Photo-chemistry of Vision

1. By-
Manish Sahu
M.Sc. Chemistry (Final)
Sp.- Physical Chemistry

2.  Introduction
 Rods and cones
 Photochemistry of vision
 Visual pigments
 Absorption spectrum of visual pigments
 Structure of Rhodopsin
 Rhodopsin- - Retinal visual cycle
 Photo transduction in Rods
 Photo receptors and their activation
 Conclusion
 References

3.  Photochemistry of vision
basically means detection of
light which strikes the eyes
and brains that receives a
signal.
 The eye of a vertebrate
consists of Lens an Retina.
 Retina is made up of two types
of light sensitive cells:-
i)Rods ii) Cones.
 Eyes of humans are made up
of photosensitive compound
i.e. PROTEINS. This proteins
are made up of two types :-
i) Opsin. ii) Retinal.

4.  Rods :-
 Rods are sensitive therefore
functions at low intensity .
 Responsible for vision in dim
light.
 Cone :-
 Comparatively low sensitivity.
 Responsible for colour
selectors
Both of them present in Retina
acts as the receptors of an Eye.

5. i.Photopic vision :-
 Daylight vision due to cones.
 Mainly for colour vision
ii. Scotopic vision :-
 Dim lights vision due to rods.
iii. Mesopic vision :-
 Full moonlight vision.
 Both rods and cones.

6.  The photosensitive pigment in the rod is known as Rhodopsin or
visual purple.
 3 cones pigments :-
Erythrolable – maximum sensitivity for RED.
Cholorable – maximum sensitivity for GREEN.
Cyanolable – maximum sensitivity for BLUE .

7. o Rhodospin
absorbs light most
of the visible
spectrum shows
absorption at
500nm.
o 3 types of cones
which contain
photo pigment that
absorbs light at
particular wave
length.
o RED CONES
o GREEN CONES
o BLUE CONES

8. Rhodopsin also known as
visual purple pigment i.e. photo
sensitive pigment in the rods.
Its opsin (protein) ,scotopsin
and 11-cis-retinal derivative of
vitamin A.
Molecular weight of a human
Rhodopsin is 41,000 and it
consists of 348 amino acids.

Location:- present in the
membrane of rod disks. It
is very sensitive light
receptors.

9. REGENERATION OF
RHODOPSIN…..
All the trans retinol is
enzymatically isomerized to all
11-cis-retinol then enzymatically
oxidized to11-cis retinal. The 11-
cis-retinal then spontaneously
combines with opsin reforming
rhodopsin.
Conversion of the retinol to
retinal is dependent on the
pigment epithelium.
Deficiency of vitamin A Prevents
regeneration of Rhodopsin, for
this reason lack of vitamin A
leads of Night blindness.

10.  Photo transduction is
a process by which
light is converted into
an electrical signals
in the rod cells and
cone cells.

PHOTOTRNSDUCTION PROCESS..

11.  Photoreceptors are the cells in the retina that respond to light.
Activation of Rods by Light..
DARK CONDITIONS..
 In dark conditions, cGMP levels are high and keep cGMP gated
sodium channels open allowing a steady inward current called
dark current. This dark current keeps the cell depolarized at
about -40mv.
LIGHT CONDITIONS..
 In photopic conditions photoreceptors hyperpolarized to potential
of -60mv. Light closes cGMP gated sodium channels reducing
influx of sodium ions and lead to hyperpolarization.

13. Photo isomerization reaction occurs as the first step in the
chemistry of vision. Photo isomerization is a molecular
behaviour in which structural change between isomers is
caused by photoexcitation.The cis-transisomerization of
rhodopsin is the primary reaction in vision.