- [Voiceover] Alright so in a previous video I introduced
a class of biological molecules called lipids.
I broke lipids down into two main categories.
We have hydrolyzable lipids, hydrolyzable,
and then not hydrolyzable.
Essentially what this means and we've kind of
talked about this is these lipids can be broken down
into smaller sections,
and these Lipids is non-hydrolyzable.
Lipids cannot be broken down into smaller sections
and in this we fit probably the most abundant lipid,
triglycerol and if you remember
triglycerol has the main function in our bodies
at least of energy storage.
So we break down triglycerol because it's a hydrolyzable
lipid to get energy.
I want to continue talking about these hydrolyzable lipids
but the other ones that I think are important
instead of kind of having a predominant energy
storage function, they have a structural function
in our cells and kind of
in the biological role, so a structural function.
So the first one I wanna talk about are phospholipids.
Phospholipids are hydrolyzable lipids,
that contain a phosphorus atom.
So this phosphorus atom usually comes in a form
of a phosphodiester bond
and I have no expectation of you to know
what a phosphodiester bond is because I don't think
we talked about it so we start with phosphoric acid,
which is H three, P O four, that's phosphoric acid
and it has a, kind of has a structure like this.
We've got phosphorus double bonded to oxygen
and then bonded three other times to O H group.
So we have four oxygens in total
and three hydrogens and a phosphorus atom.
That might look a little bit goofy
if you're paying real close attention
because this phosphorus is bonded five times
but remember that phosphorus is a third row element
unlike carbon, so it can bond more than four times.
It has, it's capable of sharing more than eight
valence electrons.
So we've got phosphoric acid right here
and if we take these two side O H groups
and replace them with O R groups,
so let me kind of redraw the base structure here.
Still have it double bonded to an oxygen
but if we replace those sides
with O R groups, say for example
in a dehydration reaction this becomes
a phosphodiester.
And it's an -ester because this phosphorus
is double bonded to an oxygen
and an O R group and it's a -diester because
it happens twice, so this is a phosphodiester.
This is the form that phosphorus is gonna come into play
for our lipids.
So if we start with triglycerol,
which is kind of that basic hydrolyzable lipid
and I guess I better go ahead and draw out triglycerol here.
So this is triglycerol and if we replace this lower
fatty acid chain, remember all of these are fatty acids,
if we replace that with a phosphodiester
it's gonna look like this.
So this would be one of the O R groups.
This whole rest of the molecule would be the other O R group
and I guess in our bodies
not to confuse you too much
but phosphodiesters had a pH in our body
of around 7.4, this hydrogen right here
is actually usually deprotonated so
usually carries a negative charge on this oxygen so
we wouldn't have an H right here
instead we'd have a negative charge.
So this right here is a phospholipid
and one of the cool things about phospholipids
is unlike triglycerols now we have
a specifically polar section, so this right here is polar
and then we have a non-polar section to this molecule.
So this is all still non-polar
and the purple part principally because of this kind of
being negative charge and just this
the polar nature of this phosphodiester
but that allows it to play a pretty cool role
in cell membranes.
You've probably heard of a phospholipid bi-layer
and what happens is you have a phosphodiester head,
so kind of a phosphodiester head here
and then you have two kind of fatty acid tails coming off
and this is polar, this purple part is polar
and the orange part would be non-polar
and you get a whole bunch of these and they kind of
line up together, the polar heads kind of line up
and the non-polar tails kind of line up.
And those non-polar tails become attracted to
another set of non-polar tails,
and another polar head right here,
and this forms a two layer membrane to our cells.
So this is kind of the thing that separates
the inside contents of our cell from the outside contents
and this whole middle section right here
is not attracted to water because of this kind of non-polar
lipid characteristic and it keeps water over here
and water over here but separates these two fluid
compartments so that's a pretty neat function,
a structural function of the phospholipid
and I don't think you can really talk about phospholipids
without stopping at least briefly to mention sphingolipids,
they're another hydrolyzable lipids.
So sphingolipid,
and you need to talk about sphingolipids
because they're another lipid that incorporate
this phosphodiester unit, but instead of
having it kind of on a base
of a triglycerol, it's gonna be on the base
of the amino alcohol sphingosine.
So let me kind of draw in sphingosine.
And so sphingosine is an amino alcohol,
it's got these O H groups, but it's also got a lot
of lipid characteristics because it's really high
molecular weight alcohol.
This is a lot of carbon here, 15 carbons
on this kind of tail, it would extend out like that
if I kind of drew them all out
but if we replace this O H group with a phosphodiester
just like we did with our triglycerol so
kind of replace it.
It also develops a structural function within cell membranes
but mostly within nerve cells and I'll show you why.
So we've got a nerve cell here
with a long axon
and nerves are surrounded by the insulation
of myelin, so we've got myelin right here.
The properties of myelin, which allowed to insulate
this nerve axon right here are really predominantly
due to the sphingolipid concentration in myelin.
So sphingolipids kind of have a similar structure function
within cell membranes but mostly within their cells
and then I guess the last hydrolyzable lipid
that I really wanna talk about
are waxes, so waxes.
Again, I'll kind of do this briefly but
waxes are also esters and they're made
of a high molecular weight alcohol.
So again a long carbon chain alcohol that might
look like this
with an R group but it'd be really long,
so it'd kind of look like this
and then they've got a fatty acid so --
A fatty acid, again really high molecular weight.
If you form an ester out of these two molecules
you end up with a product that looks a little something
like this, so a long chain,
double bonded to an oxygen with an O R group right here.
This would be the ester
and so you got an ester right here
but it's kind of a unique ester in the sense
that it's got two really long carbon chains making it
two kind of non-polar sections here.
These two non-polar sections make the molecules
very hydrophobic, waxes are very hydrophobic
and so we see waxes in nature often forming
a real barrier against water and this happens in
leaves, we put waxes on our cars to protect the surface
of our cars from rain and from the kind of the humidity
but waxes are another type of hydrolyzable lipid.
And their hydrolyzable again, I just kind of
wanna beat this point home because I've got this
ester group right here, carbon double bonded to oxygen
with an O R group that can be hydrolyzed.
You can do an ester hydrolysis reaction.
So that's with waxes, same thing with sphingolipids
they've got this, sorry let me make sure
you know that that's an oxygen,
they've got this ester group right here
that you can break down.
You can do the same thing if I go up with phospholipids.
Again we have this O, this kind of
this ester bond right here that we can hydrolyze
and we can do it with these bonds as well,
just like we could with triglycerols.
So all of these, again are lipids that can be broken down
into smaller units through hydrolysis reactions.