If a typical current density (current per unit cross-sectional area) in the cell membrane is 5 mA/cm2 when the voltage across the membrane (the action potential) is 50 mV, what is the number density of open ion channels in the membrane?

a.1/cm2
b.10/cm2
c.10/mm2
d.1/μm2

Certain bacteria (such as aquaspirillum magnetotacticum) tend to swim toward the earth’s geographic north pole because they contain tiny particles, called magnetosomes, that are sensitive to a magnetic field. if a transmission line carrying 100 a is laid underwater, at what range of distances would the magnetic field from this line be great enough to interfere with the migration of these bacteria? (assume that a field less than 5% of the earth’s field would have little effect on the bacteria. take the earth’s field to be 5.0 * 10-5 t, and ignore the effects of the seawater.)