BCH 441: TISSUE BIOCHEMISTRY ©
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. Yet the vision process is a fascinating example of how light
(such as the light reflected off of the objects that we see) can produce molecular changes with important
consequences (i.e., our ability to perceive an image). 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. Hence, the visual process requires the intricate coordination of the
eyes and the brain. 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. Almost all multicellular organisms have
specialized light receptor cells in which light energy can cause changes in a light-sensitive pigment. In
most invertebrates, the light receptors do not function as eyes and as a result, they are unable to form
images. However, they are able to perceive the presence of light and can detect any changes in light
intensity. 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. However, there are some cases in which light
receptors are arranged in such a manner as to indicate direction. One of the earliest forms of „vision‟ is
known as „phototaxis‟ which is a light-controlled motion. This phenomenon has been observed in some
photosynthetic bacteria such as Chromatium, which move selectively towards illuminated areas rather
than dark places. 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. Hence, the bacterium cannot move in dark places since there is no production of
ATP. However, the organism will start moving again as soon as it finds an illuminated area.
In higher life forms, they have more complex eyes that generally have a lens which is capable of
concentrating light onto a photosensitive area. This increases the sensitivity of the eye to dim light. It
also increases the ability of the eye to perceive direction and movement. The light from each source is
focused onto some of the receptor cells at any moment. There are basically two different types of image-
forming eyes in animals; compound eyes and camera-type eyes. Many insects and crustaceans have
compound eyes which utilise many closely packed lenses. Each lens is connected with a few sensory cells to
form a functional unit known as the ommatidium. The formation of images depends on the pattern of light
that falls onto the compound eye‟s surface. The ommatidia point in various directions and as such will be
stimulated by light from different points. 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). Various animals such as molluscs and vertebrates possess a camera-
type eye which uses a single lens system to focus light onto a photosensitive surface, known as the retina,
which functions similarly to a piece of photographic film (Keeton and Mc Fadden 1983). The recognition of
the shapes of objects involves the formation of an image on this photosensitive area.
For humans, the term „vision‟ is a complex process of information regarding the environment of a
living organism. 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. However, it is actually the brain that „sees‟.
In order to understand vision, it is necessary to know how the eye generates sensations, and then follow
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, BCH 441: TISSUE BIOCHEMISTRY ©
these signals to the visual centres of the brain, where images are perceived. Hence, this note will focus
mainly on vision in humans.
The Human Eye
The shape of an adult human eye (Figure 1) is like a globe with a diameter of approximately 2.5 cm,
that fits into the bony sockets in the skull. The globe of the eye or eyeball is a three-layered structure
which consists of sclera, choroid and retina.
Figure 1: Longitudinal section through the human eye (Clegg and Mackean, 2000).
The sclera is a tough but elastic, white outer layer of connective tissue. At the front of the eye,
the sclera becomes the transparent cornea, which allows light to enter the interior of the eye and
functions as the first constituent of the light–focusing system of the eye. 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 a layer of darkly pigmented tissue through which many
blood vessels pass and is located just inside the sclera. 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. At the immediate back of the junction between the main part of the sclera and
the cornea, the choroid becomes thicker with smooth muscles embedded; this part of the choroid is
known as the ciliary body. The front choroid forms the donut-shaped iris, which gives the eye its colour.
The iris consists of smooth muscle fibres arranged in circular and radial directions. By changing size, the
iris regulates the amount of light entering the pupil, the hole in the centre of the iris. The pupil is
reduced when the circular muscle fibres contract and it is dilated when the radial muscle contract
(Keeton and Mc Fadden 1983). Just inside the choroid, the retina forms the inner most layer of the
eyeball and contains the photoreceptors.
