CHEMISTRY OF LIPIDS
Questions on Lipid Chemistry
1.
2.
3.
4.
Classify lipids with examples
Essential fatty acid
Name the essential fatty acids and one function of EFA
Name essential fatty acids and mention their functions
5. Name the essential fatty acids and their importance
6. What are eicosanoids? Mention them with their functions.
(3/4)
(3)
(3)
(3)
(3)
(3)
7. What are eicosanoids? Mention their biomedical importance.
(3)
8. What are prostaglandins? Mention their functions.
(3)
9. What are prostaglandins? Mention their biological functions.
(3)
10. Prostaglandins
(3)
11. Polyunsaturated fatty acids
(3)
12. Triglycerides.
(3)
13. Explain the biochemical basis: a) Oils are liquid at room temperature b) Fats are solid at room
temperature.
(3)
14. Rancidity
(3)
15. What is the chemical basis of rancidity? How can it be prevented?
(3)
16. Compound lipids
(3)
17. Name three phospholipids and mention their function.
(3)
18. What are phospholipids? Give 3 examples. Mention their functions
(3)
19. What are phospholipids? Mention the composition of 3 of them with their functions. (3)
20. How are phospholipids classifieds? Give any 3 important functions of phospholipids. (3)
21. Classify phospholipids with examples. Mention their functions.
(3)
22. Chemistry of phospholipids.
(3)
23. Write short notes on a) Lecithin b) PUFA
(3)
24. Write briefly on the lung surfactant.
(3)
25. Give examples and compositions of a) Glycophospholipid b) Sphingophospholipid (3 )
26. Write the product formed due to complete hydrolysis of a) Lecithin b) Sphingomyelin (3)
27. Give one example and one function for a) phospholipid b) PUFA c) Cerebrolipid
(3)
28. Write a note on: a) Cerebrosides and b) Gangliosides
(3)
29. Give 3 examples for each of the following. a) Amphipathic lipids b) Glycolipids
(3)
30. What are lipoproteins? Describe their composition and functions
(3/4)
31. Describe the composition and functions of plasma lipoproteins.
(3/4)
32. Lipoproteins.
(3/4)
a) Define ‘Lipoprotein’. Classify lipoproteins. Explain lipoprotein structure with a diagram (3/4)
33. Classify lipoproteins. Mention their function.
(3/4)
34. Explain the biochemical basis: a) Sunflower oil gets rancid sooner than hydrogenated oils.
b) HDL-cholesterol is “good” cholesterol c) LDL-cholesterol is “bad” cholesterol.
(3/4)
1
CHEMISTRY OF LIPIDS
By Dr. Vinitha Ramanath Pai
NOTE : Superscript numbers are references for a deeper understanding , the details of which
are listed at the end of the chapter.
Definition :
Lipids are a group of heterogeneous organic biomolecules, relatively insoluble in water and
soluble in organic solvents 1.
They are hydrophobic/non-polar nature is due to the hydrocarbon chain (-CH2-CH2-).
Lipids are widely distributed in nature and present in both plants and animals
Biological Importance :
1. Fats and Oils (triacylglycerol) are concentrated form of energy. They are present in adipose
tissue found subcutaneously and around organs. Subcutaneous fat serves are shock absorber,
thermal insulator and gives contour to the body for females. The fat around organs gives
cushioning effect for protection of the organs.
2. Phospholipids and cholesterol are structural components of cell membrane. They are also
precursors of biologically important compounds.
3. In addition phospholipids serve as surfactants, detergents and as emulsifying agents (help in
lipid absorption.
Medical Importance :
1. Obesity
: excessive deposition of fat in adipose tissue.
2. Atherosclerosis: Abnormality in cholesterol metabolism. Atherosclerosis leads to myocardial
infarction.
3. Ketosis
: derangement in the metabolism of fatty acid in type-1 Diabetes mellitus.
4. Respiratory Distress Syndrome (RDS) : is due to lack of surfactant in the lungs.
5. Lipid storage disorders, essential fatty acid deficiency, dys lipoproteinemias
are some other diseases associated with lipid metabolism.
