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Membrane Lipids

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Membrane Lipids 0:26
    • Definition of Membrane Lipids
    • Five Major Classes of Membrane Lipids
  • Glycerophospholipids 5:04
    • Glycerophospholipids Overview
    • The X Group
    • Example: Phosphatidyl Ethanolamine
    • Example: Phosphatidyl Choline
    • Phosphatidyl Serine
    • Head Groups
    • Ether Linkages Instead of Ester Linkages
  • Galactolipids 23:39
    • Galactolipids Overview
    • Monogalactosyldiacylglycerol: MGDG
    • Digalactosyldiacylglycerol: DGDG
    • Structure Examples 1: Lipid Bilayer
    • Structure Examples 2: Cross Section of a Cell
    • Structure Examples 3: MGDG & DGDG

Transcription: Membrane Lipids

Hello, and welcome back to, and welcome back to Biochemistry.0000

Today, we are going to continue our discussion of lipid biochemistry by talking about membrane lipids; these are structural lipids.0004

On the last lesson, we talked about the storage lipids, the triacylglycerols.0011

Today, we are going to be talking about the structural lipids, the lipids, the fats that actually show up in biological membranes, in the cell membranes.0016

Let’s get started.0025

OK, let’s go ahead and write down a couple of things.0028

Yes, that is fine; I guess we can stick with black here.0033

Your membrane lipids are lipids that make up the biological membranes of cells, the bilayer - and we are going to be looking at some illustrations a little bit later on - in cells or reside in the membrane.0038

In the case of something like cholesterol, it is not so much that it makes up the fats in the membrane.0078

It just happens to reside there, or reside in the membrane among the other types of lipids.0085

Again, it is just sort of the other types of lipids that actually make up the membrane.0092

It is just a question of perspective, whether one considers it to be something that actually makes up the membrane or happens to just be there.0101

In another cell, it is not that important; what is important is the lipid, the structure and the function.0108

OK, they have a hydrophilic end.0114

Let’s call it polar end and a hydrophobic tail.0126

Again, it looks something like this; we generally tend to draw it like that, either a single or usually a double.0140

In the case of the lipids that we are going to be talking about today, this is the hydrophobic tail; this is the polar end.0146

And again, we will be looking at some illustrations a little bit later, after we talk about some structures.0151

OK, we are going to be talking about five major classes of membrane lipids.0157

We will discuss five major classes of the membrane lipids, and we are going to be taking about the glycerophospholipids.0166

OK, I have to warn you here.0188

As you have probably already noticed with biochemistry, the nomenclature, the number of the names, they tend to not only get very, very long, but we tend to have multiple names for the same thing.0192

For a student, it is very, very daunting because often we will be talking about, let's say we will mention 3 or 4 different names of something but we are actually talking about the same class of molecule; and I understand it is an annoyance.0203

So, sometimes we are going to be calling them glycerophospholipids; you will hear them talked about as phosphoglycerides.0216

Sometimes, the name is so completely different that you are wondering, you really think that they are different molecules and they are not.0223

I have to apologize for that; that is just the nature of biochemistry.0230

Different people from all around the world call them different things; people within a certain specific research community refer to them differently.0234

These are just names that you are going to get accustomed to hearing over and over and over again.0242

I will try my best to be consistent, but in all honesty, I think it is also a good idea to hear the multiple different names and to know that we are talking about the same class of molecule or the same molecule itself, so just a little warning; but you already figured that out.0247

OK, glycerophospholipids, and then the second major class we are going to talk about is going to be the galactolipids.0261

Let’s see, the third class, something called the tetraether lipids, and then, we have something called the sphingolipids, and then, we are going to talk about the steriles.0270

OK, let’s go ahead and start with our glycerophospholipids, and I think I will go ahead and go to blue here, so glycerophospholipids, glycerophospholipid.0293

Now, these are the ones that are also called the phosphoglycerides.0314

Now, the name pretty much says it all, glycerophospholipids; you have a glycerol backbone.0327

