8. Chloride (Cl-), like Na+, is more concentrated outside the cell than inside,

Question

8. Chloride (Cl-), like Na+, is more concentrated outside the cell than inside, but its valence (z) is negative (-1, to be exact). What can you conclude about chloride’s equilibrium potential (ECl)? 9. Use the Nernst equation to find ECl for a cell whose extracellular [Cl-] is 110 mM and whose intracellular [Cl-] is 5 mM. You may use a calculator. Does your answer match your conclusion in #8 above? IV. A graphical approach to electrochemical gradients The Nernst equation tells you the membrane potential at which the electrical and chemical gradients are exactly counterbalanced. Generally, though, we will want to know whether a specific ion will flow in or out at a specific membrane potential that is not the equilibrium potential. Here is Dr. C’s recommended method for determining the direction of an ion’s flow at any membrane potential: A. Find the ion’s equilibrium potential (E). B. Set up a graph with membrane potential on the X axis and overall driving force on the Y axis. C. Plot 2 easy points: the X-intercept (when Y=0) and the Y-intercept (when X=0). D. Connect the dots! The chemical gradient is assumed to be constant throughout this process. To see how this method actually works, let’s do an example with Cl- ions, using the information given above. 8. Chloride (Cl-), like Na+, is more concentrated outside the cell than inside, but its valence (z) is negative (-1, to be exact). What can you conclude about chloride’s equilibrium potential (ECl)? 9. Use the Nernst equation to find ECl for a cell whose extracellular [Cl-] is 110 mM and whose intracellular [Cl-] is 5 mM. You may use a calculator. Does your answer match your conclusion in #8 above? IV. A graphical approach to electrochemical gradients The Nernst equation tells you the membrane potential at which the electrical and chemical gradients are exactly counterbalanced. Generally, though, we will want to know whether a specific ion will flow in or out at a specific membrane potential that is not the equilibrium potential. Here is Dr. C’s recommended method for determining the direction of an ion’s flow at any membrane potential: A. Find the ion’s equilibrium potential (E). B. Set up a graph with membrane potential on the X axis and overall driving force on the Y axis. C. Plot 2 easy points: the X-intercept (when Y=0) and the Y-intercept (when X=0). D. Connect the dots! The chemical gradient is assumed to be constant throughout this process. To see how this method actually works, let’s do an example with Cl- ions, using the information given above. 8. Chloride (Cl-), like Na+, is more concentrated outside the cell than inside, but its valence (z) is negative (-1, to be exact). What can you conclude about chloride’s equilibrium potential (ECl)? 9. Use the Nernst equation to find ECl for a cell whose extracellular [Cl-] is 110 mM and whose intracellular [Cl-] is 5 mM. You may use a calculator. Does your answer match your conclusion in #8 above? IV. A graphical approach to electrochemical gradients The Nernst equation tells you the membrane potential at which the electrical and chemical gradients are exactly counterbalanced. Generally, though, we will want to know whether a specific ion will flow in or out at a specific membrane potential that is not the equilibrium potential. Here is Dr. C’s recommended method for determining the direction of an ion’s flow at any membrane potential: A. Find the ion’s equilibrium potential (E). B. Set up a graph with membrane potential on the X axis and overall driving force on the Y axis. C. Plot 2 easy points: the X-intercept (when Y=0) and the Y-intercept (when X=0). D. Connect the dots! The chemical gradient is assumed to be constant throughout this process. To see how this method actually works, let’s do an example with Cl- ions, using the information given above.

Explanation / Answer

8) The equilibrium potential of chloride ions is given by the formula,

RT/ZF ln ((Cl)i/(Cl)o)

R is the universal gas constant, T is temperature in kelvins, Z is the valency of ion, F is Faraday's constant.

The concentration of chloride ion is more on the outside than inside the cell. So, the ratio of (Cl)i/(Cl)o is always less than one. Log values of this ratio is always negative. The valency of chloride ion is -1. Two negative values when multiplied together becomes positive.

Therefore, we can conclude that the equilibrium potential of chloride ion is always positive.

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