Vision and Phototaxis

The vision of the shapes of objects requires the formation of an image on a photosensitive surface. In higher animals the optics of the eye make use of a lens (rather like a photographic camera), but there are some lower species which use a simple pinhole aperture (as was used, incidentally, in the early days of photography). 

The primary photochemical process of vision is therefore the cis-trans isomerization of retinal in rhodopsin. The free protein opsin then leads to the production of the nerve impulse through a secondary biochemical process 

The effect of absorption of a quantum of light by rhodopsin is to cause an influx of some 105 Na+ ions. It can be surmised that the rhodopsin molecule normally blocks ion transport across the cell membrane, but its change of shape as free opsin leaves a pore which allows ion conduction. 

The eye of higher animals is remarkably sensitive to low light intensities, but saturation can occur relatively easily at high light intensities and the recovery is then slow. After the absorption of light rhodopsin (which has a red colour) is bleached when the retinal leaves the opsin, and recovery goes through a sequence of enzyme-catalysed reactions which takes several seconds. 

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