Britannica Sources used in this answer

How does rhodopsin help rod cells detect light?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

details of the mammalian eye — Photoreception The mammalian eye has a cornea and a lens and functions as a dioptric system, in which light rays are refracted to focus on the retina.

Rhodopsin is a photoreceptor molecule located in the membrane disks of the outer segment of rod cells. It consists of a protein called opsin bound to 11-cis-retinaldehyde, a derivative of vitamin A.

When light enters the eye, photons cause the 11-cis-retinaldehyde to undergo isomerization into all-trans-retinaldehyde. This change activates the opsin protein, which then triggers a biochemical cascade by activating a G-protein called transducin.12

Transducin activates an enzyme that breaks down cGMP, a molecule that normally keeps sodium channels open in the dark. As cGMP levels drop, these sodium channels close, reducing the influx of sodium ions and causing the rod cell to become hyperpolarized (more negative). This hyperpolarization inhibits the release of neurotransmitters at the cell's synaptic region, signaling the change in light intensity to the central nervous system. In bright conditions, the breakdown of rhodopsin prevents overstimulation, while in dim light, its persistence allows for high sensitivity.12