Molecular model showing the activation of rhodopsin (purple, left) by light in a photoreceptor cell. Rhodopsin is a light sensitive G-protein coupled receptor (GPCR) with retinal (orange, top left) as a cofactor. When light hits rhodopsin, this causes a change in shape in retinal. This stimulates the G-protein transducin (blue), causing its alpha subunit (light blue) to exchange its bound molecule of GDP (guanosine diphosphate) for a molecule of GTP (guanosine triphosphate). This releases the alpha subunit, allowing it 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 the photoreceptor cell to become hyperpolarised.