Lipids

1
 Chemistry of Lipids
Definition:
- Lipids are organic compounds formed mainly
from alcohol and fatty acids combined together
by ester linkage.

O
H2O O
R CH2 OH
+ HO C R R CH2 O C R
Fatty alcohol Fatty acid Esterase (lipase) ester (lipid)
2
- Lipids are insoluble in water, but soluble
in organic solvents (ether, chloroform,
benzene, acetone).

- Lipids include fats, oils, waxes and

related compounds.

They are widely distributed in nature

both in plants and in animals.

3
Biological Importance of Lipids:
1. They are more palatable and storable.
2. they provide more energy per gram than (25%
of body needs) carbohydrates and proteins but
carbohydrates are the preferable source of
energy.
3. Supply the essential fatty acids.
4. Supply fat-soluble vitamins (A, D, E and K).
5. Tissue fat is an essential constituent of cell
membrane and nervous system.
6. A pad for the internal organs to protect them
from outside shocks.

4
 Classification of Lipids
1. Simple lipids (Fats & Waxes)
2. Compound or conjugated lipids
3. Derived Lipids

5
 Introduction

Fatty alcohols
1-Glycerol:
It is a trihydric alcohol (i.e., containing three
OH groups) and has the popular name glycerin.
It is synthesized in the body from glucose.
 It has the following properties:

6
 1. Colorless ,viscous oily, liquid with sweet
taste.
2. On heating with sulfuric acid or KHSO4
(dehydration) it gives acrolein that has a
bad odour.
 This reaction is used for detection of
free glycerol or any compound
containing glycerol.
CH2 OH 2 H2O CHO

HO CH CH
Heating, KHSO4
CH2 OH CH2
Glycerol Acrolein
7
3-It combines with three molecules of nitric acid to
form trinitroglycerin (TNT) that is used as
explosive and vasodilator.
4-On esterification with fatty acids it gives:
 Monoglyceride or monoacyl-glycerol: one fatty
acid + glycerol.
 Diglyceride or diacyl-glycerol: two fatty acids +
glycerol.
 Triglyceride or triacyl-glycerol: three fatty acids
+ glycerol.
5-It has a nutritive value by conversion into glucose
and enters in structure of phospholipids.
8
Uses of Glycerol:

1. Glycerol enters in pharmaceutical and cosmetic

preparations.
2. Reduces brain edema in cerebrovascular
disease.
3. Nitroglycerin is used as vasodilator especially
for the coronary arteries, thus it is used in
treatment of angina pectoris. Also, enters in
explosives manufacturing.
4. Glycerol is used in treatment of glaucoma
(increased intraocular pressure)due to its ability
9
to dehydrate the tissue from its water content.
 2-Sphingosine:
- It is the alcohol present in sphingolipids.
- It is synthesized in the body from serine
and palmitic acid.
It is not positive with acrolein test.

OH
CH3 (CH2)12 CH CH CH CH NH2

CH2OH
Sphingosine
10
Fatty Acids
Fatty acids are long-chain organic acids having usually from
4 to 30 carbon atoms; they have a single carboxyl group and a
long, nonpolar hydrocarbon ‘tail’, which gives most lipids
their hydrophobic and oily or greasy nature.
Fatty acids which occur in natural fats are usually
monocarboxylic and contain an even number of C atoms as
these are synthesized from 2 carbon units.
Fatty acids are stored as an energy reserve (fat) through an
ester linkage to glycerol to form triglycerides.
If free, the carboxyl group of a fatty acid will be ionized.

11
 Fatty Acids
Fatty acids are aliphatic mono-
carboxylic acids
mostly obtained from the hydrolysis of
natural fats and oils.
Have the general formula R-(CH ) -
2 n
COOH
In this formula "n" is mostly an even
number of carbon atoms (2-34) with a
few exceptions that have an odd number.
mostly have straight chain.
12
Fatty acids are classified according to several bases as
follows:
I. According to presence or absence of double bonds
A-Saturated Fatty Acids
contain no double bonds with 2-10 or more
carbons.
solid at room temperature except if they are
short chained.
may be even or odd numbered.
have the molecular formula, CnH2n+1COOH.

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Saturated fatty acids

A-Short chain Saturated F.A. (2-10 carbon).

a-Short chain Saturated volatile F.A.(2-6 carbon).

b- Short chain Saturated non volatile F.A.(7-10

carbon).

B-Long chain Saturated F.A.(more the10 carbon)

14
 a-Volatile short-chain fatty acids:
 They are liquid in nature and contain (2-6)
carbon atoms.
 water-soluble and volatile at room
temperature, e.g., acetic, butyric, and
caproic acids.
Acetic F.A. (2C ) CH3-COOH.
Butyric F.A. (4C ) CH3-(CH2)2-COOH.
Caproic F.A. (6C ) CH3-(CH2)4-COOH.

