Description
15.1 Lipids
Lipids are
• biomolecules that contain fatty acids
or a steroid nucleus
• soluble in organic solvents but not
in water
• named for the Greek word lipos,
which means “fat”
• an important feature in cell
membranes, fat-soluble vitamins,
and steroid hormones
Steroid Nucleus
Learning Goal Describe the classes of lipids.
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Types of Lipids
Lipids are characterized by their structures.
1. Lipids such as waxes, fats, oils, and phospholipids are esters
that can be hydrolyzed to give fatty acids and other molecules.
2. Steroids, also lipids, do not contain fatty acids and cannot be
hydrolyzed. They are characterized by the steroid nucleus of
four fused carbon rings.
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General Structure of Lipids
Lipids are naturally occurring compounds that are soluble in
organic solvents but not in water.
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Study Check
Lipids are characterized by the presence of fatty acids or a
steroid nucleus. Which type is described by the following?
A. contains a fused four-membered ring system
B. contains long carbon chains
C. includes carbonyl groups
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Solution
Lipids are characterized by the presence of fatty acids or a
steroid nucleus. Which type is described by the following?
A. contains a fused four-membered ring system
steroid
B. contains long carbon chains
fatty acid
C. includes carbonyl groups
fatty acid
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Study Check
Which of the following is NOT a characteristic of lipids?
A. may contain a carboxylic acid group
B. may contain a four-ring structure
C. soluble in water
D. mostly nonpolar
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Solution
Which of the following is NOT a characteristic of lipids?
A. may contain a carboxylic acid group
B. may contain a four-ring structure
C. soluble in water
D. mostly nonpolar
The answer is C.
Lipids are nonpolar and therefore are NOT soluble in water.
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15.2 Fatty Acids
We might think of saturated
fatty acids as chips with
regular shapes that stack
closely together in a can.
Similarly, irregularly shaped
chips would be like
unsaturated fatty acids that do
not fit closely together.
Learning Goal Draw the condensed structural formula for a
fatty acid and identify it as saturated or unsaturated.
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Fatty Acids
Fatty acids are
• long, unbranched carbon chains with a carboxylic acid
group at the end
• typically 12– to 18–carbon atoms long
• insoluble in water because of the long carbon chain
• saturated when they do not contain C C double bonds
in the carbon chain
• unsaturated when they contain C C double bonds in
the carbon chain
Core Chemistry Skill Identifying Fatty Acids
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Drawing Fatty Acids
• In a skeletal formula of a fatty acid, the ends and bends of
the line are the carbon atoms.
• The structural formula of lauric acid can be drawn in
several forms.
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Saturated Fatty Acids
Fatty acids can be saturated, with only C — C single bonds
in the carbon chain.
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Monounsaturated Fatty Acids
Fatty acids can be monounsaturated, with only one double
C C bond in the carbon chain.
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Polyunsaturated Fatty Acids
Fatty acids can be polyunsaturated, with at least two double
C C bonds in the carbon chain.
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Cis and Trans Unsaturated Fatty
Acids
• Unsaturated fatty acids can be drawn as cis and trans isomers.
• Oleic acid is a monounsaturated fatty acid found in olives, with
one double bond at carbon 9.
Almost all naturally occurring unsaturated fatty acids have one or more
cis double bonds.
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Essential Fatty Acids
Humans
• are capable of synthesizing some fatty acids from
carbohydrates or other fatty acids
• cannot synthesize sufficient amounts of polyunsaturated fatty
acids such as linoleic acid, linolenic acid, and arachidonic acid
Because these polyunsaturated fatty acids must be obtained from
the diet, they are known as essential fatty acids.
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Properties of Saturated Fatty Acids
Saturated fatty acids
• contain only single C — C bonds and fit close together in a
regular pattern
• have strong dispersion forces between long carbon chains
• have higher melting points and are usually solids at
room temperature
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Properties of Saturated Fatty Acids
In unsaturated fatty acids,
the cis double bonds cause
the carbon chain to bend or
kink, giving the molecules
an irregular shape and
allowing fewer interactions
between molecules.
