Chapter Nineteen
Lipids
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 (nonpolar) solvents
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Biological Roles of Lipids
•
•
•
•
•
Energy storage
Membrane/barriers
Emulsification
Messengers (hormones, etc.)
Protective coating (waxes)
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Types of Lipids
• Lipids are different from other biomolecules because there
is no common structural feature
– Lipids with fatty acids (long carbon chain carboxylic acids)
Waxes
Fats and oils (trigycerides)
Phospholipids
Sphingolipids
– Lipids without fatty acids
Steroids
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Lipids cont’d
The structural formulas
of these types of lipids
illustrate the great
structural diversity
among lipids.
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Fatty Acids
•
•
•
•
Long-chain carboxylic acids
Insoluble in water
Typically 12-18 carbon atoms (even number)
Some contain double bonds (unsaturated)
corn oil contains 86%
unsaturated fatty acids and
14% saturated fatty acids
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Lipids cont’d
The melting point of a
fatty acid depends on
the length of the carbon
chain and on the
number of double
bonds present in the
carbon chain.
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Fatty Acid Formulas
• The formulas for fatty acids are written as condensed and line-bond
formulas. For example, caprylic acid with 8 carbon atoms can be
written as:
• CH3-(CH2)6-COOH
• CH3-CH2-CH2-CH2-CH2-CH2-CH2-COOH
O
||
• CH3-CH2-CH2-CH2-CH2-CH2-CH2-C-OH
O
OH
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Saturated and Unsaturated Fatty Acids
Saturated = C–C bonds
Unsaturated = one or more C=C bonds
natural fatty acid double bonds are cis
COOH
Palmitic acid, a saturated acid
COOH
Palmitoleic acid, an unsaturated acid
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Properties of Fatty Acids
• Most Contain only single C–C bonds (Saturated)
• Closely packed
• Strong attractions between chains
• High melting points
– Melting points increase with chain length
• Solids at room temperature
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Properties of Unsaturated Fatty Acids
• Contain one or more double C=C bonds
• Nonlinear chains do not allow molecules to pack
closely
• Few interactions between chains
• Lower melting points than saturated
– Melting points increase with chain length
– And decrease with number of double
bonds
• Liquids at room temperature
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Fatty Acid Structures
Saturated fatty acids
• Fit closely in regular pattern
Unsaturated fatty acids
• Cis double bonds
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Lipids cont’d
Four 18-carbon fatty
acids, which differ in
the number of double
bonds present.
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Triple esterification reaction between glycerol and three
molecules of stearic acid (18:0 acid). Three molecules of
water are a by-product of this reaction.
Simple Triacylglycerol
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Structure of a mixed triacylglycerol in which three
different fatty acid residues are present.
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Representative triacylglycerols from (a)
a fat and (b) an oil.
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Lipids cont’d
Percentages of
saturated,
monosaturated,
and
polyunsaturated
fatty acids.
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Lipids cont’d
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Lipids cont’d
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Olestra
• Used in foods as an artificial fat
• Sucrose linked by ester bonds to several longchain fatty chains
• Not broken down in the intestinal tract
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Chemical Properties of Triacylglycerols
• The chemical reactions of triacylglycerols are
similar to those of alkenes and esters
• In hydrogenation, double bonds in unsaturated
fatty acids react with hydrogen 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
Converts double bonds to single bonds
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Product of Hydrogenation
Hydrogenation converts double bonds in oils to single
bonds. The solid products are used to make margarine
and other hydrogenated items.
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Lipids cont’d
Structural equation for the complete hydrogenation of a
triacylglycerol with oleic acid and fatty acid residue.
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Lipids cont’d
During hydrogenation, cis
double bonds can be converted
to trans double bonds.
Compare the geometry of the
trans 18:2 FA to the 18:0
saturated FA. Both have
similar geometries and can
interact with each other as
though the 18:2 was saturated.
Hydrogenated oils may be
unhealthy, and spreadable
margarine in tubs may be
preferable to stick form.
