Solution

ID: past-exam-of-the-biological-physics-course-of-the-university-of-oxford/2015/7/c/solution

Including both flat ends of the cylindrical cell, its membrane surface area is
The capacitance is . The electric charge needed for the specified depolarization is therefore
Ignoring the ends gives , only about one percent smaller. Inward movement of positive ions, or outward movement of negative ions, produces the stated positive change in the inside-minus-outside membrane potential.
The cell volume is . Distributed over this volume, the transferred number would correspond to only . Relative to an illustrative, not source-specified, internal ionic pool, that is a fraction . Thus a substantial change in membrane potential requires movement of only a tiny fraction of bulk ions; it does not require wholesale depletion of the intracellular solution. The separated electric charge is concentrated near the two membrane surfaces.

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