Figure 2.

Diagrammatic representations of the hypothesized radical pair mechanism of magnetoreception. (a) In specialized photoreceptor molecules (crytochromes), light drives an electron transfer between donor (D) and acceptor (A) molecules generating a radical pair in the singlet (arrows ↑↓) or triplet (arrows ↑↑) state. The interconversion between singlet and triplet states (blue arrows) changes under different magnetic field conditions (e.g. at orientation A (OA) or B (OB)) that, in turn, changes the ratio of singlet to triplet products (denoted by the size of the bottom arrows). (b) Light entering the eye (e.g. rays A and B) drives radical pair formation in cryptochrome molecules oriented normal to the retina surface (green arrows) at sites 1 and 2, which are oriented at different angles (θ) relative to the external magnetic field (blue lines). The anisotropy of radical pair production across the retina surface may result in the addition of a superimposed impression of the magnetic field to the animal's sight (adapted from [15,16]).