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23.1 Structure and Classification of Lipids
23.2 Fatty Acids and Their Esters
23.3 Properties of Fats and Oils
23.4 Chemical Reactions of Triacylglycerols
23.5 Phospholipids and Glycolipids
23.6 Sterols
23.7 Structure of Cell Membranes
23.8 Transport Across Cell Membranes
23.9 Eicosanoids: Prostaglandins and Leukotrienes
© 2013 Pearson Education, Inc.
Goals
1. What are the major classes of fatty acids and lipids?
Be able to describe the chemical structures and general properties of fatty acids,
waxes, fats, sterols, and oils.
2. What reactions do triacylglycerols undergo?
Be able to describe the results of hydrogenation and hydrolysis of triacylglycerols,
and, given the reactants, predict the products.
3. What are sterols?
Be able to identify sterols and their derivatives, describe their structures and roles.
4. What are the membrane lipids?
Be able to identify the membrane lipids, describe their structures, and roles.
5. What is the nature of a cell membrane?
Be able to describe the general structure of a cell membrane and its chemical
composition.
6. How do substances cross cell membranes?
Be able to distinguish between passive transport and active transport and between
simple diffusion and facilitated diffusion.
7. What are eicosanoids?
Be able to describe the general structure of prostaglandins and leukotrienes, and
some of their functions.
© 2013 Pearson Education, Inc.
23.1 Structure and Classification of Lipids
• Lipids are defined by solubility in nonpolar
solvents (a physical property) rather than
by chemical structure.
• There are a great many different kinds and
they serve a variety of functions in the
body.
• Many lipids have hydrocarbon or modified
hydrocarbon structure, properties, and
behavior.
© 2013 Pearson Education, Inc.
23.1 Structure and Classification of Lipids
© 2013 Pearson Education, Inc.
23.1 Structure and Classification of Lipids
© 2013 Pearson Education, Inc.
23.1 Structure and Classification of Lipids
Lipids that are esters or amides of fatty acids
• Waxes are carboxylic acid esters with long, straight hydrocarbon chains in
both R groups; they are secreted by sebaceous glands in the skin of animals
and perform mostly external protective functions.
• Triacylglycerols are carboxylic acid triesters of glycerol, a three-carbon
trialcohol. Triacylglycerols are found in most dietary fats and oils, and are
also the fat storage molecules in our bodies. They are a major source of
biochemical energy.
• Glycerophospholipids are triesters of glycerol that contain charged phosphate
diester groups and are abundant in cell membranes. Together with other
lipids, they help to control the flow of molecules into and out of cells.
• Sphingomyelins are amides derived from an amino alcohol (sphingosine), also
contain charged phosphate-diester groups; essential to the structure of cell
membranes and especially abundant in nerve cell membranes.
• Glycolipids are different amides derived from sphingosine, contain polar
carbohydrate groups; on cell surfaces the carbohydrate portion is recognized
and interacts with intercellular messengers.
© 2013 Pearson Education, Inc.
23.2 Fatty Acids and Their Esters
• Naturally occurring fats and oils are triesters
formed between glycerol and fatty acids.
• Fatty acids are long, unbranched
hydrocarbon chains with a carboxylic acid
group at one end. Most have even numbers
of carbon atoms.
• Those without double bonds are saturated.
• Those with double bonds are unsaturated.
• In naturally occurring fats and oils, the double
bonds are usually cis rather than trans.
© 2013 Pearson Education, Inc.
23.2 Fatty Acids and Their Esters
• Chemists use a shorthand nomenclature for fatty
acids that avoids using the common names.
• The notation uses C for carbon followed by the
number of carbon atoms present, a colon and
the number of unsaturated bonds present.
• Palmitic acid (C16:0) and stearic acid (C18:0)
are the most common saturated acids;
• Oleic (C18:1) and linoleic acids (C18:2) are the
most common unsaturated ones.
• Polyunsaturated fatty acids have more than
one carbon–carbon double bond.
