= Solution
The rising sigmoid shows activation by <depolarization>: $P_o$ is almost zero at very negative <membrane potential>, rises steeply around $-20\,\mathrm{mV}$, and approaches about $0.8$ at positive <membrane potential>. The graph describes the fraction of time a single <ion channel> is open, not a continuously variable opening amplitude. A convenient phenomenological fit is
$$
P_o(V)=\frac{P_{\max}}{1+\exp[-z_{\rm eff}e(V-V_{1/2})/(k_BT)]},
\qquad P_{\max}\simeq0.8,\quad V_{1/2}\simeq-20\,\mathrm{mV}.
$$
Here $V_{1/2}$ refers to half the maximum, about $0.4$, not absolute $P_o=0.5$. The effective gating charge describes the steepness; a sigmoid alone does not identify the microscopic gating sequence or explain why the observed maximum is below one.
A <voltage-gated potassium channel> has four pore-forming subunits. Each canonical subunit contains an S1–S4 <voltage-sensing domain> and S5–S6 pore elements. Positive charges in S4 respond to the <electric field>; their displacement during <depolarization> changes the energetic preference of sensor conformations. Coupling through the sensor–pore interface favors opening of the intracellular pore gate. The <potassium> selectivity filter is structurally distinct from this voltage sensor.
Several independent observations support this account. <Voltage clamp> detects transient gating <electric currents> before pore conduction, even when ionic conduction is blocked: charge movement precedes opening. Charge-neutralization studies change total gating charge and voltage dependence, with contributions from charged S4 residues and an S2 residue. Accessibility and voltage-sensitive optical measurements support conformational motion rather than permeating <potassium> itself acting as the sensor. Structural measurements locate the charged sensor beside the pore. These observations constrain a coupled moving-sensor mechanism, while the exact displacement cannot be deduced from this $P_o$ curve alone.<Voltage-sensing residues in the S2 and S4 segments of the Shaker K+ channel by S. A. Seoh and others>[Abstract: gating-charge measurements identify contributions from S2 and S4 charged residues]{ref}<Crystal structure of a mammalian voltage-dependent Shaker family K+ channel by Stephen B. Long and others>[Structure of Kv1.2: separate voltage-sensor and pore domains and their coupling]{ref}
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