What pigment do rods contain?

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

Answer: (A)

Rods use a specific photopigment called rhodopsin. Rhodopsin is a light-sensitive GPCR made of the protein opsin bound to a light-absorbing retinal molecule. When light hits rhodopsin, retinal changes shape (11-cis to all-trans), activating opsin, which then triggers a G-protein cascade via transducin. This leads to a drop in cGMP, closing ion channels, and hyperpolarizing the cell, reducing neurotransmitter release. This mechanism gives rods their high sensitivity in low light, supporting night vision. Opsin is just the protein part found in several pigments, melanopsin is a pigment in intrinsically photosensitive retinal ganglion cells used for non-image-forming responses, and photopsin refers to cone pigments involved in color vision.