560
UNIT 3
Regulation and Integration of the Body
15
a single layer in the membranes of each of the thousands of discs
in the rods’ outer segments. Te formation and breakdown of
rhodopsin is shown in Figure 15.16.
1
Pigment synthesis:
Rhodopsin forms and accumulates in
the dark. Vitamin A is oxidized (and isomerized) to the
11-
cis
-retinal form and then combined with opsin to form
rhodopsin.
2
Pigment bleaching:
When rhodopsin absorbs light, reti-
nal changes shape to its all-
trans
isomer, allowing the sur-
rounding protein to quickly relax like an uncoiling spring
into its light-activated form. Eventually, the retinal-opsin
along the optic nerve. Let’s look more closely at these events in
rods and in cones.
Phototransduction
Phototransduction begins when a visual pigment captures a
photon of light.
Capturing Light
Te visual pigment of rods is a deep purple
pigment called
rhodopsin
(ro-dop
9
sin;
rhodo
5
rose,
opsis
5
vision). (Do you suppose the person who coined the expression
“looking at the world through rose-colored glasses” knew the
meaning of “rhodopsin”?) Rhodopsin molecules are arranged in
11-
cis
-retinal
H
3
C
H
3
C
H
O
CH
3
CH
3
CH
3
CH
3
H
O
CH
3
CH
3
H
3
C
CH
3
11-
cis
-retinal
Pigment bleaching:
Light absorption by
rhodopsin triggers a
rapid series of steps in
which retinal changes
shape (11-
cis
to all-
trans
) and eventually
releases from opsin.
2
Pigment synthesis:
11-
cis
-retinal, derived
from vitamin A, is
combined with opsin
to form rhodopsin.
1
Rhodopsin
Opsin
and
All
-trans
-retinal
Pigment regeneration:
Enzymes slowly convert
all-
trans-
retinal to its
11-
cis
form in cells of the
pigmented layer; requires
ATP.
Dark
Light
All-
trans-
retinal
Oxidation
2H
+
2H
+
Reduction
Vitamin A
3
Figure 15.16
The formation and breakdown
of rhodopsin.
11-
cis
-retinal can either be
regenerated from all-
trans
-retinal or made from
vitamin A.
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