Download Chapter 18_CHEM 131

Spencer L. Seager
Michael R. Slabaugh
www.cengage.com/chemistry/seager
Chapter 18:
Lipids
Jennifer P. Harris
IMPORTANT FUNCTIONS OF LIPIDS
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Protective wax coatings found on some plants
Energy-rich compounds with low densities
Storage form of energy for plants and animals
Structural components, especially in cellular membrane
formation
LIPID CLASSIFICATION
• Lipids are biological compounds that are soluble only in
nonpolar solvents.
LIPID CLASSIFICATION (continued)
• Saponifiable lipids contain an ester that can undergo basic
hydrolysis.
• Triglycerides, waxes, phospholipids, and sphingolipids
• Simple lipids contain a fatty acid and alcohol.
• Triglycerides and waxes
• Complex lipids contain a fatty acid, alcohol, and other
components.
• Phospholipids and sphingolipids
• Nonsponifiable lipids do not contain an ester and cannot be
hydrolyzed.
• Steroids and prostaglandins
FATTY ACIDS
• Fatty acids:
• are the building blocks of many lipids.
• are long chain carboxylic acids.
• have long nonpolar tails responsible for fatty/oily
characteristics.
• The carboxyl group is very hydrophilic under conditions of
physiological pH (exists as –COO-).
FATTY ACIDS (continued)
• In water, fatty acids will form spherical clusters called
micelles.
• Micelles are important for biological functions, like the
transport of insoluble lipids in the blood.
FATTY ACIDS (continued)
• Fatty acids are:
• usually straight chains (no branching).
• usually from C10 to C20.
• usually an even number of carbons.
• either saturated (no C=C bonds) or unsaturated (has C=C
bonds, usually in the cis configuration).
• Examples of saturated, monounsaturated, and
polyunsaturated fatty acids containing 18 carbon atoms
include:
FATTY ACIDS (continued)
• Cis C=C causes “kinking” of the carbon chain:
FATTY ACIDS (continued)
• Fatty acids with C=C bonds
cannot pack closely together
because of shape. This leads to
decreased intermolecular
attractions and lower melting
points.
• Fatty acid melting points
decrease as the number of C=C
bonds increases.
• Most unsaturated fatty acids are
liquids at room temperature.
FATTY ACIDS (continued)
• Essential fatty acids are those needed by the body, but not
synthesized within the body in adequate amounts.
• For humans, linoleic and linolenic acid are essential, but easily
obtainable from plant and fish oils.
FATTY ACIDS (continued)
• Linoleic (an omega-3 fatty acid) and linolenic (an omega-6
fatty acid) acids:
• are used to produce hormonelike substances that regulate
a wide range of functions and characteristics, including:
• e.g. blood pressure, blood clotting, blood lipid levels,
the immune response, and the inflammation response
to injury and infection.
• can be converted to other omega-3 and omega-6 fatty
acids.
STRUCTURE OF FATS AND OILS
• Fats are:
• usually from animal sources.
• solids at room temperature.
• usually composed to a high degree of saturated fatty
acids.
• Oils are:
• usually from plant and fish sources.
• liquids at room temperature.
• usually composed of more unsaturated fatty acids than
fats.
STRUCTURE OF FATS AND OILS (continued)
• Fats and oils are triglycerides (triacylglycerols) which are
triesters of glycerol.
STRUCTURE OF FATS AND OILS (continued)
• Fatty acid components in naturally occurring triglyceride
molecules are rarely identical.
• Natural triglycerides are usually mixtures of different
triglyceride molecules.
STRUCTURE OF FATS AND OILS (continued)
• Excessive fat in the diet:
• is recognized as a risk factor influencing the development
of chronic disease.
• is a concern because of its role in raising blood
cholesterol levels.
• High cholesterol is a risk factor in the development of
coronary heart disease (leading cause of death in
Americans every year).
REACTIONS OF FATS AND OILS
• Hydrolysis is important for fat and oil digestion.
• Enzymes (lipases) can catalyze the hydrolysis process.
REACTIONS OF FATS AND OILS (continued)
• Saponification is the commercial production of the salts of fatty
acids (soaps).
REACTIONS OF FATS AND OILS (continued)
• Soaps depend on the base used for saponification.
• Sodium salts (hard salts) found in cake soap.
• Potassium salts (soft soaps) found in shaving creams and
liquid soap preparations.
• Traditional soap making:
• uses animal fat is the source of triglycerides.
• uses lye (crude NaOH) or an aqueous extract of wood
ashes is the source of the base.
• was lost with fall of Roman Empire.
• The soapless centuries AD 500-1500 resulted in
devastating plagues in an unsanitary western Europe.
REACTIONS OF FATS AND OILS (continued)
• Hydrogenation decreases the degree of unsaturation and is
used to make margarines from oils.
• Complete hydrogenation results in a hard and waxy product.
• Partial hydrogenation results in a smooth, creamy product
that is desired by consumers.
• Isomerization of cis to trans fatty acids will occur during this
process.
• Current dietary advice includes reducing the consumption of
saturated and trans fatty acids.
TRIGLYCERIDE REACTION MAP
WAXES
• Waxes are esters of fatty acids and long chain alcohols (1232 carbon atoms). They are:
• water insoluble and not easily hydrolyzed.
• often found in protective coatings.
• used commercially to make cosmetics, candles,
ointments, and protective polishes.
WAXES (continued)
PHOSPHOGLYCERIDES
• A phosphoglyceride is a phospholipid that contains glycerol,
fatty acids, phosphoric acid, and an alcohol (usually an amino
alcohol).
PHOSPHOGLYCERIDES (continued)
PHOSPHOGLYCERIDES (continued)
• Lecithins:
• contain the amino alcohol choline.