2
Classification of Lipids
Bloor's Classification : Based on the structure of lipids are classified into 3 classes :
1) Simple lipids
2) Compound lipids
3) Derived lipids
LIPIDS
Simple Lipids
Fats and Oils
Compound Lipids
Derived Lipids
Waxes
(Acylglycerols)
e.g. Myricyl palmitate
e.g. Triacylglycerol
(Tripalmitoyl glycerol)
PHOSPHOLIPIDS
Glycerophospholipids
e.g. Lecithin
Sphingophospholipids
e.g. Sphingomyelins
Cerebrosides
e.g Glucocerebroside
STEROLS AND STEROID HORMONES
e.g Cholesterol
and Testosterone
GLYCOLIPIDS
FATTY ACIDS
e.g Palmitic acid
Gangliosides
Globosides
e.g GM3
e.g Lactosyl ceramide
ALCOHOLS
e.g .Sphingosine
POLYISOPRENOIDS
e.g Vitamin K
3

Simple lipids : are esters of fatty acids with various alcohols (and carry no other
substances)

Compound lipids : are esters of fatty acids with different alcohols, but carry in addition
other substances such as phosphates, nitrogenous bases or non nitrogenous molecules
/ base and carbohydrates.

Derived lipids : are derived from simple or compound lipids by hydrolytic cleavage of
the ester bonds or are derived from isoprene units.
(Neutral lipids : Acylglycerols (e.g. triacylglycerol), cholesterol and cholesteryl esters are called
as neutral lipids . Reason : they are uncharged.)
NOTE : Some authors include ( i) lipoproteins as the third subclass of compound lipids.
However, lipoproteins is a particle and not a single covalent structure. It is a non-covalent
supramolecular aggregation of many molecules of lipids and proteins.
(ii) Glycerol in the class derived lipids. But since glycerol is water soluble it is not a lipid.
Fatty Acids (FA)
Fatty acids are components of simple, compound and derived lipids.
Chemistry : Fatty acids are aliphatic hydrocarbon chains with a terminal carboxylic group.
General formula : R-COOH
Structure of a fatty acid : They are mainly linear. (Branched and cyclic fatty acids are also
known)
CH3 - (CH2)n - COOH
where n = number of CH2 groups.
Example : CH3 - (CH2)14 - COOH
eg : Palmitic acid (C16 fatty acid).
16
15 14
13 12
11 10
9
8
7
6
5
4
3
2
1
CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2- CH2- CH2 - COOTerminal methyl group
Non Polar
Carboxyl group
Polar end
4
Fatty acids are :
 Components of simple and compound lipids. Hence fatty acids are hydrolytic products
of simple and compound lipids and belong to the class of derived lipids.
 Fatty acids exist in two forms : (i) Esterfied form (ii) Nonesterified form (NEFA) or free
fatty acids (FFA).
 They also exist freely in nature.
 They occur mainly in esterified form in various lipids such as triacylglycerols (TG),
phospholipids (PL), glycolipids, cholesteryl esters, etc.
· In plasma, free fatty acids (although they are said to be free) are bound to the plasma
protein, albumin, which transports it in blood. This is referred to as non-esterified fatty
acids (NEFA) or free fatty acids (FFA).
Naming of Carbon Atoms in fatty acids
 Starting from carboxyl group.
 Starting from the terminal methyl group
 Starting from carboxyl group: Carbon atoms of fatty acids are numbered starting from
carboxyl group and this type is called “C-system of numbering”.
(i) By C-system they are numbered as C1, C2, C3, etc. starting from carboxyl group (as shown in
figure)
(ii) The  (alpha) carbon is the carbon adjacent to the carboxyl group (carbon number 2),  is
C3 and the  (omega) is the last carbon of the chain (because omega is the last letter of
the Greek alphabet).
 Starting from the terminal methyl group :  - or n - system, carbon atoms are numbered as
1, 2, 3, etc, starting from the terminal methyl group, which is termed as  -carbon or ncarbon.
Classification of Fatty Acids 2 :.
(I ) Based on the even number or odd number of C atoms : they are classified as
5
o Even-chain : Most often fatty acids are even chain. They have even number of carbon
atoms, because they are synthesized in the body by joining 2-carbon units of acetate.
Eg. : C16 Palmitic acid.
o Odd-chained :have odd no. of carbon atoms and are rarely present in body.
Eg : C3 Propionic acid.