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23.2 Fatty Acids and Their Esters
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23.2 Fatty Acids and Their Esters
Waxes
• A wax is a mixture of fatty acid–long-chain
alcohol esters.
• The acids usually have an even number of
carbon atoms, generally from 16 to 36 carbons;
• The alcohols have an even number of carbon
atoms ranging from 24 to 36.
© 2013 Pearson Education, Inc.
23.2 Fatty Acids and Their Esters
Triacylglycerols
• Animal fats and vegetable oils are the most plentiful
lipids in nature.
• Animal fats are solid, whereas vegetable oils are liquid,
but their structures are closely related.
• All fats and oils are composed of triesters of glycerol
(1,2,3-propanetriol, also known as glycerine) with three
fatty acids.
• They are named chemically as triacylglycerols, but are
often called triglycerides.
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23.2 Fatty Acids and Their Esters
Triacylglycerols
• The fat or oil from a given natural source is a complex
mixture of many different triacylglycerols.
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23.2 Fatty Acids and Their Esters
Triacylglycerols
• Vegetable oils consist of mainly unsaturated fatty acids.
• Animal fats contain a large percentage of saturated fatty
acids.
• This difference in composition is the primary reason for
the different melting points of fats and oils.
© 2013 Pearson Education, Inc.
23.2 Fatty Acids and Their Esters
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Lipids in the Diet
Fats and oils are a popular component of our diet;
excess energy from dietary fats and oils is mostly
stored as fat in adipose tissue.
Concern for the relationships among saturated fats,
cholesterol levels, and various diseases caused a
decrease of the average calories from fats and oils in
the U.S. diet.
Several organizations recommend a diet with not more
than 30% of its calories from fats and oils.
The FDA recommends that not more than 10% of daily
calories come from saturated fat and not more than
300 mg of cholesterol be included in the daily diet.
© 2013 Pearson Education, Inc.
23.3 Properties of Fats and Oils
• The more double bonds a fatty acid has,
the lower its melting point.
• The difference in melting points between
fats and oils is a consequence of this
difference.
• Vegetable oils are lower melting because
oils generally have a higher proportion of
unsaturated fatty acids than animal fats.
© 2013 Pearson Education, Inc.
23.3 Properties of Fats and Oils
• The hydrocarbon chains in saturated acids are
uniform in shape with identical angles at each
carbon atom, and flexible chains, allowing them to
nestle together.
• Unsaturated acids have rigid kinks wherever they
contain cis double bonds. The kinks make it difficult
for such chains to fit next to each. The more double
bonds there are in a triacylglycerol, the harder it is
for it to solidify.
© 2013 Pearson Education, Inc.
23.3 Properties of Fats and Oils
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23.3 Properties of Fats and Oils
• Triacylglycerols are uncharged, nonpolar,
hydrophobic molecules that coalesce when
stored in fatty tissue.
• The primary function of triacylglycerols is
long-term storage of energy.
• Adipose tissue serves to provide thermal
insulation and protective padding.
• Color and flavor are contributed by natural
materials.
• Oxidation causes rancidity, decomposition to
products with unpleasant odors or flavors.
© 2013 Pearson Education, Inc.
23.3 Properties of Fats and Oils
Properties of the Triacylglycerols in
Natural Fats and Oils
• Nonpolar and hydrophobic
• No ionic charges
• Solid triacylglycerols (fats)—high proportion
of saturated fatty acid chains
• Liquid triacylglycerols (oils)—high proportion
of unsaturated fatty acid chains
© 2013 Pearson Education, Inc.
23.4 Chemical Reactions of Triacylglycerols
Hydrogenation
• The carbon–carbon double bonds in
vegetable oils can be hydrogenated to
yield saturated fats.
• Margarine and solid cooking fats are
produced commercially by hydrogenation
of vegetable oils.
• By controlling the extent of hydrogenation
and monitoring the composition of the
product, consistency can be controlled.
© 2013 Pearson Education, Inc.