• are an important cell membrane component.
• are emulsifying and micelle-forming agents.
• Cephalins:
• contain ethanolamine or serine as the alcohol.
• are found in most cell membranes, especially brain tissue
and blood platelets (role in blood-clotting process).
PHOSPHOGLYCERIDES (continued)
• Phosphatidylcholine (lecithin):
PHOSPHOGLYCERIDES (continued)
• Cephalin:
SPHINGOLIPIDS
• Sphingolipids are complex lipids that contain sphingosine
rather than glycerol.
• Two types of sphingolipids exist:
• Sphingomyelin contains a phosphate and choline group.
It is found in the myelin sheath surrounding nerve cells.
• Glycolipids contain a carbohydrate unit. They are often
called cerebrosides because of their abundance in brain
tissue.
SPHINGOLIPIDS (continued)
• Sphingomyelin:
SPHINGOLIPIDS (continued)
• Glycolipid:
SPHINGOLIPIDS (continued)
• Several human diseases are known to result from an
abnormal accumulation of sphingomyelins and glycolipids in
the body.
• Each of these diseases is the result of an inherited absence
of an enzyme needed to break down these complex lipids.
CELL MEMBRANES
• Prokaryotic cells are simple unicellular organisms without a
nucleus or membrane-enclosed organelles.
• Eukaryotic Cells are cells containing membrane-enclosed
organelles, particularly a nucleus.
• An organelle is a specialized structure within a cell that
performs a specific function.
• The roles of membrane include:
• external cell membrane function as a selective barrier
between the living cell and its environment;
• internal membranes surround some organelles, create
cellular compartments that have separate organization and
functions.
CELL MEMBRANES (continued)
MEMBRANE STRUCTURE
• Most membranes are 60% lipid and 40% protein.
• The lipids in membranes are phosphoglycerides,
sphingomyelin, and cholesterol.
• Lipids are organized in a lipid bilayer with hydrophobic (long
carbon chain) portions inside and hydrophilic (polar groups)
exposed to the water environment.
• When a lipid bilayer is broken and the tails are exposed to
water, the resulting repulsion causes the bilayer to reform and
the break seals spontaneously.
FLUID MOSAIC MODEL OF
MEMBRANE STRUCTURE
MEMBRANE STRUCTURE (continued)
• Membrane lipids usually contain unsaturated fatty acid
chains.
• They fit into bilayers more loosely than saturated fatty
acids.
• This increases flexibility or fluidity of the membrane.
• Proteins float in the lipid bilayer like icebergs in the sea,
some extend completely through the bilayer.
• Lipid molecules are free to move laterally within the bilayer
like dancers on a crowded dance floor.
STEROIDS
• Steroids are a broad class of compounds that have the
same structural feature:
• Like all lipids, steroids are soluble in nonpolar solvents.
STEROIDS (continued)
• Cholesterol is:
• the most abundant steroid in the human body.
• an essential component of cell membranes.
• a precursor of other important steroids.
• synthesized by liver and present in foods.
• A strong correlation exists between cholesterol blood levels
and atherosclerosis (hardening of arteries).
STEROIDS (continued)
• Bile is yellowish-brown or green, produced by the liver,
stored in the gallbladder, and emptied into the intestine.
• Bile salts emulsify lipids in the intestine by breaking apart
lipid globules.
• The increased surface area allows lipids to be more
easily hydrolyzed.
STEROIDS (continued)
• Bile salts emulsify cholesterol found in the bile.
• Gallstones (80% cholesterol, colored by bile pigments)
form when:
• the cholesterol level in bile is too high.
• the concentration of bile salts is too low.
STEROIDS (continued)
• The passage of a gallstone:
• causes excruciating pain.
• can allow stones to lodge in the duct and prevent bile from
passing into the duodenum and prevents fats from being
digested normally.
• The result is great pain, feeling nauseated and ill, skin
taking on yellow color as bile pigments are absorbed
into the blood, and stool becoming gray-colored
because of the lack of excreted bile pigments.
• Both the gallbladder and the stones can be surgically
removed.
STEROID HORMONES
• A hormone is a chemical messenger secreted by specific
glands and carried through the blood to a target tissue, where it
triggers a particular response.
• There are two major categories of steroid hormones:
adrenocorticoid hormones and sex hormones.
ADRENOCORTICOID HORMONES
• Adrenocorticoid hormones are produced by the adrenal
glands.
• The two classes of adrenocorticoid hormones are:
• mineralocorticoids, which regulate the concentration of
ions in bodily fluids.
• Example: Aldosterone, which increases absorption of
Na+ and Cl• glucocorticoids, which enhance
carbohydrate metabolism.
• Example: Cortisol, which
increases glucose
and glycogen concentrations
in the body and has powerful
anti-inflammatory effects
in the body
SEX HORMONES
• Androgens are male sex hormones produced by testes,
including testosterone.
• Estrogen and progesterone are female sex hormones
produced by ovaries, including estradiol and estrone (the most
important estrogens).
PROSTAGLANDINS
• Prostaglandins are:
• cyclical compounds synthesized from arachidonic acid.
• similar to hormones in that they are intimately involved in
a host of body processes.
• involved in almost every phase of reproduction.
• involved in blood clotting.
THERAPEUTIC POTENTIAL OF
PROSTAGLANDINS
• PGE2 and PGF2 induce labor and are used for therapeutic
abortion in early pregnancy.
• PGE2 in aerosol form is used to treat asthma because it
opens up bronchial tubes by relaxing the surrounding
muscles.
• Other prostaglandins inhibit gastric secretions and used to
treat peptic ulcers.
• Many researchers believe that when they are fully
understood, prostaglandins will be found useful for treating
a much wider variety of ailments.