Lipids are non-polar hydrophobic compounds
that have a polar head group and a hydrophobic
tail.
Now we are going to understand how these are
organized into membranes and what their functions
are.
So basically anything that is a non-polar
hydrophobic compound soluble in organic solvents
is called a lipid.
The membrane lipids that we talk about are
called amphipathic in nature.
Amphipathic means it has both a non-polar
end and a polar end to it.
The non-polar end arises due to definite chemical
moieties, definite groups and polar end again
arises due to some other groups that at are
present.
If we look at the functions of lipids they
play a very important role in the biological
cells and biological cell membranes and of
course membrane transport which will be doing
later on.
We have steroid hormones, in digestion, as
fats and triacylglycerols which give us the
fuel for our bodies and the membrane structure.
So lipids are involved in all of these activities
starting from hormones to digestion elements
that are present in the bile.
These are all lipids and their components.
The fats and triacylglycerols are lipids and
in the membrane structures, which is what
we will be doing today, we have fatty acids,
phospholipids, sphingolipids and cholesterol.
Initially we are going to study the different
nomenclature and the different types of fatty
acids.
A fatty acid is a long chain acid.
In the organic carboxylic group that we speak
about we speak of a –C=OOH being the acid
moiety.
Here we have a long chain hydrocarbon with
a -COOH attached to it.
These fatty acids are sigma-bonded carbon
chains.
Sigma bond mean we have just single bonds
here and they have at the end a carboxylic
acid moiety, a carboxylic acid functional
group.
This functional group or this moiety is going
to be used in the overall function or overall
structure of the lipids that form these membranes.
If we consider the fatty acids, the first
thing that we are going to look into is their
structure and the nomenclature as to how they
are identified and then the physical and the
chemical properties of the fatty acids.
Then we will study about the triacylglycerols
and see how they play an important role in
the structure and function and the formation
of lipid membranes.
The first thing is about the nomenclature.
The nomenclature of the fatty acids actually
follows two types.
One is called the n-designation and other
is called the delta designation.
In the n-designation you can straight away
see that the numbering is from the extreme
end away from the carboxylic acid group.
In this case the numbering is from here, 1,
2, 3 and 4.
For the delta designation the numbering begins
from the carboxylic acid end and it is this
nomenclature of this designation that we will
be using the delta designation.
If you are to write the structure of any particular
fatty acid there will be a specific nomenclature
that you will follow and that nomenclature
will be the delta designation where the numbering
will begin from the carboxylic acid end, not
the n-designation.
How does this help us?
In the next slide we will see as to how we
can write the nomenclature.
This is a fatty acid in the n-designation
and what I have below here is a fatty acid
in the delta designation.
From this nomenclature you should be able
to write the fatty acid.
We have to know what each of these numbers
mean.
The first number that you have here is the
carbon chain length.
It tells you how many carbon atoms you have.
After the colon you see another number.
This 20 means that the carbon chain length
is 20.
The number after the colon designates the
number of double bonds present.
In this case we have carbon chain length of
20 and the number of double bonds is 4.
Now you can tell me that this is actually
the position of the double bonds.
So the number that we have at the end is the
position of the double bonds.
The delta 5 means the double bond is between
5 and 6, 8 means it is between 8 and 9, 11
means between 11 and 12 and 14 means between
14 and 15.
In the n-designation there is usually just
one number put because usually when we form
or when the fatty acids are biosynthesized
what happens is they form in specific units.
If you notice here every double bond is after
three carbon atoms.
It is 5, 8, 11 and 14.
In the n-designation, only one is specified
which tells you that there is going to be
another one at 9, 12 and 15 and it is opposite
to this because this numbering is opposite
to that in the delta designation.
The double bonds are assumed to be spaced
by three carbons.
So here in the n-designation only end 6 is
specified and nothing else.
But in the delta designation the position
of the double bond is written explicitly where
we now know that if this is the nomenclature
it means that if you have a carbon chain length
of 20 there are 4 double bonds and the position
of the double bonds are 5 and 6, 8 and 9,
11 and 12 and 14 and 15.
If we look at the set of designations, another
thing that we should mention here is that
the double that we see in the fatty acid usually
have a cis configuration and most naturally
occurring fatty acids have an even number
of carbon atoms because the way that they
are biosynthesized they come in pairs of carbon
atoms.
