Fig. (4) Phototransduction steps. 1) activation of the receptor protein in rods, that is rhodopsin (1 photon → 1 rhodopsin); 2) the activated receptor protein stimulates the G-protein transducin, with GTP converted to GDP in the process (1 rhodopsin → 100 transducins); 3) in turn, activated transducin activates the effector protein phosphodiesterase, converting cGMP to GMP (1 transducin → 100 PDE/s); 4) Falling concentrations of cGMP cause the transduction channels to close, decreasing Na+ current (1 PDE → 1000 GMP/s).