Chemistry 203
Chapter 19
Lipids
Lipids
- Family of bimolecules.
- They are not defined by a particular functional
group, thus they have a variety of structures and
functions.
- They are soluble in organic solvents but not in water (nonpolar).
- They contain many nonpolar C—C and C—H
bonds and few polar bonds resulting in their water
insolubility.
Lipids
1. Store energy: fat cells
2. Chemical messengers: find in nerve fibers and hormones.
3. Parts of membranes: insoluble in water
Lipids
1. Simple lipids: (Waxes, Fats & Oils)
2. Complex lipids (Glycerophospholipids)
Store energy, insulation
Cell membrane
3. Steroid (Cholesterol & steroid hormones)
4. Eicosanoids
Pain, fever, inflammation
Chemical messenger
Cell membrane
Lipids
Lipids can be categorized as:
1. Hydrolyzable lipids can be converted into small molecules
by aqueous hydrolysis.
Lipids
Lipids can be categorized as:
2. Nonhydrolyzable lipids cannot be cleaved into smaller molecules
by aqueous hydrolysis.
Hydrolysis
Most hydrolyzable lipids contain an ester.
Hydrolysis: reaction with water.
(breaking a bond and adding the elements of water)
O
RCOR' + H2O
An ester
Heat
H+ or enzyme
O
RC- OH
A carboxylic acid
+
H-OR'
An alcohol
Fatty acids
Hydrolyzable lipids are derived from fatty acids.
Fatty acids are:
COOH Stearic acid
(mp 70°
COOH Oleic acid (1
(mp 16°C
• Long-chain unbranched carbon attached
to a carboxyl group (-COOH).
COOH Linoleic aci
(mp-5°
• Typically 12-20 carbon atoms.
COOH Linolenic ac
(mp -1
• They have an even number of C atoms.
COOH Stearic
(m
• Insoluble in water.
COOH Oleic a
(mp
Cis
COOH Linole
Fatty acids
CH3(CH2)14COOH (palmitic acid)
polar portion = hydrophillic
nonpolar portion = hydrophobic
Hydrophobic portion is much bigger than hydrophilic portion.
Insoluble in water
Saturated and unsaturated Fatty acids
Saturated fatty acids have no double bonds in their long hydrocarbon chains.
Stearic acid: CH3(CH2)16COOH
They are solids at room temperature.
Packed together  Maximum London dispersion forces
COOH
COOH
COOH
COOH
COOH
Saturated and unsaturated Fatty acids
Unsaturated fatty acids have 1 or more double bonds (generally cis) in their long
hydrocarbon chains.
Oleic acid: CH3(CH2)7CH=CH(CH2)7COOH
They are liquids at room temperature.
They can not pack together  London dispersion forces 
COOH
COOH
COOH
COOH
COOH
Fatty acids
• The human body is capable of synthesizing most fatty acids from
carbohydrates or other fatty acids.
• Humans do not synthesize sufficient amounts of fatty acids that
have more than one double bond.
• More than one double bond fatty acids are called essential fatty
acids and they must be provided by the diet.
Linoleic acid
linolenic acid
Essential Fatty acids
Omega-n acids
n: the position of the first double bond
Linoleic acid is called an omega-6 acid, because of the position
of the first C=C in the nonpolar chain.
Essential Fatty acids
Linolenic acid is called an omega-3 acid, because of the position of
the first C=C in the nonpolar chain.
Waxes
Wax is an ester of saturated fatty acid and long chain alcohol.
Long-chain alcohol
Ester bond
Acid
Fatty acid
Waxes
For example, shown below is the formation of spermaceti wax,
isolated from the heads of sperm whales.
Acid
Waxes
Because of their long nonpolar C chains, waxes are very hydrophobic.
O
Beeswax
(myricyl palmitate)
CH3(CH2)14 C
hydrophobic
region
O(CH2)29CH3
hydrophobic
region
They form protective coatings:
- In plants, they help prevent loss of water and damage from pests.
- In humans and animals, provide waterproof coating on skin and fur.
Beeswax
Carnauba
Coating
Jojoba
Lanolin from wool
lotions
Waxes
Hydrolysis reaction: like other esters, waxes are hydrolyzed.
Heat
Triacylglycerols (Triglycerides)
Triacylglycerols are:
• Fats and oils (are stored in the body).
• Triesters of glycerol.
• Produced by Fischer esterification.
• Formed when the hydroxyl groups of
glycerol react with the carboxyl groups
of fatty acids.
