 Dear students, in this topic, we shall continue to discuss the visual pigments and their photochemistry. The rhodopsin isomerization that involves the retinal molecule will be discussed here. The retinal molecule assumes two sterically distinct states in the retina. In the absence of light, retinal is in eleven cis configuration that binds to the opsin protein covalently. In the presence of light, this eleven cis retinal isomerizes into the all trans configuration. This cis trans isomerization is the light's only effect on visual pigment. The light that is perceived in the retina is the transduction event that will be the result of this change. The cis trans configuration change destabilizes the rhodopsin molecule and it starts to decompose. Because eleven cis configuration binds to the opsin and makes the rhodopsin. But the all trans configuration does not bind to the opsin and it is detached. But this does not happen at all, it is stepwise. We say that this stepwise process is decomposing and it is gradually breaking down. However, this decomposing stage of rhodopsin is actually the activated state of rhodopsin. During these changes, the color of rhodopsin changes from purple to yellow. That is why this phenomenon is also called bleaching. This change results in a series of biochemical reactions in the membrane that results in the transduction of light into electrical signals. Dear students, now we shall discuss the decomposition steps of rhodopsin. When the light hits rhodopsin, it immediately produces bethorhodopsin. This bethorhodopsin is a partially split combination of all trans retinal and opsin. Bethorhodopsin is extremely unstable and decays in a nanosecond time to lumirhodopsin. Lumirhodopsin takes about a microsecond to decay into meta-rhodopsin 1. Metarhodopsin 1 changes into metarhodopsin 2 in about a millisecond. This is how we see that the highly unstable intermediates are producing in the beginning and the ones that we are moving ahead are comparatively stable. This is after a nanosecond to a microsecond and then to a millisecond. Now, this metarhodopsin 2 is split into the opsin and all trans retinal but this occurs much more slowly and takes few seconds. Dear students, this metarhodopsin 2 is the activated rhodopsin. It excites changes in the rod cells that eventually generate graded receptor potential and transmit visual impulse into the nervous system. Now, we shall see how metarhodopsin 2 works in transduction. The metarhodopsin 2 activates a G protein which is called transducin in the membrane. This activated transducin activates the enzyme phosphodiesterase. This enzyme hydrolyzes cyclic GMP and converts it into 5' GMP. The effect of this is that cyclic GMP's concentration cell drops. This drop in cyclic GMP level results in drop in the conductance of cations. Specially, the sodium, calcium and magnesium ions through the permitting channel membranes, which they were allowing, when channels are closed, their conductance in the cations decreases. What will be the result of this? The result is that potassium channels which are open right now will flow through the potassium ions. As a result, potassium current will dominate and will cause the cell to hyperpolarize. This hyperpolarization will produce the graded receptor potential. The production of graded receptor potential due to hyperpolarization is characteristic of the photoreceptor cells. This is highly unusual in nature because receptor potentials are generally produced due to depolarization but here receptor potential is produced due to hyperpolarization.