If they come in pairs of carbon atoms all
of these are usually even numbers.
You do not see an odd numbered fatty acid
because when fatty acids bio synthesis occurs
it comes in pairs of carbon atoms.
If we look at some fatty acid and their common
names 14:0 is myristic acid.
These are all the delta configurations.
So you should be able to write what myristic
acid is.
What is it?
It’s just a long chain, a hydrocarbon chain
with 14 carbon atoms and you do not need numbering
in this case because there are no double bonds.
So we have myristic acid, palmitic acid, stearic
acid, then we go to oleic acid.
It is 18:1 cis.
The cis is not usually put in, just delta
nine is sufficient because most of them are
cis any way.
So 18:1delta 9 means that oleic acid is an
18 carbon fatty acid with 1 double bond between
9 and 10.
That is as simple as that.
The one that I had on the previous page is
actually arachidonic acid.
It was 20 with 4 double bonds at 5, 8, 11
and 14.
Eicosapentanoic acid which is an omega three
fatty acid has an additional double bond at
position 17.
So this nomenclature is sufficient to tell
you how to write a fatty acid.
This is basically the nomenclature of fatty
acids and we are going to see how we can use
these fatty acids in forming our lipids.
What happens if you have this cis double bond?
Here we have a single cis double bond.
You see how the carbon chain has now changed
direction.
If this cis or if this double bond did not
exist it would have been a nice straight chain
and they could have been rotations about the
single bond.
But when we have it in the cis configuration
then what happens is there is a break in the
chain because of the cis configuration.
You have what is called a kink in the chain.
Instead of having a normal long chain that
we would have had and free rotation about
the single bond each cis double bond causes
a kink in the chain.
If I had another cis double bond at this position
this part of the fatty acid or this part of
the chain would fold back.
So I would have a kink in the structure.
So I have a kink in the structure due to the
fact that I have double bonds in the hydrocarbon
chains.
When we have these membrane lipids, this is
something we are going to study in detail
later on, these are my fatty acid chains.
I am going to have a polar head group, we
will see what those polar head groups can
be, and my hydrophobic tail if it is a straight
chain fatty acid it will look like this.
If it happens to have one fatty acid that
has a kink to it, it is going to be shaped
like this.
We will see how I am talking about two fatty
acids linked to a single polar head group
in a moment.
But when we are talking about the polar head
group and different fatty acids when they
link together you see how you can change the
structure of the lipids because of the type
of fatty acids that is being attached to the
polar head group.
You are basically changing the structure depending
on the choice or the type of the fatty acids
that you are considering.
Basically if you look at the fatty acids,
this would be the structure where we would
have carboxylic group here.
This would be the polar part of it and if
we have a long chain it would be a smooth
long chain, a straight chain.
If you happen to have a cis bond here what
would happen?
The chain would get bend.
You would have what is called a kink and you
recognize that if you had another cis bond
here it would twist even more.
Here are some physical and chemical properties
of fatty acids.
Fatty acids are weakly acidic in nature; weakly
acidic with a pKa of 4.5 to 5 which means
that they are ionized at physiological pH.
The physiological pH is 6.7.4.
Saturated fatty acids are solids at room temperature.
The melting point is going to depend on the
chain length and definitely the number of
double bonds present; on the degree of unsaturation
and we will see how that is going to play
an important part in our lipid formation,
membrane lipids.
So we have weakly acidic fatty acids.
The saturated fatty acids are solid at room
temperature.
The melting point depends up on the chain
length and the degree of unsaturation.
With the kink in the cis bonds what happens
is it disrupts the molecular packing.
So it lowers the melting points.
If you had straight chain that would normally
completely very well organize, you would have
a higher melting point.
But due to the presence of the cis double
bond the intermolecular packing of the hydrophobic
chains is disrupted.
It is broken and that lowers the melting points.
The polyunsaturated fatty acids that you see
in a lot of vegetables oils that you consume
they say that they are pufa.
That is what it is called polyunsaturated
fatty acids; they are readily oxidized by
exposure to air and these fatty acids can
form micelles.
You know why they can form micelles?
Because they have a hydrocarbon chain and
it has a polar head group to it.
These are the basic physical and chemical
properties of fatty acids.
