copyright © houghton mifflin company. all rights reserved.3–13–1 15.1 intro to lipids lipids...
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15.1 Intro to Lipids
Lipids are biochemical molecules that are insoluble or sparingly soluble in water, but are soluble in nonpolar organic solvents.
Uses of lipids:
Storage of energy
Insulation
Protection from mechanical damage
Major component of cell membranes
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15.1 Intro to Lipids
Types of lipids:
I. Saponifiable lipids contain fatty acids bound as esters. Saponification (soap-making) is hydrolysis of the esters and frees the fatty acids. These lipids can be classified as polar or nonpolar.
II. Nonsaponifiable lipids do not contain fatty acids bound as esters.
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15.2 Structures of Fatty Acids
Fatty acids are carboxylic acids with un-branched hydrocarbon chains. They contain an even number of carbon atoms.
Fatty acids can be
saturated: contain no double bonds
monunsaturated: contain one double bond
polyunsaturated: contain several double bonds
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15.2 Structures of Fatty AcidsThe double bonds in naturally-occurring fatty
acids have cis double bonds. These put kinks in the hydrocarbon backbone.
Two of the polyunsaturated fatty acids are essential in the diet; they cannot be syn-thesized by the body. Both contain 18 carbon atoms. Fish and vegetable oils are dietary sources of these fatty acids.
Linoleic acid (2 double bonds)
Linolenic acid (3 double bonds)
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15.2 Structures of Fatty Acids
Stearic acid
Oleic acid
Linoleic acid Linolenic acid
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15.2 Structures of Fatty Acids
C
O
OH
C
O
OH
Stearic acid, a saturated fatty acid
Oleic acid, a monounsaturated fatty acid
C
O
OH
Linoleic acid, an Ω-6 polyunsaturated acid
C
O
OH
Linolenic acid, an Ω-3 polyunsaturated acid
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15.3 Properties of Fatty Acids
The melting points of fatty acids depend on the num-ber of carbon atoms and the degree of unsaturation.
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15.3 Properties of Fatty Acids
The bent shapes of unsaturated fatty acids pre-vent them from fitting together and forming high-melting solids.
Fatty acids with trans double bonds are found in “trans fats.” The trans double bonds are formed when vegetable oils are processed. These fatty acids have higher melting points than the cis isomers.
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15.3 Properties of Fatty Acids
Melting points: 70C 13C 45C
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15.4 TriglyceridesTriglycerides are triesters. They are products
of the reaction of glycerol and fatty acids. They are also called triacylglycerols, and are the nonpolar saponifiable lipids.
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15.4 TriglyceridesSimple triglycerides contain three identical
fatty acids. Mixed triglycerides contain three different fatty acids.
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15.4 Triglycerides
Triglycerides are the most abundant lipids, and are used for storage of energy.
Solid or semisolid triglycerides are called fats. They contain mainly saturated fatty acids. They are found in meats and tropical nuts.
Liquid triglycerides are called fats. They contain mainly unsaturated fatty acids. They are found in fish and plants.
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15.4 Triglycerides
The kinks associated with unsaturated fatty acids prevent the molecules from packing neatly and forming solids.
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15.4 Triglycerides
Fatty acid contents of common fats and oils.
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15.5 Reactions of Triglycerides
Complete hydrolysis of triglycerides produces the triol, glycerol, and free fatty acids.
Metabolic hydrolysis requires enzymes from the pancreas. Usually, two of the fatty acids are hydrolyzed, leaving a monoglyceride.
Triglyceride + 3 H2O Glycerol + 3 Fatty Acids
Triglyceride + 2 H2OMonoglyceride+ 2 Fatty Acids
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15.5 Reactions of TriglyceridesSaponification is hydrolysis of triglycerides in
basic solution. The fatty acid is obtained as its carboxylate salt, or soap.
The carboxylate anion has a polar end and a nonpolar hydrocarbon chain. The polar end is hydrophilic (water-loving) and the nonpo-lar chain is hydrophobic (water-fearing.)
Triglyceride + 3 OH1− Glycerol + 3 Carboxylates
CO
Opolar
"head"nonpolar "tail"
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15.5 Reactions of TriglyceridesThe two parts of the molecules associate
to form micelles and other structures.
From Wikipedia
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15.6 PhosphoglyceridesPhosphoglycerides are polar saponifiable
lipids. Two of the alcohols on glycerol are esterified with fatty acids. The third is esterified with phosphoric acid, which is esterified with an aminoalcohol.
CH
CH2
CH2
OC
O
R2
O C
O
R1
O P
O
O R3
O
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15.6 Phosphoglycerides
Different types of phosphoglycerides contain different aminoalcohols.
Phosphatadylcholines, a.k.a. lecithins, contain choline
HO CH2 CH2 N
CH3
CH3
CH3
Cephalins contain ethanolamine or serine
HO CH2 CH2 N
H
H
H
HO CH2 CH N
H
H
H
COO
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15.6 PhosphoglyceridesThe phosphate ester of lecithins is quite
polar. It has a charged phosphate group and a quaternary amine.
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15.6 Phosphoglycerides
A space-filling drawing shows its polar head and two nonpolar tails.
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15.6 Phosphoglycerides
Phosphatidylcholines form micelles and are good emulsifiers.
They are major components of cell membranes.
They are abundant in egg yolk and soybeans.
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15.6 Phosphoglycerides
Cephalins are found in heart, liver, and brain tissue.
