All right, so now we’re going to move on to the next of our four classes of essential
biomolecules, and that is lipids. A few words before we begin. Lipids are actually
extremely important, in fact may have been one of the first essential things to have
developed, which allows life to form on this planet. One of the most important
features of a cell is its ability to separate the inside of the cell from the outside of the
cell, and to control what passes between the inside and the outside, and a lot of that
is down to the lipids. The lipids form the bi-layer around the outside of the cell, and
are therefore very important. But they serve a number of other functions as well.

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Created by Brett Barbaro

Biochemistry: A
Short Course

Fourth Edition

CHAPTER 11
Lipids

Tymoczko • Berg • Gatto • Stryer

© 2019 W. H. Freeman and Company.

11.1: So first we’ll talk about fatty acids, which are a main source of fuel for us and for
other animals.
11.2: And then triacylglycerols, which are the storage form of fatty acids.
11.3: And then we’ll talk some about the membrane lipids, and the three common
types, the main types of membrane lipids.

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Chapter 11: Outline

11.1 Fatty Acids Are a Main Source of Fuel
11.2 Triacylglycerols Are the Storage Form of Fatty

Acids
11.3 There Are Three Common Types of Membrane Lipids

Now, for the purposes of this book, and remember we’re sticking to the format that is
outlined in this book {there are lots of other ways we could explore this material},
lipids are defined as molecules that are not soluble in water, but are soluble in
organic solvents, which means they are generally hydrophobic.
We’ll talk about five different classes.
The first is the free fatty acids, which is what we use as a fuel;
then the triacylglycerols, which is the storage form of fatty acids;
and then numbers three, four, and five are different types of membrane lipids.
And yes, you see steroids there, at the end of the list, polycyclic hydrocarbons with a
variety of functions. Some of those functions are cell signaling and are very important
in development of muscles and other different parts of the body.
Numbers three and four, phospholipids and glycolipids, are basically just lipids with a
phosphate attached to them and lipids with a sugar attached to them, or a
carbohydrate of some kind.

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Five Classes of Lipids

Lipids are defined as molecules that are not soluble in water,
but are soluble in organic solvents.
We will examine five classes of lipids:

1. Free fatty acids: A common fuel.
2. Triacylglycerols: Storage form of fatty acids.
3. Phospholipids: Membrane lipids.
4. Glycolipids: Membrane lipids composed in part of

carbohydrates.
5. Steroids: Polycyclic hydrocarbons with a variety of

functions.

So fatty acids are chains of hydrogen-bearing carbons that have carboxylic acid at one
end – and like carbohydrates, there’s an enormous number of these and I’m not going
to hold you responsible for knowing all the names of them. The most common ones
are about 16 and 18 carbons long, so this one we have here on this page is a fairly
good representation, but I pulled these images from the internet to just to give you a
little better idea of what these lipids look like.
There’s two main kinds. One is the saturated fatty acid, and that would be saturated
with hydrogen. There is as much hydrogen as you can attach to this molecule as
possible. An unsaturated fatty acid is missing a couple of hydrogens, and those bonds
that were formed with the hydrogens are now turned into a double bond – and you
can see the effect of that is to create a bend in the molecule, and that’s very
important for its functional characteristics.

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Section 11.1 Fatty Acids Are a Main
Source of Fuel

Learning objective 3: Describe the key chemical properties of fatty
acids.

• Fatty acids are chains of hydrogen-bearing carbon atoms that have a
carboxylic acid at one end and a methyl group at the other end.

• Fatty acids may be saturated or unsaturated.

http://www.worldaccordingtomaggie.com/photographyylms/examples-saturated-unsaturated-fatty-acids

Now, here’s a different diagram representation. This doesn’t include the hydrogens,
and that’s actually how you’ll see it drawn most of the time. You’ll just have to
imagine that the hydrogens are there.
You can see a fully saturated stearate molecule at the top, and then the unsaturated
trans-oleate and cis-oleate at the bottom. Notice these all have the same number of
carbons, but because they’re not saturated, the oleate has a different name than the
stearate.
There are two different ways that these double bonds can be formed: trans, meaning
that the carbons on either end go away from each other (like “trans” meaning across
{as in “trans-Atlantic”}). And then cis, where the carbons are on the same side.
And there is some ability to rotate around that double bond, but it’s hard to do – and
apparently, the cis form is the most common. The reason for this is that the
desaturations are catalyzed by an enzyme that plucks two hydrogens off of the same
side of the chain.

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Structures of Palmitate and Oleate

SATURATED WITH HYDROGEN:

NOT SATURATED WITH HYDROGEN:

So – as we’ve already said, fatty acids vary in chain length and degree of unsaturation.
and the only new piece of information on this slide is polyunsaturated fats.
Polyunsaturated, that means that there are more than one double bond in the carbon
chain. And the double bonds are separated by at least one methylene group {bridge}.
You don’t see two double bonds in a row.