It contains a phosphate somewhere, and its glyceride part, it basically has some fatty acids that are attached to it.0332

Let’s go ahead and look at the general structure; I think I will go ahead and do it this way.0340

We have carbon, carbon, carbon; that is going to be our glycerol backbone.0346

We have O; we have O, and let me go ahead and put this O a little bit down here.0351

We have one fatty acid; I will go ahead and call it R1.0358

It could be any particular length that could have some double bonds.0361

It could have no double bonds, the saturated or unsaturated.0366

We have a second one attached; I will go ahead and call this one R2.0370

Rather than choosing a specific example of a fatty acid, I will just go ahead and leave them as R1 and R2.0374

And here, here is where we have our phosphate; and then, of course, we, I am going to go ahead and put a little bit of an X there, this X.0380

This is the general structure of a glycerophospholipid; let me go ahead and finish with my hydrogens here, make sure I have all of those.0391

There is an H2 here, and we have an H here.0399

We have the glycerol backbone; this is your glycerol right here.0403

And then, of course, we have a fatty acid attached to one of them, a fatty attached, another one, in an ester linkage - right - C double bond O, single bond O.0410

And then, of course, to this third oxygen, we have a phosphate group attached; and then we have this thing right here, this XO.0420

I am going to put a little square around this, and the reason I am going to do that is the following.0428

We are going to be doing some specific examples of what this X group is, but it is really, really important to know that this oxygen, over here, that is attached to the phosphate and attached to the X actually comes from the X group.0433

The X is some kind of an alcohol, something that has a hydroxy group on it.0444

This oxygen actually comes from that alcohol.0448

Later on, when you study the actual biosynthesis of lipids, then you will actually see where each individual atom comes from; but for now, it is good to know that this X group, it is actually an XOH.0452

It is actually some alcohol that is attached to this phosphate- there you go.0469

And also, this oxygen right here, it actually comes from the glycerol.0475

All right, now, and, of course, here is our phosphate group right there.0479

A couple of things, now, the X group is what changes.0486

So, this is what is variable; the rest of the molecule is pretty much fixed.0493

I mean, it is true; R1 and R2 can be different.0496

Generally, there will not be a huge variety; they can be different, but what actually characterizes that particular glycerophospholipid is this XO group, the alcohol that is attached.0500

Group is what changes and gives a particular glycerophospholipid its name.0512

OK, X is the polar group, so this is the polar group; and these R1 and R2, these long hydrocarbon chains, that is going to end up being the nonpolar tail.0531

X is the polar group and is an alcohol like we said.0549

OK, now, when X = H, when X is equivalent, when it is just a hydrogen, then the molecule is called a phosphatidic acid.0566

We often refer to these as derivatives of phosphatidic acid because the X is going to change.0594

When it is just H, when it is just POOOH, it is phosphatidic acid or A phosphatidic acid.0599

OK, R1 and R2 are variable.0608

R...well, I do not need to write that; you know that already, that is why we call them R1 and R2.0613

OK, and at physiological pH, at physio pH, the phosphate group carries a -1 charge.0616

That is very, very important.0630

Charge on these lipids is actually very, very important; it is going to affect the biochemistry.0636

OK, that is why I have this -1 here; and in general, I am going to be putting circles around my charges, so that I can actually see them clearly and add them up.0642

OK, let’s do some examples here.0651

We have examples; let’s go this way.0655

I am going to draw this a little bit different; I am going to put the tails on one end, then I am going to the polar head group on the other.0662

Let me go ahead and put C, C and C.0668

I am going to put the esters on this side.0673

COO, this is R1; this is going to be one of the tail ends in ester linkage, and this is going to be that.0676

This is going to be R2, and over here, and I hope you will forgive me if I leave off my hydrogens.0684

I tend to leave off my hydrogens; if you see a carbon that has two bonds attached to it, the other 2 are going to be hydrogens.0689

That is just how it is.0695

O phosphate, O, there is - oops, let me go ahead and make that negative sign a little bit clearer, a little circle around it - O, and then if we have C, C, NH3+, this is called, so this right here - I do not know which one I should...yes, it is fine - I will go ahead and call it the whole thing.0699

