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

question 33

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

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