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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.^[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.^[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.

Channelrhodopsin-2 is a microbial light-gated cation channel originally identified in the green alga Chlamydomonas reinhardtii. Its retinal-containing membrane protein couples light absorption to channel opening. Unlike a light-driven pump that transports charge through a directed cycle, an open Channelrhodopsin-2 provides a conductive path whose net current depends on ion concentrations and membrane potential.^[ref]

The original experiments expressed the protein in frog oocytes and mammalian cells and recorded light-induced membrane currents. Testing the isolated membrane response helped show that light directly gates the conductive protein, rather than acting only through a separate intracellular signaling cascade. Responses depended on voltage and ionic conditions, and prolonged illumination did not simply maintain an unchanged initial peak current.^[ref]

A useful conceptual example is a cell held below the channel's reversal potential: opening the channel can give net inward positive charge and depolarization. Near reversal there can be little net current despite channel opening. Consequently a blue-light pulse does not guarantee an action potential; channel abundance, ionic driving force and the cell's excitability all matter.

This direct connection between light and conductance made Channelrhodopsin-2 useful for optogenetics. The measurement logic remains important: a light-controlled current is evidence about channel function, whereas precise control of firing in a particular cell type also requires testing the resulting electrical response.

Georg Nagel studies membrane proteins that respond to light. His work on channelrhodopsin-2 helped establish the molecular basis of optogenetics, in which introduced light-sensitive proteins allow light to control cellular electrical activity.

In 2003, Nagel and colleagues expressed channelrhodopsin-2 from the microalga Chlamydomonas reinhardtii in Xenopus laevis oocytes and mammalian cells. Electrical recordings showed a rapidly opening ion channel permeable to cations. Experiments on isolated membrane patches supported direct gating by light, without requiring a freely diffusible intracellular intermediary. During sustained illumination the initial electric current declined to a lower maintained level.^[ref]

This gives a useful distinction between an ion channel and a molecular pump: an open channelrhodopsin-2 permits ions to move down their electrochemical gradient. The resulting change in membrane potential depends on the cell and its ionic conditions; a flash of light does not imply an identical electrical response in every cell.^[ref]