This is ethanolamine; this molecule is called ethanolamine.0726

The name of this whole thing - OK - of the glycerophospholipid is, let me write it out, lamine, M-ethanolamine; the regular molecule without the attachment, is exactly what it sounds like.0734

It is ethanol, 2 carbons and a hydroxy and an amine group attached right there.0760

This is ethanolamine before it is actually attached.0765

Again, this oxygen actually comes from the alcohol.0772

Ethanolamine, this is called - depending on where you want to put the stress again with pronunciation - phosphatidylethanolamine or phosphatidylethanolamine.0776

It is up to you; again, pronunciation is unimportant.0784

You will often see this written as one word; I tend to write it as two words.0787

Again, it is going to be up to your teacher, about how strict they are with things like that.0791

What is important is the chemistry and the structure.0797

This part, the phosphatidyl, that is this basic structure; and then this other name right here, depends on what it is that is attached.0801

Let’s do another molecule; let’s do a C, a C, a C.0815

OK, let’s go O, C, R1, O, C.0821

This is R2, and then, we have O, P, our phosphate with a negative charge.0830

We have O, and this time, we have C, C; we have the nitrogen again, but this time, we have a CH3, a CH3 and a CH3.0837

And again, there is a positive charge on there, negative charge here.0848

There is a positive; there is a negative charge.0853

Notice the net charge on the ethanolamine derivative is 0 because the nitrogen is carrying a positive.0856

The phosphate is carrying a -1, so this is a net charge of 0; so this is a neutral glycerophospholipid.0865

This one also, it is a neutral glycerophospholipid; these things are going to be very, very important.0871

In a minute you will see an example of one that is not neutral, so positive, negative, neutral, charges- very, very important.0876

This one is called - oops - phosphatidylcholine because we have this right here.0883

This group is called choline.0898

Ethanolamine has 3 hydrogens; choline has 3 methyl groups attached to the nitrogen.0903

And again, nitrogen has 4 things attached to it, so it is carrying a formal charge of +1.0908

OK, let’s take a look at another one.0914

We have got C, and we have got C, and C, O, C.0920

This is R1, O, C; and this is R2.0929

We have our O; we have our phosphate group.0935

The oxygen is carrying a -1 charge; we have our O there.0938

And now, we have C and C; we have N there.0943

We have C here; I will go ahead and put, yes, I will go ahead and just leave it like this.0948

And then, this is going to be NH3+, and there is a negative.0956

There is a negative, and hopefully you will recognize this particular molecule right here, this particular residue.0963

This is serine; here is the N, C, C.0971

OK, this is an amino acid; and here is the C, and this is an OH, so it is an alcohol, right?0976

Serine has, its R-group has CH2 and then OH.0983

This is phosphatidylserine- that is it.0986

Now, what is the net charge on here?0994

Well, we have -1, +1, -1, so the net charge here is -1.0996

Looks different and because it is a different, it is going to have a different biochemistry- net charge, -1.1002

OK, just to reiterate that, the charges on the head groups.1009

When we say head groups, we are talking about the polar group.1024

This right here, this whole thing, is referred to as the head group; and that, right there, is going to be the tail.1027

Again, this R1 and R2 - I probably should have drawn them out - these are just long hydrocarbon chains- that is it.1038

That is all they are, the fatty acid chains, the 14, 16, 18, 20, 22, 24, 26, length, saturated, unsaturated.1043

That is what these are, so that is the tail group; this is the head group.1053

We often refer to it that way; the charges on the head groups are important and do have consequences at the layer - that is fine, I will just go ahead and write it this way - at the head group, water interface.1057

The charge is going to affect how that particular group reacts or interfaces with whether it is in the cytosol or outside of the cell.1079

The aqueous environment, it is going to have an effect on the biochemistry there.1092

OK, let me see if I should do a little, yes, let’s go ahead and give a schematic version of this.1099

