Vision consists of the following processes (see here):

1. A photon of light stimulates the membrane receptor called rhodopsin, which is an abundant membrane protein in the outer segment of rod cells in the retina.

2. A photochemical change in the structure of rhodopsin causes it to activate the G protein called transducin so that it binds GTP.

3. The transducin--GTP complex activates a specific phosphodiesterase, which cleaves a cyclic nucleotide, guanosine 3',5'-monophosphate (cyclic GMP, or cGMP).

4. Cleavage of cGMP, in turn, stimulates intracellular reactions that generate a visual signal to the brain.

Thus, the stimulated hydrolysis of cGMP is the visual analog of the stimulated synthesis of cAMP in -adrenergic responses.

1. G Protein Receptor Coupled Database

2. G Protein Coupled Receptors Point Mutation Database