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Hydrolysis
Triglycerides split into glycerol and three fatty acids (H+
or enzyme catalyst)
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Oxidation
• Carbon-carbon double bonds in unsaturated fatty
acids can be oxidized to form both short chain
aldehydes and carboxylic acids
– Oils go “rancid”
• They often have bad odors
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Saponification and Soap
•
•
•
•
Hydrolysis with a strong base
Triglycerides split into glycerol and the salts of fatty acids
The salts of fatty acids are “soaps”
KOH gives softer soaps
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Saponification
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Lipids cont’d
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Phospholipids
• A lipid that contains one or more fatty acids, a
phosphate group, a platform molecule to which the
fatty acids and the phosphate group are attached,
and an alcohol attached to the phosphate group
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Glycerolphospholipids
• The most abundant lipids in cell membranes
• Composed of glycerol, two fatty acids, phosphate,
and an amino alcohol
• Has polar and nonpolar regions
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Lipids cont’d
(a) structural formula and (b) molecular model showing the “head
and two tails” structure of a phosphatidyl-choline molecule.
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Lipids cont’d
Molecular models for (a) sphingosine and (b) sphingophosopholipid.
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Sphingophospholipids
• Sphingophospholipids are phospholipids that have
an amide bond between a fatty acid and
sphingosine, an 18-carbon alcohol
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Sphingoglycolipids
• Another type of membrane protein
• Glycosphingolipids contain monosaccharides
bonded to the –OH of sphingosine by a glycosidic
bond
• Cerebrosides contain only one monosaccharide
• Gangliosides are similar to cerebrosides, but
contain two or more monosaccharides
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Lipids cont’d
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Lipids cont’d
Structural formula and
model for the
cholesterol molecule.
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Lipids cont’d
A severely occluded
artery.
Howard Socurek / Medichrome
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Lipoproteins
• Combine lipids with proteins and phospholipids
• Are soluble in water because the surface consists
of polar lipids
LDL = low density (more fat
than protein)
HDL = high density (more
protein, relatively)
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Lipids cont’d
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Lipids cont’d
Cross section of a
lipid bilayer.
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Lipids cont’d
Space-filling
model of a
section of a lipid
bilayer.
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Lipids cont’d
The 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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Lipids cont’d
Cholesterol molecules
fit between fatty acid
chains in a lipid bilayer.
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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
• The unsaturated fatty acids make cell membranes
fluid-like rather than rigid
• Proteins and carbohydrates on the outer surface
communicate with hormones and
neurotransmitters
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Lipids cont’d
Proteins are
important
structural
components of
cell membranes.
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Transport through Cell Membranes
• Diffusion (passive transport)
– Moves particlse from a higher to a lower concentration
• Facilitated transport
– Uses protein channels to increase the rate of diffusion
• Active Transport
– Moves ions against a concentration gradient
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Three Mechanisms
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G. Siegel, B Agranoff, R.W.
Albers, and P. Molinoff
editiors. In Basic
Neurochemistry 4 th Ed.,
Raven Press, Ltd. 1989, p.
57.
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Bile Salts
• Synthesized from cholesterol and stored in the gall
bladder
• Emulsify fats and oils to give a greater surface
area for lipid digesting enzymes
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Bile Acids and Salts
Line-angle structural
formulas for
cholesterol, cholic acid,
and two deoxycholic
acids.
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Lipids cont’d
A large percentage of
gallstones are almost
pure crystallized
cholesterol.
C. James Webb / Phototake
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Steroid Hormones
• Produced from cholesterol
• Include sex hormones such as androgens (testosterone) in
males and estrogens (estradiol) in females
– Anabolic steroids are derivatives of testosterone
• Also include aldosterone (controls electrolytes and water
balance by kidneys) and cortisone (increases blood
glucose levels and stimulates the synthesis of glycogen in
the liver)
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Lipids cont’d
Selected sex
hormones and
synthetic compounds
that have similar
actions.
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Lipids cont’d
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Lipids cont’d
Relationship of the
structures of various
eicosanoids to their
precursor,
arachidonic acid.
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Lipids cont’d
A biological wax has a structure with a small, weakly polar
“head” and two long, nonpolar “tails”.
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Lipids cont’d
Plant leaves often have
a biological wax
coating to prevent
excessive loss of water.
© Kevin Schaefer / Peter Arnold, Inc.
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