Which one of the following is not a photoreceptor in vertebrate ?

Question

Accepted Answer

The correct option is 2: Chlorocruorin

To address this question, we must first understand the function of a photoreceptor. In vertebrates, photoreceptors are specialized cells in the retina (rods and cones) that contain *visual pigments*—proteins that absorb light and initiate the signal for vision. These visual pigments are primarily composed of an opsin protein bound to a light-sensitive molecule (chromophore) like retinal. **Chlorocruorin**, however, is not a visual pigment. It is a large, iron-containing **respiratory pigment**, chemically similar to hemoglobin, that functions exclusively in the transport of oxygen throughout the body. Furthermore, Chlorocruorin is characteristically found dissolved in the blood plasma of certain **marine annelid worms** (invertebrates) like sabellids and serpulids, and has no presence or function in the vertebrate eye. Since its role is respiration and its taxonomic location is invertebrate, it is unequivocally *not* a photoreceptor in a vertebrate.

The incorrect option is 1: Cyanopsin

Cyanopsin is a true photoreceptor component, specifically an opsin protein found within the cone cells of the vertebrate retina. Cone cells are responsible for photopic (daylight) and color vision, and they possess different opsins to distinguish various wavelengths of light. Cyanopsin is the photopigment that makes the cone cells sensitive to short-wavelength light, corresponding to the perception of the **blue** color spectrum. The complex of Cyanopsin with its chromophore (retinal) is directly responsible for absorbing light and initiating the neural signal for blue color perception, thus establishing it as a primary vertebrate photoreceptor.

The incorrect option is 3: Idopsin

Idopsin (more commonly spelled **Iodopsin**) is a collective term historically used for the family of visual pigments found in vertebrate cone cells, contrasting with Rhodopsin in the rod cells. It typically refers to the opsin that is sensitive to longer wavelengths of light (green/yellow/red). These opsins—such as Erythropsin and Chloropsin—bind with retinal to form the light-sensitive complex required for color vision. As the light-absorbing pigment in the cone cells, Iodopsin is functionally and structurally a key component of the vertebrate photoreceptor system.

The incorrect option is 4: Porphyropsin

Porphyropsin is a valid visual pigment found in the photoreceptor cells of many vertebrates, particularly those that live in freshwater environments, such as certain fish and amphibians. Chemically, it is a visual pigment that utilizes a different form of the Vitamin A-derived chromophore, specifically 3,4-didehydroretinal (A2), rather than the retinal (A1) used by Rhodopsin. Porphyropsin is often responsible for dim-light (scotopic) vision in these species, serving the same function as Rhodopsin but with a spectral shift suited to the light conditions of their aquatic habitats. Therefore, as a functional pigment in the rod cells, Porphyropsin is a vertebrate photoreceptor.