In rod phototransduction, which pigment is used?

• A. Rhodopsin
• B. Photopsin
• C. Melanopsin
• D. Opsin only

Answer: (A)

Rod phototransduction relies on a light-sensitive pigment in the rod outer segments. That pigment is rhodopsin, a complex of the opsin protein bound to the chromophore 11-cis-retinal. When light is absorbed, 11-cis-retinal isomerizes to all-trans-retinal, activating rhodopsin and triggering a G-protein cascade via transducin that lowers cGMP, closes the cGMP-gated channels, and hyperpolarizes the rod to send a signal. The other pigments are associated with other cells or contexts: photopsin is the cone pigment for color vision, melanopsin is in intrinsically photosensitive retinal ganglion cells for non-image-forming responses, and an opsin alone cannot function as a pigment without the bound chromophore.