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Fatty Acids Vary in Chain Length and
Degree of Unsaturation (1/2)

• Fatty acids in biological systems usually contain an even number of
carbon atoms, with the 16 and 18 carbon atom chains being the
most common.

• When double bonds are present, they are commonly in the cis
configuration.

• In polyunsaturated fatty acids, the double bonds are separated by at
least one methylene group.

bridge

NOTE: “A methylene bridge is often called methylene
group or just methylene; as in “methylene chloride”
(dichloromethane CH2Cl2). However, the term methylene
group (or “methylidene”) properly applies to the CH2
group when it is connected to the rest of the molecule by a
double bond, that gives it chemical properties very distinct
from those of a bridging CH2 group.” -Wikipedia

Now, of course, different fats have different properties, and the length of the chain
and the degree of unsaturation determine some of those properties. The shorter the
chain length, the less likely they are to group together. You basically have the one
carboxylic acid moiety at one end. (I should probably mention that “moiety” is
another way of saying functional group or some small part of a molecule. And I know
it’s not included in your text, but sometimes I use it, so it’s a good thing to know. It’s
spelled M-O-I-E-T-Y.) So you’ve got basically a balance between the {hydrophilic}
carboxylic acid and the {hydrophobic} carbon chain. The carboxylic acid will be the
same in all of these fatty acids – but the more carbons that are attached to it, the
more hydrophobic it will be and the more likely it will bunch up together. Similarly,
double bonds in fatty acids cause a bend in the fatty acid, and that bend will make it
more difficult for them to pack tightly, so that will also increase the fluidity of the
fatty acids. And we’ll talk about that a little bit more, a little bit later.

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Fatty Acids Vary in Chain Length and
Degree of Unsaturation (2/2)

• The properties of fatty acids are dependent on chain length
and degree of unsaturation.

• Short chain length and the presence of cis double bonds
enhances the fluidity of fatty acids.

Now triacylglycerols – this is the storage form of fatty acid. They are basically three
fatty acids which are attached to a molecule of glycerol – and you can see in the
diagram, on the left-hand side in red is the glycerol backbone, and then three fatty
acid chains are attached to it. And those fatty acid chains could be various lengths.

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Section 11.2 Triacylglycerols Are the
Storage Form of Fatty Acids

Learning objective 4: Identify the major lipids and
describe their biochemical functions.

• Fatty acids are stored as triacylglycerols in which three
fatty acids are esterified to one molecule of glycerol.

So soaps are actually salts of fatty acids that are generated by treating triacylglycerols
with a strong base. So you basically have your triacylglycerol, and you can break it
apart by treating it with sodium hydroxide or potassium hydroxide, which are both
extremely strong bases. And then, it will separate into its fatty acid form. So why are
fatty acids good for soaps? Well, because they have a hydrophobic part and a
hydrophilic part – and the hydrophobic part can attach to oils that are stuck to your
skin or clothes, and the hydrophilic part will interact with water and therefore help
these oils to dissolve; and actually can totally encase oils in fatty acids and have the
external surface of the oils be completely soluble in water.
The triacylglycerols are a way of making the fatty acids more compact and
manageable for the cell, and they are stored in a lipid droplet in these “adipose {fat}
cells” (there’s a picture on the next page).

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Soaps and Mammal Triacylglycerol Storage
• Soaps are the sodium or potassium salts of fatty acids

generated by treating triacylglycerols with a strong base.

• In mammals, the major site for triacylglycerol storage is
adipose tissue. Each adipocyte (adipose cell or fat cell)
contains a large lipid droplet in which the triacylglycerols are
housed.

https://www.gaiabodyworks.com/pages/real-awesome-soap

Figure 11.3 Electron micrograph of an adipocyte. A small band of cytoplasm
surrounds the large deposit of triacylglycerols. [Biophoto Associates/Photo
Researchers.]

So this would be an adipose cell or an “adipocyte”. And you see how big this lipid
droplet is – it’s basically the whole cell, with the nucleus pushed into the corner over
there on the bottom, and a few mitochondria. So this cell has a lot of energy in it. A
lot of fuel for use by the body.

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Electron Micrograph of an Adipocyte

And one of the nice things about lipids is that they are energy dense. When we talked
about the carbohydrates, like glycogen and starch, those are branched chains of
sugars – and those sugars are very hydrophilic so water will associate with them and
make them difficult to pack together. So they take up a lot of space. Whereas lipid
deposits are hydrophobic and therefore pack very tightly. In fact, as it says here, one
gram of anhydrous fat stores more than six times the energy of a gram of hydrated
glycogen. The hydration there is extremely important because that’s what makes
glycogen so big. It’s the hydration. And that’s how birds store energy for their long
flights across the ocean and stuff. They store it as triacylglycerols or lipid deposits
because they are able to store more energy with less weight.

