Molecular model showing the activation of rhodopsin by light in a photoreceptor cell. Rhodopsin (purple, top centre) is a light sensitive G-protein coupled receptor (GPCR) with retinal (red) as a cofactor. When light hits rhodopsin, this causes a change in shape in retinal. This stimulates the G-protein transducin (blue, lower left). Transducin's G-alpha subunit (light blue) exchanges its bound molecule of GDP (guanosine diphosphate, yellow) for a molecule of GTP (guanosine triphosphate, dark orange), releasing the G-alpha subunit. This G-alpha subunit is then free to activate the enzyme phosphodiesterase (PDE6). Phosphodiesterase hydrolyses cGMP, decreasing cGMP levels. This leads to the closing of sodium channels causing the outer segment membrane of a photoreptor cell to become hyperpolarised.