The reduced interactions in fatty acids with cis bonds reduces the
melting point of the molecules.
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Study Check
Consider the condensed structural formula for oleic acid.
A. Why is this molecule an acid?
B. How many total carbon atoms are in oleic acid?
C. Is this a saturated, monounsaturated, or polyunsaturated
fatty acid?
D. Is it likely to be a solid or liquid at room temperature?
E. Would it be soluble in water?
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Solution
Consider the condensed structural formula for oleic acid.
A. Why is this molecule an acid?
It contains a carboxylic acid functional group.
B. How many total carbon atoms are in oleic acid?
Oleic acid contains 18 carbon atoms.
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Solution
Consider the condensed structural formula for oleic acid.
C. Is this a saturated, monounsaturated, or polyunsaturated
fatty acid?
Oleic acid is a monounsaturated fatty acid.
D. Is it likely to be a solid or liquid at room temperature?
It is a liquid at room temperature.
E. Would it be soluble in water?
The long carbon chain makes it insoluble in water.
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Prostaglandins
Prostaglandins
• are hormone-like substances produced in cells
• are also known as eicosanoids, formed from arachidonic acid,
a polyunsaturated fatty acid with 20 carbon atoms
• differ by the substituents attached to the five-carbon ring
• have many functions, such as lowering or raising blood
pressure and stimulating contraction and relaxation of
the smooth muscle of the uterus
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Prostaglandins: NSAIDs
Nonsteroidal anti-inflammatory drugs (NSAIDs) block
production of prostaglandins, decreasing pain and inflammation.
NSAIDs include naproxen (Aleve and Naprosyn), ketoprofen
(Actron), and nabumetone (Relafen).
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Chemistry Link to Health:
Omega-6 Fatty Acids
• Unsaturated fats such as those in vegetable oils and fish are
recognized as more beneficial to health than saturated fats.
• Vegetables contain omega-6 acids, meaning the first double
bond occurs at carbon 6 counting from the methyl end of the
carbon chain. Examples of omega-6 acids are linoleic and
arachidonic acids.
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Chemistry Link to Health:
Omega-3 Fatty Acids
Fish has high levels of omega-3 acids, meaning the first double
bond occurs at carbon 3, counting from the methyl end of the
chain. Examples of omega-3 acids include linolenic,
eicosapentaenoic, and docosahexaenoic acids.
Cold-water fish are a source of omega-3 fatty acids.
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Chemistry Link to Health:
Omega-3 Fatty Acids
In atherosclerosis and heart disease,
• cholesterol forms plaques that adhere to the walls of the
blood vessels
• blood pressure rises as blood has to squeeze through a smaller
opening in the blood vessel
• more plaque forms; there is also a possibility of blood clots
blocking the blood vessels and causing a heart attack
Omega-3 fatty acids lower the tendency of blood platelets to
stick together, reducing the possibility of blood clots.
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Study Check
Label each of the following fatty acids as saturated,
monounsaturated, or polyunsaturated.
A. linolenic acid
B. palmitoleic acid
C. stearic acid
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Solution
Label each of the following fatty acids as saturated,
monounsaturated, or polyunsaturated.
A. linolenic acid
polyunsaturated
B. palmitoleic acid
monounsaturated
C. stearic acid
saturated
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15.3 Waxes and Triacylglycerols
Natural waxes are found on the
surface of fruits and on the leaves
and stems of plants, where they help
prevent loss of water and damage
from pests.
Waxes on the skin, fur, and feathers
of animals provide a waterproof
coating.
Learning Goal Draw the condensed structural formula for a
wax or triacylglycerol produced by the reaction of a fatty acid
and an alcohol or glycerol.