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Triacylglycerols Are Energy Rich DENSE*

• Triacylglycerols are ENERGY-DENSE. Because they are
hydrophobic and reduced, a gram of anhydrous fat stores
more than six times the energy of a gram of hydrated
glycogen.

• Migratory birds use fat stores to power long flights without
the opportunity to feed.

*”Rich” and “dense” mean more-or-less the same thing, but I prefer the term
“dense” in this context to emphasize that triacylglycerols contain more energy
than the same amount of carbohydrate, in terms of both weight and volume.

So here are some examples of common types of membrane lipids – and I know we say
here there’s three, that would be phospholipids, glycolipids, and cholesterol, but
we’re showing four because there’s two main types of phospholipids. There are
phospholipids that are built on a glycerol backbone – that’s three carbons with three
oxygens attached to it, and you can see on the left-hand diagram with the little labels
(sn-1), (sn-2), and (sn-3). And then there is a phospholipid based on the molecule
sphingosine, and those do not have a glycerol backbone, they just have a sphingosine
backbone. Now, the phospholipids you can see on the left are called phospholipids
because they have a phosphate attached to them. And that’s very important –
phosphate is extremely electronegative, extremely hydrophilic, and is one of those
things that is responsible for the organization of the membrane.
Glycolipids, on the other hand, have sugars attached to them – and as you can see in
this one, it looks like a glucose or some relative of glucose attached to the top of the
glycolipid there. And those are important too for signaling and for attachment of
other cells, cell-cell recognition, and things like that.
And then, the totally different oddball is the steroid. And this is cholesterol, which is
the basic, the most common steroid that you’ll find. Yes, cholesterol is a steroid,
believe it or not. And it’s a lipid because it is generally not dissolvable in water. All it’s
got on there is a little hydroxide group at the top.

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Section 11.3 There Are Three Common
Types of Membrane Lipids

http://popups.ulg.ac.be/1780-4507/index.php?id=6568

Phospholipids
Glycolipids

Cholesterol

Now we’ll zoom in on phospholipids a little bit. Phospholipids are probably the most
common lipids that you’ll run into, because they’re the primary constituents of
membranes. It says here that phospholipids are composed of four components: fatty
acids, a platform which would be like your backbone such as the glycerol, a
phosphate, and an alcohol. (I’m not really sure about that alcohol. I don’t really
understand why they call it that, so it’s not that important.)
So the two common platforms, as we mentioned, are glycerol and sphingosine. So
things that have a glycerol platform are called phosphoglycerides, and the other ones
with a sphingosine backbone are called sphingolipids. And most of the major
phospholipids come from the particular molecule called phosphatidate. (Man these
words are really long, and difficult to pronounce!)

Just as a side note here, I’d like to mention that we’re just barely grazing the surface
of lipids – I mean, you could spend an entire career studying lipids, and people do. But
we’re just covering the basics, and as long as you have a good idea of what’s going
on, don’t worry too much about these long words. I will try not to make you have to
memorize them. {You can always look them up!}

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Phospholipids Are the Major Class of
Membrane Lipids (1/2)

• Phospholipids are composed of four components:

– fatty acids (2 or more), a platform, a phosphate, and an
alcohol.

• Two common platforms are glycerol and sphingosine.

• Phospholipids with a glycerol platform are called
phosphoglycerides or phosphoglycerols.

• The major phospholipids are derived from phosphatidate.

Figure 11.6 Structure of phosphatidate (diacylglycerol 3-phosphate). The absolute
configuration of the central carbon atom (C-2) is shown.

So this is what phosphatidate looks like – and you can see the black portion here is
the glycerol backbone, and the two fatty acid groups attached to it can be of various
size, and there’s a phosphate attached to the glycerol backbone. That’s the general
structure of this entire class of molecules.

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Structure of Phosphatidate

Figure 11.7 Some common phosphoglycerides found in membranes.

Here are some other specific examples. You can see in the upper left,
phosphatidylserine is just a phosphatidate with a serine attached to it. And I hope
you’ll remember, serine is an amino acid. Phosphatidylcholine is a phosphatidate with
a choline attached to it. So there’s basically a similar pattern for all of these things.
This phosphatidylinositol you see on the right-hand side has got a sugar attached to
the phosphate, a close relative of glucose. And that is actually a very important
signaling molecule which we will discuss later.

This diphosphatidylglycerol at the bottom, I actually haven’t even seen that before so
I don’t think it’s that common. I don’t know why they put it in there.

