Rhodopsin is a complex consisting of an opsin (a protein) and a chromophore (retinal), allowing the detection of light in the rod cells of the eye.
In a similar process to bacteriorhodopsin, light energy (in the form of photons) isomerises retinal, straining the opsin. This conformational change induces the exchange of GDP for GTP in a G protein, allowing the G protein transducin to dissociate from the GPCR and activate phosphodiesterase. The phosphodiesterase reduces the concentration of cGMP present in the cell, causing cGMP-gated ion channels to close. This causes a build-up of Na+ and K+ extracellularly, causing hyperpolarisation. This induces the formation of an action potential and the subsequent release of neurotransmitter.
This is used in rod cells to allow the detection of light intensity levels by the brain.
In order to detect different wavelengths of light, the opsin requires different residues to interact with the retinal. This allows the tuning of the chromophore, which lets different wavelengths of light to be detected (in most individuals). Individuals with colour vision deficiencies (colour blindness) have mutations in these tuning amino acids, or the complete absence, impairing their ability to perceive certain or all ranges of colours. The most common form is deuteranopia (red-green colour blindness), through to achromatopia (complete lack of colour vision) which is the least common form of colour vision impairment.