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
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
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
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
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
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
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.