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PS1 Answer Key Problem 1. Membrane Potentials (1.5 points) You have learned about ionic components of resting and action potentials in the NaCl-rich oceans of Earth. Recently, images from the Cassini spacecraft provided intriguing evidence that there might be a water ocean underneath the surface ice of Saturn’s moon, Enceladus. Suppose that there is indeed life on Enceladus in the form of a marine squid, Loligo parker . Let’s postulate that Enceladus’s oceans have KCl as the primary salt (as well as a small amount of NaCl). We will then derive the ionic bases of action potentials that might have evolved there. A. (0.6 point) a. Which ion’s conductance would dominate the L. parker resting membrane potential? Since the KCl and NaCl concentrations are reversed, the conductance of SODIUM ions will dominate the resting potential in an Encedelan squid. b. Which ion would flux most during the L. parker action potential? POTASSIUM ions will flux most, flowing inwards quickly to depolarize the cell. c. Once an action potential is triggered, does the membrane potential become more positive or more negative? Explain. Since positive potassium ions flow into the cell when an action potential is triggered, the membrane potential becomes MORE POSITIVE. B. (0.3 point) Given the following cellular ion concentrations and conductances of an Enceladan L. frazier squid, calculate the resting membrane potential. Note that Enceladus’s ocean temperature is roughly 0° C. (Show your work) Extracellular (mM) Intracellular (mM) Conductance mS/cm 2 Na + 5 mM 450 mM 1 K + 100 mM 10 mM .1 Cl - 105 mM 50 mM .01 Ca 2+ 2 mM 10 -5 mM 0 Mg 2+ 2 mM 0.5 mM 0 Protein Anions 0 High 0 R = 8.314 J K -1 mol -1 F = 9.65 x 10 4 C mol -1 First calculate the electrochemical gradient for each ionic species using the Nernst potential. Using the driving force for the ions, we then use the conductance based equation from the lecture notes to calculate the membrane potential.

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