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]
Eyespot-dependent determination of the phototactic sign in Chlamydomonas reinhardtii by Noriko Ueki and others. Introduction and Fig.1, eyespot granules and membrane photoreceptors
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]
Eyespot-dependent determination of the phototactic sign in Chlamydomonas reinhardtii by Noriko Ueki and others. Abstract, mutant comparisons and Fig.5, cellular lens effect
This distinguishes detecting light from correctly interpreting its direction. Articles by others on the same topic
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