Chemistry 20 Chapters 13 Lipids

Chemistry 20
Chapters 13
Lipids
Lipids: are family of biomolecules that have the common property of being soluble in organic
solvents but not in water.
Role of lipids: they have three important roles in nature:
1. They store energy and protect and insulate internal organs. In human bodies they are stored
as fat cells and in animal bodies as starch.
2. They are found in nerve fibers and hormones as steroids, which act as chemical
messengers.
3. Because they are not soluble in water, a major function of lipids is to build the cell
membranes that separate the internal contents of cells from the surrounding aqueous
environment.
Lipids are divides into the four groups: 1. Simple lipids (waxes, fats and oils). 2. Complex
lipids (glycerophospholipids). 3. Steroid (Cholesterol and steroid hormones). 4.
Prostaglandins.
Fatty acids (review): a fatty acid contains a long chain attached to carboxylic acid group at
one end. They are insoluble in water because the size of the nonpolar portion is bigger than
the size of polar portion (carboxyl group). Normally, fatty acid contains an even number of
carbon atoms, usually between 12 and 20. They can be either saturated fatty acids with only
single bonds between the carbons or unsaturated fatty acids with one (monounsaturated fatty
acid) or more (polyunsaturated fatty acid) double bonds.
Note (review): Unsaturated fatty acids can be cis or trans (trans is rare). Unsaturated fatty
acids are liquid at room temperature because the cis double bond causes the carbon chain to
bend, which gives the molecules an irregular shape. As a result, fewer attractions occur
between carbon chain and they have lower melting point. On the other hand, saturated fatty
acids are solid at room temperature because they are linear (no bend) and they can pack
together in close parallel alignment and the have higher London dispersion forces than
unsaturated fatty acids.
Essential fatty acids: human body is capable of synthesizing most fatty acids from
carbohydrates or other fatty acids. However, humans do not synthesize sufficient amounts of
fatty acids that have more than one double bond. These fatty acids are called essential fatty
acids because they must be provided by the diet (a deficiency of essential fatty acids can
cause skin dermatitis in infants.
Waxes: are found in many plants and animals. Coating of carnauba wax on fruits and the
leaves and stems of plants help to prevent loss of water and damage from pests. Waxes on the
skin, fur, and feathers of animals and birds provide a waterproof coating. A wax is an ester of
a saturated fatty acid and a long-chain alcohol, each containing from 14 to 30 carbon atoms.
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Chemistry 20
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Long-chain alcohol
Ester bond
Fatty acid
Note: Beeswax obtained from honeycombs and carnauba wax obtained from palm trees are
used to give a protective coating to furniture, cars, and floors. Jojoba wax is used making
candles and cosmetics such as lipstick. Lanolin, a mixture of waxes obtained from wool, is
used in hand and facial lotions to aid retention of water, which softens the skin.
Fats and oils (Triacylglycerols): in the body, fatty acids are stored as fats and oils known as
triacylglycerols (or triglycerides). Triglycerides are trimesters of glycerol (a trihyroxyl
alcohol) and fatty acids.
A triacylglycerol is produced by esterification (a reaction in which the hydroxyl groups of
glycerol form ester bonds with the carboxyl groups of fatty acids. Most fats and oils are mixed
triacylglycerols that contain two or three different fatty acids.
Note: Triacylglycerols are the major form of energy storage for animals. Animal that
hibernate eat large quantities of plants, seed, and nuts that contain high levels of fats and oils.
They gain as much as 14 kilograms a week. As the external temperature drop, the animal goes
into hibernation. The body temperature drops to nearly freezing and there is a dramatic
reduction in cellular activity, respiration, and heart rate. They will hibernate for 4-7 months
and during this time, stored fat is the only source of energy.
Difference between fat and oil: a fat is a triacylglycerol that is solid at room temperature,
such as fats in meat, whole milk, butter, and cheese. Most fats come from animal sources. An
oil is a triacylglycerol that is usually liquid at room temperature. The most commonly used
oils come from plant sources. Olive oil and peanut oil are monounsaturated (one double bond
C=C). Oils from corn, cottonseed, safflower, and sunflower are polyunsaturated (two or more
double bond C=C). A few oils such as palm oil and coconut oil are solid at room temperature
because they consist mostly of saturated fatty acids (single bond C-C). Animal fats usually
contain more saturated fatty acids than do vegetable oils.
