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CHAPTER – III
LIPIDS
Introduction, occurrence, classification, biochemical functions
and physiological importance of lipids.
INTRODUCTION
Fats and their derivatives are known collectively as lipids. (Greek word: lipos= fat)
The principal component of lipid is fatty acid. The term lipid was first time given by
biochemist BLOOR in 1943.
Bloor defined lipid as ` Naturally occurring compound which are insoluble in water
and soluble in organic solvents such as benzene, chloroform, ether etc. On hydrolysis
they yield fatty acids.
OCCURRENCE
They are widely distributed throughout plant and animal kingdom. In plants, they
are present in seeds, nuts and fruits while they are present in adipose tissues in animals,
bone marrow and nervous tissues.
CLASSIFICATION
The lipids are classified in THREE groups.
1. Simple lipids : Fats , oils , wax
2. Compound lipids : Phospholipids, glycolipids, gangliosides, lipoproteins
3. Steroids: Cholesterols, ergosterols, sex hormones.
1 SIMPLE LIPID:
Simple lipids are the esters of fatty acids with various long chain alcohols.
Fatty acids are of two types.
1. Saturated Fatty acids.
2. Unsaturated fatty acids.
Saturated Fatty acids.








Butyric acid
Caproic acid
Caprylic acid
Lauric acid
Myristic acid
Palmitic acid
Stearic acid
Arachidic acid
CH3CH2CH2COOH
CH3 (CH2) 4COOH
CH3 (CH2)6 COOH
CH3 (CH2)10 COOH
CH3 (CH2)12 COOH
CH3 (CH2)14 COOH
CH3 (CH2)16 COOH
CH3 (CH2)18 COOH
Butter
Butter
Coconut, palm oil
Laurel.kernel oil
Nutmeg oil
Palm oil
Cocoa butter
Peanut oil.
Unsaturated fatty acids :
1. Oleic acid
(C18 )
2. Linoleic acid (C18 )
3. Linolenic acid (C18 )
4. Erucic acid
(C22 )
5. Arachidonic acid (C20 )
One double bond
Two double bond
Three double bond
One double bond
One double bond
Olive oil
Linseedoil,Soybean oil
Linseed oil
Mustard oil
Ground nut oil
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No. (2), (3) and (5) are the essential unsaturated fatty acids required by
the human
being.
CLASSIFICATION OF LIPIDS
Lipids
Simple lipid
Compound Lipid
Sterols


 