23.4 Chemical Reactions of Triacylglycerols
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23.4 Chemical Reactions of Triacylglycerols
Hydrolysis of Triacylglycerols
• Triacylglycerols can be hydrolyzed. In
the body, this hydrolysis is catalyzed by
enzymes (hydrolases) and is the first
reaction in the digestion of dietary fats
and oils.
• Soap-making involves the base-catalyzed
hydrolysis of triacylglycerols. Any mixture
of triacylglycerols can be used; the second
ingredient needed is lye or potash.
© 2013 Pearson Education, Inc.
23.4 Chemical Reactions of Triacylglycerols
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Detergents
Detergent is a term usually applied to synthetic materials made from petroleum
chemicals.
Like soaps, synthetic detergent molecules have hydrophobic hydrocarbon tails and
hydrophilic heads, and cleanse by forming micelles around greasy dirt.
All substances that function in this manner are described as surfactants. The
hydrophilic heads may be anionic, cationic, or non-ionic.
Soaps work as cleaning agents because the two ends of a soap molecule are so
different. The ionic end is hydrophilic (water-loving); it tends to dissolve in water. The
long hydrocarbon chain portion of the molecule, however, is nonpolar and therefore
hydrophobic (water-fearing). Because of these opposing tendencies, soap molecules
are attracted to both grease and water.
When soap is dispersed in water, the big organic anions cluster together so that their
long, hydrophobic hydrocarbon tails are in contact. By doing so, they avoid disrupting
the strong hydrogen bonds of water and create a nonpolar microenvironment.
Hydrophilic ionic heads on the surface of the cluster stick out into the water. The
resulting spherical clusters are called micelles. Grease and dirt become coated by
the nonpolar tails of the soap molecules and trapped in the center of the micelles as
they form.
Once suspended within micelles, the grease and dirt can be rinsed away.
© 2013 Pearson Education, Inc.
23.5 Phospholipids and Glycolipids
• Cell membranes separate the aqueous interior
of cells from the aqueous environment
surrounding the cells.
• The three major kinds of cell membrane lipids in
animals are phospholipids, glycolipids, and
cholesterol.
© 2013 Pearson Education, Inc.
23.5 Phospholipids and Glycolipids
• Phospholipids are built up from glycerol
(to give glycerophospholipids) or
sphingosine (to give sphingomyelins).
• Glycolipids are also derived from
sphingosine, but have an attached
carbohydrate that is a monosaccharide or
a short chain of monosaccharides.
• Cholesterol is a sterol, a class of
biomolecules that are characterized by a
system of four fused rings.
© 2013 Pearson Education, Inc.
23.5 Phospholipids and Glycolipids
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23.5 Phospholipids and Glycolipids
Phospholipids
• Similar to soap and detergent molecules in
having ionic, hydrophilic heads and hydrophobic
tails. They differ, however, in having two tails
instead of one.
Glycerophospholipids or phosphoglycerides
• Triesters of glycerol 3-phosphate, the most
abundant membrane lipids.
• Two of the ester bonds are with fatty acids.
• The third position is a phosphate ester linked to
ethanolamine, choline, or serine.
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23.5 Phospholipids and Glycolipids
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23.5 Phospholipids and Glycolipids
• Glycerophospholipids are named as derivatives
of phosphatidic acids.
• Lipids with a phosphate ester to choline are
known as phosphatidylcholines, or lecithins.
• Hydrophobic tails and hydrophilic head groups
make glycerophospholipids emulsifying agents.
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23.5 Phospholipids and Glycolipids
• In sphingolipids, the amino alcohol sphingosine provides
one of the hydrophobic tails.
• The second tail is from a fatty acid acyl group connected
by an amide link to the —NH2 group in sphingosine.
• Sphingomyelins are sphingosine derivatives with a
phosphate ester group at C1 of sphingosine. The
sphingomyelins are major components of the coating
around nerve fibers.
© 2013 Pearson Education, Inc.
23.5 Phospholipids and Glycolipids
Glycolipids
• Glycolipids are also derived
from sphingosine. They have
a carbohydrate group at C1
instead of a phosphate
bonded to an amino alcohol.
