Final answer:
The essential G-protein coupled receptor in phototransduction is rhodopsin, a molecule in rod cells that initiates a signal transduction cascade upon interaction with light.
Step-by-step explanation:
The essential G-protein coupled receptor in phototransduction is rhodopsin. Phototransduction is the process by which light is converted into electrical signals in the rod cells of the retina. Rhodopsin is the photopigment molecule found in rod cells, which consists of two components: the protein opsin and the chromophore retinal. When light strikes retinal, it undergoes a shape change from a bent (cis) form to a linear (trans) form. This conformational change activates rhodopsin and initiates a signal transduction cascade that involves the G-protein called transducin. Transducin then triggers downstream events that ultimately lead to a neural response.