Just schematically, basically, you are going to have your glycerol.1106

OK, you are going to have glycerol, and then you are going to have some fatty acid attached in ester linkage, another fatty acid attached in ester linkage.1119

You are going to have a PO3-, and then you are going to have some alcohol.1151

This is for the schematic representation of a phosphoglyceride, glycerophospholipid- that is it, glycerol, 2 fatty acids, phosphate, diester linkage here, a phosphodiester.1156

In other words, it is an ester linkage with a phosphate instead of a carbon, OC, OC.1171

This is called a phosphodiester because this is phosphodiester linkage.1180

That is your 1 ester; this is another ester.1187

That is you are phosphate group and it is attached to some alcohol; that is the general schematic for that.1190

OK, let’s see what else we have got here.1195

OK, slight variation.1203

Some glycerophospholipids have 1 of the 2 fatty acids connected to glycerol with ether linkages instead of ester linkages.1209

OK, let’s do some examples here, and I will go ahead and keep these in black.1251

Let’s see, so let’s go C, C, C; and this time, I am going to go O, C, double bond C and some R1.1260

I will go ahead and - yes that is fine - I will go ahead and leave it like that, and then this one will be O.1274

This is our ester linkage, so I will go ahead and leave this as R2.1281

And over here we have our O; we have our phosphate group.1286

So, this is the head group, and I will go and leave it as, let me see O, C, C, C; and I will go ahead and just put (CH3)3 there and this, oops, that is not a C, that is a nitrogen, because it is choline.1291

That is that; that is that.1312

OK, when you have something like this, same basic structure - you have the phosphor, you have the alcohol group - one of the fatty acids is the normal fatty acid connected in ester linkage, but one of the fatty acids, instead of an ester right here, instead of an oxygen double bonded to a carbon, what you have is these 2.1315

The first carbon and the second carbon are attached in alkene linkage.1337

OK, this right here, this is our alkene connection.1342

I do not know why I am having such a hard time writing today.1355

This is our alkene, and, of course, this right here - we will do it in blue - this is our ether linkage.1359

OK, it is not the ester linkage, the C, double bond O, single bond O, C; it is C, O, C single bond.1365

This right here, this is your ether linkage, and the general name for this class of molecule, it is still a glycerophospholipid.1373

It just happens to be called a, again, a plasmalogen or a plasmalogen, depending on where you want to put the stress; and these are characterized by the alkene linkage at no. 1, no. 2 carbon.1390

OK, let’s go ahead and move on to our second class, which is going to be the galactolipids.1404

Let me go ahead and start drawing a little bit of a line here.1413

Let me do this one in red.1417

Our second class, these are going to be our galactolipids.1421

OK, these predominate in plant cells; where the glycerophospholipids tend to predominate in animal cells, these predominate in plant cells.1430

OK, and they are characterize by having 1 or 2 galactose monomers; and you remember galactose, it is the c4-epimer of glucose.1446

Monomers is connected; it is connected to the no. 3 carbon of glycerol - yes, that is right - glycerol while the other 2 have their 2 fatty acids, while the other 2 Cs are attached to fatty acids, as usual.1462

OK, let’s go ahead and do a structure for a monogalactolipid and a digalactolipid, so 1 or 2 monomers attached.1507

Let’s go ahead and do a monogalactolipid; this time, let’s do it this way.1519

We will go C, C, and we will do C here.1525

Here we have our normal fatty acid linkage; we will call it R1.1530

This one is going to be R2, and now, this is carbon.1535

So, we have oxygen - wait, let me do this a little bit differently, carbon, no, that is alright, I will go ahead and do it this way - oxygen, OK, there, there, there, there.1545

And, of course, we have our galactose, so this down.1564

This is up; this is up and this is up.1568

This is our galactose monomer, and let me go ahead and draw those in, just for the heck of it.1573

OK, here we go; we have our glycerol backbone- 1 fatty acid, 2 fatty acids.1585

And here, instead of phosphate, the sugar is just directly attached, so this is your anomeric carbon right here, right?1593

