= Ion channel
{wiki}
An <ion channel> is a <membrane protein> that provides a selective pathway for <ions> across a <cell membrane>. An open pore permits <passive transport> down the relevant <electrochemical gradient>; it does not supply the <free energy> for uphill transport as a pump involved in <active transport> does. Two things are needed for a net <electric current>: an available pathway and an <electrochemical potential> difference. Opening a pore alone need not produce a net <electric current>. See <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Ion Channels and the Electrical Properties of Membranes>.
The <selective permeability> of an <ion channel> depends on interactions between its pore and the passing <ions>, not simply on a hole's diameter. Some <ion channels> strongly favor <potassium> over <sodium>; others admit several kinds of positively charged <ions>. Thus selective does not necessarily mean exclusive to one <ion> species. See https://www.ncbi.nlm.nih.gov/books/NBK11123/[Purves et al., Neuroscience, 2nd edition (2001): molecular structure of ion channels] and the mixed-channel example in https://www.ncbi.nlm.nih.gov/books/NBK11013/[Purves et al.: principles derived from studies of the neuromuscular junction].
\Image[/ion-channel-selective-passage.png]
{title=An open <ion channel> supplies a selective passive pathway}
{description=Original model schematic: selected positive <ions> can traverse a pore in a <membrane protein>, while the differently shaped particles are excluded from this modeled pore. The arrow shows one possible net direction when the corresponding <electrochemical gradient> favors entry; the pore does not enforce that direction. Shapes, particle counts and dimensions are illustrative, not physical microscopic data or a universal selectivity rule.}
Gating is the opening and closing of an <ion channel>'s pathway. A <voltage-gated ion channel> responds to <membrane potential>; an <ionotropic receptor> can respond to a bound <ligand>. Gating changes the available <electrical conductance>, while the <electrochemical gradient> supplies the drive for <passive transport>. For a fixed <reversal potential> in the simple model below, opening more identical <ion channels> increases the magnitude of the <electric current> at a fixed nonzero drive, without choosing its sign independently of <membrane potential>. The <two-state ion-channel gating> model describes how the open fraction can change. See the Alberts reference above.
For a worked approximation, hold an open population's <electrical conductance> at $g=10\,\mathrm{nS}$ and its <reversal potential> at $E_{\mathrm{rev}}=-80\,\mathrm{mV}$. Use $V=V_{\mathrm{inside}}-V_{\mathrm{outside}}$ for <membrane potential> and \b[inward-positive <conventional current>]:
$$
I_{\mathrm{in}}=g(E_{\mathrm{rev}}-V),\qquad g\geq0.
$$
At $V=-100,-80,-60\,\mathrm{mV}$, respectively, the <electric currents> are $+200,0,-200\,\mathrm{pA}$, since $1\,\mathrm{nS}\times1\,\mathrm{mV}=1\,\mathrm{pA}$. Positive <electric current> means net positive <electric charge> entering, or net negative <electric charge> leaving; it does not mean that every <ion> moves inward. For a pore passing only positive <ions>, the three results mean net inward passage, no net passage, and net outward passage. These stipulated values describe an illustrative Ohmic approximation, not measured properties of a particular <ion channel>; see <reversal potential> for the current plot and limitations.
The useful connection is between a molecular gate and a controllable <electric current>. A <ligand> can change <electrical conductance> through an <ionotropic receptor>, or <membrane potential> can change it through a <voltage-gated ion channel>, connecting recognition and electrical signaling. In a historical landmark, Erwin Neher and Bert Sakmann reported currents from individual <ion channels> in frog muscle membranes in 1976. See https://doi.org/10.1038/260799a0[Neher and Sakmann, Single-channel currents recorded from membrane of denervated frog muscle fibres, Nature 260, 799–802 (1976)].
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