Fat Oil Wax
Cholesterol
Carotenoids
Ergosterol
Sex hormone


Phosho
Glyco
Lipid
lipid


Lecithin Cerebroside
Cephalin
Hydrocarbon

Vitamin
A, D,E & K


Ganglioside
Lipo
|
Protein


N-acetyl
Protein+
Neuramic
triglyceride
Acid +
+ cholesterol
Fattyacid +
or phospho
hexose
lipid
COMPARISON OF FATS AND OILS:
FAT
OILS
1. Esters of fatty acids + glycerol
1. Esters of fatty acids + glycerol
2. Solid at room temperature
2. Liquid at room temperature
3. Low molecular weight
3. High molecular weight
4. More saturated fatty acids
4. Higher unsaturated fatty acids
5. High saponification value
5. Low saponification value
6. Low iodine value
6. High iodine value
CHEMICAL PROPERTIES OF SIMPLE LIPIDS:
1. HYDROLYSIS: They are hydrolysed by super heated steam, strong alkali,
dilute mineral acid or lipase enzyme.
Lipids + H2O ------------------------- Fatty acid
Lipase enzyme
+ glycerol
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2. SAPONIFICATION: when fat or oil reacts with NaOH or KOH, they yield salts
of fatty acid (soap) and glycerol.
Neutral fat + NaOH/KOH ------ Soap + glycerol
3. HYDROGENATION: Unsaturated fatty acids react with hydrogen in presence
of nickel at 150-190 0 C to give saturated fatty acids. Liquid oil is converted to
solid fat. Hydrogenation is the process used for preparing the artificial ghee
(vegetable oil).
Unsaturated fatty acid + H2 --------- Saturated fatty acid
CH3 (CH2)7 CH = CH CH2COOH
CH3 (CH2)7 CH2 CH2CH2COOH
4. HALOGENATION: When unsaturated fat is treated with halogens (I2 / Br2 /Cl2 ),
the fat is converted to halogenated compounds.
E.g. Iodine value is a useful tool to know the amount of unsaturation in the fat.
CH3 (CH2)7 CH= CH CH2COOH + I2 / Br2 /Cl2 --
CH3 (CH2)7 CH - CH CH2COOH
I/Br/Cl I/Br/Cl
Halogenated compound
5. RANCIDITY: When butter/oil/fat is stored for a long time, they often become
rancid due to hydrolysis/oxidation. The rancid fat gives off flavour, bad smell and
unpleasant taste. The rancidity can be defined as the production of bad
unpleasant smell in oil or fat is called rancidity.
The rancidity is of two types.
(A) Hydrolytic rancidity
(B) Oxidative rancidity
Aldehydic rancidity
Ketonic rancidity
A) Hydrolytic rancidity: This is developed by the action of microbial lipase
enzyme which split the triglycerides and produce low molecular weight (C 4 – C10 )
fatty acid such as butyric acid. This butyric acid produces bad smell in the butter.
Butter or butterfat + Lipase + 3H20 ----- Glycerol
+ Butyric acid
B) Oxidative rancidity: This type of rancidity develops due to oxidation of
unsaturated fats to produce either the cleavage or polymerisation of fatty acid.
Two types of oxidative rancidity a) Aldehydic rancidity b) Ketonic rancidity
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1) Aldehydic rancidity: There is production of peroxides when these peroxides
are converted into aldehyde which gives bad smell in fat.
O -- O
HC = CH
+ O2
--- - C - C ------
H
Unsaturated fatty acid
H
Peroxide
C=O
H
Aldehyde
2) Ketonic rancidity: When ketones are formed from the β oxidation of
saturated fatty acid, then keto acids are formed. Elimination of CO 2 leads to
rancid ketone formation to produce bad smell.
CH3 (CH2) n COOH --
Saturated fatty acid
CH3 CH2- CH2 – C – COOH 
||
Keto acid
O
CH3CH2CH2COCH3
Ketone (bad smell)
CLASSIFICATION OF OILS ON THE BASIS OF THEIR DRYING
CHARACTERS
OILS
DRYING OILS
Oxidation of unsaturated
Fatty Acids lead to tough
Water-Proof film
e.g linseed oil, oil paint
SEMIDRYING OILS
NON DRYING OILS
Dries slowly in presence of
oxygen
Remain unaffected
by Presence of
oxygen
e.g. Oleic acid, Linoleic acid
e.g. greasing oil
Present in cotton seed
for lubrication.
And soybean
QUALITATIVE TESTS (ANALYTICAL CONSTANTS) TO DETERMINE PURITY
OF OIL / FAT:
It is very essential to keep the oil in the pure, hygienic form. Upon storage
or adulteration (malpractice by the oil millers), there is a chance of adulteration in
the purity of oils. Analytical constants help in judging the quality of oil.
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1. SAPONIFICATION VALUE: It can be defined as milligrams of NaOH / KOH
required saponifying one gram of oil or fat is called saponification value. This
test suggests the average chain length of fatty acids in the oil.
2. ACID VALUE: It can be defined as the milligram of KOH to neutralise the
FREE FATTY ACIDS present in one gram of oil/fat. This test suggests us the
amount of free fatty acids. When the fat becomes rancid due to hydrolysis free
fatty acids are produced.
3. IODINE NUMBER: It can be defined as number of centi grams (1 centigram
= 10 mg.) of iodine taken up by one gram of oil. Iodine value suggests the
amount of unsaturated fatty acids present in the oil. More iodine value = better
the oil for consumption.
4. REICHERT- MEISSL NUMBER (R.M.Number): It can be defined as the
number of ml of 0.1 N alkali required to neutralise WATER SOLUBLE and
volatile fatty acid contained in 5 grams of oil / fat. If the RM value decrease /
increase beyond the range, there is a chance of adulteration.
5. POLENSKE NUMBER (P NUMBER): It can be defined as the number of ml
of 0.1 N alkali required to neutralise WATER INSOLUBLE and volatile fatty
acid contained in 5 grams of oil / fat. If the P value decrease / increase
beyond the range, there is a chance of adulteration.
6. KRISCHNER VALUE (K VLUE): It can be defined as the number of ml of 0.1
N alkali required to neutralise WATER SOLUBLE and volatile fatty acid
contained in 5 grams of oil / fat. If the K value decrease / increase beyond the
range, there is a chance of adulteration. The quantity of butyric acid can be
measured by this test.
IMPORTANCE OF GLYCERIDES IN EVERYDAY LIFE:

Manufacturing vegetable ghee (Hydrogenation)

Soap Industry (Saponification)

In Paint and Varnish Industry (Drying oils)

Candle Industry ( Wax)

Medicine (Castor oil, coconut oil, cod liver oil)

Cosmetics Industry ( Toilet soap, nail polish, perfumes)

