<P> When light hits photoreceptive pigments within the photoreceptor cell , the pigment changes shape . The pigment , called rhodopsin ( conopsin is found in cone cells ) comprises a large protein called opsin ( situated in the plasma membrane ) , attached to which is a covalently bound prosthetic group : an organic molecule called retinal ( a derivative of vitamin A ) . The retinal exists in the 11 - cis - retinal form when in the dark , and stimulation by light causes its structure to change to all - trans - retinal . This structural change causes an increased affinity for the regulatory protein called transducin ( a type of G protein ) . Upon binding to rhodopsin , the alpha subunit of the G protein replaces a molecule of GDP with a molecule of GTP and becomes activated . This replacement causes the alpha subunit of the G protein to dissociate from the beta and gamma subunits of the G protein . As a result , the alpha subunit is now free to bind to the cGMP phosphodiesterase ( an effector protein ) . The alpha subunit interacts with the inhibitory PDE gamma subunits and prevents them from blocking catalytic sites on the alpha and beta subunits of PDE , leading to the activation of cGMP phosphodiesterase , which hydrolyzes cGMP ( the second messenger ) , breaking it down into 5 ' - GMP . Reduction in cGMP allows the ion channels to close , preventing the influx of positive ions , hyperpolarizing the cell , and stopping the release of the neurotransmitter glutamate ( Kandel et al. , 2000 ) . Though cone cells primarily use the neurotransmitter substance acetylcholine , rod cells use a variety . The entire process by which light initiates a sensory response is called visual phototransduction . </P>
<P> When light hits photoreceptive pigments within the photoreceptor cell , the pigment changes shape . The pigment , called rhodopsin ( conopsin is found in cone cells ) comprises a large protein called opsin ( situated in the plasma membrane ) , attached to which is a covalently bound prosthetic group : an organic molecule called retinal ( a derivative of vitamin A ) . The retinal exists in the 11 - cis - retinal form when in the dark , and stimulation by light causes its structure to change to all - trans - retinal . This structural change causes an increased affinity for the regulatory protein called transducin ( a type of G protein ) . Upon binding to rhodopsin , the alpha subunit of the G protein replaces a molecule of GDP with a molecule of GTP and becomes activated . This replacement causes the alpha subunit of the G protein to dissociate from the beta and gamma subunits of the G protein . As a result , the alpha subunit is now free to bind to the cGMP phosphodiesterase ( an effector protein ) . The alpha subunit interacts with the inhibitory PDE gamma subunits and prevents them from blocking catalytic sites on the alpha and beta subunits of PDE , leading to the activation of cGMP phosphodiesterase , which hydrolyzes cGMP ( the second messenger ) , breaking it down into 5 ' - GMP . Reduction in cGMP allows the ion channels to close , preventing the influx of positive ions , hyperpolarizing the cell , and stopping the release of the neurotransmitter glutamate ( Kandel et al. , 2000 ) . Though cone cells primarily use the neurotransmitter substance acetylcholine , rod cells use a variety . The entire process by which light initiates a sensory response is called visual phototransduction . </P>
the rod cells in the retina contain a light absorbing pigment called
the rod cells in the retina contain a light absorbing pigment called
rhodopsin
rhodopsin