This document discusses the rhodopsin cycle in the rods of the eye. It begins by defining photoreceptors as the end organs of vision that transform light energy into visual impulses. It then describes rods and cones, focusing on the structure of rods which contain the visual pigment rhodopsin in their outer segments. Rhodopsin is composed of the protein opsin and the chromophore retinal. Upon absorption of light, rhodopsin undergoes a chemical change where retinal is separated from opsin, in a process called bleaching. The bleached rhodopsin is then regenerated through a series of reactions that reform the bond between retinal and opsin, maintaining the visual cycle that is

1. RHODOPSIN CYCLE
BY :
REEMA V. DANDAVATE
F.Y. B.OPTOM

2. WHAT ARE
PHOTORECEPTORS ?
They are the end organs
of vision which transform
light energy into visual impulse.

3. PHOTORECEPTORS
•RODS (scotopic vision)
•CONES (photopic vision)

4. THE ROD CELL
•40-60 millimicron long.

5. •Outer segment of rod :
cylindrical, highly refractile, transversly
striated, contains visual purple
•Composed of : lipid protein lamellar
disc stacked one on top of the other and
surrounded by a cell membrane.
•No. of discs :600-1000/rod
•Thickness of disc :22.5-24.5nm
•90% visual pigment is present in disc
•Remaining pigment scattered on surface
of plasmalemma .

6. •Inner segment of rod : thicker then
outer segment.
•Consists of 2 regions: ellipsoid
and myoid
•Ellipsoid : outer portion adjacent to
the outer segment, contains
abundant number of mitochondria.
•Myoid : inner portion contains
glycogen as well as other organelles.

7. VITAMIN A AND VISUAL PIGMENTS
•DIETARY SOURCES OF RETINOL
•ABSORPTION AND STORAGE
•TRANSPORT FROM LIVER TO THE EYE
•UTILIZATION FOR SYNTHESIS OF
RHODOPSINE

10. VISUAL PIGMENTS
•CONE PIGMENTS
•RHODOPSIN

11. WHAT ARE CONE PIGMENTS ?
•Respond to specific wavelengths of light
giving rise to colour vision.
•The peak absorbance wavelengths lies:
Blue 435nm
Green 535nm
Red 580nm
•The relative sizes of population of the three types of
cones are not well established, though several lines
of evidence indicate that blue sensitive cones are
least prevalent.
•As yet the molecular details of the cone pigment
protein remain obscure.

12. WHAT IS RHODOPSIN ???
Also known as VISUAL PURPLE due to its
pinkish colour and is a light-sensitive receptor
protein.

13. Opsin and retinal
Rhodopsin is made up of the
protein opsin with the
chromophore, retinal,
covalently attached
The linkage occurs at
Lys-296

14. RHODOPSIN
•Photosensitive visual pigment.
•Present in : discs of rods outer segment.
•Consists of: protein opsin and carotenoid
retinal.
•molecular weight : 40,000
•Insoluble in water but can be taken into
solution if detergent is added.
•Sensitive to heat and chemical agents.
•Absorption spectrum of rhodopsin : 493-
505nm.

15. Structure of rhodopsin
The protein opsin of
the human rhodopsin
is a 348 amino acid
protein that crosses
the disc membrane 7
times.
The amino acid
sequences for all
human cone opsins is
almost identical, the
difference in
spectral absorbance
being determined by
a few different
amino acids.

16. Where is it found ???
It is a biological pigment in photoreceptor
cells of the retina.
Rhodopsin is the primary pigment found in the
rods photoreceptors.

17. The eye
Rhodopsin is found in the rods
that are located in the eye
Rods are composed of stacked
disks
Rhodopsin is densely packed
into each disk
Rods are responsible for black and white vision
Rhodopsin works best at dim light, responsible
for night vision (too much light will saturate
the protein)
Rod density is found greater on the outer
edges of the retina (peripheral vision)

18. LIGHT-induced changes
Light falling upon retina -
absorbed by - Photosensitive
pigments - present in - Rods
& Cones - & initiates -
photochemical changes - in
this way - process of vision
starts

19. The photochemical reactions
studies in the rod outer
segments can be described
under three headings:
RHODOPSIN BLEACHING
RHODOPSIN REGENERATION
VISUAL CYCLE

20. RHODOPSIN BLEACHING
•Rhodopsin consists of protein opsin and a
carotenoid called retinene.
•The light absorbed by the rhodopsin converts
its
11-cis-retinal into all-trans-retinal.
•The all-trans-retinal can no longer remain in
combination with opsin and thus there occurs a
separation of all-trans-retinal and opsin.
•The process of separation is called
photodecomposition and the rhodopsin is said
to be bleached by the action of light.

22. Rhodopsin regeneration
The first stage of reformation of rhodopsin, is
isomerization of all-trans-retinal back to 11-
cis-retinal.
The process is catalyzed by enzyme retinal
isomerase.
Energy for the regeneration process is
supplied by the overall metabolic pool of the
photoreceptor outer segment.

23. Visual cycle
In the retina of living animals, under constant
light stimulations, a steady must exist under
which the rate at which the photochemicals
are bleached is equal to the rate at which they
are regenerated.
This equilibrium between the
photodecomposition and the regeneration of
visual pigments is referred to as visual cycle.

25. Reference :
A.K.Khurana
http://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision
www.wikipedia.visualcycle.com