(II) Based on the chain length (i.e., number of carbon atoms present) can be classified as
o Short chain
(2 to 6 carbon atoms) eg. Butyric acid (C4)
o Medium chain (8 to 14 carbon atoms) eg. Lauric acid (C12)
o Long chain
(16 to 24 carbon atoms) eg. Stearic acid (C18)
NOTE : Fatty acids having  C10 are water soluble .
(III) Based on the presence or absence of double bond, fatty acids are classified as
o Saturated
: no double bonds eg. Stearic acid (C18) CH3 – (CH2)16- COOH
o Unsaturated : having one or more double bonds.
Eg. : Oleic acid (C18 with one double bond),
CH3 – (CH2)7 - CH = CH - (CH2)7- COOH
NOTE : All short and medium chain fatty acids are saturated and
all unsaturated fatty acids are long chained (C16 and above).
Unsaturated FA maybe
1) Monounsaturated fatty acids
: containing single double bond eg. Oleic acid C18:1:9
2)Polyunsaturated fatty acids (PUFA) :
containing 2 or more double bonds.
Eg. Linoleic acid C18 : 2 : 9,12
Isomerism in unsaturated fatty acids :
Due to the presence of double bonds, unsaturated fatty acids show Cis – Trans isomerism.
 Cis-Form : is most common. In Cis form the 2 hydrogen atoms, of the carbon atoms
linked by the double bond are on the same side of the double bond.
 Trans- form : the H atoms are on the opposite sides of the carbons linked by double
bond.
6
Cis form
H
H
H
C = C
CH2
Trans form
- CH2 - CH2- C = C - CH2 - CH2 CH2
H
NOTE : (i ) Cis-double bond induces a bend in the fatty acid molecule
Reason : the hydrophobic interactions between the 2 hydrocarbon chains which are
on the same side of the double bond in the fatty acid chain.
(ii) Fatty acid molecule with trans-double bond is a straight chain.
Oleic acid (Cis Isomer)
Eladic acid (Trans Isomer)
Biological importance of Cis – trans isomerism in fatty acids :
Cis :
o Cis double bonds render fluidity to the cell membrane
o One of the reasons for oils being liquids at room temperature is due to presence of Cis
double bonds in the fatty acid component of oil.
Trans :
o Not as common as Cis type fatty acids.
o (I )Found in hydrogenated oils (ii) product of bacterial synthesis
7
Position of the double bond in the fatty acid chain:
Position of the double bond in the fatty acid chain is denoted by ‘’
For example, 9 means double bond is located between C9 and C10.
Example :


CH3-CH2-CH2-CH2-CH2-CH2-CH2 = CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH
16
10 9
1
No. of carbon atoms is
:
16
(C16)
No. of double bonds
:
1
(monounsaturated)
Position of the double bonds
:
between C9 and C10 (9)
Structure
Name of the fatty acid
:
:
CH3-( CH2 )5CH = CH-(CH2)7 –COOH
Palmitoleic acid
C16 : 1 : 9
It is also referred to as - 7 ( position of double bond is at 7 starting from the  end ) series or
n- 7 series.
 Series of unsaturated fatty acids :
 This is based on the position of the terminal double bond from the  end in unsaturated
fatty acids, it maybe -3 , -6, -7, -9 positions.
 Unsaturated fatty acids belonging to -3 and -6 series or family (n-3 or n-6 series /
family) are of biological importance. (i.e., Essential Fatty Acids)
Essential Fatty Acids (EFA):
Definition : Essential fatty acids are not synthesized in the body , but are essential for the
normal functioning of the body and therefore, they have to be taken in the diet.
Examples : Linoleic acid
(C18 : 2 : 9,12
-6)
Linolenic acid
(C18 : 2 : 9,12,15 ,
-3) and
5,
8,
11,
14,
Arachidonic acid (C20 : 4 : .
, -6)
These are polyunsaturated fatty acids (PUFA).
Body requires a balanced intake of -3 (linolenic acid) and -6 (linoleic acid and arachidonic
acid) essential fatty acids
Sources of essential fatty acids :
Linoleic acid and Linolenic acid : Good sources are mainly vegetable oils such as corn oil,
linseed oil, peanut oil, olive oil, cottonseed oil, soyabean oil.
In addition, cod liver oil and animal fats are also good sources of linoleic acid.
8
Arachidonic acid : peanut oil and phospholipids in animal.
