474
UNIT THREE
In darkness, sodium channels in portions of the receptor
cell membranes are kept open by a nucleotide called
cyclic
guanosine monophosphate
(cGMP). When rhodopsin mol-
ecules absorb light, they change shape and release opsin,
in trillionths of a second. The released opsin then becomes
an active enzyme, which activates a second enzyme (trans-
ducin), which, in turn, activates another enzyme (phospho-
diesterase). The third enzyme of this series breaks down
cGMP, and as the concentration of cGMP decreases, sodium
Discs of
membrane
within cell
Mitochondria
Nucleus
Synaptic
ending
Rod cell
FIGURE 12.39
Rhodopsin is embedded in
discs of membrane stacked in the rod cells.
Many sensory
nerve fibers
Cones
Retinal pigment
epithelium
Rods
(b)
Single sensory
nerve fiber
(a)
Rod
Cone
(c)
FIGURE 12.38
Rods and cones. (
a
) A single sensory nerve F
ber
transmits impulses from several rods to the brain. (
b
) Separate sensory
nerve F
bers transmit impulses from cones to the brain. (
c
) Scanning
electron micrograph of rods and cones (1,350×).
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