Which photopigment is responsible for vision in dim light conditions and is most sensitive to blue-green wavelengths (507 nm)?

Explanation

## Photopigments of the Retina **Key Point:** Rhodopsin is the visual pigment found in rod cells and is the primary photopigment for scotopic (dim light) vision. ### Structure and Function Rhodopsin consists of: - **Opsin** — a G-protein coupled receptor (GPCR) - **11-cis retinal** — the chromophore (vitamin A aldehyde) ### Spectral Sensitivity - **Wavelength of maximum absorption:** 507 nm (blue-green light) - **Function:** Rod vision in low-light conditions - **Sensitivity:** Approximately 1000× more sensitive than cone photopigments ### Comparison with Cone Pigments | Photopigment | Location | Wavelength (nm) | Light Condition | Chromophore | | --- | --- | --- | --- | --- | | **Rhodopsin** | Rods | 507 (blue-green) | Scotopic (dim) | 11-cis retinal | | **Iodopsin (S-cone)** | Blue cones | 420 (blue) | Photopic (bright) | 11-cis retinal | | **Chlorolabe (M-cone)** | Green cones | 531 (green) | Photopic (bright) | 11-cis retinal | | **Erythrolabe (L-cone)** | Red cones | 558 (red) | Photopic (bright) | 11-cis retinal | **High-Yield:** All photopigments use 11-cis retinal as the chromophore; they differ only in the opsin protein structure, which determines wavelength sensitivity. **Mnemonic:** **ROD = Rhodopsin, Outer segment, Dim light** — helps recall that rhodopsin is the rod photopigment for scotopic vision. **Clinical Pearl:** Vitamin A deficiency leads to loss of rhodopsin regeneration, causing night blindness (nyctalopia) — the earliest sign of vitamin A deficiency. ## Photochemical Cascade When rhodopsin absorbs light: 1. 11-cis retinal → all-trans retinal (isomerization) 2. Conformational change in opsin → activated rhodopsin (metarhodopsin II) 3. Activation of transducin (G-protein) 4. Cascade leads to hyperpolarization of rod cell 5. Decreased neurotransmitter release (glutamate) → signal to bipolar cells