Functions of essential fatty acids :
o Essential fatty acids are precursors of a group of biologically important compounds
called eicosanoids, which include – prostaglandins, prostacyclins, thromboxanes and
leukotrienes having diverse functions in the body.
o Unsaturated fatty acids increase the fluidity of the cell membrane.
o Dietary PUFA (that also include the essential fatty acids) has the effect of lowering blood
cholesterol level.
o EFA prevent fatty liver i.e., act as liportropic factor.
Functions of fatty acids other than essential fatty acids :
o Fatty acids are concentrated source of energy. They are stored as TAG , which are esters
of fatty acids with glycerol).
o Unsaturated fatty acids increase the fluidity of the cell membrane.
o They are constituents of waxes, compound lipids and cholesteryl esters, in the esterified
form.
o Eicosapentaenoic acid (Timnodonic acid) which is present in many fishes, can
completely replace the dietary requirement for linolenic acid.
o Docosahexaenoic acid (DHA) (Cervonic acid) is an -3 fatty acid that is a primary
structural component of the human brain and retina.
Chemical structure : C22 : 6 - cis double bonds,  - 3 series.
Source : fish oils are rich in DHA.
Deficiency of essential fatty acids :
Seen mainly in children and infants fed on low fat milk formula. Rarely seen in adults, except in
patients maintained on prolonged IV nutrition low in essential fatty acids.
In children, dietary deficiency of essential fatty acids can produce phrynoderma (toad skin)
characterized by rough, thickened, horny skin with keratinized papules, particularly on arms,
buttock and thighs, loss of hair, poor wound healing prevented and fatty liver.
Prevention of deficiency : by intake of EFA; i.e., 1-2 % of total calorie requirement.
Eicosanoids :
Include – prostaglandins, prostacyclins, thromboxanes and leukotrienes.
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Eicosanoids are derived from C20 PUFA. Ex: Arachidonic acid.
Functions of eicosanoids
• act as local hormones. They have actions on
1. smooth muscle of blood vessels, bronchioles , GIT and uterus
2. Platelet aggregation
3. Inflammatory and allergic reactions.
Use of eicosanoids as potential therapeutic agents :
(i) Induction of labor and termination of pregnancy.
(ii) Prevention of conception (contraceptives).
(iii) Bronchodilator in asthma and as decongestant in nasal congestion.
(iv) Treatment of gastric ulcer.
Classes of lipids :
1. Simple lipids :
Inculde 1) fats and oils and 2) waxes.
2) Waxes :
Esters of long-chain fatty acids with higher monohydric aliphatic alcohols.
Waxes are widely distributed in nature, such as secretions of certain insects (like bee-wax),
protective coatings of the skins and furs of animals and leaves and fruits of plants.
Examples : Bee wax (ester of myricyl alcohol and palmitic acid ),
wax like compound lanolin (from wool).
NOTE – Waxes have no nutritional value
Uses of waxes :
Waxes are useful as a base in the manufacture of lotions, cosmetics, ointments, lubricants and
polishes.
1. Fats and Oils (Acylglycerols)
At room temperature, fats are solid and oils are liquid.
Naturally occurring fats and oils are a mixture of acylglycerols.
Acylglycerols are esters of one to three fatty acids with glycerol (a trihydric alcohol).
10
The COOH group of fatty acids are linked to –OH group of glycerol by ester bonds.
() 1 CH2 – OH
HO–CH 2 ()
( ’)3 CH2 – OH
Glycerol
Monoacylglycerol (MAG)
1
2
1-MAG
2-MAG
Diacylglycerol (DAG)
Triacylglycerol (TAG)
1
1
2
3
1,2 – DAG
1,3 - DAG
TAG, are the major constituents in fats and oils. They are the storage form of lipids mainly in
the adipocytes of the adipose tissue.
The commonest fatty acids in animal fats are oleic, palmitic and stearic acids.
The three fatty acids in TAG maybe either of the same type (e.g., tristearrin) or 2 / 3 different
types (E.g. 1, 3 dipalmitoyl-2-olein).
• Oils are TAG which is liquid at room temperature. E.g. all vegetable oils e.g., mustard oil
ground nut oil, etc., and fish oils e.g., cod liver, shark liver.
• Fats are TAG which is solid at room temperature. E.g. Lard and tallow (solid animal fats).
