Which photopigment is responsible for scotopic vision and has maximum absorption at 507 nm?

Explanation

## Photopigments of the Retina **Key Point:** Rhodopsin is the visual pigment present in rod cells and is the primary photopigment for scotopic (low-light) vision. ### Structure and Composition Rhodopsin consists of: - **Opsin** — a G-protein coupled receptor (apoprotein) - **11-cis retinal** — the chromophore (derived from vitamin A) ### Spectral Properties | Photopigment | Location | Vision Type | λ max (nm) | Sensitivity | | --- | --- | --- | --- | --- | | **Rhodopsin** | Rod cells | Scotopic | 507 | Very high; detects single photons | | Iodopsin (M-cone) | M-cones | Photopic | 530 | Lower; requires more photons | | Photopsin (L-cone) | L-cones | Photopic | 560 | Lower; requires more photons | | S-cone pigment | S-cones | Photopic | 420 | Lower; requires more photons | **High-Yield:** Rhodopsin's λ max of 507 nm (blue-green light) explains why rod vision is most sensitive to this wavelength and why scotopic vision appears "colorless" — rods cannot distinguish wavelengths. ### Mechanism of Vision 1. **Light absorption** → 11-cis retinal isomerizes to all-trans retinal 2. **Conformational change** in opsin activates transducin (G-protein) 3. **Cascade amplification** via cGMP hydrolysis 4. **Hyperpolarization** of rod cell → signal to bipolar cells **Clinical Pearl:** Vitamin A deficiency leads to impaired rhodopsin regeneration, causing night blindness (nyctalopia) — one of the earliest signs of vitamin A deficiency. **Mnemonic:** **ROD = 507** (Rhodopsin, Opsin, 507 nm — scotopic vision in dim light).