= Channelrhodopsin
A <channelrhodopsin> is a light-gated <ion channel> associated with a <retinal> chromophore. Absorbing light changes the <protein>'s conducting state, connecting photoreception directly with <ion> movement across a <cell membrane>. Naturally occurring members in <Chlamydomonas reinhardtii> participate in responses to illumination.<Channelrhodopsin-2, a directly light-gated cation-selective membrane channel by Georg Nagel and others>[2003, abstract and introduction]{ref}
The landmark <channelrhodopsin-2> study expressed the <protein> in <Xenopus laevis> <oocytes> and mammalian cells and recorded light-evoked <electric currents>. Altering ionic conditions showed passage of multiple <cations>; continued illumination produced an initial response followed by a smaller sustained conductance. Expressing the channel in another cell type helped establish that light gating was a property of this molecular machinery.<Channelrhodopsin-2, a directly light-gated cation-selective membrane channel by Georg Nagel and others>[Abstract: heterologous expression, cation permeability and desensitization]{ref}
In an illustrative Ohmic model with outward-positive <electric current>, $I=g(V-E_{\mathrm{rev}})$. If light produces $g=1\,\mathrm{nS}$ at $V=-60\,\mathrm{mV}$ and $E_{\mathrm{rev}}=0$, then $I=-60\,\mathrm{pA}$: net inward positive <electric charge>. These invented values illustrate the driving force. Light opens a route whose direction depends on <membrane potential> and ionic conditions, a principle used in <optogenetics>.
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