Problem 8
Typical $K^+$ ion concentrations are 140 mM inside the cell, and 5 mM outside of the cell. Given these $K^+$ ion concentrations, an equilibrium potential of -90 mV is given by the Nernst equation. Assume that you have cells in culture with $K^+$ leak channels, but no other ion channels.
1. (2 pts) At a cell membrane potential of -30 mV, which direction are $K^+$ ions being pushed by the electric field? (A) Into the cell. (B) Out of the cell. (C) Neither
2. (2 pts) At -30 mV, what is the direction of net $K^+$ ion transport? (A) Into the cell. (B) Out of the cell. (C) Neither.
After allowing the membrane potential to reach -90 mV, you add a solution of KCl to the medium surrounding your cells, adding both $K^+$ ions and $Cl^-$ ions. The concentration of $K^+$ ions outside of the cell is raised to 10 mM, but the concentration inside the cell is not affected initially.
3. (2 pts) After adding the KCl solution, the diffusion of $K^+$ ions... (Choose ONE)
(A) Increases in magnitude. (B) Decreases in magnitude. (C) Remains the same.
4. (2 pts) After adding the KCl solution, at a membrane potential of -90 mV, what is the direction of net $K^+$ ion transport? (Choose ONE) (A) Into the cell. (B) Out of the cell. (C) Neither
5. (2 pts) After adding the KCl solution, the equilibrium potential... (Choose ONE)
(A) Becomes more negative. (B) Becomes less negative. (C) Remains unchanged.
6. (2 pts) You add the KCl solution and wait for a period of time. The cell membrane potential is now stable. What is the $K^+$ ion concentration inside the cell? (A) Significantly higher than 140 mM. (B) Significantly lower than 140 mM. (C) Not significantly changed, remaining at 140 mM.
7. (2 pts) As you add the KCl solution, what is the instantaneous effect on the electric field across the cell membrane, prior to any ion transport across the cell membrane? (A) Electric field is increased. (B) Electric field is decreased. (C) Electric field is unaffected.
Suppose that your cells have $Cl^-$ leak channels instead of $K^+$ leak channels, and assume $Cl^-$ concentrations of 140 mM inside the cell, and 5 mM outside of the cell.
8. (2 pts) At a cell membrane potential of 0 mV, what is the direction of net $Cl^-$ ion transport? (A) Into the cell. (B) Out of the cell. (C) Neither.
9. (2 pts) Once the equilibrium potential has been reached, which direction is the electric field pointing? (A) Into the cell. (B) Out of the cell. (C) Neither.
