Chapter 21
Lipids
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Lipids
Lipids: A heterogeneous class of naturally occurring
organic compounds classified together on the basis of
common solubility properties.
• Insoluble in water, but soluble in organic solvents
including diethyl ether, dichloromethane, and acetone.
Lipids include:
• Fatty acids, triacylglycerols (triglycerides),
sphingolipids, phosphoacylglycerols, and glycolipids
• Lipid-soluble vitamins
• Prostaglandins, leukotrienes, and thromboxanes
• Cholesterol, steroid hormones, and bile acids
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Triglycerides
Triglyceride: A triester of glycerol with three fatty acids.
• In most triglycerides, two or three different fatty acid
components are present.
• The hydrophobic character of triglycerides is caused by
the long nonpolar hydrocarbon chains of the fatty acid
components.
• The ester groups, although polar, are buried within a
nonpolar environment, which makes triglycerides
insoluble in water.
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Triglycerides
• In triglycerides, all three hydroxyl groups of glycerol are
esterified with fatty acids.
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Fatty Acids
The fatty acid components of triglycerides have certain
things in common:
1. Practically all are unbranched carboxylic acids.
2. They range in size from approximately 20 carbons.
3. They contain an even number of carbon atoms.
4. Apart from the –COOR ester groups, triglycerides have
no functional groups, except that some have one or more
carbon-carbon double bonds in the fatty acid
hydrocarbon chains.
5. In most fatty acids that have carbon-carbon double
bonds, the cis isomers predominate.
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Triglycerides
The physical properties of triglycerides depend on the fatty
acid components.
• Melting points of fatty acids increases as the number of
carbons in the hydrocarbon chains increases and as
the number of double bonds decreases.
• Triglycerides rich in unsaturated fatty acids are
generally liquid at room temperature and are called oils
• Triglycerides rich in saturated fatty acids are generally
semisolids or solids at room temperature and are
called fats.
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Triglycerides
Hardening: The reduction of some or all of the carboncarbon double bonds of an unsaturated triglyceride using
H2/transition metal catalyst, which converts a liquid
triglyceride to a semisolid.
• In practice, the degree of hardening is carefully
controlled to produce fats of a desired consistency.
• The resulting fats are sold for kitchen use (Crisco,
Spry, Dexo, and others).
• Margarine and other butter substitutes are produced by
partial hydrogenation of polyunsaturated oils derived
from corn, cottonseed, peanut, and soybean oils.
• The hardening process is the source of trans fatty
acids (see Chemical Connections 18A).
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Triglycerides
• Saponification: the base-promoted hydrolysis of fats and
oils in aqueous NaOH produces glycerol and a mixture of
fatty acid sodium salts called soaps.
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Complex Lipids
Phospholipids
• Contain an alcohol, two fatty acids, and a phosphate
ester.
• In glycerophospholipids, the alcohol is glycerol.
• In sphingolipids, the alcohol is sphingosine.
Glycolipids
• Complex lipids that contain a carbohydrate.
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Complex Lipids
Figure 21-1 Schematic diagram of simple and complex
lipids.
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Biological Membranes
Complex lipids form the membranes around cells and
around small structures within cells.
In aqueous solution, complex lipids spontaneously form
into a lipid bilayer, with a back-to-back arrangement of
lipid monolayers.
• Polar (hydrophilic) head groups are in contact with the
aqueous environment.
• Nonpolar (hydrophobic) tails are buried within the
bilayer and shielded from the aqueous environment.
• The major force driving force for the formation of lipid
bilayers is hydrophobic interaction.
• The arrangement of hydrocarbon tails in the interior
can be rigid (if rich in saturated fatty acids) or fluid (if
rich in unsaturated fatty acids).
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Glycerophospholipids
• Glycerophospholipids, also called phosphoglycerides are
the second most abundant group of naturally occurring
lipids.
• Found almost exclusively in plant and animal
membranes, which typically consist of 40% -50%
phosphoacylglycerols and 50% - 60% proteins.
• The most abundant glycerophospholipids are derived
from phosphatidic acid, a molecule in which glycerol is
esterified with two molecules of fatty acid and one of
phosphoric acid.
• The three most abundant fatty acids in phosphatidic
acids are palmitic (16:0), stearic (18:0), and oleic (18:1).
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Glycerophospholipids
• A phosphatidic acid
• The fatty acid on carbon 2 of glycerol is always
unsaturated.
• Further esterification with a low-molecular-weight
alcohol (screen 16) gives a glycerophospholipid.
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Glycerophospholipids
The structure of glycerophospholipids is very similar to that
of fats.
• The alcohol is glycerol.
• Two of the three hydroxyl groups are esterified with fatty
acids. The third hydroxyl group is esterified with
phosphoric acid, which is also esterified with another
alcohol.
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Glycerophospholipids
• If the other alcohol is choline, the glycerophospholipid is
called a phosphatidylcholine(common name lecithin).
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Glycerophospholipids
Figure 21-4 Space-filling model
of complex lipids in a bilayer.
The hydrophobic tails point
toward the middle of the bilayer
and the hydrophilic heads line
both the inner and outer
surfaces of the membrane.
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Glycerophospholipids
Cephalins: Another group of glycerophospholipids in
which the additional phosphate ester is provided by either
ethanolamine or serine.
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Glycolipids
• Glycolipid: a complex lipid that contains carbohydrates
and ceramides.
• The carbohydrate is either glucose or galactose.
• The cerebrosides are ceramide mono- or
oligosaccharides.
• The following is a glucocerebroside.
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