11
Reason : Fats are solid and oils are liquid at room temperature because
fat contain more of long chain, saturated fatty acids, and oils contain more of short and
medium chain, unsaturated fatty acids with cis double bonds.
Butterfat (milk fat) is fairly fluid in spite of containing mostly saturated fatty acids because of
short chain fatty acids and higher water content.
Chemical Properties of TAG ( Important Reactions of TAG)
1. Hydrolysis :
On hydrolysis, each molecule of TAG yields 1 molecule of glycerol and 3
molecules of fatty acids. MAG and DAG are intermediates of fat hydrolysis.
TAG + 3 H2O → Glycerol + 3 Fatty acid
Catalysed by : super heated steam, acid, alkali or enzyme lipase.
Lipases catalyze the hydrolysis of fat during its digestion in the GI tract.
2. Saponification
Hydrolysis of fats and oils by alkali is called saponification3. The products are glycerol and the
alkali salts of the fatty acids called soaps.
Triacylglycerol
+
NaOH → Glycerol + Na-salt of fatty acid (a soap)
3. Hydrogenation :
Hydrogenation of unsaturated oil in the presence of a catalyst (nickel) is known as ‘hardening’.
It is commercially valuable as a method of converting oils (liquid) of plant origin into solid fats
as margarines, vegetable ghee, etc. Hydrogenation increases the shelf life and prevents
rancidity of the fat.
H2
-CH2-CH2-CH=CH-CH2-CH2
-CH2-CH2-CH2-CH2-CH2-CH2Nickel, high temperature, pressure.
Trans fatty acids are byproducts of hydrogenation which are harmful to the body.
The effect of trans fatty acids on the serum lipoprotein profile is, they raise LDL cholesterol (bad
cholesterol) levels and lower HDL cholesterol (good cholesterol) levels increasing the risk of
coronary heart disease.
4. Iodine Number
It is defined as the number of grams of iodine taken up by 100g of fat or oil. It gives degree of
unsaturation of fatty acids in oils .
12
I2
-CH2-CH2-CH=CH-CH2-CH2
I
I
CH2-CH2-CH -CH-CH2-CH2-
i.e. Higher the iodine number, higher is the degree of unsaturation.
E.g. Coconut oil 7 – 10 ;
Butter 20 –30 ;
Ground nut oil 85 – 100
5. Perodixation of fats and oils :
Rancidity
Definition : Refers to deterioration of fats and oils when exposed to air. Rancid fats / oils have
unpleasant odor or taste and are unsuitable for human consumption. Rancidity happens
because of auto-oxidation of lipids (lipid peroxidation) when exposed to atmospheric oxygen/
light/ moisture/ bacterial or fungal contamination and/or heat.
Rancidity occurs because of i) auto-oxidation (ii) hydrolysis by enzymes i.e., lipases.
• Oxidative rancidity : Lipid peroxidation occurs due to oxidation of the double bonds in
unsaturated fatty acids of oils, which produces free radicals.
Free radicals (ROO, RO, OH, etc.) are molecules or molecular species containing one or more
unpaired electrons. Free radicals are highly reactive compounds.
Lipid peroxidation is a chain reaction (involving three phases – chain initiation, propagation and
termination) generating a continuous supply of free radicals that initiate further peroxidation.
In living cells, lipids undergo oxidation to produce free radicals and peroxides, which can
damage the tissues and may be the cause for cancer, inflammatory diseases, atherosclerosis,
aging, etc. Cell membrane lipids, DNA and proteins are the main targets of free radical action.
Prevention of rancidity is achieved by addition of anti-oxidants.
Antioxidants :
Antioxidants are compounds which prevent or reduce lipid peroxidation.
Examples : Vitamin C and vitamin E.
13
Functions of TAG :
TAG are stored in areas such as adipocytes (fat cells) of the adipose tissue, found in the
subcutaneous layer and around soft organs. TAG is stored in the form of globules.
• They are concentrated storage form of energy in the body. 1 gm of fat provides 9 kilo calorie
(or Calorie) of energy in the body, compared to 4 kcal for carbohydrates and proteins.
• Subcutaneous fat also serves as shock absorber, thermal insulator and gives contour or
shape – rounder limbs, fuller breasts and gluteal region – to the body of females (a
secondary sexual characteristic).
