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Chapter 15
Lipids
15.1 Lipids
15.2 Fatty Acids
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.”
• extracted from cells using organic solvents.
2
Types of Lipids
The types of lipids containing fatty acids are
• waxes
• fats and oils called triglycerides (triacylglycerols)
• glycerophospholipids
• prostaglandins
The types of lipids that do not contain fatty acids are
• steroids
3
Classes of Lipids
Triglycerides
4
Fatty Acids
Fatty acids are
• long-chain carboxylic acids.
• typically 12-18 carbon atoms.
• insoluble in water.
• saturated or unsaturated.
Olive oil contains 84%
unsaturated fatty acids and
16% saturated fatty acids.
5
Saturated and Unsaturated Fatty
Acids
Fatty acids are
• saturated with
all single C–C
bonds.
• unsaturated
with one or
more double
C=C bonds.
O
C OH
palmitic acid, a saturated acid
O
C OH
palmitoleic acid, an unsaturated acid
6
Properties of Saturated Fatty Acids
Saturated fatty acids
• contain only single C–C bonds.
• are closely packed.
• have strong attractions between chains.
• have high melting points.
• are solids at room temperature.
COOH
COOH
COOH
7
Properties of Unsaturated Fatty
Acids
Unsaturated fatty acids
• contain one or more cis double C=C
bonds.
• have “kinks” in the fatty acid chains.
• do not pack closely.
• have few attractions between chains.
• have low melting points.
• are liquids at room temperature.
HOOC
H
H C
COOH
C
“kinks” in
chain
H
C
C
H
8
Melting Points of Some Saturated
Fatty Acids
9
Melting Points of Some
Unsaturated Fatty Acids
10
Learning Check
Assign the melting points of –17 °C, 13 °C, and 69 °C
to the correct fatty acid. Explain.
stearic acid (18 C)
oleic acid
(18 C)
linoleic acid (18 C)
saturated
one double bond
two double bonds
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Solution
Stearic acid is saturated and would have a higher
melting point than the unsaturated fatty acids.
Because linoleic has two double bonds, it would have
a lower mp than oleic acid, which has one double
bond.
stearic acid mp 69 °C
saturated
oleic acid
mp 13 °C
linoleic acid mp -17 °C most unsaturated
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Chapter 15
Lipids
15.3
Waxes, Fats, and Oils
13
Waxes
O
C
O
Waxes are
• esters of saturated fatty acids and long-chain alcohols.
• coatings that prevent loss of water by leaves of plants.
(pronounced “ho-HO bah”
14
Fats and Oils: Triglycerides or
Triacylglycerols
Fats and oils are
• also called triglycerides.
• esters of glycerol.
• produced by esterification.
• Formed when the hydroxyl
groups of glycerol react with the
carboxyl groups of fatty acids.
15
Triglycerides (Triacylglycerols)
In a triglyceride, glycerol forms ester bonds with three
fatty acids.
Ester Bonds
16
Formation of a Triglyceride
Glycerol
+
three fatty acids
triglyceride
O
CH2
OH
CH
OH
CH2
OH
+
HO C
O
(CH2)14CH3
HO C
O
(CH2)14CH3
HO C
(CH2)14CH3
O
CH2 O C
(CH2)14CH3
O
CH O C
(CH2)14CH3 + 3H2O
O
CH2 O C
(CH2)14CH3
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Melting Points of Fats and Oils
A fat
• is usually solid at room temperature.
• is prevalent in meats, whole milk, butter, and cheese.
An oil
• is usually liquid at room temperature.
• is prevalent in plants such as olive and safflower.
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Oils with Unsaturated Fatty Acids
Oils
• have more unsaturated fats.
• have cis double bonds that cause “kinks” in the fatty
acid chains.
• with “kinks” in the chains do not allow the
triacylglycerol molecules to pack closely.
• have lower melting points than saturated fatty acids.
• are liquids at room temperature.
19
Diagram of Triglyceride with
Unsaturated Fatty Acids
Unsaturated
fatty acid
chains with
kinks cannot
pack closely.
20
21
Percent Saturated and Unsaturated
Fatty Acids In Fats and Oils
22
7
8
9
61
32
77
15
16
75
12
16
72
13
29
58
15
15
19
27
43
51
68
91
75
23
10
62
33
48
54
19
47
10
11
38
4
28
7
2
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Chapter 15
Lipids
15.4
Chemical Properties of Triglycerides
24
Chemical Properties of
Triglycerides
The chemical reactions of triglycerides are similar
to those of alkenes and esters.
• In hydrogenation, double bonds in unsaturated
fatty acids react with H2 in the presence of a Ni or
Pt catalyst.
• In hydrolysis, ester bonds are split by water in
the presence of an acid, a base, or an enzyme.
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Hydrogenation of Oils
The hydrogenation of oils
• adds hydrogen (H2) to
the carbon atoms of
double bonds.
• converts double bonds
to single bonds.
• increases the melting
point.
• produces solids, such as
margarine and shortening.