15
 b-Non-volatile short-chain fatty acids:
They are solids at room temperature and
contain 7-10 carbon atoms.
 They are water-soluble and non-volatile at
room temperature include caprylic and
capric F.A.

Caprylic (8 C ) CH3-(CH2)6-COOH.
Capric (10 C ) CH3-(CH2)8-COOH.

16
B-Long-chain fatty acids:
They contain more than 10 carbon atoms.
They occur in hydrogenated oils, animal fats,
butter and coconut and palm oils.
They are non-volatile and water-insoluble
Include palmitic, stearic, and lignoceric F.A.

 Palmitic(16C) CH3-(CH2)14-COOH
Stearic (18 C ) CH3-(CH2)16-COOH
lignoceric (24C ) CH3-(CH2)22-COOH
17
B-Unsaturated Fatty Acids:
Based on Degree of Unsaturation they are
 Monoethenoid acids: They contain one double bonds .
Molecular Formula :(CnH2n-1 COOH) Ex: Oleic acid.
 Diethenoid acids : They contain two double bonds .
Molecular Formula :(CnH2n-3 COOH) Ex: Linoleic acid.
 Triethenoid acids : They contain three double bonds .
Molecular Formula :(CnH2n-5 COOH) Ex: Linolenic acid.
 Tetraethenoid acids : They contain two double bonds .
Molecular Formula :(CnH2n-7 COOH) Ex: Arachidonic acid.
 Monoethenoid acids are commonly called as
monounsaturated fatty acids (MUFAs) and the remaining
18
ones as polyunsaturated fatty acids (PUFAs).
linoleic, linolenic and arachidonic acid - On
account of the important physiological role, these 3
acids are collectively called as essential fatty acids
(EFA)
1-Monounsaturated fatty acids:
1-Palmitoleic acid :
It is found in all fats.
It is C16:1∆9, i.e., has 16 carbons and one
double bond located at carbon number 9 and
involving carbon 10.

19 CH3-( CH2 )5CH = CH-(CH2)7 –COOH

 2-Oleic acid

Is the most common fatty acid in natural

fats.
It is C18:1∆9, i.e., has 18 carbons and one
double bond located at carbon number 9
and involving carbon 10.

CH3-(CH2)7- CH=CH – (CH2)7-COOH

20
3-Nervonic acid
(Unsaturated lignoceric acid).
 It is found in cerebrosides.
 It is C24:115, i.e., has 24 carbons and one
double bond located at carbon number 15
and involving carbon 16.

CH3 – (CH2)7 CH= CH – (CH2)13- COOH

21
2-Polyunsaturated fatty acids :
(Essential fatty acids):
Definition:
 They are essential fatty acids that can
not be synthesized in the human body
and must be taken in adequate amounts
in the diet.

 They are required for normal growth

and metabolism.

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Source: vegetable oils such as corn oil, linseed oil,

peanut oil, olive oil, cottonseed oil, soybean oil and

many other plant oils, cod liver oil and animal
fats.
 Deficiency: Their deficiency in the diet leads to

nutrition deficiency disease.

Its symptoms include: poor growth and health
with susceptibility to infections, dermatitis,
decreased capacity to reproduce, impaired
transport of lipids, fatty liver, and lowered
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resistance to stress.
 Function of Essential Fatty Acids:
1. useful in the treatment of atherosclerosis
(transporting blood cholesterol and triglycerides).
2. The hormones are synthesized from them.
3. They enter in structure of all cellular and
subcellular membranes and the transporting plasma
phospholipids.
4. They are essential for skin integrity, normal growth
and reproduction.
5. They have an important role in blood clotting
(intrinsic factor).
6. Important in preventing and treating fatty liver.
7. Important role in health of the retina and vision.
8. They can be oxidized for energy production.
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1-Linoleic:
C18:29, 12.
 It is the most important since other essential
fatty acids can be synthesized from it in the
body.

CH3-(CH2)4-CH = CH-CH2-CH=CH-(CH2)7-
COOH

25
 2-Linolenic acid:
C18:39, 12, 15,
in corn, linseed, peanut, olive, cottonseed
and soybean oils.

CH3-CH2-CH=CH-CH2-CH=CH-CH2-
CH=CH-(CH2)7-COOH

26
3-Arachidonic acid:
C20:45, 8, 11, 14.
 It is an important component of phospholipids
in animal and in peanut oil from which
prostaglandins are synthesized.