Esterification
glycerol
three fatty acids
triacylglycerol
O
CH2
OH
HO C
O
(CH2)14CH3
CH
OH
HO C
O
(CH2)14CH3
CH2
OH
HO C
(CH2)14CH3
O
CH2 O C
(CH2)14CH3
O
Acid
CH O C
(CH2)14CH3 + 3H2O
O
CH2 O C
(CH2)14CH3
Triacylglycerols (Triglycerides)
CH2
OH
CH
OH
CH2
OH
Glycerol
O
CH2
O
C
(CH2)5CH CH(CH2)7CH3
O
CH
O
CH2
O
C (CH2)5CH CH(CH2)7CH3
O
C
(CH2)5CH CH(CH2)7CH3
G
L
Y
C
E
R
O
L
Fatty acid
Fatty acid
Fatty acid
Produced by esterification of glycerol (a trihydroxyl alcohol).
Triacylglycerols (Triglycerides)
Simple triacylglycerols have three identical fatty acid side chains.
Mixed triacylglycerols have two or three different fatty acids.
Triacylglycerols (Triglycerides)
Saturated triacylglycerols contain only saturated fatty acids.
Monounsaturated triacylglycerols have 1 C=C bond.
Polyunsaturated triacylglycerols have many C=C bonds.
Increasing the number of double bonds in the fatty acid chain
decreases the melting point of the triacylglycerol.
Triacylglycerols (Triglycerides)
Fat: is a triacylglycerol that is solid at room temperature.
Made by more saturated fatty acids (Saturated triacylglycerols).
Meat, milk, butter and cheese (animal sources).
Oil: is a triacylglycerol that is liquid at room temperature.
Made by more unsaturated fatty acids (Unsaturated triacylglycerols).
Corn, cotton seed, safflower and sunflower (plant sources).
Both are colorless, odorless, and tasteless.
Fat & Health
- Fats are used to build cell membranes, insulate the body, and
store energy for later use.
- It is recommended that no more than 20-35% of a person’s caloric
intake should come from lipids.
- A high intake of saturated triacylglycerols is linked to heart disease.
- Saturated fats stimulate cholesterol synthesis in the liver, which
can lead to cholesterol plaques building up inside arteries.
- The result is high blood pressure, heart attack, and even stroke.
- Unlike other vegetable oils, oils from palm and coconut trees are
very high in saturated fats.
Fat & Health
- Unsaturated triacylglycerols (omega-3 fatty acids from fish) lower the risk
of heart disease by decreasing the level of cholesterol in the blood.
- However, if the double bond of the unsaturated triacylglycerol is
trans, the beneficial effect is lost.
- Trans fats, which are primarily synthesized instead of naturally
occurring, act like saturated fats and increase the cholesterol levels in the
blood.
1- Hydrogenation
- Hydrogen adds to the double bonds of unsaturated fats (using transition
metal catalyst such as Ni).
H H
_
- Melting point is increased.
- Liquid oils are converted to semi-solid fats.
H H
Ni
C=C + H2 → _C_C_
_
H H
1- Hydrogenation
O
CH2
O
C
(CH2)5CH CH(CH2)7CH3
Ni
O
CH
O
CH2
O
_
C (CH2)5CH _ CH(CH2)7CH3
O
C
+ 3H2
(CH2)5CH CH(CH2)7CH3
glyceryl Trioleate
(triolein)
O
CH2
O
C
(CH2)14CH3
O
CH
CH2
O
O
C (CH2)14CH3
O
C
(CH2)14CH3
glyceryl tristearate
(tristearin)
2- Hydrolysis
Triacylglycerols are hydrolysis (split by water) in the presence of strong acid
or lipase (digestive enzyme).
O
CH2 O C
(CH2)14CH3
O
CH O C
(CH2)14CH3
+ 3H2O
O
CH2 O C
(CH2)14CH3
CH2 OH
H
O
+CH OH
O C
H +Na3H
H+ or Lipase
CH2 OH
H
(CH2)14CH3
Metabolism of tricaylglycerols
- Humans store energy as triacylglycerols in adipose cells below the
surface of the skin, in the breast area, and surrounding internal organs.
- The number of adipose cells is constant; weight gained or lost causes
them to swell or shrink, but not decrease or increase in number.
- To metabolize triacylglycerols for energy, the esters are hydrolyzed by
enzymes called lipases.
- Complete metabolism of a triacylglycerol yields CO2, H2O, and a
great deal of energy.
3- Saponification (Basic Hydrolysis)
- Is the process of forming “soaps” (salts of fatty acids).
- Is the reaction of a fat with a strong base (NaOH).
- Splits triacylglycerols into glycerol and the salts of fatty acids.
- With KOH or the oils that are polyunsaturated gives softer soaps
(liquid soaps).
