Absorption of how many photons does it take to bleach rhodopsin?

• A. 0 photons
• B. 2 photons
• C. 10 photons
• D. 1 photon

Answer: (D)

Rhodopsin bleaching happens the moment a photon is absorbed by the pigment. The 11-cis retinal bound to opsin is isomerized to all-trans retinal, driving a change to the active form of the protein. This single photochemical event converts that pigment molecule into a state that no longer responds in the same way to light until it’s regenerated. So, for one rhodopsin molecule, absorbing one photon is enough to bleach it. The other options describe either no bleaching or more photons than are needed for a single molecule; the key point is that a single photon suffices to bleach an individual rhodopsin molecule.