• A layer of TAG protects the internal organs by providing cushioning effect.
2. Compound Lipids
Definition : are esters of fatty acids with different alcohols, but in addition carry other
substances such as phosphates and nitrogenous bases or non nitrogenous molecules / base or
carbohydrates . Compound lipids are constituents of cell membrane.
In general compound lipids have
 polar head (phosphate, nitrogenous base / non-nitrogenous molecule / carbohydrate.
 non-polar hydrocarbon tail/s (i.e., the side chains of the fatty acids).
Molecules carrying both polar and non-polar groups are called amphipathic molecules.
Diagrammatically represented as
Classification of Compound Lipids :
Depending on the additional groups they are further divided into,
A) phospholipids and B) glycolipids. (as shown in Fig. Classification of Compound Lipids ).
A. Phospholipids (PL)
Phospholipids are main lipid constituents of membranes. They have also other important
functions
Structure: phospholipids are composed of:
1. Fatty alcohol
2. Fatty acids
3. Phosphoric acid
4. Non lipid – groups : Nitrogenous base / Non-nitrogenous molecule/ carbohydrate.
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Compound Lipids
PHOSPHOLIPIDS
GLYCOLIPIDS
Sphingolipids
Glycerophospholipids
e.g. Lecithin
Sphingophospholipids
e.g. Sphingomyelins
Cerebrosides
e.g Glucocerebroside
Gangliosides
e.g GM3
Globosides
e.g Lactosyl ceramide
Fig : Classification of Compound Lipids
Depending on the alcohol group present phospholipids may be:
1) Glycerophospholipids (glycerol) and
2) Sphingophospholipids (sphingosine)
1) Glycerophospholipids :
The alcohol in glycerophospholipids is glycerol. In addition they contain fatty acids, phosphate
and a nitrogenous base or a non-nitrogenous molecule.
Glycerophospholipids often contain an unsaturated fatty acid at C2 of the glycerol moiety.
Important glycerophospholipids :
With Nitrogenous bases :
1. Lecithins
( phosphatidyl choline )
2. Cephalins
(phosphatidyl ethanolamine)
3. Phosphatidyl serine
With non nitrogenous molecules :
4. Inositides.
(Phosphatidyl inositol)
15
5. Cardiolipin
( Phosphatidyl glycerol)
6. Plasmalogens
Glycerophospholipids : With Nitrogenous bases :
Sl.No.
Name of Phospholipid
Base
With Nitrogenous bases :
1.
Cephalin 4- phosphatidyl
Ethanolamine
ethanolamine
2.
Lecithin 5 - Phosphatidyl choline Choline
3.
Phosphatidyl serine 6
With non nitrogenous molecules :
4.
Phosphatidyl inositol 7
5.
Serine
Inositol
Cardiolipin 8 - Diphosphatidyl
glycerol
Glycerol
phosphatidic acid
Function of phospholipid
is present more in brain and liver
major phospholipids of cell
membrane .
Dipalmitoyl lecithin is a lung
surfactant
found in most tissues
precursor of second messenger
for hormone action (1,2 – DAG
and Inositol triphosphate)
Mitochondrial membrane is rich
in cardiolipin
2. Sphingophospholipids
These are phospholipids containing an amino alcohol sphingosine9 instead of glycerol.
The combination of sphingosine plus fatty acid is known as ceramide10 (acyl sphingosine), a
structure also found in the glycolipids.
Ceramide
=
Sphingosine
Fatty acid
Sphingomyelins11 contain ceramide, phosphate group and choline.
Sphingosine
Fatty acid
Phosphate
Choline
Sphingomyelin
16
Sphingomyelins are found in large quantities in all nervous tissues brain and peripheral nervous
tissues (important constituent of myelin).
Functions of Phospholipids
Amphipathic nature of phospholipids enables it to bind both polar and non-polar substances
simultaneously and is the basis for most of its functions:
1. Phospholipids are the major component of cell membrane. (They comprise about 40%
of lipids in the RBC membrane and over 75% in the inner mitochondrial membrane.)
2. Lecithin along with bile salts help in the intestinal absorption of digested products of
dietary lipids.
3. They are essential for the formation of lipoproteins, which help in the transport of lipids
in blood.
4. Dipalmitoyl lecithin acts as lung surfactant, which prevents alveolar collapse. The
absence of the surfactant increases the surface tension of the membrane resulting in
respiratory distress syndrome in infants.
