An opsin is a protein found in the retina that is sensitive to light. Many opsins are involved in transmitting signals from light to become visual images. Other types of these proteins were discovered in the 1990s. These have non-visual, and often unknown, roles in physiology, as of 2010. A non-visual opsin of known function is melanopsin, which is involved in setting circadian rhythms.
The retina is the inner layer of the eye that contains photoreceptor cells. These cells are specialized types of nerve cells that convert light into a chain of biological reactions. The most commonly known are rods, which function in dim light and result in black and white vision, and cones, which are responsible for color vision.
The different types of photoreceptor cells have different types of opsins. Rod cells have a compound called rhodopsin, which is composed of the protein opsin and a Vitamin A-like compound known as retinal. Since it contains retinal, also known as retinaldehyde, it is known as a retinylidene protein. The retinal responds to green-blue light by changing conformation, and this activates a receptor on the cell’s surface. This activation then triggers a cascade of changes within the cell, and results in night vision.
Cone cells, also retinylidene proteins, have opsins with slight structural variations from rhodopsin, and this shifts the wavelengths of light at which they will absorb. These cells have one of three types of opsin, known as photopsin that take in light at different wavelengths. They absorb yellow-green, green, and blue-violet, and the signals are then combined by the brain to give a view in color. Cone cells are less sensitive to light than rod cells.
The transmission of visual information to the brain is only one type of signal relayed by opsins. Another type is relayed by melanopsin, an opsin more similar to invertebrate opsins. It responds to light and transmits a non-visual signal that translates the need for sleep to a 24 hour light-and-dark cycle, known as the circadian rhythm. Blind people with functioning retinas can still adhere to this cycle.
Opsins are widespread throughout the animal kingdom, and are even found in the skin of amphibians. Over 1,000 types of opsin have been identified so far. They are also found in bacteria, and these proteins are used to harvest the energy from light to convert carbon dioxide to sugars. The opsins in these primitive organisms are thought to have evolved separately from those in more advanced creatures.