rhodopsin

Meaning: Rhodopsin is the light-sensitive pigment in rod cells of the retina. It consists of a protein, opsin, bound to retinal, a molecule derived from vitamin A. A photon converts retinal from the 11-cis form to all-trans, which triggers a G protein cascade. cGMP levels fall, Na⁺ channels close, and the rod hyperpolarizes, so it releases less glutamate. In bright light rhodopsin is bleached; it regenerates in the dark, which underlies dark adaptation. Rods respond to dim light, give no color information, and are dense in the periphery but absent from the fovea. Cones use related pigments (photopsins) of three types, which support color vision.

On the MCAT: Expect a passage on night vision or dark adaptation. On a dark adaptation curve, sensitivity improves in two phases: cones adapt quickly, then rods take over and reach a lower threshold after more time. You may be asked why vitamin A deficiency impairs vision in dim light, or which cell type mediates peripheral, low-light vision.

Common confusion: Students expect light to depolarize a photoreceptor, but the reverse happens. In darkness rods are depolarized and release more glutamate; light hyperpolarizes them. Also, rhodopsin belongs to rods, not cones.