Opsin is a class of protein that, together with retinal, constitutes what?

• A. Enzymes
• B. Retinal
• C. Lipids
• D. Photopigments

Answer: (D)

Light detection in photoreceptors begins when opsin binds retinal to form a photopigment. Opsin provides the protein framework in the cell membrane, while retinal is the light-absorbing molecule that changes shape when it captures a photon. This change in retinal triggers a conformational shift in opsin, starting a signaling cascade that converts light into an electrical signal for the brain. The combined complex of opsin and retinal is what we call a photopigment, which is why that option is the best choice. The other options don’t fit: enzymes are catalytic proteins, retinal by itself is just the chromophore without the protein scaffold, and lipids are a different class of biomolecules.