively charged ion, Ion A-, is more concentrated on the outside of the cell membrane, yet is at electrochemical equilibrium (i. e., there is no net ion flux across the membrane, even though the membrane is permeable to Ion A-).

Definition of terms;

Vm - is the membrane potential

VDF - is the electrochemical driving force acting on the ion of interest. (Used to predicting the direction of ion flow across the plasma membrane (i. e., into or out of the cell).

Veq. - is the equilibrium potential for the ion of interest

At electrochemical equilibrium;

Vm = Veq., i. e. VDF = Vm - Veq. = 0.

Meaning that there is no net movement of the ion across the plasma membrane into or out of the cell (simply put there is no net flux of ion).

Examining the scenario we see Vdf <0 negative driving force predicts ion movement into the cell (influx) because negatively charged ion, Ion A-, is more concentrated on the outside of the cell membrane.

The scenario is impossible because the electrochemical driving force on ion is not equals to zero.

VDF = Vm - Veq.

Where VDF = Electrochemicall driving force

Vm=Membrane potential

Veq.=Equilibrium potential for the ion of interest

Ideally when VDF=0 at Electrochemical equilibrium, i. e. Vm=Veq, there is no net movement of ions across the membrane.

So, Answer is (D) This scenario is impossible.