CH3-(CH2)4-CH=CH-CH2-CH=CH-CH2-
CH=CH-CH2-CH=CH-(CH2)3-COOH

27
 I. Simple Lipids
A. Neutral Fats and oils (Triglycerides)
They are called neutral because they are
uncharged due to absence of ionizable groups in
it.
The neutral fats are the most abundant lipids in
nature.
They are esters of glycerol with various fatty
acids. Since the 3 hydroxyl groups of glycerol
are esterified, the neutral fats are also called
“Triglycerides”.

Esterification of glycerol with one molecule of

fatty acid gives monoglyceride, and that with 2
28 molecules gives diglyceride.
 O O
HO C R1 CH2 OH H2C O C R1
O
O
HO C R2 + HO C H R2 C O C H
O
O
CH2 OH 3 H2O H2C O C R3
HO C R3
Glycerol Triglycerides
Fatty acids (Triacylglycerol)

29
 Types of triglycerides
1-Simple triglycerides: If the three fatty acids
connected to glycerol are of the same type the
triglyceride is called simple triglyceride, e.g.,
tripalmitin.
2-Mixed triglycerides: if they are of different
types of fatty acids connected to glycerol, it is
called mixed triglycerides, e.g., stearo-diolein
and palmito-oleo-stearin.
 Natural fats are mixtures of mixed
triglycerides with a small amount of simple
30 triglycerides.
 O
CH2 O C (CH2)14 CH3
O
CH3 (CH2)14 C O C H
O
CH2 O C (CH2)14 CH3
Tripalmitin
(simple triacylglycerol)
O
CH2 O C (CH2)16 CH3
O
CH3 (CH2)7 CH CH (CH2)7 C O C H
O
CH2 O C (CH2)7 CH CH (CH2)7 CH3
1-Stearo-2,3-diolein
(mixed triacylglycerol)
O
CH2 O C (CH2)14 CH3
O
CH3 (CH2)7 CH CH (CH2)7 C O C H
O
CH2 O C (CH2)16 CH3
1-palmito-2-oleo-3-stearin
31 (mixed triacylglycerol)
  The commonest fatty acids in animal
fats are palmitic, stearic and oleic acids.

The main difference between fats and

oils is for oils being liquid at room
temperature, whereas, fats are solids.

 This is mainly due to presence of

larger percentage of unsaturated fatty
acids in oils than fats, that has mostly
saturated fatty acids.
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Physical properties of fats and oils:
1. Freshly prepared fats and oils are colorless,
odorless and tasteless.
 Any color, or taste is due to association with other
foreign substances, e.g., the yellow color of body
fat or milk fat is due to carotene pigments(cow
milk).
2. Fats have specific gravity less than 1 and,
therefore, they float on water.
3. Fats are insoluble in water, but soluble in
organic solvents as ether and benzene.
4. Melting points of fats are usually low, but
higher than the solidification point.
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Chemical Properties of fats and oils:
1-Hydrolysis:
They are hydrolyzed into their constituents (fatty
acids and glycerol) by the action of super heated
steam, acid, alkali or enzyme (e.g., lipase of
pancreas).
- During their enzymatic and acid hydrolysis
glycerol and free fatty acids are produced.
O O
CH2 O C R1 H2C OH R1 C OH
O Lipase or Acid O
R2 C O C H HO C H + R C OH
2
O
O
CH2 O C R3 3 H2O H2C OH
R3 C OH
Triacylglycerol Glycerol Free fatty acids
34
2-Saponification. Alkaline hydrolysis produces
glycerol and salts of fatty acids (soaps).
Soaps cause emulsification of oily material this
help easy washing of the fatty materials

O O
CH2 O C R1 H2C OH R1 C ONa
O O
R2 C O C H HO C H + R C ONa
2
O
O
CH2 O C R3 3 NaOH H2C OH
R3 C ONa
Triacylglycerol Glycerol Sodium salts of
fatty acids (soap)

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3-Halogenation
Neutral fats containing unsaturated fatty acids
have the ability of adding halogens (e.g., iodine
or iodination) at the double bonds.
 It is a very important property to determine the
degree of unsaturation of the fat or oil that
determines its biological value.
CH3 (CH2)4 CH CH CH2 CH CH (CH2)7 COOH
Linoleic acid
2 I2

CH3 (CH2)4 CH CH CH2 CH CH (CH2)7 COOH

I I I I
Stearate-tetra-iodinate
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4-Hydrogenation or hardening of oils:
It is a type of addition reactions accepting
hydrogen at the double bonds of unsaturated
fatty acids.
The hydrogenation is done under high pressure
of hydrogen and is catalyzed by finely divided
nickel or copper and heat.
It is the base of hardening of oils (margarine
manufacturing), e.g., change of oleic acid of fats
(liquid) into stearic acid (solid).
It is advisable not to saturate all double bonds;
otherwise margarine produced will be very
hard, of very low biological value and difficult
37 to digest.
 Oils Hydrogen, high pressure, nickel Hard fat
(liquid) (margarine, solid)
(with unsaturated (with saturated
fatty acids, e.g., oleic) fatty acids, e.g., stearic)