• Glycolipids reside in cell
membranes with their
carbohydrate segments
extending into the fluid
surrounding the cells.
© 2013 Pearson Education, Inc.
23.5 Phospholipids and Glycolipids
• Cerebrosides contain a single monosaccharide.
• They are abundant in nerve cell membranes in the brain
with the monosaccharide D-galactose.
• In other membranes, the sugar unit is D-glucose.
• Gangliosides are glycolipids in which the carbohydrate is a
small polysaccharide rather than a monosaccharide.
• Over 60 different gangliosides are known.
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23.6 Sterols
• Sterols are lipids whose structure is
based on a tetracyclic (four-ring) carbon
skeleton.
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23.6 Sterols
Cholesterol
• Cholesterol is the most abundant animal sterol.
• It is a component of cell membranes and the starting
material for all other sterols.
• Within a cell membrane, nearly-flat cholesterol molecules
are distributed among the tails of phospholipids.
• Because they are more rigid than the hydrophobic tails,
the cholesterol molecules help to maintain the structural
rigidity of the membrane.
• Approximately 25% of liver cell membrane is cholesterol.
© 2013 Pearson Education, Inc.
23.6 Sterols
• Bile acids are essential for the emulsification of fats
during digestion.
• Synthesized in liver cells from cholesterol and stored in
the gall bladder, these molecules have a polar end and a
nonpolar end.
• Solubility of bile acids is increased by conjugation with
either taurine, a cysteine derivative, or glycine, which
increases solubility and enhances the formation of
micelles.
• Micelles are essential for the digestion of dietary fat.
• The two most common bile acids are cholic acid and
chenodeoxycholic acid. In the intestinal tract these acids
are ionized to anions and referred to as bile salts.
© 2013 Pearson Education, Inc.
23.6 Sterols
• Steroid Hormones are divided into three types.
• Mineralocorticoids, such as aldosterone,
regulate the cellular fluid balance between Na+
and K+ ions.
• Glucocorticoids help to regulate glucose
metabolism and inflammation. Anti-inflammatory
ointments contain hydrocortisone to reduce
swelling and itching.
• Sex hormones—The two most important
androgens, are testosterone and androsterone.
Estrone and estradiol, the estrogens, are
synthesized from testosterone.
© 2013 Pearson Education, Inc.
23.6 Sterols
Butter and Its Substitutes
• It has become medically accepted that butter can contribute to
elevated blood cholesterol, which is to be avoided because of
cholesterol’s role in heart disease.
• Margarine, which is made from vegetable oils, contains no
cholesterol and much less saturated fat than butter.
• Margarine contains what might be an even more unhealthful
ingredient—trans fatty acids. Oils are catalytically hydrogenated to
give them a firmer consistency. During the partial hydrogenation,
some of the cis double bonds are converted to trans double bonds.
• Numerous studies have linked the quantity of trans fatty acids in a
person’s diet to a greater risk for heart disease and cancer.
• Meat and dairy products contain a very small amount of trans fatty
acids (about 0.2% in butter), but the quantities in foods containing
hydrogenated oils are much higher—up to 40% in the stiffer
margarines.
© 2013 Pearson Education, Inc.
23.7 Structure of Cell Membranes
• Phospholipids provide the basic structure
of cell membranes, where they aggregate
in a closed, sheet-like, double leaflet
structure—the lipid bilayer.
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23.7 Structure of Cell Membranes
• The bilayer is highly ordered and stable, but still
flexible.
• When phospholipids are shaken vigorously with
water, they spontaneously form liposomes—small
spherical vesicles with a lipid bilayer surrounding an
aqueous center.
• Water-soluble substances can be trapped in the
center of liposomes, and lipid-soluble substances
can be incorporated into the bilayer.
• Liposomes are potentially useful as carriers for drug
delivery because they can fuse with cell membranes
and empty their contents into the cell.
© 2013 Pearson Education, Inc.
