English: Cryptochrome protein as magnetoreceptor.

A. Cryptochrome proteins found in bird's retinal cells contain a flavin adenine dinucleotide molecule (FAD) and a tryptophan amino acid (Trp), and they are electrically neutral in the ground state. When exposed to blue light, a photon causes an electron to move from Trp to FAD, creating a radical pair with unpaired electrons spinning in opposite directions (singlet state). B. The protein oscillates between the singlet state and the triplet state, where unpaired electrons spin in the same direction, a million times per second for about a few microseconds, influenced by Earth's magnetic field.

C. Both states can undergo chemical reactions leading to a signaling state, involving the addition of a hydrogen ion to FAD and a change in the protein's structure. The signaling state triggers a biochemical cascade, leading to the release of neurotransmitters in the retina and conveying magnetic information to the bird's brain. Cryptochrome eventually returns to the ground state, initiating the process once again.