Fototranszdució

Fototranszdució is the biochemical process by which photoreceptor cells in the retina, the rods and cones, convert light energy into electrical signals. This process is fundamental to vision.

In the rods, which are responsible for vision in low light conditions, the photopigment is rhodopsin. When a photon of light strikes rhodopsin, it causes a conformational change in the molecule retinal, which is a chromophore attached to a protein called opsin. This change activates opsin, initiating a cascade of events involving a G-protein called transducin.

Activated transducin then activates an enzyme called phosphodiesterase. Phosphodiesterase breaks down cyclic guanosine monophosphate (cGMP), a molecule that normally keeps ion channels open in the cell membrane. When cGMP levels drop, these ion channels close, reducing the influx of sodium ions and causing hyperpolarization of the cell. This hyperpolarization is the electrical signal that is eventually transmitted to the brain.

Cones, responsible for color vision and vision in bright light, employ a similar mechanism but use different photopigments (opsins sensitive to different wavelengths of light) and have a faster response time. The overall principle of light causing a cascade of molecular events leading to a change in membrane potential remains the same. This conversion of light into a neural signal is the essence of phototransduction.