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Structures of Some Common
Phosphoglycerides Found in Membranes

Aside from the phospholipids that are built on the glycerol platform, we have
phospholipids built on the sphingosine platform, or sphingolipids, an example of
which is sphingomyelin. And you’ve probably heard of myelin sheath that covers your
axons in your brain – that’s one of the most important components of that myelin.

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Phospholipids Are the Major Class of
Membrane Lipids (2/2)

• Phospholipids built on a sphingosine platform are called
sphingolipids.

• Sphingomyelin is a common membrane sphingolipid and is
especially common in the myelin sheath of nerve cells.

DID YOU KNOW?
Niemann–Pick disease can result from an
accumulation of sphingomyelin owing to the lack
of sphingomyelinase, an enzyme that degrades
sphingomyelin. Symptoms of Niemann–Pick
disease include cognitive disability, seizures, eye
paralysis, ataxia, and retarded growth.

Figure 11.8 Structures of sphingosine and sphingomyelin. The sphingosine moiety of
sphingomyelin is highlighted in blue.

At the top here you see the general diagram of sphingosine, and it’s basically just a
bunch of carbons with a little hydroxide group sticking off it and then an amine group
and another hydroxide group, very similar in some senses to glycerol. You can see
those three carbons on the right would be your glycerol, and instead of having a
hydroxyl group coming off the central carbon, you have +H3N, your amino group. And
that amino group can attach to other fatty acids, very much like the hydroxyl group
does in glycerol. The third carbon, the terminal carbon there, is attached to a
phosphate, and various groups can be attached to that phosphate.

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Structures of Sphingosine and Sphingomyelin

Now glycolipids – and don’t get those confused with the glycerolipids; these are not
based on glycerol – this is “glyco-”, which is sugar, so they will contain carbohydrates.
And yes, most of these carbohydrates are on the exterior surface of the cell – so if you
were wondering how these carbohydrates get attached to the outside of the cell,
that’s how they get attached. They have this hydrophobic element that is embedded
in the cell membrane, and that’s why they stick there. The simplest glycolipids are
called cerebrosides.

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Membrane Lipids Can Include
Carbohydrates

• Glycolipids are carbohydrate-containing lipids.

• The carbohydrate components of glycolipids are on the
extracellular surface of the cell membrane, where they play
a role in cell–cell interactions.

• Cerebrosides are the simplest glycolipids.

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So this is an example of a cerebroside. It’s a sphingolipid, as you can see. It’s not built
on a glycerol backbone, but on a sphingosine backbone. And there at the right-hand
side, you can see the sugar units such as glucose or galactose being attached directly
to the sphingosine backbone. There’s no phosphate involved in this, so that’s one of
the things that makes these glycolipids different from the phospholipids.

Now steroids! Steroids are all built on this common structure which you see here in
this diagram called the steroid nucleus. This is three six-membered rings and one five-
membered ring.

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Steroids Are Lipids That Have a Variety of
Roles

• Steroids share a common structure called the “steroid
nucleus”.

As you can see in this diagram, there are several things built onto the nucleus of the
steroid. This is cholesterol, the most common steroid, and it has a hydroxyl group on
the left, a couple of methyl groups on it, and then some goofy thing up on the top.
Also note there is a double bond down near the bottom, and that is also something
that you’ll find in steroids. Cholesterol is a very important component of lipid bilayers.
It helps to maintain rigidity, and also in some cases, helps to maintain fluidity. And is
also the precursor to all of the steroid hormones that you know about.

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Structure of Cholesterol

• Cholesterol is the most common steroid and plays a role in
maintaining membrane fluidity.

• Cholesterol is also a precursor to steroid hormones.

Figure 11.10 Membrane anchors. Membrane anchors are hydrophobic groups that
are covalently attached to proteins (in blue) and tether the proteins to the
membrane. The green circles and blue square correspond to mannose and GlcNAc,
respectively. R groups represent points of additional modification.

So we talked a bit about how carbohydrates can be embedded in the lipid bilayer by
having a hydrophobic fatty acid tail attached to them, and the same is true of
proteins. Oftentimes proteins need to be attached to a membrane in order to serve
their purpose. That can be accomplished by attaching fatty acids to them. And then
the fatty acids become embedded in the membrane, and the proteins are stuck.
Here are just a few examples of these membrane “anchors”, so-to-speak. The top two
we see attached to cysteine residues in the protein, and in the bottom we see a
slightly more complex arrangement with sugars, and phosphates, and all kinds of
things attached to the carboxyl terminus of the protein. But the key point in all of
these is that the long hydrocarbon chain is hydrophobic, and as a hydrophobic
element it will go to the membrane. That’s where hydrophobic stuff goes.

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Some Proteins Are Modified by the Covalent
Attachment of Hydrophobic Groups

• Proteins are sometimes covalently bound to lipids to
localize the protein to the cell membrane.