Question

In its resting state, the membrane surrounding a neuron is permeable to potassium ions but only slightly permeable to sodium ions. Thus, positive K ions can flow through the membrane in an attempt to equalize K concentration, but Na ions cannot as quickly. This leads to an excess of Na ions outside of the cell. If the space outside the cell is defined as zero electric potential, then the electric potential of the interior of the cell is negative. This resting potential is typically about 80 . A schematic of this situation is shown in the figure:

  

In response to an electrical stimulus, certain channels in the membrane can become permeable to Na ions. Due to the concentration gradient, Na ions rush into the cell and the interior of the cell reaches an electric potential of about 40 . This process is termed depolarization. In response to depolarization, the membrane again becomes less permeable to Na ions, and the K ions flow out of the interior of the cell through channels established by the positive electric potential inside of the cell. This then reestablishing the resting potential. This is termed repolarization. Only a small percentage of the available Na and K ions participate in each depolarization/repolarization cycle, so the cell can respond to many stimuli in succession without depleting its "stock" of available Na and K ions. A graph of an electric potential inside a cell vs. time is shown in the next figure  (Intro 2 figure)  for a single depolarization/repolarization cycle.

 

Part B

During the resting phase, what is the electrical potential energy of a typical K ion inside of the cell?

40  meV   +40  meV   80  meV   +80  meV   0 meV

Part C

During depolarization, what is the work done (by the electric field) on the first few Na ions that enter the cell?

40 meV   +40 meV   80 meV   +80 meV   120 meV   +120 meV   0 meV

Part D

During repolarization, what is the work done (by the electric field) on the first few K ions that exit the cell?

40 meV   +40 meV   80 meV   +80 meV   120 meV   +120 meV   0 meV

Answer 1

A) 0 meV

Because it does not have electric potential outside of the cell membrane.
U=qV=eV

B) -80meV

C) +80meV

D)+40meV