Light sensitive molecules, called chromophores are held by proteins. During darkness, a protein called rhodopsin, in a pigment called visual purple, accumulates in the rods. The amount of visual purple determines the sensitivity of the eye to light, and mostly reaches maximum after thirty minutes, with some improvement of up to two hours. Cone proteins are very similar to rhodopsin, in fact, differing in only one key amino acid. This single difference in the amino acid at position 122 results in the cone proteins resetting 100 times faster than rhodopsin after absorbing light.
Visual purple sensitizes our eyes by many thousands of times.
The change in light sensitivity when the iris opens and closes is very slight in comparison. Each rod cell contains 2000 stacked disks which contain up to 100 million modules of light sensitive pigment rhodopsin. Each metarhodopsin II molecule activates hundreds of molecules of transducin, a protein. Each of these activates an enzyme, phosphodiesterase, which alters the structure of thousands of molecules of the neurotransmitter cGMP, cyclic guanosine monophosphate. Levels of cGMP which in darkness are high, are thus reduced, closing channels which allow sodium ions to flow through the cell membrane.