What we have to remember is that they are
weakly acids, saturated fatty acids are usually
solid at room temperature and the melting
point is going to depend up on the number
of carbon atoms you have and on the degree
of unsaturation and the more the number of
cis bonds that you have the lower the melting
point is going to be because you are going
to disrupt the intermolecular packing between
the hydrophobic chains.Now we are going to
come to what are called Glycerophospholipids.
We have glycerol.
Glycerol is CH2OH CHOH-CH2OH.
Glycerophospolipids are what comprise lipid
membranes.
They form or they are the constituents of
cellular membranes.
You recognize that these -OH groups that you
have here can be esterified by acids.
What is an esterfication reaction?
We have ROH and RCOOH.
With the removal of water, we form a -OCO
an ester formation.
That means that these H’s if they react
with fatty acids can be esterified and I can
have to this glycerol a long chain attached
to either this hydrogen or this hydrogen or
this hydrogen.
Usually there are two fatty acids attached
to it which is why I have two lines sticking
out from the polar head groups.
We have the hydroxyls at C1 and C2 that are
esterified with the fatty acids.
We have our glycerol.
The one that I show you here is triacylglycerols
which is what comprises the fat droplets that
we have in cells.
If you see there are triacylglycerols or triglycerides
test that have to be performed in blood to
see whether you have appropriate triacylglyceride
content.
If you have more fat droplets then you have
fat restricted diet.
This is what a triacylglycerol would look
like.
Here is the structure of glycerol.
We have three -OH groups here.
If each of them number 1, number 2 and number
3 are each esterified, this is what it is
going to look like.
So in the first carbon atom and in the third
carbon atom we have straight chain fatty acids
that have been used to esterify the -OH groups
of the glycerol.
The extreme -OH groups of the glycerol have
been esterified with straight chain fatty
acids here.
In the middle we straight away know that this
has now not only one but it has two cis double
bonds which is why it is even bent further
than the one I showed you previously.
If you look at the structure here this is
linolinic acid that has been used.
We have one cis bond here and another cis
bond here.
So it has changed the structure of the hydrocarbon
chain into making it more disrupted.
It’s more kinked in a sense.
We are going to see how we can change the
properties of the groups here and then see
what the lipids or the glycerophospholipids
are actually made of.
What we have are called phospholipids.
What are these phospholipids?
In the two classes of phospholipids that are
present these form cell membranes.
We have glycerolphospholipids that have a
glycerol backbone just like I showed you.
We have sphingomyelin that forms from a spingosine
backbone and in this case this actually forms
a lot of the membranes and these phospholipids
are usually refered to as PL.
What is essential of these?
They are extremely important for membrane
structure.
They are found in membrane lipids.
We will see what these structures actually
are.
What we need to know is there are two types
of phospholipids and they are essential for
the membrane structure and they are found
in membrane lipids.
This is the break up.
What we have here is we have storage lipids.
Storage lipids like storage in terms of fat
droplets.
What are the fat droplets?
They are triacyl glycerols.
We need to know is that the storage lipids
which are neutral in nature have three fatty
acids attached to the glycerol.
We have membrane lipids.
In the membrane lipids we have phospholipids
and glycolipids.
In phospholipids we can have glycerophospholipids
or sphingolipids.
It is just a break up tree.
The membrane lipids are polar in nature because
they have a phosphate group attached we will
see what that means in a minute.
Phospholipids are glycerophospholipids or
sphingolipids and glycolipids are other sphingolipids.
Glyco means you have sugar.
Whenever the word glyco comes in a prefix,
glyco means there is sugar present.
If we go back to the phospholipids break up
we have storage lipids that are triacylglycerols
fatty acids.
We have membrane lipids that are phospholipids
or glycolipids.
The breakup of phospholipids is glycerolphospholipids
or sphingolipids where the backbone basically
different.
Now we will study this in a bit more detail.
The black part here is part of glycerol CH2OH
CHOH CH2OH.
What has happened at the first two carbons
is the C1 and the C2 have been esterified
by fatty acids.
We have long chain fatty acids in both cases.
The third carbon has been esterified with
the phosphate.
Remember that is also an acid.
We have two of the carbons esterified with
fatty acid chains and one with the phosphate.
This is called a phosphatidate.
What is the basic structure?
The basic structure is glycerol.
