Cryptochromes are blue light photoreceptors in plants and animals. They are a family of flavoproteins that regulate germination, elongation, photoperiodism, and other responses in higher plants. Cryptochromes are involved in the circadian rhythm of plants and animals, and in the sensing of magnetic fields in a number of species. The genes coding for two cryptochromes, CRY1 and CRY2 active in the regulation of the mammalian circadian clock, are found in many species - including in humans on chromosomes 12 and 11.
Cryptochromes possess two chromophores that may absorb a photon, which probably mediates the phosphorylation of a certain domain in the cryptochrome. The conformational change in the molecule may trigger the transduction of the light signal from the environment into a biochemical signal that affects gene regulation of the circadian clock in the cell nucleus. In corals cryptochromes are part of the mechanism that triggers coordinated spawning for a few nights after a full moon in the spring.
A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can be absorbed by exciting an electron from its ground state into an excited state. In biological molecules that capture or detect light energy, the chromophore is the part of the molecule that causes a conformational change of the molecule when hit by light. The retinal molecule in the figure straightens when a photon of the correct wavelength hits it.Some chromophores occur as a porphyrin ring attached to a metal ion involving the splitting of d-orbitals. Examples of such chromophores can be seen in chlorophyll (used by plants for photosynthesis), hemoglobin (in the blood of vertebrates), and hemocyanin (in the blood of many invertebrates).
Excerpted and adapted from: Cryptochrome, Chromophore