Lubricant Industry ( Greasing oil, castor oil)
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WAX
These are the esters of aliphatic monohydric alcohol with higher fatty acids.
E.g.
Bee wax -----------
Neoceryl alcohol
Carnauba wax-------
Carnauba plant
Spermacetic Wax --
Sperm whale
n- octacosanol -----
Wax of wheat blade
Waxes are also found in plants (particularly aquatic) and microorgs where they
form a protective covering (in leaves and fruit) and also found in insect
secretions.
COMPOUND LIPIDS
They are the complex esters of aliphatic mono carboxylic fatty acid with
aliphatic alcohols having nitrogenous base such as choline, ethanolamine etc.
1. PHOSPHOLIPIDS: These are the esters produced by the combination of
fatty acid, glycerol, phosphoric acid and a nitrogenous base.
E.g. Lecithin, Cephalin, Plasmlogens, Phosphoinositis, Sphingomyelins
LECITHIN:
They are found in the growing tissues, egg yolk, brain, kidney,
soybean, nervous tissues, blood etc.
They are occurring in two isomeric forms.
a) α - Lecithin
and b) β – Lecithin
STRUCTURE OF α - LECITHIN
CH2 CO – R’ (STEARIC ACID)
|
|
CHO CO – R” (OLEIC ACID)
|
O
CH3
|
CH2O –O – P – O ---- CH2-CH2—N – CH3
OH
Phosphoric
Acid
OH
CH3
Choline residue
STRUCTURE OF β - LECITHIN
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CH2 CO – R’ (STEARIC ACID)
CH3
|
O
|
CHO - ---- O – P – O ---- CH2-CH2—N – CH3
|
|
OH
OH
CH3
|
|
CH2O –CO – R” (OLEIC ACID)
FUNCTIONS OF LECITHIN

Controls fat metabolism

Used as food product in the form of emulsifying agent

Acts as anti-oxidant to prevent rancidity

Regulate the permeability of cell membrane
CEPHALIN:
The difference between lecithin and cephalin is only in the structure.
Choline residue is replaced with the cholamine, attached to glycerol. Their main
function is in nervous tissues.
Plasmalogen, phosphoinositides and sphingomyelins are components in the
human tissues and have little importance in the plants.
2. GLYCOLIPID: It contains carbohydrate moiety particularly galactose in the
structure.
E.g. Cerebroside is present in brain tissues and act as neuro transmitters.
STEROLS
Sterols are the solid alcohols with high molecular weight having many cyclic rings
in the structure. ( Steros = solid ) , ( ol = alcohol ). They are widely distributed in
plant, animals and micro organisms in conjugated form.
1. CHOLESTEROL : It is a animal sterol with a molecular formula of C27H45 OH
2. ERGOSTEROL: It is a mycosterol with a molecular formula C28H43 OH.
3. Sistosterols or Stigmsterols : It is a phytosterol of plant origin.
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METABOLISM OF LIPIDS:
1. ANABOLISM:
Glucose
Glucose
Glycolysis
Glycolysis
Glyceraldehyde
Pyruvuc acid
Glycerol
Acetyl Co –A
Fatty acid
Glycerol
+
synthesis
Fatty acid -----------> Lipid
2. CATABOLISM:
Lipid -------------> Fatty acid + Glycerol
Lipase
β –oxidation
Acetyl co-A
Glycolysis
Pyruvic acid ------> Acetyl co -A
TCA cycle
TCA cycle
CO2 + H2O + ENERGY
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PHYSIOLOGICAL FUNCTIONS AND IMPORTANCE OF LIPIDS:
1. The main function of fat is that it serves as a source of energy. One gram of
fat gives 9 Kcal. Excess foodstuff is converted to fat in the adipose tissues
in the human.
2. They are structural components of cells. Phospholipids and sterols are
present in biological membrane. They control the movement of materials
into and out of the cell. Lipo proteins also play important role.
3. It acts as a carrier of essential compounds such as fat soluble vitamins (A,
D, E and K). Lipid act as solvent in the body fluid.
4. Their main function is to provide essential fatty acids such as Linoleic acid,
Linolenic acid and Arachidonic acid.
5. Enzyme activation: They sometimes act as enzyme activator. E.g.
Phosphatase requires presence of lipid for maximum activity.
6. Synthesis of hormone such as adrenal corticoid, sex hormones and vitamin
D3 (Cholecalciferol) from lipid derivatives.
7. Heat Insulator: Presence of fats in subcutaneous tissues provide heat
insulation e.g. Whale. It also provides heat insulation to animals of Antarctic
regions and animals living in cold environments.
8. Mechanical Protection: It provides mechanical protection to vital organs
such as heart, brain etc. Because of the presence of waxy materials, it
protects the surface from the attack of microorganisms and insects.
9. Terpenes have medicinal use as mild local irritant.
10. They are used in the perfume and flavour industry because of less solubility
in water.
11. It reduces the bulk of the diet.
12. Some essential oils make the food unpalatable to animals while some
aroma is released to attract the insect for the cross-pollination
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