The two carbons of glycerol have been esterified
with two fatty acids and the third with phosphate.
We have a glysphosphatidate or otherwise glycerophospholipids.
This is what it looks like.
We have a fatty acid on the first carbon,
we have a fatty acid on the second carbon
and we have a phosphate on the third carbon.
The phosphate again is esterified.
If the phosphate again is esterfied by this
X group this is where we can change the type
of glycerophospholipids that we have.
So where can we make the changes?
We have the basic structure of glycerophospholipids
that is going to be the glycerol.
We have one fatty acid linked on the first
carbon, a second carbon linking another fatty
acid.
So we can change the type of fatty acids that
we have.
As soon as I change the type of fatty acid
the type of lipid is going to change.
Then I have an esterification on the third
carbon atom with phosphate and I have a polar
head group here and we will see how that affects
it.
Where is my polar head group?
It is here.
Here are all the oxygens and the phosphates
and where are my chains?
R1and R2.
I have the basic structure of the lipid that
is going to look like a polar head, that is
this part here, and the R1 and the R2 are
these chains.
You understand what the structure looks like
now?
We have the overall glycerol, two of the –OH’s
have been esterified with long chain fatty
acids which is why I have two legs to this
polar head group and the polar head group
forms because of the phosphate esterfication
and an additional polar head group.
This is the basic structure of a glycerophospholipid.
What can I change here?
I can change type R1.
I can change R2, I can change X.
In that I will be changing the complete type
of glycerophospholipid that I have and we
are going to see how we can do that.
This is our structure.
So we have the Pi-OH.
What is happening to this –OH?
It has been esterified again.
It can be esterified with serine, choline,
ethanolamine, glycerol or inositol.
There are different groups that can be used
to esterify the phosphate in the phospoglyceryl
and the two fatty acids that we have R1 and
R2 are usually not the same.
We will see why later.
How can they be different?
They can be different in their length, they
can be different in the number of double bonds
and they can be different in the location
of these double bonds.
That is where we have a difference R1, R2,
X.
So what are the differences?
This is phosphatidylinositol.
You recognize now the glycerol moiety.
Here is a glycerol moiety.
So this is one carbon atom, this is the other
carbon atom and this is the third carbon atom.
What was our glycerol structure?
-COH then we have CH-OH and again we have
CH2OH.
This has been esterified and this has been
esterified.
We have long chains here and in this case
we have it esterified with the phosphate.
Linked to the phosphate again now is another
group.
This is X that I showed in the previous slide.
So it is this group X that in this case is
inositol.
When you have all this number of OH here and
the phosphate here and the negative charge
here what does this comprise?
It comprises the polar head groups.
It is this part that forms the polar head
groups and what is R1 and R2 forming?
The tail.
It can be different depending on the type
of R1 and the type R2.
This is the basic structure of a glycerophospholipid.
Now what can we change?
We can change inositol, make it something
else.
Let us see what we can change it to.
We can make it choline.
Again the basic structure is exactly the same.
I have R1, R2.
I have the phosphate.
Linked to the phosphate I have another X.
What is that X?
Choline.
So I have instead of phosphatidylinositol
I have phosphatidylcholine.
I can also have phosphatidylethanolamine.
I can have phosphatidylserine.
In each of these the difference is going to
be just in the X group in this case.
We can have identical R1’s and R2’s for
phosphatidylchloine, phosphatidylserine and
phosphatidylethanolamine.
These are the different types of glycerophospholipids
that we can have.
What you need to remember is the basic structure
is a glycerol, you have two fatty acids R1,
R2, you have a phosphate and the phosphate
is linked again to another polar head group
that is going to result in a polar head group
to your lipid.
What is a sphingolipid?
This is the structure.
It is based on the structure called sphingosine.
This is the structure of spingosine.
You have a -CH2OH.
You have in the middle CH-NH3+ and in the
last carbon you have a –CH-OH to which is
linked a long hydrocarbon tail.
This is the basic structure of sphingosine.
We have a long carbon chain as it is.
By default sphingosine comes with a long hydrocarbon
chain.
It has a polar region here that constitutes
an amino group.
It has a NH3+.
What can happen with a NH3+ and a fatty acid?
It can form an amide.
In the part here we have an RC=O-NH.
Just like an amino acid linked to NH3+would
give you an amide, you can have an amide formed
here.
