Which molecule is the light-sensitive pigment in rods?

Which molecule is the light-sensitive pigment in rods?

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

Answer: (B)

Rods use a photopigment called rhodopsin to detect light. Rhodopsin is made of the protein opsin bound to a light-absorbing molecule called retinal. In darkness, retinal is in the 11-cis form and rhodopsin is inactive. When light hits retinal, it changes shape to all-trans retinal, activating rhodopsin and triggering a signaling cascade through a G-protein. This cascade lowers cGMP levels, closes the cGMP-gated channels, and causes the rod to hyperpolarize, sending a signal toward the brain for vision in dim light. Photopsin, by contrast, is the photopigment used in cones for color and daylight vision, not in rods. Opsin is just the protein part of the photopigment, and melanin is a pigment unrelated to the phototransduction process in rods.

Rods use a photopigment called rhodopsin to detect light. Rhodopsin is made of the protein opsin bound to a light-absorbing molecule called retinal. In darkness, retinal is in the 11-cis form and rhodopsin is inactive. When light hits retinal, it changes shape to all-trans retinal, activating rhodopsin and triggering a signaling cascade through a G-protein. This cascade lowers cGMP levels, closes the cGMP-gated channels, and causes the rod to hyperpolarize, sending a signal toward the brain for vision in dim light. Photopsin, by contrast, is the photopigment used in cones for color and daylight vision, not in rods. Opsin is just the protein part of the photopigment, and melanin is a pigment unrelated to the phototransduction process in rods.