photoresceptor

A photoreceptor is a light-sensitive organelle or molecule that, when it absorbs a photon of light, undergoes a chemical change. This change initiates a signal transduction cascade that eventually leads to a response. In biology, photoreceptors are crucial for vision and are found in a wide variety of organisms, from bacteria to humans.

In animals, photoreceptors are specialized cells found in the retina of the eye. There are two main types: rods and cones. Rods are highly sensitive to light and are responsible for vision in low-light conditions, allowing us to see in shades of gray. Cones, on the other hand, are less sensitive to light but are responsible for color vision and are most effective in bright light. Humans have three types of cones, each sensitive to different wavelengths of light (red, green, and blue).

The molecular basis of photoreception involves photopigments, such as rhodopsin in rods and iodopsins in cones. These molecules consist of a protein called opsin and a light-absorbing chromophore, usually a derivative of vitamin A called retinal. When a photon strikes the photopigment, retinal changes its shape, triggering a series of conformational changes in the opsin protein. This cascade of events ultimately leads to a change in the electrical potential of the photoreceptor cell, which is then transmitted to other neurons in the retina and eventually to the brain, where it is interpreted as visual information.

Beyond vision, photoreceptors play roles in other biological processes, such as photoperiodism (the response of organisms to seasonal changes in day length) and circadian rhythm regulation. In plants, photoreceptors like phytochromes and cryptochromes are involved in regulating growth, development, and flowering in response to light.