23.7 Structure of Cell Membranes
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23.7 Structure of Cell Membranes
• The overall structure of cell membranes is represented
by the fluid-mosaic model. The membrane is fluid
because molecules can move around within it, and as a
mosaic because it contains many kinds of molecules.
• 20% or more of a membrane consists of protein
molecules.
• Peripheral proteins are associated with just one face of
the bilayer and are held by noncovalent interactions with
the hydrophobic lipid tails or the hydrophilic head groups.
• Integral proteins extend completely through the
membrane and are anchored by hydrophobic regions
that extend through the bilayer.
• The carbohydrate parts of glycoproteins and glycolipids
mediate the interactions of the cell with outside agents.
© 2013 Pearson Education, Inc.
23.7 Structure of Cell Membranes
• Because the bilayer membrane is fluid rather
than rigid, it is not easily ruptured.
• Proteins move sideways in the membrane layers
continuously, not unlike floating on a pond; this
is an energetically neutral motion.
• Membrane components do not flip from the
inside of the membrane to the outside.
• Small nonpolar molecules can easily enter the
cell through the membrane and individual lipid or
protein molecules can diffuse rapidly from place
to place.
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23.8 Transport Across Cell Membranes
• There are two modes of passage across the
membrane.
• Passive transport, in which substances move
across the membrane freely by diffusion from
regions of higher concentration to regions of
lower concentration.
• Active transport, in which substances cross the
membrane only when energy is supplied
because they go from lower to higher
concentration regions.
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23.8 Transport Across Cell Membranes
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23.8 Transport Across Cell Membranes
• Some solutes enter and leave cells by simple
diffusion. Small, nonpolar molecules and lipidsoluble substances, including steroid hormones,
move through the hydrophobic lipid bilayer in
this way.
• In facilitated diffusion, solutes are helped
across the membrane by proteins. The molecule
binds to a membrane protein, which changes
shape so that the transported molecule is
released on the other side of the membrane.
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23.8 Transport Across Cell Membranes
• Maintaining concentration
gradients requires an
expenditure of energy.
• Energy from the conversion
of ATP to ADP is used to
change the shape of the
sodium/potassium pump,
simultaneously bringing two
K+ ions into the cell and
moving three Na+ ions out
of the cell.
© 2013 Pearson Education, Inc.
23.8 Transport Across Cell Membranes
Properties of cell membranes
• Cell membranes are composed of a fluid-like phospholipid
bilayer.
• The bilayer incorporates cholesterol, proteins (including
glycoproteins), and glycolipids.
• Small nonpolar molecules cross by simple diffusion through
the lipid bilayer.
• Small ions and polar molecules diffuse across the membrane
via protein pores (simple diffusion).
• Glucose and certain other substances (including amino acids)
cross with the aid of proteins and without energy input
(facilitated diffusion).
• Na+, K+, and other substances that maintain concentration
gradients across the cell membrane cross with expenditure of
energy and the aid of proteins (active transport).
© 2013 Pearson Education, Inc.
23.9 Eicosanoids: Prostaglandins and Leukotrienes
• Eicosanoids are a group of compounds
derived from 20-carbon unsaturated fatty acids
(eicosanoic acids) and synthesized throughout
the body. They function as short-lived chemical
messengers that act near their points of
synthesis (“local hormones”).
• Prostaglandins and leukotrienes are differ
somewhat in their structure; prostaglandins
contain a five-membered ring, which the
leukotrienes lack.
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23.9 Eicosanoids: Prostaglandins and Leukotrienes
• Prostaglandins and leukotrienes are synthesized from
arachidonic acid, which is itself synthesized from
linolenic acid.
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23.9 Eicosanoids: Prostaglandins and Leukotrienes
• Prostaglandins can lower blood pressure, influence
platelet aggregation during blood clotting, stimulate
uterine contractions, and lower the extent of gastric
secretions. In addition, they are responsible for some
of the pain and swelling that accompany inflammation.
• The anti-inflammatory and fever-reducing (antipyretic)
action of aspirin results in part from its inhibition of
prostaglandin synthesis.
