Abstract
When a rod cell of the retina absorbs light as a primary photoevent, an isomerization of the photoreceptor chromophore 11-cis-retinal to 11-trans-retinal occurs within picoseconds. The photoreceptor cells that make it possible to form black and white images in dim light, i.e., the rod cells, are exquisitely sensitive detectors. When a photon strikes the retina, the rhodopsin molecule (the macromolecule holding the retinal) that is struck reports the event with high efficiency, whereas the millions of other rhodopsin molecules in the cell remain silent. The photoreceptor of rod cells, rhodopsin, located in the rod disks as a transmembrane protein, has two components: cis-retinal, an organic molecule derived from vitamin A, and opsin, a protein that has the capacity to act as an enzyme. Retinal is nested at the center of a complex and highly structured protein environment, responsible for “tuning” retinal by influencing the spectrum of radiation it can absorb. When a photon is absorbed by cis-retinal, the energy of light straightens the bend in the retinal carbon chain, a bend that is because of the presence of hydrogen atoms attached to the C-11 and C-12 on the same side of the chain. In the transduction process that follows, a cascade of reactions results in a nerve signal. The chapter also explains signal transduction between the disk membrane and the rod outer membrane.
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