The photoreceptors are of two types, referred to as rods and cones. 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 which are short sensory neurons synapse with the photoreceptors in the retina. The
bipolar cells synapse in the retina with longer neurons, i.e. ganglion cells, whose axons form the optic nerve
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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. Yet the vision process is a fascinating example of how light
(such as the light reflected off of the objects that we see) can produce molecular changes with important
consequences (i.e., our ability to perceive an image). 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. Hence, the visual process requires the intricate coordination of the
eyes and the brain. 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. Almost all multicellular organisms have
specialized light receptor cells in which light energy can cause changes in a light-sensitive pigment. In
most invertebrates, the light receptors do not function as eyes and as a result, they are unable to form
images. However, they are able to perceive the presence of light and can detect any changes in light
intensity. 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. However, there are some cases in which light
receptors are arranged in such a manner as to indicate direction. One of the earliest forms of „vision‟ is
known as „phototaxis‟ which is a light-controlled motion. This phenomenon has been observed in some
photosynthetic bacteria such as Chromatium, which move selectively towards illuminated areas rather
than dark places. 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. Hence, the bacterium cannot move in dark places since there is no production of
ATP. However, the organism will start moving again as soon as it finds an illuminated area.
In higher life forms, they have more complex eyes that generally have a lens which is capable of
concentrating light onto a photosensitive area. This increases the sensitivity of the eye to dim light. It
also increases the ability of the eye to perceive direction and movement. The light from each source is
focused onto some of the receptor cells at any moment. There are basically two different types of image-
forming eyes in animals; compound eyes and camera-type eyes. Many insects and crustaceans have
compound eyes which utilise many closely packed lenses. Each lens is connected with a few sensory cells to
form a functional unit known as the ommatidium. The formation of images depends on the pattern of light
that falls onto the compound eye‟s surface. The ommatidia point in various directions and as such will be
stimulated by light from different points. 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). Various animals such as molluscs and vertebrates possess a camera-
type eye which uses a single lens system to focus light onto a photosensitive surface, known as the retina,
which functions similarly to a piece of photographic film (Keeton and Mc Fadden 1983). The recognition of
the shapes of objects involves the formation of an image on this photosensitive area.
For humans, the term „vision‟ is a complex process of information regarding the environment of a
living organism. 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. However, it is actually the brain that „sees‟.
In order to understand vision, it is necessary to know how the eye generates sensations, and then follow
1|Page
, BCH 441: TISSUE BIOCHEMISTRY ©
these signals to the visual centres of the brain, where images are perceived. Hence, this note will focus
mainly on vision in humans.
The Human Eye
The shape of an adult human eye (Figure 1) is like a globe with a diameter of approximately 2.5 cm,
that fits into the bony sockets in the skull. The globe of the eye or eyeball is a three-layered structure
which consists of sclera, choroid and retina.
Figure 1: Longitudinal section through the human eye (Clegg and Mackean, 2000).
The sclera is a tough but elastic, white outer layer of connective tissue. At the front of the eye,
the sclera becomes the transparent cornea, which allows light to enter the interior of the eye and
functions as the first constituent of the light–focusing system of the eye. 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 a layer of darkly pigmented tissue through which many
blood vessels pass and is located just inside the sclera. 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. At the immediate back of the junction between the main part of the sclera and
the cornea, the choroid becomes thicker with smooth muscles embedded; this part of the choroid is
known as the ciliary body. The front choroid forms the donut-shaped iris, which gives the eye its colour.
The iris consists of smooth muscle fibres arranged in circular and radial directions. By changing size, the
iris regulates the amount of light entering the pupil, the hole in the centre of the iris. The pupil is
reduced when the circular muscle fibres contract and it is dilated when the radial muscle contract
(Keeton and Mc Fadden 1983). Just inside the choroid, the retina forms the inner most layer of the
eyeball and contains the photoreceptors.
The photoreceptors are of two types, referred to as rods and cones. 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 which are short sensory neurons synapse with the photoreceptors in the retina. The
bipolar cells synapse in the retina with longer neurons, i.e. ganglion cells, whose axons form the optic nerve
2|Page
