A photo pigment is a molecule in a photoreceptor
cell (a rod or cone) that can trap the energy in
visible light and convert it into a signal that
can trigger a nerve cell. When the nerve cell is
triggered, your nervous system knows that light is
hitting that particular photoreceptor. The way
pigments usually work is by having a 3-dimensional
structure with a special arrangement of chemical
bonds. When light of the correct wavelength hits
the pigment, the electrons in the bonds are moved
around, and this changes the 3-D structure of the
pigment. When the pigment changes shape, it
causes the photoreceptor cell to send a nerve
impulse to the brain. The pigment molecule then
has to return to its original 3-D shape to be able
to be stimulated again.
absorb light of different wavelengths. The cone
photoreceptors in our eyes have 3 different
pigments that absorb 3 different wavelengths, so
the range of wavelengths that our cones can 'see'
is quite broad -- ranging from blue to red with a
peak at green (remember -- the spectrum goes
Red-Orange-Yellow-Green-Blue-Indigo-Violet -- ROY
G BIV). This range of the visible spectrum that
the cones can absorb is called the photopic
response. The range of the spectrum that the rods
can absorb is called the scotopic response, and it
is much narrower -- really just in the blue-violet
range. It is much narrower because rods only have
one pigment, and it absorbs blue-violet light.
Rods are much more sensitive in low-light
conditions than cones are. So at nighttime when
there is very little light, we mostly see with our
rods. But because the scotopic response is very
narrow, we can only see blues, not the entire
spectrum of visible light. This is why everything
has a blueish tinge when you are trying to see in
very dark conditions.
I hope this answered your
The photo pigmet in rods is called RHODOPSIN. As I
mentioned previously, the function of this
photosensitive molecule is to convert light into
electrochemical energy, which is neural activity.
When a photon (basic unit of light) strikes a
rhodopsin molecule and is absorbed by it, the
molecule changes shape in a way that alters the
flow of electric current in and around the pigment
From your question, I am guessing
that you might me a little confused about the
actual number of photo pigments in the rods
(rhodopsin molecules). It is true that there is
only ONE TYPE of photo pigment in the rods. We
have about 120million rods so it is safe to assume
there is more than one rhodopsin molecule.
large number of rhodopsin molecules helps us dark
adapt in scotopic conditions. Dark adaptation is
the process of becoming LESS sensitive to
dark,which means we are MORE sensitive to light.
Dark adaptation happens when we are in dark places
over a long period of time.
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