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Waxes
Waxes are
• esters of saturated fatty acids and long-chain alcohols, each
containing from 14 to 30 carbon atoms
• coatings that prevent loss of water by leaves of plants
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Typical Waxes
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Triacylglycerols
In the body, fatty acids are stored as triacylglycerols
(triglycerides), which are
• esters of glycerol (a trihydroxy alcohol) and fatty acids
• formed when three hydroxyl groups of glycerol react with the
carboxyl groups of three fatty acids
• named by changing glycerol to glyceryl and naming the fatty
acids as carboxylates
Core Chemistry Skill Drawing Structures for Triacylglycerols
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Triacylglycerol: Glyceryl Tristearate
The name of the fatty acid formed with three stearic acids
becomes glyceryl tristearate.
The common name of this compound is tristearin.
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Mixed Triacylglycerols
Triacylglycerol may contain different fatty acids, such as the
triacylglycerol made from stearic, oleic, and palmetic acids.
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Triacylglycerols: Energy Storage
Triacylglycerols are the major form of energy storage for
animals.
• Animals that hibernate eat large quantities of plants, seeds,
and nuts that are high in calories.
• As the external temperature drops, the animal goes into
hibernation and the body temperature drops to nearly
freezing, reducing cellular activity, respiration, and heart rate.
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Triacylglycerols: Energy Storage
• During hibernation, the
animal’s stored fat in the
form of triacylglycerols is
its only source of energy.
Prior to hibernation, a polar bear
eats food with a high caloric content.
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Study Check
What are the fatty acids in the following triacylglycerol?
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Solution
What are the fatty acids in the following triacylglycerol?
stearic acid
oleic acid
myristic acid
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Drawing the Structure of a
Triacylglycerol
Draw the condensed structural formula for glyceryl
tripalmitoleate (tripalmitolein).
SOLUTION:
Glyceryl tripalmitoleate (tripalmitolein) is the triacylglycerol that
contains ester bonds between glycerol and three palmitoleic
acids acid molecules.
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Drawing the Structure of a
Triacylglycerol
Draw the condensed structural formula for glyceryl
tripalmitoleate (tripalmitolein).
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Study Check
Draw the condensed structural formula for the triacylglycerol
containing three molecules of myristic acid (14:0).
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Solution
Draw the condensed structural formula for the triacylglycerol
containing three molecules of myristic acid (14:0).
O
CH2—O—C—(CH2)12 —CH3
O
CH—O—C—(CH 2)12 —CH3
O
CH2—O—C—(CH2)12 —CH3
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Melting Points of Fats and Oils
A fat
• is usually solid at room temperature
• usually comes from animal sources such as meat, whole milk,
butter, and cheese
An oil
• is usually liquid at room temperature
• is usually obtained from a plant source such as palm oil and
coconut oil
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Oils with Unsaturated Fatty Acids
Oils from
• olive and peanut are monounsaturated; they contain large
amounts of oleic acid
• corn, cottonseed, safflower seed, and sunflower seed are
polyunsaturated because they contain large amounts of
fatty acids with two or more double bonds
• palm and coconut are solids at room temperature because
they consist mostly of saturated fatty acids
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Saturated and Unsaturated
Melting Points
Saturated fatty acids
• have higher melting
points than unsaturated
fatty acids because they
pack together more
tightly
• are usually found in
animal fats and in
coconut and palm oils
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Percent Saturated and Unsaturated
Fatty Acids in Fats and Oils
Vegetable oils have low melting points because they have a
higher percentage of unsaturated fatty acids than do animal fats.
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15.4 Chemical Properties of
Triacylglycerols
Many soft margarines, stick margarines, and solid shortenings
are produced by the partial hydrogenation of vegetable oils.
Learning Goal Draw the condensed structural formula for
the product of a triacylglycerol that undergoes
hydrogenation, hydrolysis, or saponification.
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Triacylglycerols: Hydrogenation
In hydrogenation reactions,
• double bonds in unsaturated fatty acids react with hydrogen
gas to produce carbon–carbon single bonds
• hydrogen gas is bubbled through the heated oil, typically in
the presence of a nickel catalyst
Core Chemistry Skill Identifying the Products for the
Hydrogenation, Hydrolysis, and Saponification of a
Triacylglycerol
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Hydrogenation Reactions
When hydrogen adds to all of the double bonds of glyceryl
trioleate (triolein) using a nickel catalyst, the product is the
saturated fat glyceryl tristearate (tristearin).
