inner segments contain nuclei, mitochondria, and other structures necessary
for the functioning of the cell.This is a profound phenomenon, and the origin of the metaphor of
the blind spot: an area of one’s knowledge, belief, or behavior in which
there is some significant ignorance to which one is oblivious.
Transitioning to the subcellular and molecular level of description
of the visual system, consider the rod cell. It consists of a rodlike
segment containing numerous lipid bilayer membrane disks, with
each disk containing numerous rhodopsin photoreceptor proteins
embedded in the disk membranes (Fig. 14.4). Cone cells have a similar
structure, with a cone-shaped segment containing cone opsin pho-
toreceptor proteins embedded in a highly infolded cell membrane.
Each rod cell contains about one hundred million (108) rhodopsin
molecules. Because there are one hundred million rod cells in a retina,
there are approximately 10! (ten quadrillion) rhodopsin molecules
in each of our two retinas—a very large number!
Each rhodopsin or cone opsin protein is composed of about 350
amino acids, joined into a long chain by covalent chemical bonds. The
chain is embedded in lipid bilayer membrane in the outer segment of
a photoreceptor cell and winds itself back and forth across the bilayer
seven times (Fig. 14.5). In the sections where the polypeptide chain
crosses the membrane, it forms alpha-helical structures composed of
amino acids that are more hydrophobic and thus agreeable to being
inside the hydrophobic core of the lipid bilayer.
Within a rhodopsin or cone opsin there is another molecule—a
small molecule, although not an amino acid—attached to the opsin
protein via a covalent bond with a nitrogen atom in a specific lysine
amino acid within the protein. This molecule is called retinal, named
after the retina. It is the retinal molecule that absorbs the light and be-