100 rhodopsins are bleached with photons of light. After 10 minutes, how many have recovered and are now unbleached?

• A. 50
• B. 60
• C. 80
• D. 75

Answer: (D)

When light hits rhodopsin, it bleaches into opsin and all-trans retinal, and it must go through the visual cycle to be rebuilt as rhodopsin again. The regeneration process involves converting all-trans retinal to all-trans retinol, transporting it to the retinal pigment epithelium, converting it back to 11-cis retinal, and rejoining it with opsin. This recovery happens over minutes, not instantly. After about 10 minutes, a substantial portion will have been rebuilt, roughly three-quarters, so about 75 of the original 100 rhodopsin molecules are unbleached. The rest remain bleached because some molecules still need more time to complete the regeneration steps, a process that can vary with bleaching intensity and metabolic conditions.