Transcript Chapter 15 - faculty at Chemeketa

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.
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Types of Lipids
The types of lipids containing fatty acids are
• waxes
• fats and oils (triacylglycerols)
• glycerophospholipids
• prostaglandins
The types of lipids that do not contain fatty acids are
• steroids
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Classes of Lipids
Triglycerides
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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.
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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
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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
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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
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Melting Points of Some Saturated
Fatty Acids
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Melting Points of Some
Unsaturated Fatty Acids
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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
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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”
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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.
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Triglycerides (Triacylglycerols)
In a triglyceride, glycerol forms ester bonds with three
fatty acids.
Ester Bonds
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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.
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Diagram of Triglyceride with
Unsaturated Fatty Acids
Unsaturated
fatty acid
chains with
kinks cannot
pack closely.
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21
Percent Saturated and Unsaturated
Fatty Acids In Fats and Oils
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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
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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
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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
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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.
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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
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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
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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
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Structure and Polarity of a
Glycerophospholipid
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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
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Steroid Nucleus
A steroid nucleus
consists of
• 3 cyclohexane rings.
• 1 cyclopentane ring.
• no fatty acids.
steroid nucleus
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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
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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.
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.
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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
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Lipoproteins
Lipoproteins
• combine lipids
with proteins
and
phospholipids.
• are soluble in
water because
the surface
consists of
polar lipids.
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Types of Lipoproteins
Lipoproteins
• differ in density, composition, and function.
• include low-density lipoproteins (LDLs) and highdensity lipoproteins (HDLs).
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Steroid Hormones
Steroid hormones are
• chemical messengers in cells.
• sex hormones.
- androgens in males (testosterone)
- estrogens in females (estradiol)
• Adrenocortical hormones from adrenal glands.
- mineralocorticoids (electrolyte balance)
- glucocorticoids (regulate glucose level)
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Steroid Hormones
Steroid hormones
• are produced from
cholesterol.
• include sex
hormones such as
androgens
(testosterone) in
males and estrogens
(estradiol) in
females.
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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.
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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.
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Adrenal Corticosteroids
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Learning Check
Identify each lipid as a
(1) fatty acid, (2) steroid, or (3) triglyceride.
A. cholesterol
B. stearic acid
C. glyceryl tristearate
D. contains no fatty acids
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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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Chapter 15
Lipids
15.7
Cell Membranes
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Cell Membranes
Cell membranes
• separate cellular contents from the external
environment.
• consist of a lipid bilayer made of two rows of
phospholipids.
• have an inner portion made of the nonpolar tails of
phospholipids with the polar heads at the outer and
inner surfaces.
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Fluid Mosaic Model of Cell
Membranes
The lipid bilayer
• contains proteins, carbohydrates, and cholesterol.
• has unsaturated fatty acids that make cell
membranes fluid-like rather than rigid.
• has proteins and carbohydrates on the surface
that communicate with hormones and
neurotransmitters.
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Fluid Mosaic Model of Cell
Membranes
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Transport Through Cell Membranes
The transport of substances through cell membranes
involves
• simple diffusion (passive transport), which
moves particles from a higher to a lower
concentration.
• facilitated transport, which uses protein channels
to increase the rate of diffusion.
• active transport, which moves ions against a
concentration gradient.
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Transport Pathways Through Cell
Membranes
Copyright © 2009 by Pearson Education, Inc.
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Learning Check
The transport of particles across a cell membrane from
high concentration to low concentration is called
1. facilitated transport.
2. simple diffusion.
3. active transport.
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Solution
The transport of particles across a cell membrane from
high concentration to low concentration is called
2. diffusion.
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