Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

BIOCHEMISTRY OF VISION
Overview
For most of us, vision is such an everyday occurrence that we seldom think to wonder how we are
able to see the objects that surround us
...
e
...
The eyes receive the light and contain the molecules
that undergo a chemical change upon absorbing light, but it is the brain that actually makes sense of the
visual information to create an image
...
How do these organs work together in order to allow us to see the light-reflecting
objects around us as a visual image?

The Sense of Vision
In general, all living things respond to the stimulus of light
...
In
most invertebrates, the light receptors do not function as eyes and as a result, they are unable to form
images
...
As a result, some of these receptors can give no indication of the direction of the light source
and hence the animal responds mainly by random movements
...
One of the earliest forms of „vision‟ is
known as „phototaxis‟ which is a light-controlled motion
...
The exact mechanism of this movement is unknown; however, it is likely that the energy
needed to move is provided by the light which produces adenosine triphosphate (ATP) in the
photosynthetic process
...
However, the organism will start moving again as soon as it finds an illuminated area
...
This increases the sensitivity of the eye to dim light
...
The light from each source is
focused onto some of the receptor cells at any moment
...
Many insects and crustaceans have
compound eyes which utilise many closely packed lenses
...
The formation of images depends on the pattern of light
that falls onto the compound eye‟s surface
...
Therefore, the brain integrates all the messages received from
the various ommatidia and it apparently creates an image that corresponds to the total of many smaller
images (Keeton and Mc Fadden 1983)
...
The recognition of
the shapes of objects involves the formation of an image on this photosensitive area
...
The human eye is capable of detecting a variety of colours, forming images of objects
miles away, and responding to as little as one photon of light
...

In order to understand vision, it is necessary to know how the eye generates sensations, and then follow

1|Page

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

these signals to the visual centres of the brain, where images are perceived
...


The Human Eye
The shape of an adult human eye (Figure 1) is like a globe with a diameter of approximately 2
...
The globe of the eye or eyeball is a three-layered structure
which consists of sclera, choroid and retina
...

The sclera is a tough but elastic, white outer layer of connective tissue
...
A delicate layer of epithelial
cells forms a mucous membrane, known as the conjunctiva, which covers the outer surface of the sclera
and helps to keep the eye moist
...
The choroid is important since it provides blood to
other parts of the eye and it functions as a light absorbing layer which prevents internally reflected light
from blurring the image
...
The front choroid forms the donut-shaped iris, which gives the eye its colour
...
By changing size, the
iris regulates the amount of light entering the pupil, the hole in the centre of the iris
...
Just inside the choroid, the retina forms the inner most layer of the
eyeball and contains the photoreceptors
...
The rod cells are abundant
toward the periphery of the retina while the cone cells are abundant in the central portion of the retina
...
The
bipolar cells synapse in the retina with longer neurons, i
...
ganglion cells, whose axons form the optic nerve

2|Page

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

that runs to the visual centres of the brain
...
There
are no rods and cones present in the optic disc, and as such this region on the lower outside of the retina
is a blind spot, i
...
light focused on that part is not detected
...
The fovea or “yellow spot” lies on the optical axis and is the
place of most acute vision
...

The second constituent of the light-focusing system of the eye is the lens which is suspended just
behind the pupil by a suspensory ligament attached to the ciliary body
...
The ciliary body constantly produces the clear, watery aqueous humour
that fills the front cavity of the eye
...
The aqueous and vitreous humour function as liquid lines that
helps focus light onto the retina
...

The eye is similar to a camera
...
The iris regulates the opening of the lens while the
eyelids prevent light from entering and also prevents any possible damage to the surface of the cornea
...
The focusing of light onto the retina can be accomplished by this mechanism and also by the
curvature of the cornea
...
38; the lens is 1
...
33
...
The delicate and accurate control is
achieved by the lens which acts as a fine adjustment
...
These are referred to as rods and cones and
the names of these cells come from their individual shapes
...
The rod cells are abundant toward the periphery of the retina while the
cone cells are abundant in the central portion of the retina
...

The visual pigment in the rods is built into the membranes of the flattened vesicles in the outer
segment and is referred to as rhodopsin
...
They are
thought to be three types of cone cells which contain different forms of iodopsin and as such they
respond to light of different wavelengths
...
In general, colours are detected as a result of the relative degree of stimulation of the three types
of cones
...

The cone cells are concentrated in a central part of the retina, called the fovea, and as a consequence a
person can only perceive the colour of an object if its image falls close to the fovea or in the direct line
of vision
...
edu
...

The brain receives information through an intermediary optical nerve and has no direct contact
with the photoreceptor cells
...
Rod cells
are more sensitive to small amounts of light
...
It is possible for a number of rod cells to share the same nerve fibers, in some
cases as many as 150
...
In some cases, few cone cells may share the same one
...
This image is
precisely analyzed by the tightly packed cones which individually or in small groups send separate impulses
to the brain
...

Therefore, when the brain receives an impulse from the fibers connected to the rods, it has no way of
determining precisely from which one of the rod cells the impulse originated
...
The signals from the rods are combined where as those from the
cones are distributed between many nerve fibers
...