5. Phospholipid is are required for clotting of blood
6. Phospholipids are activating factors for lipoprotein lipase (involved in lipoprotein
metabolism) and pancreatic lipase (involved in digestion of fat in the intestine).
7. Sphingomyelin is the essential component of the myelin sheath of the nerves.
8. Plasmalogens (platelet activating factor) are involved in platelet aggregation.
9. Thomboplastin (coagulation factor III), needed to initiate clotting, is composed of
phospholipids and tissue factor.
10. Phosphatidylinositol is a precursor of second messenger for the action of certain
hormones.
11. Membrane phospholipids are sources of essential fatty acids (usually esterified at the
C2 position) required for the synthesis of eicosanoids.
12. Lecithin plays a role in the esterification of free cholesterol to form cholesteryl ester
and thus help in the transport of cholesterol from extra hepatic tissue to the liver by the
high density lipoprotein (HDL).
B. Glycolipids (Glycosphingolipids)
These are esters of fatty acid with alcohol sphingosine9 with an additional carbohydrate group.
They are widely distributed in every tissues of the body.
17
They do not contain phosphate and are important constituent of nerve tissues and cell
membrane.
Glycolipids particularly occur in the outer layer of the bilipid layer of the plasma membrane,
where the carbohydrate units are seen projecting towards the exterior of the cell membrane
(cell surface carbohydrates).
Functions of Glycolipids :
1. Glycolipids are important component of the cell membrane and abundantly found in nervous
tissue.
2. Glycolipids of RBC membrane are responsible for blood group antigens.
According to the nature of carbohydrate glycolipids are further classified as follows :
Glycolipids
Cerebrosides
Gangliosides
Ceramide +
Ceramide + oligosaccharide
Glucose / Galactose
with Sialic acid
e.g., Glucocerebroside
e.g., GM3
Globosides
Ceramide + Oligosaccharide
without Sialic acid
e.g., Lactosyl ceramide
Sulfated cerebrosides - Sulfatides
Subclasses of Glycolipids
1) Cerebrosides
Cerebrosides are ceramide + carbohydrate i.e., monosaccharide (glucose/galactose)
Cerebroside = Sphingosine - Fatty acid
Monosaccharide
E.g. : Glucosyl ceramide (glucocerebroside)
– [ceramide + glucose]
Predominant , simple glycosphingolipids in tissues, but found in small amounts in the brain
tissue.
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Galactosyl ceramide (galactocerebroside) – [ceramide + galactose]
This is the major glycolipid of brain and other nervous tissues, but is found in low amounts
in other tissues.
Sulfatides are sulfated cerebrosides. E.g. -sulfogalactocerebroside.
Ceramide - Galactose sulfate
2) Gangliosides
Ceramide + oligosaccharide units bearing one or more sialic acid residues in the
oligosaccharide.
E.g. GM3 : Structure: Ceramide–Glucose–Galactose–N-acetylneuraminic acid (a sialic acid).
They are present in the nervous tissues in high concentrations.
3) Globosides
These are also ceramide oligosaccharide complexes. No sialic acid in the oligosaccharide part.
[ceramide + oligosaccharide unit, without sialic acid]
E.g. Lactosyl ceramide (ceramide + galactose + glucose) is an important component of RBC
membrane.
3. Derived Lipids
Derived lipids are derived from simple or compound lipids by hydrolytic cleavage of the ester
bonds or derived (synthesized in the living organisms) from isoprene units12.
Classification of Derived Lipids
Derived Lipids
Sterols and Steroids
e.g Cholesterol
and Testosterone
Fatty Acids
e.g Palmitic acid
Alcohols
e.g Sphingosine
Polyisoprenoids
e.g Vitamin K
Subclasses of derived lipids are – a) sterols (and steroids), b) fatty acids, c) alcohols and d)
polyisoprenoids.
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a) Sterols and Steroids
Sterols have cyclopentano-perhydro-phenanthrene ring having one or more hydroxyl groups
and no carbonyl or carboxyl groups (see structure of cholesterol).
E.g., Cholesterol, ergosterol (present in plants and yeast, a precursor of vitamin D).
Coprostanol (occurs in feces, derived in the intestine by bacterial action on dietary cholesterol).
