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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.

An eyespot is a subcellular light-sensing apparatus that helps a motile organism determine the direction of illumination. In Chlamydomonas reinhardtii, it combines carotenoid-rich granules in the chloroplast with channelrhodopsin proteins in the adjacent cell membrane. Its optical arrangement gives the light-sensitive machinery a directional response; it does not form a visual image.^[ref]

During swimming, flagella propel the rotating biological cell, so the eyespot samples changing orientations to the light. Differences in light response can then guide phototaxis. Experiments with mutants lacking the visible pigment layers found that they could still show phototaxis, but with the direction reversed under the tested conditions. Optical observations showed that the cell body could concentrate light onto the rear of the receptors; pigment-layer shielding helps determine the direction of the response.^[ref] This distinguishes detecting light from correctly interpreting its direction.

Noriko Ueki investigates how swimming green algae respond to light. A 2016 study led by Ueki examined Chlamydomonas reinhardtii mutants lacking a normal eyespot and found that their direction of phototaxis could reverse relative to normal cells under the same chemical conditions. The result made the optical arrangement of the biological cell part of the explanation for its behavior.^[ref]

The carotenoid layers of an eyespot help control which direction of illumination reaches light-sensitive channelrhodopsins. Without that shielding, the cell body can act as a convex lens, concentrating light arriving from the rear. The investigators demonstrated the lens effect by imaging a small letter through the cells with a microscope. This is an illuminating example of a physical structure changing the meaning of a sensory signal: the same light-sensitive machinery can receive its strongest illumination from a different direction. Comparing mutants, a rescued strain and normal cells connected the optical effect with the observed phototaxis.^[ref]