Advantages for hydrogenated oil or fat are as follows:

1. It is more pleasant as cooking fat.
2. It is digestible and utilizable as normal animal fats
and oils.
3. It is less liable to cause gastric or intestinal irritation.
4. It is easily stored and transported and less liable to
rancidity.
Disadvantages of hydrogenated
 Fats include lack of fat-soluble vitamins (A, D, E and
K) and essential fatty acids
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5-Oxidation(Rancidty)
This toxic reaction of triglycerides leads to
unpleasant odour or taste of oils and fats
developing after oxidation by oxygen of air,
bacteria, or moisture.

Also this is the base of the drying oils after

exposure to atmospheric oxygen.
Example is linseed oil, which is used in paints
and varnishes manufacturing

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 Rancidity
Definition:
It is a physico-chemical change in the
natural properties of the fat leading to the
development of unpleasant odor or taste or
abnormal color particularly on aging after
exposure to atmospheric oxygen, light,
moisture, bacterial or fungal contamination
and/or heat.

 Saturated fats resist rancidity more than

unsaturated fats that have unsaturated
40
double bonds.
Types and causes of Rancidity:
1. Hydrolytic rancidity
2. Oxidative rancidity
3. Ketonic rancidity
1-Hydrolytic rancidity:
 It results from slight hydrolysis of the
fat by lipase from bacterial
contamination leading to the liberation
of free fatty acids and glycerol at high
temperature and moisture.
 Volatile short-chain fatty acids have
unpleasant odor.
41
 O O
CH2 O C R1 H2C OH R1 C OH
O O
Lipase
R2 C O C H HO C H + R C OH
2
O
O
CH2 O C R3 3 H2O H2C OH
R3 C OH
Triacylglycerol Glycerol Free fatty acids
(volatile, bad odor)

42
2-Oxidative Rancidity:
It is oxidation of fat or oil catalyzed by
exposure to oxygen, light and/or heat
producing peroxide derivatives which on
decomposition give substances, e.g., peroxides,
aldehydes, ketones and dicarboxylic acids that
are toxic and have bad odor.

This occurs due to oxidative addition of oxygen

at the unsaturated double bond of unsaturated
fatty acid of oils.

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 Polyunsaturated fatty acid
Oxidant, O2

Peroxyradical

Cyclic peroxide Hydroperoxide

Aldehydes Hydroxy fatty acid

such as malondialdehyde
Other fragments
such as dicarboxylic acids

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3-Ketonic Rancidity:
It is due to the contamination with
certain fungi such as Asperigillus niger on
fats such as coconut oil.

 Ketones, fatty aldehydes, short chain

fatty acids and fatty alcohols are formed.

Moisture accelerates ketonic rancidity.

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 Prevention of rancidity is achieved by:
1. Avoidance of the causes (exposure to light,
oxygen, moisture, high temperature and
bacteria or fungal contamination).
 By keeping fats or oils in well-closed
containers in cold, dark and dry place (i.e., good
storage conditions).
2. Removal of catalysts such as lead and copper
that catalyze rancidity.
3. Addition of anti-oxidants to prevent
peroxidation in fat (i.e., rancidity). They
include phenols, naphthols, tannins and
hydroquinones.
 The most common natural antioxidant is vitamin
E that is important in vitro and in vivo.
46
Hazards of Rancid Fats:
1. The products of rancidity are toxic, i.e.,
causes food poisoning and cancer.
2. Rancidity destroys the fat-soluble vitamins
(vitamins A, D, K and E).
3. Rancidity destroys the polyunsaturated
essential fatty acids.
4. Rancidity causes economical loss because
rancid fat is inedible.