The amino group of the sphingosine that can
form an amide bond with the fatty acid gives
you what is called a ceramide.
Have you heard the word ceramide before?
They sell you shampoos with ceramide in it.
If you look at the advertisement of shampoos
they will tell you that ceramides are present
in it.
This is what a ceramide is.
What do you have in a ceramide?
You basically have a sphingosine.
What is a sphingosine?
Sphingolipids are going to be derivatives
of this.
It has nothing but a long hydrocarbon chain,
there is NH3+ attached to it and an -OH attached
to it.
This already has the long hydrocarbon chain
attached to it.
You can have this NH3+ form an amide with
another fatty acid.
You are going to have two long carbon chains
here.
We have ceramides that usually include a polar
head group and they are esterified to the
terminal -OH of the sphingosine.
We will see what it is.
You recognize the basic structure of the sphingosine
now?
This is the basic structure of the sphingosine,
the one in black.
This is the long 
carbon chain.
This is the sphingosine moiety.
This is the fatty acid attached to it to form
an amide and we had an -OH here.
So this can attach to another group forming
a sphingomyelin.
We have what is called a sphingomyelin which
is a ceramide with the phosphocholine.
What is a ceramide?
A ceramide is when you have -OH here and the
amide here and the sphingosine as it is.
So in the basic structure of the sphingosine
if you have the fatty acid linked to form
an amide it is called a ceramide.
In this case when you have phosphocholine
to form a head group here this forms what
is called a sphingomyelin.
So this is after the formation of a ceramide.
So you have an -OH group, initially this was
an –OH.
This is now linked to phosphocholine to form
what is called a sphingomyelin.
There is another thing that we need to know.
We remember what a sphingomyelin or what a
ceramide is?
What is a ceramide?
A sphingosine with the fatty acid with –OH.
Now you have to recognize that that H can
be replaced.
If you replace this H with phosphocholine
you have sphingomyelin.
If you replace it with a sugar you have what
is called a cerebroside.
It’s just nomenclature.
All you need to know is you have a glycerol,
you have a sphingosine.
These are 
the two backbones.
That’s it.
You have three -OH groups here.
If three of them are fatty acids you have
a triglyceride.
If two of them are fatty acids and one of
phosphate you have glycerophospholipids.
If that phosphate again is attached with another
X, you have series of lipids.
In sphingosine you have different types.
Basically you have a shape like that.
You have a long carbon chain in the structure
of sphingosine itself.
You have fatty acid attached in forming a
ceramide.
Then this -OH can be linked to sugar, to form
a cerebroside.
It can be linked to phosphocholine forming
a sphingomyelin.
That is the basic structure of all these.
So we can have a cerebroside if you had just
simple sugar or you can have ganglioside when
you have complex oligosaccharide attached
to it.
It’s just the nomenclature.
They are usually found in the membrane bilayer
which is why we have to consider all the different
types of possibilities of the sphingolipids
of the lipid themselves.
If we look at the molecular arrangements of
the lipids you know that they can associate
with water because the cytosol of the cell
itself is embedded in water, embedded in the
blood, embedded in the cytosol.
The hydrophobic tails will never be in the
cytosol.
We have to have what is called a bilayer.
So it is this polar part that is going to
be outside the cell and this polar part that
is going to be inside the cell.
So we have a bilayer that has polar head groups
in either directions.
So the amphipathic lipids in association with
water will form complexes in which the polar
regions are in contact with water and the
hydrophobic regions are away from the water.
So it is a very smart way of forming the lipids,
where you have a strong bilayer which is not
going to allow everything in and out of the
cell but at the same time it is going to be
extremely important in the characterization
of the lipid bilayer.
There is another way we can organize this.
How is that?
In a spherical manner, we form what are called
spherical micelles.
What are these micelles?
These micelles have polar head groups outside
and we have the hydrophobic tails inside.
We can also have what is called a reverse
micelle, where you have the opposite of this.
If you put this spherical micelle with the
polar head groups in an organic solvent it
is going to reverse and the polar head groups
are now going to be in the center and the
hydrophobic tails are going to be facing the
hydrophobic or the organic solvent.
So we have what we call the reverse micelles
or the normal micelles.
When we have a cerebroside or a ganglioside,
some structure like this form a micelle then
we have a long fatty acid chain at the R group.