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Chemistry 20
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Chemical properties of triacylglycerols: the chemical reactions of the triacylglycerols (fats
and oils) are the same as we discussed for alkenes, carboxylic acids and esters:
1. Hydrogenation: hydrogenation of unsaturated fats converts carbon-carbon double bonds to
single bonds. The hydrogen gas bubbled through the heated oil the presence of a nickel
catalyst (or another transition metal).
Note: Most of the time the liquid oil hydrogenate to produce solid oil. This process has two
advantages: increasing the period of consumption and transporting easer the solid product. In
commercial hydrogenation, the addition of hydrogen is stopped before all the double bonds in
an oil become completely saturated. As a result, we can obtain a soft and semisolid product
instead of a brittle product. This control of hydrogenation gives the various products such as
soft margarines, solid stick margarine, and solid shortenings.
2. Hydrolysis: triacylglycerol are hydrolysed (split by water) in the presence of strong acids
or digestive enzymes called lipases. The products of hydrolysis of the ester bonds are glycerol
and three fatty acids.
3. Saponification: when a fat is heated with a strong base such as sodium hydroxide,
saponification of the fat gives glycerol and the sodium salts of the fatty acids, which are
soaps. KOH produces a softer, liquid soap. Oils that are polyunsaturated produce softer soaps.
Names like “coconut” or “avocado shampoo” tell you the sources of the oil used in the
reaction.
Fat or oil + Strong base → glycerol + salts of fatty acids (soaps)
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Chemistry 20
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Glycerophospholipids: are similar to triacylglycerols except that one hydroxyl group of
glycerol is replaced by the ester of phosphoric acid and amino alcohol, bonded through a
phosphodiester bond.
G
L
Y
C
E
R
O
L
Fatty acid
G
L
Y
C
E
R
O
L
Fatty acid
Fatty acid
Triacylglycerol (triglyceride)
Fatty acid
Fatty acid
phosphate
amino
alcohol
Glycerophospholipid
Three amino alcohols found in glycerophospholipids are clorine, serine, and ethanolamine.
Because these compounds exist in the body at physiological pH of 7.4, they are ionized.
CH3
HO-CH2-CH2-N+-CH3
+
NH3
HO-CH2-CH-COO-
O
+
HO-CH2-CH2-NH3
CH3
Choline
O
P
O
OSerine
Ethanolamine
Phosphate
Glycerophospholipids contain both polar and nonpolar regions, which allow them to interact
with both polar and nonpolar substances. The ionized alcohol and phosphate portion called
“the head” is polar and can hydrogen bond with water. The two fatty acids connected to the
glycerol molecule represent the nonpolar “tails” of the phospholipids. Glycerophospholipids
are the most abundant lipids in cell membranes, where they play an important role in cellular
permeability. In the body fluids, they combine with the less polar triglycerides and cholesterol
to make them more soluble as they are transported in the body.
Steroids: are compounds containing the steroid nucleus, which consists of three cyclohexane
rings and one cyclopentane ring fused together (no fatty acids). The four rings in the steroid
nucleus are designated A, B, C, and D. The carbon atoms are numbered beginning with the
carbons in ring A and, in types of steroids like cholesterol, ending with two methyl groups
(attached groups make a wide variety of steroid compounds).
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Chemistry 20
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Cholesterols: is one of the most important and abundant steroids in the body. Cholesterol has
methyl groups, alkyl chain and hydroxyl group (-OH) attached to the steroid nucleus.
Cholesterol in the body: Cholesterol is a component of cellular membranes, myelin sheath,
and brain and nerve tissue. It is also found in the liver, bile salts, and skin, where it forms
vitamin D. In the adrenal gland, it is used to synthesize steroid hormones. Cholesterol in the
body in obtained from eating meats, milk, and eggs, and it is also synthesized by the liver
from fats, carbohydrates, and proteins. There is no cholesterol in vegetable and plant products.
If the diet is high in cholesterol, the liver produces less. A typical American daily diet
includes 400-500 mg of cholesterol (one of the highest in the world). However, we should
consume no more than 300 mg of cholesterol a day.