47
 Analysis and Identification of fats and oils
(Fat Constants)
Fat constants or numbers are tests used for:
1. Checking the purity of fat for detection of
adulteration (mixing of impurity).
2. To quantitatively estimate certain properties
of fat.
3. To identify the biological value and natural
characteristics of fat.
4. Detection of fat rancidity and presence of
toxic hydroxy fatty acids.
48
1-Iodine number (or value):
Definition: It is the number of grams of iodine
absorbed by 100 grams of fat or oil.
Uses: It is a measure for the degree of unsaturation
of the fat, as a natural property for it.
Unsaturated fatty acids absorb iodine at their
double bonds, therefore, as the degree of
unsaturation increases iodine number and hence
biological value of the fat increase.
It is used for identification of the type of fat (no. of
I no. of double bonds), detection of adulteration and
determining the biological value of fat.
49
2-Saponification number (value):
Definition: It is the number of milligrams of
KOH required to completely saponify one gram
of fat.
Uses:
Since each carboxyl group of a fatty acid reacts
with one mole of KOH during saponification,
therefore, the amount of alkali needed to
saponify certain weight of fat depends upon the
number of fatty acids present per weight.
Thus, fats containing short-chain acids will
have more carboxyl groups per gram than long
chain fatty acids and consume more alkali, i.e.,
50
will have higher saponification number.
3-Acids Number (value):
Definition:
It is the number of milligrams of KOH
required to neutralize the free fatty acids
present in one gram of fat.
Uses:
It is used for detection of hydrolytic
rancidity (bad odour due to
FFAs)because it measures the amount of
free fatty acids present.
51
4-Reichert- Meissl Number (value):
Definition: It is the number of milliliters of 0.1
N KOH required to neutralize the water-
soluble fatty acids distilled from 5 grams of fat.
Short-chain fatty acid (less than 10 carbons) is
distillated by steam.
Uses: This studies the natural composition of
the fat and is used for detection of fat
adulteration.
 Butter that has high percentage of short-chain
fatty acids has highest Reichert-Meissl number
52
compared to margarine.
5-Acetyl Number (or value):
Definition: It is number of milligrams of KOH
needed to neutralize the acetic acid liberated
from hydrolysis of 1 gram of acetylated fat
(hydroxy fat reacted with acetic anhydride).
Uses: The natural or rancid fat that contains
fatty acids with free hydroxyl groups are
converted into acetylated fat by reaction with
acetic anhydride.
 Thus, acetyl number is a measure of number
of hydroxyl groups present.
 It is used for studying the natural properties of
the fat and to detect adulteration and rancidity.
53
B. Waxes
Definition: Waxes are solid simple lipids
containing a monohydric alcohol (with a higher
molecular weight than glycerol) esterified to long-
chain fatty acids. Examples of these alcohols are
palmitoyl alcohol, cholesterol, vitamin A or D.
Properties of waxes: Waxes are insoluble in water,
but soluble in fat solvents and are negative for
acrolein test.
Waxes are not easily hydrolyzed as the fats and
are indigestible by lipases and are very resistant to
rancidity.
Thus they are of no nutritional value.
54
Type of Waxes:
Waxes are widely distributed in nature such as
the secretion of certain insects as bees-wax,
protective coatings of the skins and furs of
animals and leaves and fruits of plants. They are
classified into true-waxes and wax-like
compounds as follows:

A-True waxes: include:

Bees-wax is secreted by the honeybees that use it
to form the combs. It is a mixture of waxes with
55 the chief constituent is mericyl palmitate.
 O O
C15H31 C OH + C30H61OH C15H31 C O C30H61

Palmitic Mericyl Mericyl

H2O
acid alcohol palmitate

B-Wax-like compounds:
 Cholesterol esters: Lanolin (or wool fat) is
prepared from the wool-associated skin
glands and is secreted by sebaceous glands of
the skin.
 It is very complex mixture, contains both free
and esterified cholesterol, e.g., cholesterol-
palmitate and other sterols.
56
 Differences between neutral lipids and waxes:

Waxes Neutral lipids

1.Digestibility: Indigestible (not hydrolyzed Digestible (hydrolyzed by lipase).
by lipase).
2-Type of alcohol: Long-chain monohydric alcohol + Glycerol (trihydric) + 3 fatty acids
one fatty acid.

3-Type of fatty Fatty acid mainly palmitic or Long and short chain fatty acids.
acids: stearic acid.

4-Acrolein test: Negative. Positive.

5-Rancidability: Never get rancid. Rancidible.

6-Nature at room Hard solid. Soft solid or liquid.

temperature.

7-Saponification Nonsaponifiable. Saponifiable.

8-Nutritive value: No nutritive value. Nutritive.

9-Example: Bee & carnuba waxes. Butter and vegetable oils.

57
 II. Compound Lipids
Definition:
 They are lipids that contain additional substances, e.g.,
sulfur, phosphorus, amino group, carbohydrate, or proteins
beside fatty acid and alcohol.
 Compound or conjugated lipids are classified into the
following types according to the nature of the additional
group:

