Voltage-Gated Cation Channels in the Plasma Membrane of a Neuron

Multiple Choice

Voltage-gated cation channels in the plasma membrane of a neuron open when the membrane potential passes a threshold value that is about 20 mV above the resting potential. This corresponds to a change of about 4 × 10⁶ newtons per coulomb in the magnitude of the electric field across the membrane compared to the resting state. The tetrameric channels have a voltage-sensing helix S4 in each of their four subunits. Most S4 helices contain seven residues with positively charged side chains. The charge carried by each such residue is about 1.6 × 10-¹⁹ coulombs. By reaching threshold depolarization, what are the magnitude and the direction of the collective force that the four S4 helices in the channel experience as a result of the change in the electric field? The force that particles of charge q in an electric field E experience is calculated as F = q.E.

A) 18 pN, toward the cell interior
B) 18 pN, toward the cell exterior
C) 90 pN, toward the cell interior
D) 90 pN, toward the cell exterior
E) 45 pN, toward the cell interior