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Hydrogenation Reactions
In commercial hydrogenation,
• the addition of hydrogen is
stopped before all the double
bonds in a liquid vegetable oil
become completely saturated
• the partial hydrogenation of a
liquid vegetable oil changes it
to a soft, semisolid fat
• the more saturated product has
a higher melting point
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Study Check
What products are obtained from the complete hydrogenation
of glyceryl trioleate?
A. glycerol and three oleic acids
B. glyceryltristearate
C. glycerol and three stearic acids
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Solution
What products are obtained from the complete hydrogenation
of glyceryl trioleate?
B. glyceryltristearate
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Chemistry Link to Health:
Trans Fatty Acids
Unsaturated fatty acids can be
• cis, with bulky groups on the same side of C
• trans, with bulky groups on opposite sides of C
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C
C
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Hydrogenation: Trans Fatty Acids
During hydrogenation,
• double bonds are converted to C — C single bonds
• a small number of the cis double bonds are converted to more
stable trans double bonds, causing a change in the overall
structure of the fatty acids
In the body, trans fatty acids
• behave like saturated fatty acids
• are estimated to be 2–4% of our total calories
• raise LDL-cholesterol and lower HDL-cholesterol
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Hydrogenation: Trans Fatty Acids
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Trans Fatty Acids in Foods
Foods containing naturally occurring trans fatty acids include
milk, eggs, and beef.
Foods that contain trans fatty acids from the hydrogenation
process include
• deep-fried foods
• bread, baked goods, and cookies
• crackers and chips
• stick and soft margarines
• vegetable shortening
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Study Check
Which of the following statements are true and which are false?
A. There are more unsaturated fats in vegetable oils.
B. Vegetable oils have higher melting points than fats.
C. Hydrogenation of oils converts some cis double bonds to
trans double bonds.
D. Animal fats have more saturated fats.
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Solution
Which of the following statements are true and which are false?
A. True There are more unsaturated fats in vegetable oils.
B. False Vegetable oils have higher melting points than fats.
C. True Hydrogenation of oils converts some cis double bonds
to trans double bonds.
D. True Animal fats have more saturated fats.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Study Check
Draw the condensed structural formula for the product of a
glyceryl tripalmitoleate that undergoes hydrogenation.
O
CH2—O—C—(CH 2)7 —CH
O
CH—(CH 2)5 —CH3
CH2—O—C—(CH2)7 —CH
O
CH—(CH2)5 —CH3 + 3H2
CH2—O—C—(CH 2)7 —CH
CH—(CH 2)5 —CH3
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Ni
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Solution
Draw the condensed structural formula for the product of a
glyceryl tripalmitoleate that undergoes hydrogenation.
O
CH2—O—C—(CH 2)14 —CH3
O
+ 3H2
glyceryl
tripalmitoleate
Ni
CH2—O—C—(CH 2)14 —CH3
O
CH2—O—C—(CH 2)14 —CH3
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Hydrolysis
In hydrolysis,
• triacylglycerols split into glycerol and three fatty acids
• a strong acid, HCl or H2SO4, or digestive enzymes called
lipases are required
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Saponification and Soap
Saponification
• is the reaction of a fat with a strong base such as NaOH in
the presence of heat
• splits triacylglycerols into glycerol and the sodium salts of
fatty acids
• is the process of forming “soaps” (salts of fatty acids)
• with NaOH gives solid soaps that can be molded into
different shapes
• with KOH gives softer, liquid soaps
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Saponification and Soap
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Organic and Lipid Reactions
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Study Check
Write the equation for the reaction catalyzed by the enzyme
lipase that hydrolyzes glyceryl trilaurate (trilaurin) during the
digestion process.