1. Phospholipids

2. Glycolipids.

3. Lipoproteins

4.58 Sulfolipids and amino lipids.

 1-Phospholipids
Definition: Phospholipids or phosphatides are compound lipids, which
contain phosphoric acid group in their structure.
Importance:
1. They are present in large amounts in the liver and brain as well as blood.
Every animal and plant cell contains phospholipids.
2. The membranes bounding cells and sub-cellular organelles are composed
mainly of phospholipids. Thus, the transfer of substances through these
membranes is controlled by properties of phospholipids.
3. They are important components of the lipoprotein coat essential for
secretion and transport of plasma lipoprotein complexes. Thus, they are
lipotropic agents that prevent fatty liver.
4. Myelin sheath of nerves is rich with phospholipids.
59
5-Important in digestion and absorption of neutral lipids and
excretion of cholesterol in the bile.
6-Important function in blood clotting and platelet aggregation.
7-They provide lung alveoli with surfactants that prevent its
irreversible collapse.
8-Important role in signal transduction across the cell membrane.
9-Phospholipase A2 in snake venom, hydrolyses membrane
phospholipids into hemolytic lysolecithin or lysocephalin.
10-They are source of polyunsaturated fatty acids for synthesis of
eicosanoids.

60
 Sources: They are found in all cells (plant and animal), milk
and egg-yolk in the form of lecithins.

Structure: phospholipids are composed of:

1. Fatty acids (a saturated and an unsaturated fatty acid).

2. Nitrogenous base (choline, serine, threonine, or

ethanolamine).

3. Phosphoric acid.

4. Fatty alcohols (glycerol, inositol or sphingosine).

61
 Classification of Phospholipids are classified into 2 groups

according to the type of the alcohol present into two types:

A-Glycerophospholipids: They are regarded as derivatives of
phosphatidic acids that are the simplest type of phospholipids
and include:
1. Phosphatidic acids.
2. Lecithins
3. Cephalins.
4. Plasmalogens.
5. Inositides.
6. Cardiolipin.
B-Sphingophospholipids: They contain sphingosine as an alcohol
and are named Sphingomyelins.
62
A-Glycero-phospholipids
1-Phosphatidic acids: They are metabolic intermediates in
synthesis of triglycerides and glycero-phospholipids in the
body and may have function as a second messenger. They
exist in two forms according to the position of the phosphate
O
Saturated
 CH2 O C R1
Polyunsaturated O fatty acid

fatty acid R2 C O C H
O
 CH2 O P OH Phosphate
OH
-Phosphatidic acid
O
 CH2 Saturated
O C R1
O fatty acid

Phosphate HO P O C H
OH O
Polyunsaturated
 CH2 O C R2 fatty acid
63 -Phosphatidic acid
2-Lecithins:
Definition:
Lecithins are glycerophospholipids that contain
choline as a base beside phosphatidic acid. They
exist in 2 forms - and -lecithins.
Lecithins are a common cell constituent obtained
from brain (-type), egg yolk (-type), or liver
(both types).
Lecithins are important in the metabolism of fat by
the liver.
Structure: Glycerol is connected at C2 or C3 with a
polyunsaturated fatty acid, at C1 with a saturated
fatty acid, at C3 or C2 by phosphate to which the
choline base is connected.
The common fatty acids in lecithins are stearic,
64 palmitic, oleic, linoleic, linolenic, clupandonic or
Lysolecithin causes hemolysis of RBCs. This partially explains
toxic effect of snake venom,. The venom contains
lecithinase, which hydrolyzes the polyunsaturated fatty
acids converting lecithin into lysolecithin.
Lysolecithins are intermediates in metabolism of phospholipids.

O
CH2 O C R1
O
R2 C O C H
O CH3
CH2 O P O CH2 CH2 N
+ CH3
OH Choline CH3
-Lecithin
O
CH2 O C R1
CH3 O
CH3
+N CH2 CH2 O P O C H
OH O
CH3 Choline
CH2 O C R2
 -Lecithin
65
3-Cephalins (or Kephalins):
Definition: They are phosphatidyl-ethanolamine

or serine. Cephalins occur in association with

lecithins in tissues and are isolated from the
brain (Kephale = head).
Structure: Cephalins resemble lecithins in
structure except that choline is replaced by
ethanolamine, serine or threonine amino acids.

66
 Certain cephalins are constituents of the complex mixture of
phospholipids, cholesterol and fat that constitute the lipid component of
the lipoprotein “thromboplastin” which accelerates the clotting of blood
by activation of prothrombin to thrombin in presence of calcium ions.
O
CH2 O C R1
O
R2 C O C H
O
CH2 O P O CH2 CH2 NH2 Ethanolamine
OH HO CH CH COOH Serine
2
-Cephalin
NH2
HO CH CH COOH Threonine
CH3 NH2
67
4-Plasmalogens:
Definition: Plasmalogens are found in the cell

