00:02
In this lecture, I’m going to
describe vitamins D and A,
two of the fat-soluble vitamins
and some of the very different
functions that each one has.
00:11
Now, vitamins D and A, as I
said, are fat-soluble vitamins.
00:15
They are important in the
case of vitamin A for vision.
00:18
In the case of vitamin D,
for calcium metabolism.
00:20
But that’s not the only thing
that these vitamins do.
00:24
Vitamin A is also important
for gene expression
and differentiation that occurs within
organism during development.
00:30
Vitamin D is also important for
controlling gene expression
and essential for a
healthy immune system.
00:36
As we will also see, it has some
anticancer properties as well.
00:40
Now, one of the things
to be careful about with
vitamins A and D is that
because they’re fat soluble,
excessive amounts of either
one of them could be harmful
because they get stored in fat tissue and
can be released over a long period of time.
00:52
So these are two vitamins you don’t
want to take in excess amounts.
00:56
Vitamin A, as I said, is fat soluble.
00:59
It is toxic at high doses and it’s
actually possible for a person to overdose
on eating too much liver from
certain organisms like polar bears,
so stay away from polar bears.
01:08
Vitamin A is stored in the liver and
it occurs in three forms in the body.
01:14
The alcohol form is known as retinol and
you can see it outlined on the screen.
01:19
On the far right of the molecule,
there’s an alcohol group
and that’s what gives it the
–ol ending of its name.
01:25
Retinal is a related
compound that differs only
in containing an aldehyde group
at the end of its structure.
01:31
And finally, retinoic acid has
a carboxyl group at its end.
01:35
Now these three different molecules have
three very different
functions within our body.
01:41
Now, retinol is the storage
form of vitamin A.
01:45
To store vitamin A, retinol
is esterified to a fatty acid
as you can see on the
structure on the top.
01:51
Now, vitamin A comes in a
variety of isomeric forms.
01:54
And the form that you see on the top for
retinol is known as the all-trans form,
that is all the double bonds are
in the trans configuration.
02:01
Those double bonds can isomerize
in the presence of light,
meaning that light can actually
physically change their structure.
02:09
The summarization of retinal is what
gives rise to vision as we shall see.
02:13
And one of those isomers you
can see in this structure
which is the 11-cis retinal isomer.
02:19
And it’s this isomer that
is stored within our eyes.
02:22
Retinoic acid is important
for cell differentiation.
02:25
Without retinoic acid, we won’t
form into the organisms that we do.
02:30
Now, vitamin A is produced in the body
using beta carotene as a precursor.
02:34
And beta carotene is shown
in the figure at the bottom.
02:37
Basically, if you cut
beta carotene in half,
you will get one of the forms of vitamin
A that you see on the screen here.
02:45
Now, 11-cis retinal, as I said,
is important for vision.
02:49
So I need to describe to you that
process by which this occurs.
02:52
First, 11-cis retinal is bound
to a protein called opsin.
02:56
And it’s through protein called opsin
that retinal provides us
with vision as we shall see.
03:02
The absorption of a photon of light
by the 11-cis form of retinal
causes the 11-cis form to flip
back to the trans configuration.
03:12
So we can see this flipping
process occurring here
where we see it flipping
on the top to the bottom
or from the bottom
back up to the top.
03:19
and this happens very readily
with this form of vitamin A.
03:23
It’s the change in structure,
the change in form,
that actually provides the
very first signal in our eyes
that light has been detected.
03:32
So before I talk about the
biochemistry of vision,
I’ll just say a little bit
about the actual physiology
and the cell structure
that gives rise to vision.
03:40
So vision happens as a
result of light detection
that occurs in specialized cells
in our eye known as retina cells.
03:47
The opsin that I described earlier
is the protein that actually holds
vitamin A containing
compound, the retinal,
that allows us to have vision.
03:55
Now, notice that retina and retinal
are pretty similar in terms of name,
but they’re very
different things.
04:02
We have two types of cells in
our eye that detect light.
04:05
The most abundant cells that
we have are known as rod cell.
04:08
And they provide very basic
functions of light detection.
04:11
They don’t provide for
differences in color,
but rather simply detection
of a photon of light.
04:18
The photo pigment that
they contain, the opsin,
contain links to the retinal,
is known as rhodopsin.
04:24
The other cells that we have in
our eyes are known as cone cells.
04:27
And they actually provide the color detection
that we see when we see a well lit room.
04:32
They also have retinal linked to an opsin.
04:35
But the opsin there is
a little bit different
and so we call those
photopigments, photopsins.
04:41
There are three types of cone cells.
04:43
One type specialized for the absorption
of red wavelengths of light.
04:47
One type of cone cells specialized for
the detection of green type of light
and one specializes for
blue types of light.
04:55
Now, to give you an idea of abundance,
there are about a hundred and
twenty million rod cells per eye
and about 6 million
cone cells per eye.
05:02
So eyes have pretty good
resolution in terms of
being able to detect
small amounts of light.
05:10
The rod cells can detect.
05:11
They’re so sensitive that they can
detect individual photons of light.
05:15
Now, that detection comes at a price.
05:17
They can’t tell the color of that light,
but they can tell whether or
not light impinged upon them.
05:23
The opsin as I said earlier
contains the 11-cis retinal
and in that form, it’s
known as rhodopsin.
05:29
The retinal in rhodopsin isomerizes
in response to the light
and that isomerization causes
the retinal to change form.
05:37
So instead of being bent as we saw earlier
in the structure, it straightens out.
05:42
Well, that straightening out of the retinal
that happens in the presence of light
causes the rhodopsin that contains
it to also change its structure.
05:51
Now, one of the things we’ve seen in
other lectures that I’ve given here
relates to small modifications
in protein structure.
05:58
Small modifications in protein
structure can have very big effects
on what the protein actually does.
06:03
And that’s very much the
case here with rhodopsin.
06:06
So these retina cells that I’m
describing to you are very sensitive
and that sensitivity allows them to
very easily send signals to the brain
that they detected something.
06:17
Those signals happen as a result of chemical
signals that arise in the nerve cells.
06:22
Now, this chemical process
is kind of complicated.
06:24
So I’m going to take
you through it slowly.
06:27
The unstimulated optic nerve cells
are what we call unpolarized,
meaning that they have an
even distribution of ions
on the outside and on
the inside of the cell.
06:37
This is very different
from other nerve cells
because they start out
in a polarized fashion.
06:43
Light stimulation that happens on the
nerve cells of the eye, however,
caused hyperpolarization.
06:50
This is exactly the opposite
of other nerve cells.
06:53
Other nerve cells start out hyperpolarized
and stimulation causes them
to become unpolarized.
06:59
So we see the reverse of the process that’s
happening with nerve cells in our eyes.