Biology Reference
In-Depth Information
19
Relationship between
amount of rhodopsin and
sensitivity during dark
adaptation
On the discoveries of Boll (1877) and Kühne (1877, 1878,
1879
) that
rhodopsin in rods is engaged in reversible cycles of bleaching and
regeneration, Parinaud (
1885
) had suggested that changes in visual
sensitivity were due to variation in the amount of rhodopsin. This
view had a great impact. Thus, for a long period it became generally
accepted that the alteration of visual sensitivity in light and dark
adaptation reflected changes in the concentration of the visual
pigments and hence their capacity to absorb light.
19.1 Results of Tansley
Tansley (
1931
) appears to be the first to measure quantitatively
the change in rhodopsin concentration during dark adaptation. She
light adapted albino rats almost completely and then measured the
quantity of rhodopsin extracted after varying times (from 2.5 to 1140
min) in the dark. The results obtained could be explained both by
bimolecular and monomolecular reactions, although the monomo-
lecular reaction was found to fit slightly better. In accordance with
Parinaud's (
1885
) assumption, she obtained a striking similarity
between the regeneration curve of rhodopsin of the albino rat and the
dark-adaptation curve measured in humans. Hence, she suggested
that the sensitivity during dark adaptation was proportional to the
amount of rhodopsin present in the retina.
19.2 Results of Granit
This simple photochemical theory of dark adaptation, however,
eventually met with serious difficulties. Thus, evidence put forward