membrane phospholipids fraction of brain and

muscle (10% of it is plasmalogens), liver, semen
and eggs. Properties: Similar to lecithins.
lecithins
-Unsaturated
CH2 O CH CH R1 fatty alcohol
O
R2 C O C H
O CH3
CH2 O P O CH2 CH2 N CH3
+
OH CH3
68 -Plasmalogen
5-Inositides:
Definition:
- They are phosphatidyl inositol.
Structure: They are similar to lecithins or cephalins but
they have the cyclic sugar alcohol, inositol as the base.
They are formed of glycerol, one saturated fatty acid, one
unsaturated fatty acid, phosphoric acid and inositol.
O
CH2 O C R1
O
R2 C O C H
O OH OH
2 3
CH2 O P O H H H
1 OH 4
OH H
H OH
6 5
-Phosphatidylinositol OH H
69
6-Cardiolipins:
Definition: They are diphosphatidyl-glycerol. They
are found in the inner membrane of mitochondria
initially isolated from heart muscle (cardio). It is
formed of 3 molecules of glycerol, 4 fatty acids and
2 phosphate groups.
Function: Used in serological diagnosis of
autoimmune diseases.
O OH
CH2 O C R1 CH2 O P O CH2
O
O
R2 C O C H H C OH H C O C R3
O
O
CH2 O P O CH2 R4 C O CH2
OH O
70 Cardiolipin
B-Sphingophospholipids
1-Sphingomyelins
Definition: Sphingomyelins are found in brain and
nerves ,in lung, spleen, kidney, liver and blood.
Structure: Sphingomyelins differ from lecithins and
cephalins in that they contain sphingosine as the
alcohol instead of glycerol, they contain two
nitrogenous bases: sphingosine itself and choline.

71
Ceramide is the part of sphingomyelin in which the
amino group of sphingosine is attached to the fatty acid
by an amide linkage.
Ceramides have been found in the free state in the spleen,
liver and red cells.
Ceramide

Sphingosine
Fatty acid
OH O
CH3 (CH2)12 CH CH CH CH NH C R1

CH2 Choline
O CH3
O P O CH2 CH2 N
+ CH3

OH CH3
Phosphate
72 Sphingomyelin
 2-Glycolipids
 Definition: They are lipids that contain carbohydrate
residues with sphingosine as the alcohol and a very
long-chain fatty acid (24 carbon series).
 They are present in cerebral tissue, therefore are called
cerebrosides
 Classification: According to the number and nature of
the carbohydrate residue(s) present in the glycolipids
the following are
1. Cerebrosides. They have one galactose molecule
(galactosides).
2. Sulfatides. They are cerebrosides with sulfate on the
sugar (sulfated cerebrosides).
3. Gangliosides. They have several sugar and sugar-
amine residues.
73
1-Cerebrosides:
Occurrence: They occur in myelin sheath of nerves and
white matter of the brain tissues and cellular membranes.
They are important for nerve conductance.
Structure: They contain sugar, usually -galactose and
may be glucose or lactose, sphingosine and fatty acid, but
no phosphoric acid. Ceramide

Sphingosine
Fatty acid
OH O
CH3 (CH 2)12 CH CH CH CH NH C R1

CH2

CH2OH O
OH O
H
Galactose
OH H
H H

H OH

Psychosin
74
Cerebroside
2-Sulfatides:
Sulfatides are usually present in the brain, liver, muscles
and testes.
OH O
CH3 (CH2)12 CH2 CH CH CH NH C R1

CH2

CH2OH
O O
OH
H
OSO3H H
H H
H OH
Sulfatides (sulfated cerebroside)
75
3-Gangliosides:
They are more complex glycolipids that occur in the gray
matter of the brain, ganglion cells, and RBCs.
They transfer biogenic amines across the cell membrane
and act as a cell membrane receptor.
The most simple type of them is the
monosialoganglioside. It works as a receptor for cholera
toxin in the human intestine.
Ceramide-Glucose-Galactose-N-acetylgalactosamine-Galactose
Sialic acid
Monosialoganglioside

