Rhodopsin vs. Opsin — What's the Difference?

Rhodopsin, also known as visual purple, is essential for night vision and enables the eye to detect light in dark conditions. It consists of a protein called opsin and a light-sensitive retinal molecule. Opsin, in contrast, refers to a group of protein molecules found in photoreceptor cells of the retina, playing a key role in the formation of various photopigments by combining with different chromophores, including retinal.

When rhodopsin absorbs light, it undergoes a biochemical change that initiates the signal transduction pathway, leading to vision. This process, known as phototransduction, involves the conversion of light into electrical signals by the retina. Opsin, on the other hand, does not directly trigger phototransduction but is integral to the structure and function of rhodopsin and other photopigments by binding with chromophores to detect light.

The sensitivity of rhodopsin to low light makes it critical for scotopic vision, or vision under dim light conditions, enabling humans and animals to see at night. Opsins, while a fundamental part of rhodopsin and other photopigments, vary in type and function, contributing to the diversity of light detection, including color vision in cone cells through different opsin variations.

Rhodopsin's role is specific to rod cells in the retina, which are responsible for black and white vision in low-light conditions. Opsins are more broadly involved in the visual system, including in cone cells, where they are part of photopigments responsible for color vision and high-acuity vision in bright light.

The regeneration of rhodopsin after it is bleached by light exposure is a critical aspect of maintaining night vision. This process involves the recombination of opsin with retinal. The versatility of opsin proteins across different types of photoreceptor cells underlines their essential role in adapting vision to various light environments.