photopigments

Photopigments are light-absorbing molecules that enable vision and other light-driven processes by converting photons into chemical signals. In animals, most photopigments consist of a light-absorbing chromophore bound to a protein called an opsin. The chromophore is typically retinal, derived from vitamin A. In darkness, retinal is in the 11-cis configuration within the opsin. When a photon is absorbed, retinal isomerizes to all-trans, triggering a conformational change in the opsin. This activates a G protein-coupled signaling cascade that alters ion channel activity and changes the cell’s membrane potential, ultimately providing the neural signal for vision.

Vertebrate retinas contain rods and cones. Rods carry rhodopsin, which is highly sensitive and supports motionless, dim-light vision, while cones carry photopsins that are sensitive to short, middle, and long wavelengths, supporting color vision. The spectral sensitivities of these pigments are determined by the specific opsin protein, which tunes the same retinal chromophore to different wavelengths. After light exposure, photopigments bleach and must be regenerated through a retinoid cycle to restore light sensitivity.

Some photopigments serve non-visual roles. Melanopsin, found in intrinsically photosensitive retinal ganglion cells, contributes to circadian entrainment and pupil reflexes. In plants and algae, photopigments include chlorophylls and carotenoids, which drive photosynthesis, as well as photoreceptors such as phytochromes, cryptochromes, and phototropins that regulate growth, development, and orientation to light.