Living cells “pump” singly ionized sodium ions, Na+, from the inside of the cell to the outside to maintain a membrane potential ΔVmembrane = Vin−Vout = -70 mV It is called pumping because work must be done to move a positive ion from the negative inside of the cell to the positive outside, and it must go on continuously because sodium ions “leak” back through the cell wall by diffusion.
A) W = |q||ΔV| = 1.6×10^-19 x 0.070 = 1.05X10^-20 J
B) In one second, pumping N sodium ions uses 15J so:
N = 15/(1.05X10^-20) = 1×10^21 (to 1 sig. fig.)
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