In order to return to the dark state, a photoreceptor must undergo recovery. Recovery involves many steps. How does each step of phototransduction return to the dark state?
A. cGMP increase
B. Inactivation of transducin
C. Deactivation of rhodopsin
D. Retinal configuration

A.Light causes cGMP levels to decrease, which closes the CNG channels. When these channels close there is a decrease in the intracellular concentration of Ca²⁺. Guanylate cyclase activating protein (GCAP) activates guanylate cyclase in the absence of Ca²⁺, which increases cGMP levels.
B. Transducin is active when it is bound by Tα-GTP. Tα-GTP has intrinsic GTPase activity, which converts the alpha subunit to the GDP bound state and therefore no longer binds to transducin, which inactivates the protein. The conversion of GTP to GDP is facilitated by RGS9 (a GTPase activating protein), which activates the GTPase.
C. Rhodopsin is deactivated by rhodopsin kinase, which specifically binds to the phosphorylated rhodopsin protein. In addition arrestin competes for the Tα binding site on phosphorylated rhodopsin and prevents rhodopsin from activating transducin.
D. All-trans retinal must be converted back to 11-cis retinal. This happens in the pigment cells in the pigment epithelium.