O
CH2—O—C—(CH2)10 —CH3
O
Lipase
CH2—O—C—(CH2)10 —CH3 + 3H2O
O
CH2—O—C—(CH2)10 —CH3
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Solution
Write the equation for the reaction catalyzed by the enzyme
lipase that hydrolyzes glyceryl trilaurate (trilaurin) during the
digestion process.
O
CH2—OH
glyceryl
trilaurate
Lipase
+ 3H2O
CH2—OH
HO—C—(CH2)10 —CH3
O
+
HO—C—(CH2)10 —CH3
O
CH2—OH
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HO—C—(CH2)10 —CH3
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15.5 Phospholipids
The phospholipids are a
family of lipids similar in
structure to
triacylglycerols; they
include
glycerophospholipids
and sphingomyelin.
Fatty acid
G
l
y
c
e
r
o
l
Glycerophospholipids are
major constituents of
membranes and are
responsible for the
membrane being a
bilayer.
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Fatty acid
PO4
Amino
alcohol
Glycerophospholipid
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Glycerophospholipids
Glycerophospholipids contain
• two fatty acids that form
ester bonds with the first and
second hydroxyl groups of
glycerol
• a hydroxyl group that forms
an ester with phosphoric
acid, which forms another
phosphoester bond with an
amino alcohol
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
Fatty acid
G
l
y
c
e
r
o
l
Fatty acid
PO4
Amino
alcohol
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Sphingomyelin
Sphingomyelin
• contains sphingosine
instead of glycerol
• contains a fatty acid,
phosphate, and an
amino alcohol
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
S
p
h
i
n
g
o
s
i
n
e
Fatty acid
PO4
Amino
alcohol
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Amino Alcohols
Amino alcohols found in glycerophospolipids
• are choline, erine, and ethanolamine
• are ionized at physiological pH of 7.4
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Lecithin and Cephalin
Lecithin and cephalin are types of glycerophospholipids that are
• abundant in brain and nerve tissues
• found in egg yolk, wheat germ, and yeast
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Structure and Polarity of a
Glycerophospholipid
Glycerophospholipids
• have both polar and nonpolar regions that allow them to
interact with polar and nonpolar substances
• have a polar head containing the ionized amino alcohol
and phosphate portion, which is strongly attracted to
water
• have a nonpolar hydrocarbon tail portion soluble only in
nonpolar substances such as lipids
• are the most abundant lipids in cell membranes and play
an important role in cellular permeability
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Structure and Polarity of a
Glycerophospholipid
(a) The components of a typical
glycerophospholipid: an amino alcohol,
phosphoric acid, glycerol, and two fatty
acids. (b) In a glycerophospholipid, a polar
“head” contains the ionized amino alcohol
and phosphate, while the hydrocarbon
chains of two fatty acids make up the
nonpolar “tails.” (c) A simplified drawing
indicates the polar region and the nonpolar
region.
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Study Check
Identify each of the following as a fatty acid, a triacylglycerol, an
amino alcohol, or a glycerophospholipid.
A. glyceryl trioleate
B. cephalin
C. choline
D. palmitic acid
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Solution
Identify each of the following as a fatty acid, a triacylglycerol, an
amino alcohol, or a glycerophospholipid.
A. glyceryl trioleate
B. cephalin
triacylglycerol
glycerophospholipid
C. choline
D. palmitic acid
amino alcohol
fatty acid
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Study Check
Draw the condensed structural formula of a cephalin that contains
stearic acid and serine.
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Solution
Draw the condensed structural formula of a cephalin that contains
stearic acid and serine.
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Sphingomyelin
In a sphingomyelin,
• the amine group of sphingosine forms an amide bond to a
fatty acid
• the hydroxyl group forms an ester bond with phosphate, which
forms another phosphoester bond to choline or ethanolamine
Sphingomyelins are abundant in the white matter of the myelin
sheath, a coating surrounding the nerve cells that increases the
speed of nerve impulses and insulates and protects the nerve cells.
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Sphingomyelin
Structure of a sphingomyelin containing myristic acid and
choline.
