Cell Biology, A Short Course - Third Edition

Answer: A, Inward. No calculation is necessary: both the concentration and electrical forces are inward, so the electrochemical gradient, the sum of the two forces, must be inward.

Answer: B, Outward. The concentration force is outward but the voltage force is inward. We therefore need to do a calculation to find out what the net electrochemical gradient is. The equilibrium voltage for a particular ion is given by the Nernst equation (In Depth 14.2 on book page 234): V_eq = (RT/F) (1/z) log_e (concentration out/concentration in). We are considering a cell in a human, so the temperature is 37°C and RT/F has the value 0.027. z, the charge on a potassium ion, is 1. Under the concentration conditions stated, the equilibrium voltage for potassium is therefore 0.027 log_e (5/130) = -0.088 volts or -88 mV. This tells us that the membrane voltage would have to be more negative than the actual resting voltage of -80mV for the two forces to balance. At -80mV, the inward voltage force is weaker than at -88mV, so we can deduce that at -80mV the electrochemical gradient for potassium is outward.

Answer: B, Outward. The concentration force is inward but the voltage force is outward (because this is a negatively charged ion). We therefore need to do a calculation to find out what the net electrochemical gradient is. The equilibrium voltage for a particular ion is given by the Nernst equation (In Depth 14.2 on book page 234) : V_eq = (RT/F) (1/z) log_e (concentration out/concentration in). We are considering a cell in a human, so the temperature is 37°C and RT/F has the value 0.027. z, the charge on a chloride ion, is -1. Under the concentration conditions stated, the equilibrium voltage for potassium is therefore -0.027 log_e (100/6) = -0.076 volts or -76 mV. This tells us that the membrane voltage would have to be more positive than the actual resting voltage of -80mV for the two forces to balance. At -80mV, the outward voltage force is stronger than at -76mV, so we can deduce that at -80mV the electrochemical gradient for chloride is outward. In the book, we state that in a typical cell the electrochemical gradient for chloride is zero. This is true, but there are plenty of exceptions.

Answer: A, Inward. The concentration force is outward but the voltage force is inward. We therefore need to do a calculation to find out what the net electrochemical gradient is. The equilibrium voltage for a particular ion is given by the Nernst equation (In Depth 14.2 on book page 234): V_eq = (RT/F) (1/z) log_e (concentration out/concentration in). We are considering a cell in a human, so the temperature is 37°C and RT/F has the value 0.027. z, the charge on a hydrogen ion, is 1. Under the concentration conditions stated, the equilibrium voltage for hydrogen ions is therefore 0.027 log_e (40/60) = -0.011 volts or -11 mV. This tells us that the membrane voltage would have to be more positive than the actual resting voltage of -80mV for the two forces to balance. At -80mV, the inward voltage force is stronger than at -11mV, so we can deduce that at -80mV the electrochemical gradient for hydrogen ions is inward. This is a typical situation for all the cells of the body: hydrogen ions are actively pumped out of most eukaryotic cells.

Answer: A, Inward. No calculation is necessary: both the concentration and electrical forces are inward, so the electrochemical gradient, the sum of the two forces, must be inward. Lead ions are actively removed from human cells. If you did calculate the equilibrium voltage, you should have obtained the answer +24 mV. If you got +48 mV, you forgot to include z=2 in the calculation.