So the process that I’ve just described to
you for the transmission of information
from the eye cell to the brain is
known pretty much for the rod cells.
Less is known about how the
process occurs in the cone cells
but it is believed to happen in a pretty
much the same way as in the rod cells.
Now, as I said earlier, the
cone cells have pigments
that have varying sensitivity to
different wavelengths of light.
Cone cells need direct light and
they need direct strong light
in order to detect the
colors that they detect.
Cone cells as I said
need more intense light.
What would stimulate rod cell
might not stimulate a cone cell.
The detection process as I said,
however, is pretty much the same.
Now, the cone cells that have various
maxima in the red region, for example,
are more likely to fire if they
get red light shined upon them
whereas those that have maxima in the
green, they get fire when the get green
and those in the blue more
likely when they get blue.
The colors detected by the
cone cells and by the brain,
not by the sensitivity
of any individual cells,
but rather by a polling of groups of cells.
So a group of blue cone
cells in one cluster
that all send a signal to
the brain about blue color
then tell the brain, “Yes, we
really did get a blue color
and that wasn’t an aberrant
signal that we got.
The brain then paints
that image that we see
as a result of these actions
of these individual cells.
Now, after vitamin A has been isomerized
from the 11-cis to the trans form,
it has to reconverted
back to the 11-cis form.
Now, I told you that light can cause
that interconversion to occur
that is it can go from cis
to trans and trans to cis.
But it turns out that that’s not what the
cell does to get the trans form back.
And it turns out, it’s kind
of a complicated process.
The process is known
as the visual cycle.
And I’m going to describe it to you here.
So first we have the absorption of
a photon that causes the 11-cis
to move out to the all-trans form
and that all-trans form of retinal now
is converted back into the 11-cis form
in this visual cycle that I’m
going to describe to you.
The all-trans retinal now
first of all is released
after it has been converted
into the all-trans form
That release happens in the figure
that you can see in the lower right.
So after the all-trans retinal
form has been formed,
it has to be converted
back to the 11-cis form
and that process is
kind of complicated.
First of all, the retinal
all-trans form is reduced
to retinol to make the
all-trans retinol form.
The all-trans retinol is esterified to
a fatty acid as I have shown before
and then that esterified
retinol fatty acid form
is converted back to the 11-cis form by
an enzyme known as retinol isomerase.
Now, the retinol isomerase then
releases the 11-cis form of retinol.
The fatty acid is cleaved off
and the aldehyde, the alcohol,
is oxidized back to an alcohol.
So there’s a lot of
chemistry that’s going on
to recreate that 11-cis retinal so that
another cycle of vision can occur.
The 11-cis retinal after it’s
been formed by this process
that I’ve just described to you
is then linked to the opsin
and cause the formation of rhodopsin
in the case of rod cells or photopsin
as in the case of cone cells.
Now, another form of vitamin
A is retinoic acid.
And it has a function that’s completely
different from that in vision.
First of all, the retinoic acid is created
from the retinal by an oxidation reaction.
This oxidation reaction that creates
retinoic acid is non-reversible.
That is we can’t make retinoic
acid back into retinal.
So it’s for this reason we call it
a nonstorage form of vitamin A.
The intracellular signalling system
in the embryo uses retinoic acid
as a way to determine the anterior
and posterior region of the embryo
for the development
through Hox genes.
Hox genes play very important roles
during the development of organisms
that have many different
limbs and features as we do.
Retinoic acid is therefore
meaning it strongly
So retinoic acid
exerts its functions
through a retinoic acid
receptor known as RAR
or the retinoic X
receptor known as RXR.
These receptors allow the cell
to activate the transcription
of genes that are
known as rare genes.
And I’ll describe those in just a moment.
So as I said, retinoic
acid acts through RAR,
and RAR is a transcription factor,
meaning it’s a protein that will bind
to DNA and activate certain genes.
The sequence that RAR binds
to is known as rare, R-A-R-E,
or retinoic acid
RARE genes are genes that are involved
in the process of development
and it’s for this reason that retinoic
acid is so strongly teratogenic.
The retinoic acid response element control
genes involved in differentiation.
As I said earlier, Hox
genes that are involved
in controlling differentiation
that are like we are, are regulated
ultimately by retinoic acid.
This retinoic acid response
element, as I said,
are sequences located upstream of
genes that control differentiation.
Now, some of these genes are involved
in the Hox gene transcription
as retinoic acid is involved
in controlling them.
These are regulated by retinoic
acid acting through the RAR
binding to these RARE
sequences as we see here.
Now, because retinoic acid is so strongly
teratogenic and causing differentiation,
that’s one way to treat cancer.
And so there are derivatives of vitamin
A known as isotretinoin or allretinoin
as you can see on the screen here
that are used to treat cancer
and in some cases to
treat acne because they
actually have pretty strong
effects on the skin.