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Study Check
Palmitic acid, the 16-carbon saturated fatty acid, is the most
common fatty acid found along with the ionized amino alcohol
choline in the sphingomyelin of eggs.
Draw the condensed structural formula for this sphingomyelin.
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Solution
Palmitic acid, the 16-carbon saturated fatty acid, is the most
common fatty acid found along with the ionized amino alcohol
choline in the sphingomyelin of eggs.
Draw the condensed structural formula for this sphingomyelin.
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15.6 Steroids: Cholesterol, Bile Salts,
and Steroid Hormones
High- and low-density lipoproteins transport cholesterol
between the tissues and the liver.
Learning Goal Describe the structures of steroids.
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Steroid Nucleus
A steroid nucleus consists of
• three cyclohexane rings and one
cylopentane ring, fused together
• rings designated as A, B, C,
and D
• numbered carbon atoms
beginning in ring A
• two methyl groups at positions
18 and 19
Steroid nucleus
Core Chemistry Skill Identifying the Steroid Nucleus
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Cholesterol
Cholesterol
• is the most important and
abundant steroid in the body
• has an hydroxyl group (— OH)
on (carbon 3)
• has a double bond between
carbons 5 and 6
• has methyl groups at carbons 10
and 13
• has an alkyl chain at carbon 17
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
Cholesterol
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Cholesterol in the Body
Cholesterol
• is obtained from meats, milk,
and eggs
• is synthesized in the liver
• is needed for cell membranes,
brain and nerve tissue, steroid
hormones, and vitamin D
• clogs arteries when high levels
form plaque
Cross-sections of arteries show
how plaque clogs the arteries.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
A normal, open artery.
An artery clogged by
cholesterol plaque
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Cholesterol in Foods
Cholesterol
• is considered elevated if plasma cholesterol exceeds 200 mg/dL
• is synthesized in the liver and obtained from foods
A diet that is low in foods containing cholesterol and saturated fats
appears to be helpful in reducing the serum cholesterol level.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Cholesterol in Foods
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Study Check
Match the components of the cholesterol molecule with
the following:
___ carbon chain
___ hydroxyl group
___ steroid nucleus
___ methyl group
D
B
A
C
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Solution
Match the components of the cholesterol molecule with
the following:
D carbon chain
A hydroxyl group
C steroid nucleus
B methyl group
D
B
A
C
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Bile Salts
Bile salts
• are synthesized in the liver from
cholesterol and stored in the
gallbladder
• have polar and nonpolar regions
that act like soaps to make fat
soluble in water
• help in absorption of cholesterol
When large amounts of cholesterol
accumulate in the gallbladder,
gallstones are formed.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
Gallstones form in the gallbladder
when cholesterol levels are high.
© 2015 Pearson Education, Inc.
Bile Salts
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Lipoproteins: Lipid Transport
Lipids are nonpolar and made more soluble by combining them
with glycerophospholipids and proteins to form water-soluble
complexes called lipoproteins.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Lipoproteins
Lipoproteins
• surround nonpolar
lipids with polar
lipids and protein
for transport
to cells
• are soluble in
water because the
surface consists of
polar lipids
A spherical lipoprotein particle
surrounds nonpolar lipids with polar
lipids and protein for transport to
body cells.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Types of Lipoproteins
Lipoproteins
• differ in density, composition, and function
• include low-density lipoproteins (LDLs) and high-density
lipoproteins (HDLs)
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Transport of Lipoproteins in the Body
Lipoproteins such as HDLs and LDLs transport nonpolar lipids and cholesterol
to cells and the liver.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Chylomicrons
•Chylomicrons are the largest and most dense lipoprotein.
Chylomicrons enter the lymphatic system and travel throughout
the body, breaking down and getting smaller as triglycerides are
detached from the chylomicron and distributed to cells and
adipose sites. Most matter in chylomicrons is used during this
process, and the remaining matter in the chylomicron returns to
the liver and is recycled as more triglyceride or is inserted into
another lipoprotein.
