Download Classification of Lipids

Lipids
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INTRODUCTION
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The lipids are a heterogeneous group of
compounds. They have the common property of
being (1) relatively insoluble in water and (2)
soluble in nonpolar solvents such as ether,
chloroform, and benzene.
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The lipids include fats, oils, steroids, waxes, and
related compounds.
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Lipids are important dietary constituents not
only because of their high energy value but also
because of the fat-soluble vitamins and the
essential fatty acids contained in the fat of
natural foods.
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In the body fat serves as an efficient source of
energy and stored in adipose tissue. It serves as a
thermal insulator in the subcutaneous tissues
and around vital organs.
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Nonpolar lipids act as electrical insulators
allowing rapid propagation of depolarization
waves along myelinated nerves.
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The fat content of nerve tissue is very high.
Combinations of fat and protein (lipoproteins)
are important cellular constituents, occurring in
the cell membrane and in the mitochondria
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CLASSIFICATION
Lipids are classified as simple lipid, compound lipid and
derived lipids.
1. Simple Lipids: They are esters of fatty acids with
various alcohols.
a. Fats: They are esters of fatty acids with glycerol. A fat
in the liquid state at room temperature is known as oil.
b. Waxes: Esters of fatty acids with higher molecular
weight monohydric alcohols.
2. Compound lipids:
Compound lipids are esters of fatty acids containing groups in
addition to an alcohol and a fatty acid.
a.
Phospholipids: Lipids containing fatty acids, an alcohol and a
phosphoric acid residue. They frequently have nitrogen containing
bases and other constituents.
i) Glycerophospholipids : In glycerophospholipids, the alcohol is
glycerol.
e.g. Phosphatidyl choline, Phosphatidyl ethanolamine,
Phosphatidylserine, Phosphatidyl inositol and cardiolipin
ii) Glycolipids or Sphingophospholipids: In sphingophospholipids,
the alcohol is sphingosine.
e.g. Sphingomyeline
b. Glycolipids (glcosphingolipids):
Lipids containing a fatty acid, sphingosine and
carbohydrate. e.g. Cerebrosides, Gangliosides
c. Other compound lipids: Other compound
lipids include sulfolipids, proteolipids
and
steroids.
3. Derived lipids: These include free fatty
acids, glycerol, sterols and alcohols. Fatty
aldehydes, ketone bodies, hydrocarbons, lipidsoluble vitamins, and hormone, acyglycerols
(glycerides), cholesterol, and cholesteryl esters
are also termed as lipids, because they are
uncharged.
FATTY ACIDS
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Fatty acids are the building block components of most
lipids.
Almost all naturally occurring lipids yield fatty acids
upon hydrolysis.
Fatty acids are aliphatic carboxylic acids.
They are usually straight-chain derivatives and contain
an even number of carbon atoms because they are
synthesized from two-carbon units.
They may be saturated containing no double bonds or
unsaturated containing one or more double bonds.
They occur mainly as esters in natural fats and oils and
are part of other lipids.
They may also occur as free fatty acids.
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Classification of fatty acids
Fatty acids are classified into saturated and
unsaturated fatty acids.
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i) Saturated Fatty acids: Many different kinds
of saturated fatty acids have been isolated from
the lipids of various compounds.
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Fatty acids with 16 and 18 carbons are the most
abundant. Table 6.1 give some important
saturated fatty acids found in naturally occurring
lipids.
No. of Common
Carbon Name
Atoms
2
Acetic acid
3
Propionic
acid
4
Butyric acid
5
Valeric acid
Structure
Occurrence
CH3COOH
A major end product of
carbohydrate fermentation
by rumen organisms
CH3CH2COOH An
end
product
of
carbohydrate fermentation
by rumen organisms
CH3
In certain fats, in small
(CH2)2COOH amounts in milk (especially
butter). An end product of
CH3 (CH2)3
carbohydrate fermentation
COOH
by rumen organisms.
6
Caproic
acid
CH3 (CH2)4
COOH
Present in milk fat
8
Caprylic
(Octanoic)
CH3 (CH2)6
COOH
In small amounts in many
fats
(including butter),
especially those of plant
origin.
10
Capric acid
CH3 (CH2)8
COOH
Major components of many
millets
and
seed
triglycerides
12
Lauric acid
CH3 (CH2)10
COOH
Spermaceti, cinnamon, palm
kernel, coconut oils, laurels,
butter
14
Myristic
acid
CH3 (CH2)12
COOH
Nutmeg, palm kernel,
coconut oils, butter
16
Palmitic
acid
CH3 (CH2)14
COOH
Common in all animal and
plant fats
18
Stearic
acid
CH3 (CH2)16
COOH
Ground nut (arachis)oil
20
Arachidic
acid
CH3 (CH2)18
COOH
22
Behenic
acid
CH3 (CH2)20
COOH
Seed waxes
24
Lignoceric
CH3 (CH2)22
COOH
Cerebrosides, peanut oil
and plant waxes
Palmitic acid (C16)
CH3- CH2- CH2- CH2-CH2-CH2- CH2- CH2- CH2CH2-CH2- CH2-CH2-CH2- CH2- COOH
Stearic acid (C18)
CH3- CH2- CH2- CH2- CH2- CH2- CH2- CH2- CH2CH2- CH2- CH2- CH2- CH2- CH2- CH2- CH2- COOH
FATTY ACIDS
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Natural fatty acids may be saturated or
unsaturated, and as the following data indicate,
the saturated acids have higher melting points
than unsaturated acids of corresponding size.
The double bonds in the unsaturated
compounds listed on the right are all cis (or Z).
Saturated
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Formula
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CH3(CH2)10CO2H
CH3(CH2)12CO2H
CH3(CH2)14CO2H
CH3(CH2)16CO2H
CH3(CH2)18CO2H
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Common Name Melting Point
lauric acid
myristic acid
palmitic acid
stearic acid
arachidic acid
45 ºC
55 ºC
63 ºC
69 ºC
76 ºC
Unsaturated
Formula
Common Name
CH3(CH2)5CH=CH(CH2)7CO2H -palmitoleic acid
CH3(CH2)7CH=CH(CH2)7CO2H -oleic acid
CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H
linoleic acid
CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H
linolenic acid
CH3(CH2)4(CH=CHCH2)4(CH2)2CO2H
arachidonic acid
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The higher melting points of the saturated fatty acids
reflect the uniform rod-like shape of their molecules.
The cis-double bond(s) in the unsaturated fatty acids
introduce a kink in their shape, which makes it more
difficut to pack their molecules together in a stable
repeating array or crystalline lattice.
The trans-double bond isomer of oleic acid, known as
elaidic acid, has a linear shape and a melting point of
45 ºC (32 ºC higher than its cis isomer).
Stearic acid & Oleic acid
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The shapes of stearic and oleic acids are
displayed in the models below.
“Essential fatty acid
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Two polyunsaturated fatty acids, linoleic and linolenic,
are designated "essential" because their absence in the
human diet has been associated with health problems,
such as scaley skin, stunted growth and increased
dehydration.
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These acids are also precursors to the prostaglandins, a
family of physiologically potent lipids present in minute
amounts in most body tissues.