A nerve cell is shaped like a cylinder. The membrane wall of the cylinder has a +0.07-Vpotential difference from the inside to the outside of the wall. To help maintain this potential difference, sodium ions are pumped from inside the cell to the outside. For a typical cell, 109 ions are pumped each second.

a) Determine the change in chemical energy each second required to produce this increase in electric potential energy. Express your answer to two significant figures and include the appropriate units.

b) If there are roughly 7×1011 of these cells in the body, how much chemical energy is used in pumping sodium ions each second? Express your answer to two significant figures and include the appropriate units.

c) Estimate the fraction of a person's metabolic rate used to pump these ions. Assume the metabolic rate to be 100 W. Express your answer using two significant figures.