Very Low-Density Lipoproteins
•Very low-density lipoproteins (VLDL) travel throughout the body.
VLDLs have a smaller concentration of fat to protein at the beginning,
with more protein than fat. However, as they travel throughout the
body and give off matter, the concentration of cholesterol to protein
increases, and they essentially become low-density lipoproteins
(LDL) once they increase their concentration of fat to protein enough.
Low-Density Lipoprotein
•Low-density lipoproteins, or LDLs, transport cholesterol to
replenish cell cholesterol supply. LDLs are then lysed in the liver
and secreted into the gallbladder and the intestinal system for
waste. Excess LDLs in the bloodstream may break down and
release cholesterol into bloodstream, causing plaquing of the
arteries. A high ratio of LDLs to HDLs is more than 4 to 1, while a
normal ratio is around 3 to 1.
High-Density Lipoproteins
•High-density lipoproteins (HDLs) transport excess cholesterol
out of the bloodstream and bring it into liver, where it is
destroyed or recycled. HDLs are seen as “healthy” lipoproteins
because of their “job” of clearing cholesterol out of the
bloodstream.
Steroid Hormones
Steroid hormones are
• chemical messengers that serve as a communication system
for the body
• produced from cholesterol
• male sex hormones, testosterone and androsterone
• female sex hormones, estrogens and progesterone
• adrenal corticosteroids from adrenal glands
– mineralocorticoids (electrolyte balance)
– glucocorticoids (regulate glucose level)
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Structures of Steroid Hormones
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Chemistry Link to Health:
Anabolic Steroids
Anabolic steroids
• are derivatives of testosterone
• are used illegally to increase muscle mass
• have side effects including fluid retention, hair growth, sleep
disturbance, and liver damage
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Adrenal Corticosteroids
Steroid hormones called adrenal corticosteroids
• are produced by the adrenal glands located on the top
of each kidney
• include aldosterone, which regulates electrolytes and
water balance by the kidneys
• include cortisone, a glucocorticoid, which increases
blood glucose level and stimulates the synthesis of
glycogen in the liver
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Adrenal Corticosteroids
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Study Check
Identify each of the following as a fatty acid, steroid,
or triacylglycerol.
A. cholesterol
B. stearic acid
C. glyceryl tristearate
D. estradiol
E. contains no fatty acids
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Solution
Identify each of the following as a fatty acid, steroid,
or triacylglycerol.
A. cholesterol
B. stearic acid
steroid
fatty acid
C. glyceryl tristearate
D. estradiol
E. contains no fatty acids
triacylglycerol
steroid
steroid
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Chapter 16 Amino Acids, Proteins,
and Enzymes
Many physician assistants take on
the role of the primary caregiver.
Their duties may include
• obtaining patient medical
records and histories
• diagnosing illnesses
• educating and counseling
patients
• referring the patient, when
needed, to a specialist
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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16.1 Proteins and Amino Acids
Protein molecules, compared
with many of the compounds
we have studied, can be
gigantic.
The horns of animals are
made of proteins.
Learning Goal Classify proteins by their functions. Give the
name and abbreviation for an amino acid and draw its
ionized structure.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Functions of Proteins
Proteins
• in the body are polymers made from 20 different amino acids
• differ in characteristics and functions that depend on the
order of amino acids that make up the protein
• form structural components such as cartilage, muscles, hair,
and nails
• function as enzymes to regulate biological reactions such as
digestion and cellular metabolism
• such as hemoglobin and myoglobin transport oxygen in
the blood
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Structural Classification of Proteins
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Amino Acids
Amino acids, the molecular building blocks of proteins,
• have a central carbon atom called the α-carbon bonded to
two functional groups: an ammonium group (— NH3+) and a
carboxylate group (— COO−)
• have a central carbon atom bonded to a hydrogen atom and
R group or side chain in addition to the carboxylate and
ammonium groups
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Classification of Amino Acids
Amino acids are classified as
• nonpolar (hydrophobic) with hydrocarbon side chains
• polar (hydrophilic) with polar or ionic side chains
Nonpolar
Valine
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
Polar
Asparagine
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Nonpolar Amino Acids
An amino acid is nonpolar when the R group is H, alkyl,
or aromatic.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Polar Amino Acids
An amino acid is polar when the R group is an alcohol, a thiol,
or an amide.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Structural Formulas of Amino Acids
An amino acid has
• an α-carbon atom that is attached to three components:
— NH3+, — COO−, and — H group
• a fourth component, an R group that differs for each
particular amino acid (see Table 16.2)
• a three-letter or one-letter abbreviation derived from its name
Core Chemistry Skill Drawing the Ionized Form for an
Amino Acid
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Drawing Amino Acids
• All amino acids have — NH3+, — COO−, and — H on the
α-carbon.