- Soaps are typically made from lard (from hogs), tallow (from cows
or sheep), coconut oil, or palm oil.
- All soaps work in the same way, but have different properties
depending on the lipid source, length of C chain, and degree of
unsaturation.
3- Saponification (Basic Hydrolysis)
O
CH2 O C
(CH2)14CH3
O
CH O C
(CH2)14CH3
+ 3NaOH
O
CH2 O C
(CH2)14CH3
CH2 OH
H
O
Heat
CH OH
H
+
3Na
Na++--O C
(CH2)14CH3
“soap”
Salt of fatty acid
CH2 OH
H
n)
Soaps
Hydrophobic part: nonpolar
Hydrophilic part: polar (remains in contact with environment)
O
+
3 RCO N a
Sodium soaps
Soaps
When soap is mixed with dirt (grease, oil, and …), soap
micelles “dissolve” these nonpolar, water-insoluble molecules.
phospholipids
Phospholipids are lipids that contain a P atom.
Two common types:
phospholipids
1. Phosphoacylglycerols:
They are the main component of most cell membranes.
Structurally, they resemble a triacylglycerol, except the third fatty acid
has been replaced with a phosphodiester bonded to an alcohol.
Fatty acid
Fatty acid
Amino alcohol
phospholipids
1. Phosphoacylglycerols:
There are two types of phosphoacylglycerols:
Ethanolamine
Choline
phospholipids
2. Sphingomyelins: They differ in two ways:
1. They do not contain a glycerol backbone, they have a sphingosine backbone instead.
sphingosine
2. They do not contain an ester; their single fatty acid is bonded to the backbone
by an amide bond.
phospholipids
2. Sphingomyelins:
The myelin sheath, the coating that surrounds nerve cells, is rich in sphingomyelins.
phosphoacylglycerols
Nonpolar
O
R
Polar
Interact with both polar and nonpolar substances.
1. Most abundant lipids in cell membranes (semipermeable).
2. Combine with less polar triglycerides and cholesterol to make them soluble.
Cell Membrane
Carbohydrate
Phospholipid
bilayer
Nonpolar
Hydrophobic
Polar
Hydrophilic
Semipermeable: selected nutrients can enter and waste products can leave.
Fluid mosaic model
Cell Membrane
- Peripheral proteins are embedded within the membrane and
extend outward on one side only.
- Integral proteins extend through the entire bilayer.
- Sometimes carbohydrates are attached to the exterior of the cell
forming glycolipids and glycoproteins.
Transport Across a Cell Membrane
Simple Diffusion: Small molecules like O2 and CO2 can diffuse through the cell
membrane, traveling from higher to lower concentration.
Facilitated Transport: Larger polar molecules (glucose) and ions (Cl- and
HCO3-) travel through integral protein channels.
Active Transport: Other ions, Na+, K+, and Ca2+, move against the concentration
gradient; this required energy input.
Steroids
Steroids have:
• A steroid nucleus which is
4 carbon rings.
• Attached groups that make the
different types of compounds.
(steroid nucleus)
• No fatty acids.
Cholesterol
Cholesterol:
• Is the most abundant steroid in the body.
• Insoluble in water (need a water soluble carrier).
• Has methyl CH3- groups, alkyl chain, and -OH
attached to the steroid nucleus.
CH3
CH3
CH3
HO
CH3
CH3
Cholesterol
Cholesterol:
• Is obtained from meats, milk, and eggs.
• Is synthesized in the liver from fats,
carbohydrates and proteins.
• Is needed for cell membranes, brain and
nerve tissue, steroid hormones, and
Vitamin D.
At artery clogged by
cholesterol plaque
• Clogs arteries when high levels form
plaque (because it is insoluble in blood).
• No cholesterol in vegetable and plants.
Gallstones form in gallbladder
Lipoproteins
Triacylglycerols
Water-soluble form of lipids
(soluble in blood)
Spherical particles
Polar surface and nonpolar inner
Transporting lipids through the bloodstream to tissues where they are stored,
Used for energy, or to make hormones.
Lipoproteins
VLDL: very-low-density lipoprotein
Triglycerides and Cholesterol
LDL: low-density lipoprotein (bad Cholesterol)
Cholesterol
HDL: high-density lipoprotein (good Cholesterol)
Cholesterol
Recommended levels are: HDL > 40 mg/dL, LDL < 100 mg/dL, total serum cholesterol < 200 mg/dL.
Chylomicrons
Triglycerides and Cholesterol
VLDL
Liver
Intestine
and
elimination
Heart and
muscles
Fat storage
cells
HDL
LDL
Energy
Steroid Hormones
A hormone is a molecule that is synthesized in one part of an
organism, which then elicits a response at a different site.
