Biology Reference
In-Depth Information
cells. The densest concentration of cone cells occurs at a point called the fovea,
which is directly on the optical axis of the lens and provides the sharpest image.
A rod cell generates a nerve impulse after absorbing just one photon of light,
although the brain requires impulses from six photons to perceive the signal. This
remarkable sensitivity is mediated by a light-absorbing molecule, a protein-pigment
complex called rhodopsin. This complex consists of a membrane protein called
opsin, bound covalently to a pigment called retinal that absorbs photons. Retinal
is made in the retina from vitamin A, so that a diet deficient in this vitamin leads
to a condition called night-blindness, in which the patient experiences difficulty in
seeing in dim light. Absorption of a photon of light by rhodopsin results in a very
rapid and reversible change in the conformation of the retinal that triggers confor-
mational changes in other proteins bound to the retinal membrane. These changes
in protein shape generate an electrical impulse that travels down the axon of the rod
cell to the next nerve cell in the relay.
Cone cells contain three slightly different kinds of opsin protein, the differences
being in a few of the amino acids in the polypeptide chain, close to where the retinal
is bound. These changes in some amino acids result in different absorption spectra
of the attached retinal pigment because they alter the electronic distribution in the
retinal molecule. The absorption maxima of these three rhodopsins are at 560, 530
and 426 nm, resulting in cones sensitive to red, green or blue wavelengths of light.
Each cone contains only one type of these three rhodopsins, an arrangement resem-
bling that found in the photosensors of digital cameras. The rhodopsin in rod cells
has its absorption maximum at 500 nm, which is the most abundant wavelength in
solar radiation at ground level.
Many of the details of the “inimitable contrivances” that allow the eye to function
so well were discovered after Darwin's time, but he know enough to appreciate
the problem of explaining its origin in term of natural selection. After pointing out
how well the eye is adapted to its function, Darwin goes on to say in his chapter
“Difficulties with the Theory”:
Reason tells me, that if numerous gradations from a simple and imperfect eye to one com-
plex and perfect can be shown to exist, each grade being useful to its possessor, as is
certainly the case; if further, the eye varies and the variations are inherited, as is likewise
certainly the case; and if such variations be useful to any animal under changing condi-
tions of life, then the difficulty of believing that a perfect and complex eye could be formed
by natural selection, though insuperable to our imagination, should not be considered as
subversive of the theory.
In the reminder of his discussion about the evolution of eyes, Darwin points
out that much simpler eyes than the ones than mammals possess occur in animals
such as starfish and lancelets. These “eyes” consist only of some pigmented cells
shielding some photosensitive cells on one side They can sense the direction of
light, but are unable to form images because they lacks a lens, and so are more
accurately called “eyespots”. He suggests that the existence today of these eyespots
supports his idea that “simple and imperfect eyes” could be ancestors of modern
eyes. Since his time, many more examples of biological light-detection structures
have been found, and I discuss some of these in the next section.
