gins the cascade of events leading to a neural signal.Figure 14.5. Diagram of a rhodopsin molecule in a lipid bilayer membrane,
with individual amino acids drawn as small circles. Something of rhodopsin’s
secondary structure is depicted: seven alpha-helix regions where the polypep-
tide chain snakes back and forth through bilayer membrane. This diagram
doesn’t reveal much about tertiary structure; within an actual membrane the
polypeptide chain would be clumped together much more closely than shown
here.
The same retinal molecule occurs in rhodopsin and in the various
different cone opsins, and yet these proteins respond to light of quite
different wavelengths. This occurs because the different amino acid
sequences and arrangements in the various opsin proteins produce
different electronic environments, altering the molecular energy lev-
els in the attached retinal and thereby changing its light absorption
spectrum.