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Hydrogenation
O
CH2
O
C
Ni
(CH2)5CH CH(CH2)7CH3
+ 3H2
O
CH
O
CH2
O
C (CH2)5CH CH(CH2)7CH3
CH2
O
C
(CH2)5CH CH(CH2)7CH3
Glyceryl tripalmitoleate
(tripalmitolean)
CH
CH2
O
O
C
(CH2)14CH3
O
O
O
C (CH2)14CH3
O
C
(CH2)14CH3
Glyceryl tripalmitate
(tripalmitin)
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OlestraTM, a Fat Substitute
Olestra is
• used in foods as an artificial fat.
• sucrose linked by ester bonds to several long-chain fatty
chains.
• not broken down in the intestinal tract.
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Cis and Trans Fatty Acids
Unsaturated fatty acids can be
• cis with bulky groups on same side of C=C.
CH3─ (CH2)5
(CH2)7─ COOH cis
C=C
H
H
• trans have bulky groups on opposite sides of C=C.
CH3─ (CH2)5
H
C=C
H
trans
(CH2)7─ COOH
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Hydrogenation and Trans Fatty
Acids
Most naturally occurring fatty acids have cis double
bonds.
• During hydrogenation, some cis double bonds are converted to
trans double bonds.
• In the body, trans fatty acids behave like saturated fatty acids.
• It is estimated that 2-4% of our total Calories is in the form of
trans fatty acid.
• Several studies reported that trans fatty acids raise LDLcholesterol and lower HDL-cholesterol.
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Hydrogenation of Unsaturated
Fats
31
Learning Check
(1) True or (2) 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
(1) True or (2) false:
A. T There are more unsaturated fats in vegetable oils.
B. F Vegetable oils have higher melting points than fats.
C. T Hydrogenation of oils converts some cis-double
bonds to trans- double bonds.
D. T Animal fats have more saturated fats.
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Hydrolysis
In hydrolysis,
• triglycerides split into glycerol and three fatty acids.
• an acid or enzyme catalyst is required.
O
CH2
CH
CH2
O
O
O
C
O
(CH2)14CH3
C (CH2)14CH3 + H2O
O
C
(CH2)14CH3
H+
CH2
OH
CH
OH
CH2
OH
O
+
HO C
(CH2)14CH3
34
Saponification and Soap
Saponification
• is the reaction of a fat with a strong base.
• splits triacylglycerols into glycerol and the salts of fatty
acids.
• is the process of forming “soaps” (salts of fatty acids).
• with KOH gives softer soaps.
35
Saponification
O
CH2 O C
(CH2)14CH3
O
CH O C
(CH2)14CH3
O
CH2 O C
+ 3NaOH
CH2 OH
(CH2)14CH3
CH OH
O
+ 3Na+ -O C (CH2)14CH3
“soap”
CH2 OH
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Learning Check
Write the product of the following reaction:
+3
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Solution
O
CH2
O
C (CH2)14CH3
O
CH
O
C (CH2)14CH3
O
CH2
O
C
(CH2)14CH3
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Chapter 15
Lipids
15.5
Glycerophospholipids
Fatty acid
Glycerol
Fatty acid
PO4
Amino
alcohol
39
Glycerophospholipids
Glycerophospholipids are
• the most abundant lipids in cell membranes.
• composed of glycerol, two fatty acids, phosphate,
and an amino alcohol.
Fatty acid
Glycerol
Fatty acid
PO4
Amino
alcohol
40
Polarity of Glycerophospholipids
A glycerophospholipid has
• two nonpolar fatty acid chains.
• a phosphate group and a polar amino alcohol.
CH3
│+
+
HO−CH2−CH2−N−CH3
HO−CH2−CH2−NH3
│
Choline
CH3
Ethanolamine
+
NH3
│
HO−CH2−CH−COO− Serine
Amino alcohols
41
Structure and Polarity of a
Glycerophospholipid
42
Lecithin and Cephalin
Lecithin and cephalin are glycerophospholipids
• abundant in brain and nerve tissues.
• found in egg yolk, wheat germ, and yeast.
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Chapter 15
Lipids
15.6
Steroids: Cholesterol and Steroid
Hormones
CH3
CH3
CH3
CH3
CH3
HO
44
Steroid Nucleus
A steroid nucleus
consists of
• 3 cyclohexane rings.
• 1 cyclopentane ring.
• no fatty acids.
steroid nucleus
45
Cholesterol
Cholesterol
• is the most abundant steroid in the body.
• has methyl CH3- groups, alkyl chain, and -OH
attached to the steroid nucleus.
CH3
CH3
CH3
CH3
CH3
HO
46
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.
A normal, open artery.
An artery clogged
by cholesterol
plaque
47
Cholesterol in Foods
Cholesterol
• is considered
elevated if
plasma
cholesterol
exceeds 200
mg/dL.
• is synthesized in
the liver and
obtained from
foods.
48
Learning Check
Match the components of the cholesterol molecule with
the following:
___ carbon chain
___hydroxyl group
___ steroid nucleus
___methyl group
CH3
CH3
B
D
CH3
CH3
CH3
A
HO
C
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Solution
Match the components of the cholesterol molecule with
the following:
_D carbon chain
_A_hydroxyl group
_C steroid nucleus
_B_methyl group
CH3
CH3
B
D
CH3
CH3
CH3
A
HO
C
50
Solution
Identify each lipid as a
(1) fatty acid, (2) steroid, or (3) triglyceride.
A. 2 cholesterol
B. 1 stearic acid
C. 3 glyceryl tristearate
D. 2 contains no fatty acids
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