And notice, you have - let me do this in blue - this is the beta-1.1602

The beta configuration is actually going up.1609

This is our galactose monomer, and a couple of words, R1 and R2 are generally the same.1612

They can be different, but they are generally the same; and they tend to be linoleic acid, which is better represented as 18:2 delta 9, 12- there is that.1626

This is often called an MGDG stands for monogalactodiacylglycerol- that is it.1646

Diacyl, 2, monogalacto has one sugar- that is it.1665

Now, a couple of things you want to note to be in a net neutral.1670

The charge on this is neutral; nothing is ionized here.1678

Nothing is carrying a charge, so it is net neutral.1683

There is no charge on the head group, and, of course, the beta configuration at the anomeric carbon of the galactose.1686

OK, now, let’s go ahead and do a digalactolipid.1693

I will go ahead and draw that down here; let me do this in blue.1699

Actually, let me do it in red again.1703

I have got C; I have got C.1705

I have got C; let me make sure I have enough room here.1708

Yes, I should have enough room; this is O, C.1711

It is really interesting; when you actually draw these structures out by hand, it is kind of nice to look at illustrations and see them passively because when you are drawing them out by hand, especially these ester linkages, you will often forget an oxygen or forget a carbon or something like that.1717

It is really kind of, it is interesting; but it a good idea to be able to reproduce them by hand.1731

We have C; also, hopefully, I am not forgetting any of my atoms here because that would be really bad.1738

OK, let’s go here.1745

And we said we have O, and we have this, that O.1751

That is one of them, and then we have our CH2; and I will go ahead and put that there.1762

This is going to be O; I will go ahead and do it this way, that, that, that, that, that, that, that.1771

I think that is right, so we have a down.1781

This is going to be up; this is going to be up.1785

That is one galactose monomer, and notice, 1, this is the anomeric carbon.1789

Notice we have a - let me do this in black - this is going to be beta-1 configuration.1794

Over here, 1, 2, 3, 4, 5 connected to the 6 carbon.1800

OK, this is going to be a, this is an alpha, so we are on alpha-(1,6) glycosidic bond, between 1 galactose monomer and another galactose monomer.1808

And, let me go ahead and put the rest of the substituents in here.1817

This down; this is up.1821

This is up, and, of course, we have our CH2OH, which is up; and I hope that you will confirm this for me, but I think I have got everything here.1824

R1, this is R2, not R1, R2.1835

Again, they tend to be the same, but that is OK; we will go ahead and leave it as R2.1840

This is called a DGDG- digalactodiacylglycerol.1842

Again, just names, not all together that important.1851

Notice net neutral beta-1 configuration connected to glycerol, alpha-(1,6) configuration connected from 1 galactose monomer to the other galactose monomer, galactose, galactose, c4-epimer.1855

So, this is up instead of down like it would be for glucose- that is it, just a little variation, that is all.1870

OK, now, what I am going to do is, I am just going to go ahead and jump to some illustrations.1879

Instead of drawn out by hand, I would like you to actually see what it is that they are going to look like in your book or in any other media source that you happen to consult.1885

Let’s take a look at some of these things, so we have a better sense of what is going on.1895

OK, first of all, let’s take a look at a nice, general...we are going to work our way down.1900

We are going to look a little bit deeper in magnification.1910

We have our general lipid bilayer; this is a nice cell membrane.1913

We see some integral proteins; we see the lipid bilayer.1917

This is our top layer here; this is our bottom layer there.1924

We have outside of the cell; we have inside of the cell.1929

We would call this the outer leaflet; We would call this the inner leaflet, and you can see this little blue thing, this is our head group, and you can see the tails a little bit.1931

So, when we magnify this, now, we are going to come over to this level, so nice cross-sectional view.1942

We have our head groups; these are the head groups.1949

These are the tails; notice, there are 2 tails attached.1954

These are our glycerophospholipids; in general, you have the 2 fatty acid tails attached.1958

They are going to arrange themselves like this, again, because polar with polar, nonpolar with nonpolar.1965