We have another long hydrocarbon chain in
the sphingosine moiety …. because of the
sphingosine structure itself.
When this forms a micelle we expect this part
to be the polar head groups on the surface
and these two to be the legs of the structure.
The structure that we have here is basically
going to be the bilayer with two long chains.
In case of a glycerophospholipid both of these
are fatty acid chains.
But in case of a sphingolipid one part is
the hydrocarbon chain that belongs to the
sphingosine and this is the ceramide, the
amide part that has been linked with the amide
of the sphingosine to form an amide.
So we have one fatty acid and one sphingosine
hydrocarbon chain and the same thing that
we would have here.
What are the structures that you can have?
This is a micelle.
A micelle is not two dimensional it is three
dimensional.
It actually looks like this.
This is just like half of it cut of.
We have different types of micelle structures.
All these red balls that we see here, red
spheres along the surface are all polar head
groups and all the chains that are inside
are all the hydrophobic chains that we see.
Consider a liposome and the way transport
of material occur in the body.
If you have a micelle and you have a polar
ingredient or a polar substance that has to
be transported, you understand that it is
not going to be possible for this particular
micelle to transport it.
Why?
Because it has a hydrophobic core to it and
it would be possible to transfer a hydrophobic
component but if it happened to be a polar
part it would be difficult to do so.
So we have the formation of liposomes.
What happens here is you see there is a lipid
bilayers sort of a thing that forms the membrane
and inside we have a polar center.
The reason why I am telling you this is this
is used for a lot of drug delivery.
When you have drug delivery or when you are
creating or making drugs you have to ensure
that the drug is water soluble.
If you want it to interact with blood plasma
you have to have one that is going to be easily
solubilized, which is a problem with drugs
that they are not easily solubilized.
The transport of a lot of material takes place
through these liposomes.
You understand that in the center here we
can have any polar moiety.
Any favorable ionic interaction that might
occur will hold the drug in this position
and it will transfer it to where it has to
go.
It will circulate in the blood and then be
able to transfer itself.
In the case of a bilayer is we have the most
stable configuration for amphipathic liquids.
This is the possible structure and this is
usually used in transport but this is sort
of a confined structure.
If we have a lipid bilayer then we have the
polar groups forming a sheet on one end and
the polar group forming the sheet on the other
end.
We have a lipid bilayer and it is this bilayer
that is going to result in all of the transportation,
all of the lipids structure, all of the membrane
structures that we will be seeing in the next
class.
Basically what we have learnt is that we have
our glycerol.
In the glycerol we have storage lipids.
What are storage lipids?
Storage lipids are those that are fat droplets.
What are these fat droplets?
The fat droplets 
are triacylglycerols or triglycerides.
What is the basic structure in this case?
We have our three carbon glycerol and we have
three –OH.
We 
have this replaced by a fatty acid, this replaced
by fatty acid, this esterfied to fatty acid
and each of these esterified to fatty acids
is going to give our triacylglycerols.
They can be esterified by a series of fatty
acids and we learnt that if we have such a
nomenclature delta 5, 8, 11, 14 we know how
we can write this.
We have normally a long carbon chain where
we have the -COOH attached in a normal fatty
acid.
When we have a nomenclature in the delta designation
we have 18:4.
The 18 stands for the number of carbon atoms,
the 4 stands for the number of double bonds
and these are the positions of the double
bonds from 5to 6, 8 to 9, 11 to 12 and 14
to 15.
Once we have these fatty acids we can have
a cis configuration to the double bond.
As soon as we have this in a cis configuration
this changes the direction.
As soon as this changes the direction then
what happens is I have a kink.
This gives rise to a kink in the structure
and in glycerophospholipid I have H2, I have
R2 here, I have R1 here, I have my phosphate
and I have to this, linked an X.
This is my polar part.
What can happen is if this forms R2 and for
R1 I have straight chain carbon fatty acids
and this is my polar head groups.
I will have a polar head group that is this
part, I will have a long carbon chain which
is this part if I happen to have no double
bond formation and if I happen to have double
bond formation I will bend it like this.
I have specific properties of the fatty acids
that tell me that the pKa’s are around 4.5
to 5 making them or ionizing them at physiological
pH and I have specific melting points for
these depending on the length of the chain
and on the number of the double bonds that
we have here.
