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Channelrhodopsins are ion channels that are directly opened by light. They therefore may be very useful molecules, enabling the use of light to control intracellular acidity, calcium influx, and electrical excitability.

Two channelrhodopsins are currently known: Channelrhodopsin-1 is a light-activated hydrogen ion channel, and Channelrhodopsin-2 is a light-activated cation channel. Structurally, channelrhodopsins are seven-transmembrane proteins like rhodopsin, and contains the light-isomerizable vitamin A derivative all-trans-retinal. However, whereas most opsins are G-protein coupled receptors that open other ion channels indirectly via messengers, channelrhodopsins contain not only the light-activatable transduction mechanism, but the channel pore itself. This makes cellular depolarization extremely fast, robust, and useful for bioengineering and neuroscience applications, including photostimulation.