These are nonpolar tails attaching with nonpolar tails- outer leaflet, inner leaflet.1970

This is something called a micelle; often times, they will arrange themselves.1978

One layer of these can arrange itself in a circle and create this little container, if you will, where the inside is actually nonpolar; the outside is polar.1981

The soaps and detergents that you use, they often do it like this; they trap the oil inside here, but they interact with water outside here because these things are polar.1996

OK, now, let’s magnify a little bit more.2006

Now, we have an idea of what it is that they look like.2010

This is what you might call the, well, separation; this is what they are showing.2012

What you have here, in this particular case, we have our glycerol right there.2022

OK, this is our glycerol; we have one of the fatty acids attached.2030

We have another fatty acid attached; here is our phosphate group.2033

And then, of course, in this particular case, we have C, C, N; we have C, C, C, so this is choline.2038

This is our polar head group.2045

OK, that is that, and these down here, that is that.2048

In this particular case, same thing, except we have ethanolamine instead of choline, but we have our phosphate; and here, we have our serine.2058

Again, that is the tail; that is the tail, and this is our phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine.2070

This is the general structure; this is what you see.2078

This is one of these, is one of these- there you go.2082

Again, you have seen this stuff before, should be reasonably familiar.2088

Now, we are just getting to the actual structure of these lipids.2092

OK, this is a nice blown up view of a cross section of a cell.2097

In the outer leaflet, the inner leaflet - let me go ahead and go to blue - in the outside of the cell, the inside of the cell, this is going to be, let see, sphingomyelin, cholesterol, phosphatidylcholine.2106

A couple of these, we actually have not talked about yet; they are going to be the other classes that we are going to talk about, the sphingolipids and the sterols, but this one, we have talked about.2122

Here, you have your glycerol; you have your phosphate.2132

You have your choline.2138

OK, this is the outer leaflet of the membrane, inner leaflet of the membrane; and then, of course, you have your tail group, and, of course, you have your tail group.2141

And these are just some other lipids, some other fats that are actually - well, we should not call them fats, let’s just call them lipids - some other lipids that are in the outer leaflet, which we will talk about in just a little bit.2153

Here we have our phosphatidylserine; here we have another cholesterol.2164

We have phosphatidylethanolamine, and we have phosphatidylinositol, which again, we will talk about in subsequent lessons- that is it.2169

It is just an arrangement of these lipids, the various lipid classes arranging themselves and making up the cell membrane- that is all.2177

OK, a couple of structures here, so here, we have a monogalactolipid.2191

This one is a monogalactolipid; this is the MGDG, OK, or the monogalactodiacylglycerol.2197

This is the digalactodiacylglycerol; this is our galactose monomer, down, up, down, down.2210

Now, notice in this particular case, they actually show you the stereochemistry in a different way.2220

Instead of looking at it in Howarth projection, they are looking at it directly like this.2224

Again, just another way of looking at the molecular structure of something.2230

You are going to often see structures like this; that is the thing with biochemistry- different projections, different views give us different bits of information.2235

I just wanted you to see something a little bit than a Haworth projection, back, forward, forward, forward.2245

Here, we have our 2 galactose monomers; they are connected in what we said was alpha-(1,6) because this is the 1, 2, 3, 4, 5, 6 carbon, right there.2252

This is going to be a beta-1 connection to glycerol- that is it.2265

I just wanted you to see what it is that they are actually look like.2273

These are our acyl groups, and again, R1 and R2 tend to be the same, but they do not have to be the same, so we left it that way.2276

We have our glycerol, which was carbon 1, carbon 2, carbon 3 or 1, 2, 3, however you want to arrange it.2285

1, 2, 3 ester linkage, ester linkage, direct linkage, glycosidic bind, O linked, ester linkage, ester linkage, O linked, glycosidic bond, directly to a sugar, so notice, there is no phosphate in these galactolipids- that is all.2293

OK, let’s see, do we have, nope, that is it.2316

OK, well, thank you so much for joining us here at, and we will see you next time for a further discussion of these membrane lipids, bye-bye.2323