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 molecule, top left) as a cofactor. When light hits rhodopsin, this causes a change in shape in retinal. This stimulates the G-protein transducin, causing its alpha subunit (light blue) to exchange its bound molecule of GDP (guanosine diphosphate, orange centre) for a molecule of GTP (guanosine triphosphate, yellow centre left). This releases the alpha subunit, leaving it free to activate the enzyme phosphodiesterase (PDE6, darker blue right). Phosphodiesterase hydrolyses cGMP (red molecule, centre), decreasing cGMP levels. This leads to the closing of sodium channels causing the outer segment membrane of the photoreceptor to become hyperpolarised.