Which describes the rod phototransduction cascade steps?

• A. Light activates rhodopsin (opsin + retinal) -> activates transducin -> activates phosphodiesterase -> reduces cGMP -> closes cGMP-gated Na+/Ca2+ channels -> hyperpolarization and decreased glutamate release.
• B. Light activates cone opsins -> increases cGMP -> opens channels -> depolarizes the cell.
• C. Dark adaption involves photopigments that do not respond to light.
• D. Photoisomerization of opsin causes immediate glutamate release.

Answer: (A)

When light hits a rod, the photopigment rhodopsin is activated as the retinal component changes shape from 11-cis to all-trans. The activated rhodopsin then turns on a G protein called transducin, which in turn activates the enzyme phosphodiesterase 6. This enzyme lowers the level of cyclic GMP inside the rod by hydrolyzing it to GMP. With less cGMP around, the cGMP-gated Na+/Ca2+ channels close, reducing positive ion inflow. The cell becomes hyperpolarized, which decreases calcium entry at the synaptic terminal and lowers the release of glutamate onto bipolar cells. In darkness, cGMP is high, keeping channels open and the cell depolarized with ongoing glutamate release. The light-triggered isomerization of the photopigment starts this cascade, leading to a decrease in glutamate rather than an immediate release.

The other statements don’t fit because activating cone opsins would not produce an increase in cGMP (the light response reduces cGMP in both rods and cones), photopigments do respond to light (dark adaptation involves how photopigments reset and respond to light, not nonresponsiveness), and the cascade results in decreased glutamate release after phototransduction—immediate glutamate release is not what occurs.