440 Chapter 11. Machines in membranes[[Student version, January 17, 2003]]
a. Table 11.1 on page 416 shows that sodium ions are far from equilibrium in the resting state.
Find the conductance per area for these ions, using the value 5Ω−^1 m−^2 for the total membrane
conductance per area and the ratios of individual conductances given in Equation 11.9 on page 420.
b. Using the Ohmic hypothesis, find the initial charge flux carried by sodium ions just after the
pumps have been shut off. Reexpress your answer as charge per time per unit length along a giant
axon, assuming its diameter to be 1mm.
c. Find the total charge per unit length carried by all the sodium ions inside the axon. What would
the corresponding quantity equal if the interior concentration of sodium matched the fixed exterior
concentration?
d. Subtract the two values found in (c). Divide by the value you found in (b) to get an estimate
for the time scale for the sodium to equilibrate after the pumps shut off.
e. Chapter 12 will describe a nerve impulse as an event that passes by one point on the axon in
about a millisecond. Compare to the time scale you just found.