Note: When cholesterol exceeds its saturation level in the bile, gallstones may form. High
levels of cholesterol are also associated with the accumulation of lipid deposits (plaque) that
line and narrow the coronary arteries. Some research indicates that saturated fats in the diet
may stimulate the production of cholesterol by the liver.
Steroid hormones: hormones are chemical messengers that serve as a kind of communication
system from one part of the body to another. The steroid hormones, which include the sex
hormones are closely related in structure to cholesterol and depend on cholesterol for their
synthesis. Two important male sex hormones, testosterone and androsterone, promote the
growth of muscle and of facial hair and the maturation of the male sex organs and of sperm.
The estrogens, a group of female sex hormones, direct the development of female
characteristics: the uterus increases in size, fat is deposited in the breasts, and the pelvis
broadens. Progesterone prepares the uterus for the implantation of a fertilized egg.
Lipoproteins: in the body, lipids must be transported through the bloodstream to tissues
where they are stored, used for energy, or to make hormones. However, most lipids are
nonpolar and insoluble in the aqueous environment of blood. They are made more soluble by
combining them with phospholipids and proteins to form water-soluble complexes called
lipoproteins.
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Chemistry 20
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In general, lipoproteins are spherical particles with an outer surface of polar proteins and
phospholipids that surround hundreds of nonpolar molecules of triacylglycerols and
cholesteryl esters (formed by the esterification of the hydroxyl group in cholesterol with a
fatty acid).
Triacylglycerols
There are several types of lipoproteins that differ in density, lipid composition, and function:
1. Chylomicrons 2. Very-low-density lipoprotein (VLDL) 3. Low-density lipoprotein
(LDL) 4. High-density lipoprotein (HDL).
The chylomicrons formed in the mucosal cells of the small intestine and the VLDLs formed in
the liver transport triacylglycerol, phospholipids, and cholesterol to the tissues for storage or
to the muscles for energy. The LDLs transport cholesterol to tissues to be used for the
synthesis of cell membranes, steroid hormones, and bile salts. When the level of LDL exceeds
the amount of cholesterol needed by the tissues, the LDLs deposit cholesterol in the arteries
and/or myocardial infarctions (heart stacks). This is why LDL cholesterol is called “bad”
cholesterol. The HDLs remove excess cholesterol from the tissues and carry it to the liver
where it is converted to bile salts and eliminated. When HDL levels are high, cholesterol that
is not needed by the tissues is carried to the liver for elimination rather than deposited in the
arteries, which gives the HDLs the name of “good” cholesterol. Most of the cholesterol in the
body is synthesized in the liver, although some comes from the diet. Higher HDL levels are
found in people who exercise regularly and eat less saturated fat.
VLD
L
Liver
Intestine
and
elimination
Heart
and
muscle
Fat
storage
cells
Energy
HDL
LDL
Cell membranes: the membrane of a cell separates the contents of a cell from the external
fluids. It is semipermeable so that nutrients can enter the cell and waste products can leave.
There are two rows of phospholipids in a cell membrane, that they are arranged like a
sandwich. Their nonpolar tails, which are hydrophobic (water-fearing), move to the center,
while their polar heads, which are hydrophilic (water-loving) align on the outer edge of the
membrane. This double row arrangement of phospholipids is called a lipid bilayer.
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Chemistry 20
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Most of the phospholipids in the lipid bilayer contain unsaturated fatty acids. Due to the kinks
in the carbon chains at the cis double bonds, the phospholipids do not fit closely together. As
a result, the lipid bilayer is not a rigid, fixed structure, but one that is dynamic and fluid-like.
In this liquid-like bilayer, there are also proteins, carbohydrates, and cholesterol molecules.
For this reason, the model of biological membranes is referred to as the fluid mosaic model
of membranes.
Note: In the fluid mosaic model, proteins known as peripheral proteins emerge on just one of
the surface, outer or inner. The integral proteins extend through the entire lipid bilayer and
appear on both surfaces of membrane. Some carbohydrates are attached to proteins and lipids
and they are responsible for cell recognition and communication with chemical messengers
such as hormones.
Carbohydrate
Phospholipid
bilayer
Nonpolar
Polar
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Chemistry 20
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