• Amino acids differ by their R groups.
O–
O
O
C
CH2
NH2
C
R Group
+
H3 N––C––COO –
H
Aspartic acid (Asp, D)
pH 2.8
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
CH2
+
H3 N––C––COO –
H
Asparagine (Asn, N)
pH 5.4
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Acidic Amino Acids
An amino acid is acidic when the R group is a carboxylic acid.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Basic Amino Acids
An amino acid is basic when the R group is an amine.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Study Check
Identify each of the following amino acids as polar or nonpolar:
+
A. H3N––CH2––COO –
Glycine
CH3
CH––OH
+
B. H3N––CH––COO –
Threonine
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Solution
Identify each of the following amino acids as polar or nonpolar.
+
A. H3N––CH2––COO –
Glycine
nonpolar
Threonine
polar
CH3
CH––OH
+
B. H3N––CH––COO –
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Study Check
Draw the ionized form for each of the following amino acids,
and write the three-letter and one-letter abbreviations:
A. valine
B. cysteine
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Solution
Draw the ionized form for each of the following amino acids,
and write the three-letter and one-letter abbreviations:
CH3 CH3
A. valine
(Val, V)
CH
+
H3N––C––COO –
SH
H
CH2
+
B. cysteine
(Cys, C)
–
H3N––C––COO
H
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Amino Acid Stereoisomers
All the α-amino acids except
for glycine are chiral.
• The α-carbon is attached
to four different atoms.
• The — NH3+ group
appears on the right or
left of the chiral carbon to
give D or L enantiomers.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Study Check
Identify the following amino acids as D or L enantiomers:
A.
COO –
+
H3N––C––H
B.
H
CH2
COO –
+
C ––NH3
CH2
OH
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Solution
Identify the following amino acids as D or L enantiomers:
A.
COO –
+
H3N––C––H
B.
CH2
H
COO –
+
C ––NH3
CH2
OH
L-Phenylalanine
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
D-Serine
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Chemistry Link to Health:
Essential Amino Acids
Of the 20 amino acids used to build the proteins in the body,
• only 11 can be synthesized in the body
• the other 9 amino acids are essential amino acids that must
be obtained from the proteins in the diet
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Chemistry Link to Health:
Essential Amino Acids
Complete proteins such as
eggs, milk, meat, and fish
contain all of the essential
amino acids.
Incomplete proteins from
plants such as grains, beans,
and nuts are deficient in one
or more essential amino acids.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Isoelectric Point
The isoelectric point of an amino acid is the pH at which
• the charged groups on an amino acid are balanced
• the amino acid is neutral
An amino acid can exist as
• a positive ion if a solution is more acidic (lower pH)
than its pI
• as a negative ion if a solution is more basic (higher pH)
than its pI
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
© 2015 Pearson Education, Inc.
Ionized Forms of Amino Acids
The pI values for nonpolar and
and polar neutral amino acids
are from pH 5.1 to 6.3.
Alanine has a zero overall charge at its pI of 6.0 with a carboxylate
anion (— COO−) and an ammonium cation (— NH3+).
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Twelfth Edition
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Ionized Forms of Amino Acids
Alanine adds an H+ to the carboxyl group (— COO−) when the
solution is more acidic than its pI (pH
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