How do rhodopsin and retinal help rod cells detect light?

Rod cells detect light through a specialized photochemical process involving the protein rhodopsin, which is located in the membrane disks of the cell's outer segment. Rhodopsin is composed of a protein called opsin bound to a photosensitive molecule known as 11-cis-retinaldehyde (or retinal).12

When light enters the eye, it triggers a photochemical reaction called photoisomerization. During this process, the absorption of a photon causes the 11-cis-retinal molecule to twist into a straighter shape called all-trans-retinal. This structural change activates the opsin protein, which then initiates a biochemical signaling cascade.314

Specifically, the activated opsin triggers a G-protein called transducin, which activates an enzyme that reduces the concentration of the second messenger cGMP. In the dark, cGMP keeps sodium channels open; however, when light reduces cGMP levels, these channels close. This causes the rod cell to become hyperpolarized, inhibiting the release of neurotransmitters and ultimately signaling the central nervous system that light has been detected.14