Because the more the number of double bonds,
you are disrupting the organized structure
that it would have.
What would happen if it would look perfect?
This is the way they would be organized.
But if you had a kink you would have say,
one that was shaped like that, one that was
disrupted like that.
So what would happen to this?
This would melt easier than this.
So the more the number of double bonds the
more the disruption in the structure; you
would have lower melting point which is why
a saturated fatty acid is solid at room temperature.
Then we went on to study all the different
types of lipids that we could have and these
are the different types of molecular arrangements
of lipids that we can have and in the next
class we will see how we can organize this
into an actual lipid bilayer in the protein
and we will see how proteins are embedded
and how they can help in the transfer of materials
inside and outside the cell.
Thank you.
We continue our discussion on lipids and membranes
and in the last class we learnt what comprises
lipid of the membranes.
We learnt that we can have a glycerophospholipids
or a spingolipids that will look or could
assemble into basically bilayers.
Now we are going to look at the properties
of the lipid bilayer and see how it forms
and what are the basic physical and chemical
properties of this bilayer?
We will learn about the transportation later
on from one end of the cell to the other.
If you look at the properties of the lipid
bilayer they are usually impermeable to polar
molecules or ions.
You understand why that is so because you
have the polar head groups on the surface
only and if we have to have a transportation
from the inside of the cell to the outside
or from the outside of the cell to the inside
then it is not possible because of the hydrophobic
chains that are present in traversing the
whole membrane.
Normally the membranes are impermeable to
the polar molecules or ions unless we have
some specific proteins that facilitate the
transfer.
We will see how we can have active transport
or passive transport.
The membranes themselves are flexible but
they are very strong and they are durable.
They do not just rupture.
The membranes can be about 40 A thick or even
more than that and membranes are associated
with specific proteins that have definite
activities.
The basic properties of the bilayer are that
they do not allow the transport of the ions
unless assisted by a protein.
They are flexible; they are quite thick 40
A and they are associated with proteins that
have specific activity associated with it.
When we speak of the interior of the lipid
bilayer it means we are speaking about the
hydrophobic tails.
That is what the interior of the bilayer is.
The exterior of the bilayer is the polar portion,
the polar head groups that forms the inside
and the outside and the interior is highly
fluid and we will see how that fluidity occurs,
why it occurs and what its usefulness is.
In the liquid crystal state, the hydrocarbon
chains of the phospholipids are disordered.
They are in constant motion because they are
suspended.
The cells are suspended in the plasma.
There is a cytosol to it there is an extra
cellular matrix to it.
So they are all suspended.
Because of their constant movement they are
in constant motion and they are highly disordered
as well.
polar head groups - that is the way we can
bring about a change in the curvature of the
two layers by changing the size and by changing
the packing.
We change the packing by changing the fatty
acids.
So this is what we can do.
What we have here is the cytosol.
What is the cytosol?
It is inside the cell.
Extra cellular space means it is outside the
cell.
All these are different membranes.
Red ones, the black ones and blue one with
sugar attached to them are more preferred
on the surface and a few red and black ones
in the inner leaflets but populated more by
green and yellow ones.
We have different polar head groups.
The green, the yellow, the blue and the black
of the different polar head groups and the
chains are also going to be different depending
on the type of fatty acids that we have.
Phosphatethyl ethanolamine that is PE and
phosphatethyl serine are usually in the inner
layer.
So the ethanolamine type and the serine type
are preferred in the inner layer and in the
outer layer we have the sphingomyelin and
the choline type, the phosphatethyl choline.
We have more of the phosphatethyl choline
and the sphingomyelin on the outside and more
of the phosphatethyl ethanolamine and serine
on the inside.
We have to look at membrane proteins.
What are membrane proteins going to do?
They are going to help in the transfer of
ions.
They are usually of three types.
You can have peripheral proteins, integral
proteins or ones that have a lipid anchor.
The peripheral proteins are the ones that
are marked in green here that are on the periphery
of the membrane.
The ones marked in red are integral proteins
that are sort of embedded in the bilayer.
The lipid anchor ones have a lipid chain attached
to them that the lipid chain of which interacts
with the hydrocarbon chains of the lipid fatty
acids.
These are the three types of membranes proteins
that we can have and we will see the properties
of the membrane proteins.