76
 3-Lipoproteins
 Definition: Lipoproteins are lipids combined with
proteins in the tissues.
 The lipid component is phospholipid, cholesterol or
triglycerides. The holding bonds are secondary bonds.
 They include:
1. Structural lipoproteins: These are widely distributed in
tissues being present in cellular and subcellular
membranes.
 In lung tissues, alveolar acting as a surfactant in a
complex of a protein and lecithin. In the eye,
rhodopsin of rods is a lipoprotein complex.
2. Transport lipoproteins:
 These are the forms present in blood plasma. They are
composed of a protein called apolipoprotein and
different types of lipids. (Cholesterol, cholesterol
esters, phospholipids and triglycerides). As the lipid
content increases, the density of plasma lipoproteins
77
decreases.
 Plasma lipoproteins can be separated by two
methods:
1. Ultra-centrifugation: Using the rate of floatation
in sodium chloride solution leading to their
sequential separation into chylomicrons, very low
density lipoproteins (VLDL or pre--lipoproteins),
low density lipoproteins (LDL or -lipoproteins),
high density lipoproteins (HDL or -lipoproteins)
and albumin-free fatty acids complex.
2. Electrophoresis: is the migration of charged
particles in an electric field either to the anode or to
the cathode. It sequentially separates the
lipoproteins into chylomicrons, pre--, -, and -
lipoprotein and albumin-free fatty acids complex.
78
a) Chylomicrons: They have the largest diameter and the
least density.
 they contain 1-2% protein and 98-99% fat. The main lipid
fraction is triglycerides absorbed from the intestine and
they contain small amounts of the absorbed cholesterol
and phospholipids.
b) Very low-density lipoproteins (VLDL) or pre--
lipoproteins: Their diameter is smaller than
chylomicrons.
 They contain about 7-10% protein and 90-93% lipid. The
lipid content is mainly triglycerides formed in the liver.
They contain phospholipid and cholesterol more than
chylomicrons.
c) Low-density lipoproteins (LDL) or -lipoproteins:
They contain 10-20% proteins in the form of apolipoprotein B.
 Their lipid content varies from 80-90%. They contain about 60% of
79
total blood cholesterol and 40% of total blood phospholipids. As
their percentage increases, the liability to atherosclerosis increases.
d) High-density lipoproteins (HDL) or -
Lipoproteins: They contain 35-55% proteins in the form
of apolipoprotein A.
They contain 45-65% lipids formed of cholesterol (40% of
total blood content) and phospholipids (60% of total blood
content).
They act as cholesterol scavengers, as their percentage
increases, the liability to atherosclerosis decreases.
They are higher in females than in males. Due to their high
protein content they possess the highest density.

e) Albumin-free fatty acids complex:

It is a proteolipid complex with 99% protein content associated
with long-chain free fatty acids for transporting them.
80
 III. Derived Lipids

Prostaglandins
Synthesized from arachidonic acid
Several metabolic functions
Steroids
Cholesterol, ergosterol, bile acids
Terpenes
Made by plants
Carotenoids, xanthophylls

81
 Cholesterol Vitamin D3
(a sterol) (cholecalciferol)

Testosterone
(a steroid hormone) Stigmasterol
(a phytosterol)
82
Cholesterol:
Importance: -
It is the most important sterol in animal tissues as free alcohol
or in an esterified form (with linoleic, oleic, palmitic acids).
 Steroid hormones, bile salts and vitamin D are derivatives
from it.
 Tissues contain different amounts of it that serve a structural
and metabolic role, e.g., adrenal cortex ( 10%,), brain ( 2%),
others 0.2-0.3%.
Source: - It is synthesized in the body from acetyl-CoA
(1gm/day)
 cholesterol does not exist in plants.
It is also taken in the diet (0.3 gm/day as in, butter, milk, egg
yolk, brain, meat and animal fat).
83
 Physical propeties: It has a hydroxyl group on C3, a
double bond between C5 and C6, 8 asymmetric carbon
atoms and a side chain of 8 carbon atoms.
It is found in all animal cells, corpus luteum and adrenal
cortex, human brain .
In the blood (the total cholesterol amounts about 200
mg/dL of which 2/3 is esterified, chiefly to unsaturated
fatty acids while the remainder occurs as the free
cholesterol. CH3
18
CH3
12 CH3 CH3
16 CH3
19 11 13 17
CH3 C D CH3
1 9 14 15
2
A 5 10 B 8
HO 3 4 6 7
HO
Steroid ring
84 Cholesterol
Chemical properties Intestinal bacteria reduce
cholesterol into coprosterol and dihydrocholesterol.
- It is also oxidized into 7-Dehydrocholesterol and further
unsaturated cholesterol with a second double bond
between C7 and C8. When the skin is irradiated with
ultraviolet light 7-dehydrocholesterol is converted to
vitamin D3. This explains the value of sun light in
preventing rickets. CH
CH3 3
CH3 CH3
CH3 CH3
CH3 CH3
CH3 CH3

HO HO

H H
Coprosterol, Dihydrocholesterol,
in feces in blood and other tissues
85
Ergosterol differs from 7-dehydrocholesterol in the side
chain. Ergosterol is converted to vitamin D2 by
irradiation with UV. Ergosterol and 7-
dehydrocholesterol are called Pro-vitamins D or
precursors of vitamin D.
- It was first isolated from ergot, a fungus then from
yeast. Ergosterol is less stable than cholesterol (because
of having 3 double bonds). CH
CH3 3
CH3 CH3
CH3 CH3
CH3 CH3 CH3
CH3 CH3

HO HO

86
7-dehydrocholesterol Ergosterol