The photoisomerization of retinal in rhodopsin is a crucial process in vision, as it is the primary event that converts light into a biological signal. This process occurs in the retina of the eye, where the protein rhodopsin, also known as visual pigment, is embedded in the membrane of rod cells. Rhodopsin consists of a protein called opsin and a chromophore called retinal, which is a derivative of vitamin A. The retinal molecule is responsible for the light-absorbing properties of rhodopsin.The mechanism or pathway for the photoisomerization of retinal in rhodopsin can be described in the following steps:1. Absorption of light: When a photon of light is absorbed by the retinal molecule, its energy causes the molecule to undergo a conformational change. In the dark, retinal is in the 11-cis configuration, which means that the molecule has a kink in its structure. Upon absorption of light, the molecule undergoes isomerization, changing from the 11-cis configuration to the all-trans configuration. This change straightens the molecule and occurs extremely fast, within picoseconds 10^-12 seconds .2. Activation of rhodopsin: The isomerization of retinal induces a conformational change in the opsin protein as well. This change activates the rhodopsin molecule, converting it from its inactive form also known as the ground state or R to its active form, called metarhodopsin II MII .3. Signal transduction: The activated rhodopsin MII interacts with a G-protein called transducin Gt . This interaction causes the exchange of GDP for GTP in the Gt protein, activating it. The activated Gt then dissociates into two subunits: G and G.4. Activation of phosphodiesterase PDE : The G subunit binds to and activates an enzyme called phosphodiesterase PDE . PDE catalyzes the conversion of cyclic guanosine monophosphate cGMP to guanosine monophosphate GMP . This leads to a decrease in the concentration of cGMP in the cell.5. Closing of cGMP-gated ion channels: The decrease in cGMP levels causes the cGMP-gated ion channels in the rod cell membrane to close. This leads to a decrease in the influx of sodium and calcium ions, causing the cell to hyperpolarize.6. Transmission of the signal: The hyperpolarization of the rod cell leads to a decrease in the release of the neurotransmitter glutamate at the synapse between the rod cell and the bipolar cell. This change in neurotransmitter release is detected by the bipolar cell, which then transmits the signal to the ganglion cells and, ultimately, to the brain via the optic nerve.In summary, the photoisomerization of retinal in rhodopsin is a key process in vision, as it initiates a cascade of events that ultimately leads to the conversion of light into a neural signal that can be interpreted by the brain.