They are important in the clotting of blood.
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15.7 Sphingolipids
Sphingolipids are polar saponifiable lipids based on sphingosine rather than glycerol.
A fatty acid is bonded to the nitrogen as an amide.
CH3 (CH2)12 CC CH
H
H
OH
CH NH2
CH2 OH
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15.7 SphingolipidsSphingomyelins contain a phosphate ester
bonded to choline.
CH3 (CH2)12 CC CH
H
H
OH
CH NH
CH2
C
O
R
O
PO
O
O CH2 CH2 N
CH3
CH3
CH3
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15.7 Sphingolipids
Sphingomyelins are found in cell membranes, and are important structural components of the myelin sheath of nerves.
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15.7 SphingolipidsCerebrosides contain a monosaccharide
rather than a phosphate.
CH3 (CH2)12 CC CH
H
H
OH
CH NH
CH2
C
O
R
OO
OH
OH
OH
CH2OH
Galactose
Glycoside linkage
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15.8 Steroids
Steroids are nonsaponifiable lipids that possess the steroid skeleton.
A B
C D
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15.8 Steroids
Cholesterol is the most abundant steroid in the human body.
HO
CH3 H
CH3
HH
H3C
CH3
CH3
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15.8 SteroidsCholesterol is the most abundant
steroid in the human body.
HO
CH3 H
CH3
HH
H3C
CH3
CH3
H
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15.8 SteroidsCholesterol is the most
abundant steroid in the human body.
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15.8 Steroids
Cholesterol, when mixed with a fat or oil, enables the substance to absorb a fair amount of water.
Cholesterol is abundant in wool fat, a.k.a. lanolin.
Cholesterol is used as a grease vehicle in ointments when water-soluble com-ponents are required.
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15.8 Steroids
Cholesterol is a precursor to other important steroids
Bile salts
Hormones
Vitamin D
Cholesterol is a risk factor for atherosclerosis, a buildup of plaque, in the arteries.
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15.8 SteroidsBile salts have a polar group at the end of the
hydrocarbon at carbon 17. They emulsify lipids in the digestive system.
HO
CH3 H
CH3
HH
H3C
HOH
H
O
NH
O O
sodium glycocholate
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15.8 SteroidsHormones are messenger molecules
produced in ductless glands. They are carried to specific tissues where they trigger a response.
Steroid hormones regulate “sugar, salt, and sex!”
Adrenocortical steroids regulate sugar and salt. They are secreted by the adrenal glands. Some are used as anti-inflammatory drugs.
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15.8 Steroids
Hormones are messenger molecules produced in ductless glands. They are carried
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15.8 Steroids
Estrogens are steroid hormones that cause development of secondary sex characteristics in females and regulate the menstrual cycle.
Progestins are steroid hormones that regulate pregnancy.
Androgens promote development of secondary sex characteristics in males and promote muscle growth. They are sometimes called anabolic steroids.
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15.8 Steroids
Estrogens are steroid hormones that cause development of secondary sex characteristics in females and regulate the menstrual cycle.
Progestins are steroid hormones that regulate pregnancy.
Androgens promote development of secondary sex characteristics in males and promote muscle growth.
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15.8 Steroids
Many synthetic steroids are available as drugs. Anabolic steroids cause the body to increase protein synthesis, especially in the muscles. They also prevent the breakdown of muscle tissue and affect cell differentiation, favoring muscle over fat cells.
Side effects include acne, liver disease, infer-tility, and development of inappropriate sec-ondary sex characteristics. They can stunt growth in puberty and affect sex character-istics of unborn infants.
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15.8 Steroids
Tamoxifen and raloxifene are selective estrogen receptor modulators, a.k.a. SERMs, that block estrogen receptors and slow or prevent growth of breast cancer.
ON
Tamoxifen (NolvadexTM)
ON
O
S
HO
HO
Raloxifene (EvistaTM)
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15.8 Steroids
Diethylstilbestrol, a.k.a. DES, was used in 1940’s and 1950’s to prevent miscarriage.
It caused vaginal cancer in daughters of women who took it. (Rare)
It caused premature labor in daughters of women who took it. (Common)
OH
HO
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Prostaglandins
Prostaglandins are lipids derived from arachidonic acid.
They were first isolated from prostate glands.
Prostaglandins have hormone-like prop-erties and are involved in inflammation.
Aspirin blocks production of prostaglandins.
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Prostaglandins
Prostaglandins are lipids derived from arachidonic acid. They have hormone-like properties. They are involved in inflammation; aspirin blocks production of prostaglandins.
C
O
OHCH3
O
HO
CH3
OH
C
O
OH
arachidonic acid
PGF2α
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15.9 Cell Membranes
Cell membranes separate the aqueous environment inside the cell from the external aqueous environment.
The membrane also controls movement of substances into and out of the cell.
Phospholipids are a major component of cell membranes. They form lipid bilayers.
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15.9 Cell Membranes
The membrane also controls movement of substances into and out of the cell.
Phospholipids are a major component of cell membranes. They form lipid bilayers.
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15.9 Cell MembranesThe kinks associated with cis double bonds
in fatty acid chains prevent tight packing of the lipid molecules in a lipid bilayer.
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15.9 Cell MembranesCholesterol molecules fit between fatty acid
chains in a lipid bilayer. They regulate the rigidity of the structure.
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15.9 Cell MembranesProteins are also components of the membrane.
They help move nutrients, and act as receptors for hormones and neurotransmitters.