• Leukotriene release has been found to trigger the
asthmatic response, severe allergic reactions, and
inflammation.
© 2013 Pearson Education, Inc.
Chapter Summary
1. What are the major classes of fatty acids and lipids?
• Fatty acids are carboxylic acids with long, straight
(unbranched) hydrocarbon chains; they may be
saturated or unsaturated.
• Waxes are esters of fatty acids and alcohols with long,
straight hydrocarbon chains.
• Fats and oils are triacylglycerols—triesters of glycerol
with fatty acids. In fats, the fatty acid chains are mostly
saturated; in oils, the proportions of unsaturated fatty
acid chains vary. Fats are solid because the saturated
hydrocarbon chains pack together neatly; oils are liquids
because the kinks at the cis double bonds prevent such
packing.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
2. What reactions do triacylglycerols undergo?
• The principal reactions of triacylglycerols are
catalytic hydrogenation and hydrolysis.
• Hydrogen adds to the double bonds of unsaturated
hydrocarbon chains in oils, thereby thickening the
consistency of the oils and raising their melting
points.
• Treatment of a fat or oil with a strong base
such as NaOH hydrolyzes the triacylglycerols
to give glycerol and salts of fatty acids. Such
saponification reactions produce soap, a mixture
of fatty acid salts.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
3. What are sterols?
•
The unifying feature of sterols is a fused four-ring
system.
•
Sterols include cholesterol, an important participant
in membrane structure.
•
Bile acids and salts, necessary for the emulsification
of fats during digestion, are synthesized from
cholesterol.
•
The third major group of sterols includes steroid
hormones, including the sex hormones, which
function as signaling molecules.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
4. What are the membrane lipids?
• The membrane lipids include phospholipids and
glycolipids (which have hydrophilic, polar head
groups and a pair of hydrophobic tails) and
cholesterol (a steroid).
• Phospholipids, which are either
glycerophospholipids (derived from glycerol) or
sphingomyelins (derived from the amino alcohol
sphingosine), have charged phosphate diester
groups in their hydrophilic heads.
• Sphingolipids, which are either sphingomyelins or
glycolipids, are sphingosine derivatives. The
glycolipids have carbohydrate head groups.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
5. What is the nature of a cell membrane?
• The basic structure of cell membranes is a bilayer of
lipids, with their hydrophilic heads in the aqueous
environment outside and inside the cells, and their
hydrophobic tails clustered together in the center of the
bilayer.
• Cholesterol molecules fit between the hydrophobic tails
and help maintain membrane structure and rigidity.
• The membrane also contains glycoproteins and
glycolipids (with their carbohydrate segments at the cell
surface, where they serve as receptors), as well as
proteins.
• Some of the proteins extend through the membrane
(integral proteins), and others are only partially
embedded at one surface.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
6.
How do substances cross cell membranes?
•
Small molecules and lipid-soluble substances can cross the lipid
bilayer by simply diffusing through it.
Ions and hydrophilic substances can move through aqueous fluidfilled channels in membrane proteins.
Some substances cross the membrane by binding to an integral
protein, which then releases them inside the cell.
These modes of crossing are all passive transport—they do not
require energy because the substances move from regions of higher
concentration to regions of lower concentration.
Passive transport takes the form of simple diffusion, crossing the
membrane by passing through it unimpeded, or facilitated diffusion,
crossing the membrane with the aid of a protein embedded in the
membrane.
Active transport, which requires energy and is carried out by certain
integral membrane proteins, moves substances against their
concentration gradients.
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Chapter Summary, Continued
7. What are eicosanoids?
• The eicosanoids are a group of compounds
derived from 20-carbon unsaturated fatty acids.
• They are local hormones—that is, they act near
their point of origin and are short-lived.
• Prostaglandins, which contain a five-membered
ring, have a wide range of actions (such as
stimulating uterine contractions and causing
inflammation).
• Leukotrienes, which do not contain a fivemembered ring, trigger the asthmatic response,
severe allergic reactions, and inflammation.
© 2013 Pearson Education, Inc.