Figure 1. Rhodopsin (R), the photoreceptor molecule present in rod photoreceptors, at the centre of two pathways: phototransduction and the retinoid cycle. In phototransduction, R triggers a series of reactions (T, transducin; PDE, cGMP phosphodiesterase; CNG, cGMP-gated channel), which return to the inactive state during the recovery phase. In the retinoid cycle, the chromophore of rhodopsin (11-cis-retinal) is regenerated.

Figure 2. Bleaching of rhodopsin, a G-protein-coupled receptor consisting of seven transmembrane domains and the 11-cis-retinal chromophore, produces all-trans-retinal. Rhodopsin is depicted as immersed in the ROS membrane phospholipid bilayer, where the blue circles represent polar phospholipid head groups. The chromophore retinal (shown on the right) is covalently attached to the opsin polypeptide and buried inside rhodopsin in a tightly fitting hydrophobic pocket.

Figure 3. Schematic representation of the retinoid cycle. For simplicity, only the main pathway (blue arrows) and the major components are shown. Red arrows point to genes in which mutations have been associated with human retinal diseases. Two naturally occurring canine models with retinoid cycle gene defects are shown in blue boxes. RESTs, or retinosomes, retinyl ester storage particles. ROS, the cellular organelle where rhodopsin is located. RPE, retinal pigment epithelium, where most components of the retinoid cycle are located. IPM, interphotoreceptor matrix, separating RPE and ROS.