Helpful Hints for Applying the Nernst Equation to Membrane Potentials: 1) If the potential difference measured across a membrane is equal to the potential difference calculated from the Nernst equation for a particular ion, then that particular ion is in electrochemical equilibrium across the membrane, and there will be no net flow of that ion across the membrane. 2) If the measured electrical potential is of the same sign as that calculated from the Nernst equation for a particular ion but is larger in magnitude, then the electrical force is larger than the concentration force, and net movement of that particular ion will tend to occur in the direction determined by the electrical force. 3) When the electrical potential difference is of the same sign but is numerically less than that calculated from the Nernst equation for a particular ion, then the concentration force is larger than the electrical force, and net movement of that ion tends to occur in the direction determined by the concentration difference. 4) If the electrical potential difference measured across the membrane is of the sign
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