The rods located at the outermost edge of the retina are not able to form images
...

Photochemistry of Vision and Signal Transduction
The process of vision is triggered by the photochemical isomerization of the light absorbing
pigment molecule retinal, which is the aldehyde derived from Vitamin A, bonded to a membrane protein
called an opsin
...
edu
...

Rhodopsin consists of 348 amino acid residues that are grouped mainly into seven hydrophobic,
alpha helix segments which pass between the two sides of the photoreceptor membrane
...
The 11-cisretinal absorbs light in ultraviolet region
...


Figure 3: The reaction which links 11-cis-retinal to opsin (Atkins and de Paula 2002)
...
This occurs when a photon promotes
the π electron in a π→π* excitation
...
This photoisomerization occurs in about
200 femtoseconds and causes the conjugated carbon chain to become straightened
...
These light
induced changes in 11-cis-retinal and opsin are referred to as the "bleaching" of rhodopsin
...
The bathorodopsin is not stable
enough to stay in this arrangement for long
...
As a result, the protein begins to change its
shape in a very short period of time (10-9 s)
...
Eventually the protein expels the all trans-retinal, to give free
opsin and all trans-retinal
...
The shift will be larger for stronger interaction
...


5|Page

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

Figure 4: The photoisomerization of 11-cis-retinal all trans-retinal (Casiday and Frey 2000)
...

Name of Pigment

max (nm)

Rhodopsin

498

Bathorhodopsin

543

Lumirhodopsin

497

Metarhodopsin I

487

Metarhodopsin II

380

all trans-retinal (free)

370

The most important intermediate produced is the yellow all trans-metarhodopsin II (Figure 5),
which is formed when the light absorbed causes the isomerization of the cis-double bond in rhodopsin and
it also triggers a nerve impulse
...
As a result the carbon–nitrogen bond becomes exposed and
can be readily hydrolyzed to produce all trans-retinal and opsin
...


6|Page

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

Figure 5: The reaction scheme showing the formation of trans-metarhodopsin II which
hydrolyzes to all trans-retinal, which is reconverted to 11-cis-retinal, and opsin
...
Therefore, Na+ ions are continuously diffused
into the outer segment and across the narrow stalk of the inner segment
...
In this state , the rods cells releases glutamate and regulates the
“firing” of two different classes of bipolar cells that have opposite responses to glutamate
...
The synaptic terminal of the rods slows
down the release of glutamate which consequently enhances the activity of one class of bipolar cells and
suppresses the activity of the other type
...
edu
...

Each of the opsin is related to over a hundred regulatory proteins known as G proteins
...
This transducin activates the enzyme phosphodiesterase, which catalyses the hydrolysis of
the intracellular messenger, cyclic guanosine monophosphate (cGMP) (Figure 7)
...

Rhodopsin is inactive in the dark and the cGMP which is bound to Na+ ion channels in the plasma
membrane of the rod keeps those channels open
...

The hydrolysis of cGMP by phosphodiesterase to guanosine monophosphate (GMP) causes the Na+ channels
in the plasma membrane of the rod to close
...

When a large charge difference across the membrane builds up, the cell becomes hyperpolarized and this
prevents the release of the glutamate
...
This
electrical impulse is then passed onto the adjoining nerve cell which transmits the impulse to the brain by

8|Page

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

means of the optic nerve
...

The all trans-retinal is isomerized back by a number of slow thermal reactions to the 11- cisretinal which can combine with opsin to reform rhodopsin
...
The 11-cis-retinal reattaches to free opsin to reform rhodopsin which then waits
for the next photon to begin the process again
...
For example, the blue
light filtering by the carotene-like pigment xanthophyll, present in the tinted region called the “macular
pigment”, which prevents damage to photoreceptor molecules, is not well explained in the literature
...
The deactivation mechanism
for the excited xanthophyll molecule is another area which is unclear
...
Hence, in a way, this note attempts to emphasize the very fact that vision, particularly human
vision, is not fully understood and further elaborative high level research work is an absolute necessity
for a clearer picture to emerge
...


9|Page

BCH 441: TISSUE BIOCHEMISTRY

©mphilippe@madonnauniversity
...
ng

References
Atkins, P
...
and de Paula, J
...
Physical Chemistry (7th Edition)
...

Beckett, B
...
(1976)
...
London: Oxford University Press
...
and Reece, J
...
Biology (6th Edition)
...

Casiday, R
...
(2000)
...


http://www
...
wustl
...
html
Clegg, C
...
and Mackean, D
...
2000
...
London: John Murray
Ltd
...
M
...
1987
...
Cambridge: Cambridge University
Press
...
T
...
H
...
Elements of Biological Sciences (3rd Edition)
...
W
...

Kimball, J
...
1983
...
New York: Addison Wesley Publishing Company Inc
...
H
...
B
...
Biology (4th Edition)
...

Suppan, P
...
Chemistry of Light
...

Vines, A
...
and Rees, N
...
Plant & Animal Biology: Volume 2 (4th Edition)
...

Wayne, C
...
and Wayne, R
...
1996
...
New York: Oxford Science Publications

---