Final answer:
The essential heterotrimeric G-protein involved in phototransduction is transducin, which is activated upon the light-induced isomerization of retinal, leading to a series of events that convert light into a neural signal.
Step-by-step explanation:
The essential heterotrimeric G-protein in phototransduction is transducin. During this process, when light strikes the retina, it changes the shape of the retinal molecule from a cis to a trans form. This molecular change activates rhodopsin, which is composed of retinal and the trans-membrane protein opsin. The activated rhodopsin then triggers transducin, initiating a cascade of downstream events that ultimately convert light into a neural signal.