Two types of steroids hormones:
1. Sex hormones
Estrogens & progestins in females
Androgens in males
2. Adrenal Cortical Steroids
Sex Hormones
Estrogens (Female Sex Hormones):
The estrogens estradiol and estrone control development of secondary
sex characteristics, regulate the menstrual cycle, and are made in the
ovaries.
Sex Hormones
Progestins (Female Sex Hormones):
The progestin progesterone is called the “pregnancy hormone”; it is
responsible for the preparation of the uterus for implantation of a fertilized
egg.
Sex Hormones
Androgens (Male Sex Hormones):
Testosterone and Androsterone are androgens made in the testes.
They control the development of secondary sex characteristics in males.
Sex Hormones
- Synthetic androgen analogues, called anabolic steroids, promote
muscle growth.
- They have the same effect as testosterone, but are more stable, so
they are not metabolized as quickly.
- They have come to be used by athletes and body builders, but
are not permitted in competitive sports.
- Prolonged use of anabolic steroids can cause physical and
psychological problems.
Sex Hormones
Some examples of anabolic steroids:
(THG or the Clear)
Adrenal Cortical Steroids
Aldosterone regulates blood pressure and volume by controlling the
concentration of Na+ and K+ in body fluids.
Cortisone and cortisol serve as anti-inflammatory agents, which also
regulate carbohydrate metabolism.
aldosterone
cortisone
cortisol
Adrenal Cortical Steroids
Cortisone are used to suppress organ rejection after transplant surgery
and to treat many allergic and autoimmune disorders.
Prolonged use of these steroids can have undesired side effects,
including bone loss and high blood pressure.
Prednisone, a synthetic alternative, has similar anti-inflammatory
properties but can be taken orally.
Vitamins
They are organic compounds required in small quantities for normal
metabolism.
They must be obtained from the diet (our cells cannot synthesize them).
Vitamins are either water soluble or fat soluble.
The four fat-soluble vitamins (A, D, E, and K) are lipids and
nonpolar.
They are found in fruits, vegetables, fish, liver, and dairy products.
Excess vitamins are stored in adipose cells to be used when needed.
Vitamins
Vitamin A
It is found in liver, fish, and dairy products, and is made from β-carotene
(the orange pigment in carrots).
It is needed for vision and for healthy mucous membranes.
Vitamin A deficiency causes night blindness and dry eyes and skin.
Vitamins
Vitamin D
Vitamin D can be synthesized from cholesterol.
It can be obtained in the diet from many foods, especially milk, and
helps regulate Ca and P metabolism.
A deficiency of vitamin D causes rickets (bone malformation).
Vitamins
Vitamin E
Vitamin E is an antioxidant, protecting unsaturated side chains in
fatty acids from unwanted oxidation.
Deficiency of vitamin E causes numerous neurological problems,
although it is rare.
Vitamins
Vitamin K
Vitamin K regulates the synthesis of clotting proteins (prothrombin),
and deficiency of this leads to excessive or fatal bleeding.
Eicosanoids
Prostaglandins and Leukotrienes are two types of eicosanoids (20
C atoms derived from the fatty acids).
- All eicosanoids are very potent compounds, which are not stored
in cells, but rather synthesized in response to external stimulus.
- Unlike hormones they are local mediators, performing their
function in the environment in which they are synthesized.
Prostaglandins
Prostaglandins are carboxylic acids that contain a five-membered ring
and have a wide range of biological activities.
Prostaglandins
Prostaglandins are responsible for inflammation.
- Aspirin and ibuprofen relieve pain and inflammation by blocking the
synthesis of these molecules.
- Prostaglandins also decrease gastric secretions, inhibit blood
platelet aggregation, stimulate uterine contractions, and relax
smooth muscles.
- There are two different cylcooxygenase enzymes responsible
for prostaglandin synthesis called COX-1 and COX-2.
Prostaglandins
COX-1 is involved in the usual production of prostaglandins.
COX-2 is responsible for additional prostaglandins in inflammatory
diseases like arthritis.
- Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and
ibuprofen inactivate both COX-1 and -2, but increase risk for stomach
ulcer formation.
- Drugs sold as Vioxx, Bextra, and Celebrex block only the COX-2
enzyme without affecting gastric secretions.
Leukotrienes
Asthma is characterized by chronic inflammation, so inhaled
steroids to reduce this inflammation are commonly used.
Leukotrienes are molecules that contribute to the asthmatic
response by constricting smooth muscle of the lung.
New asthma drugs act by blocking the synthesis of leukotriene C4,
which treat the disease instead of just the inflammation symptoms.