7-dehydro cholesterol (an intermediate in cholesterol synthesis and precursor of vitamin D
which is synthesized in the skin, when irradiated with ultraviolet light).
Steroids are metabolically derived from sterols and have carbonyl or carboxyl groups and other
side chains.
E.g., steroid hormones [glucocorticoids, mineralocorticoids (both are adrenocortical hormones)
and sex hormones], etc.
b) Fatty Acids (discussed earlier)
c) Alcohols
E.g. Sphingosine and cetyl alcohol are derived from sphingolipids and cetyl palmitate (a wax),
respectively.
d) Polyisoprenoids
These are linear or cyclic compounds made of 2 or more, C5 hydrocarbon units – isoprene
units12.
Examples: fat soluble vitamins A, D, E, K, -carotene (provitamin A), ubiquinone, rubber,
camphor, etc.
Cholesterol – Chemistry and Functions
Cholesterol is the best known sterol because hypercholesterolemia (increase of cholesterol
concentration in blood) causes atherosclerosis (thickening of artery leading to narrowing of its
lumen and consequent myocardial infarction).
Cholesterol is widely distributed in all cells of the body.
 It’s a constituent of cell membrane and lipoproteins.
 It occurs in animal fats but not in plant fats/oils.
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Chemistry
Cholesterol and all other sterol and steroids have the same parent ring structure – namely,
cyclo-pentano-perhydrophenanthrene ring 13.
Cholesterol
Cholesteryl Ester
Cholesterol molecule has 27 carbon atoms, a hydroxyl group at C3, a double bond between C5
and C6 and a side chain of 8 carbons at C17.
Cholesterol exists both as free cholesterol and cholesteryl ester, where hydroxyl group at C3 is
esterified with a long chain fatty acid.
Functions of Cholesterol :

Cholesterol is a constituent of cell membranes (cholesterol hinders the hydrophobic
interactions between fatty acid chains and thus increases the fluidity of cell
membranes). Cholesterol is present in large amounts in nerves and functions as an
insulating cover for transmission of electrical impulses.
Biologically Important Compounds Formed from Cholesterol
Bile salts/bile acids
Bile salts, derived from cholesterol help in digestion and absorption of dietary fat and
oil. (explained in Digestion of Fat and Oils)
Steroid hormones
Cholesterol is precursor of steroid hormones.
Three major classes of steroid hormones are glucocorticoids, mineralocorticoids (both
are adrenocortical hormones) and sex hormones.
Vitamin D
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Vitamin D is derived from cholesterol (from 7-dehydro cholesterol, an intermediate in
cholesterol biosynthesis) in the skin when exposed to sunlight (ultraviolet light).
Amphipathic Lipids
Molecules carrying both polar and non-polar groups are called amphipathic molecules.
In general amphipathic lipids have a polar head (-OH groups on phosphate or nitrogenous/nonnitrogenous residues or carbohydrate residues ) and non-polar hydrocarbon tail/s.
All compound lipids are amphipathic and among derived lipids, cholesterol (free cholesterol and
not esterified cholesterol) and fatty acids and soap are also amphipathic molecules. The polar
part of cholesterol is its hydroxyl group at C3 and rest of the molecule is hydrophobic.
Amphipathic nature of lipids enables them to bind both polar and non-polar substances
simultaneously. i.e., The polar part of amphipathic lipids interact with water phase and the nonpolar tails interact with the oil phase.
Amphipathic lipids self-orient (spontaneously self-assemble) at oil:water interfaces and form
several types of molecular aggregates in water such as lipid monolayer, lipid bilayer
(membranes), micelles, emulsions and liposomes.
Lipid Monolayer and Bilayer
Polar head
Aqueous Phase
(Hydrophilic)
Oil (Hydrophobic) Phase
Non-polar tail
Representation of an Amphipathic lipid
Lipid Monolayer
Surfaces
Aqueous medium
(Hydrophilic)
Hydrophobic membrane core
Lipid Bilayer (cross section)
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Amphipathic lipids can self-assemble in aqueous medium into bilayer sheets (lipid bilayer)
with their hydrophobic parts (non-polar tails) of each layer facing and interacting with
each other forming a hydrophobic membrane core and their hydrophilic parts (polar
heads) facing towards the two surfaces interacting with the aqueous medium.
Lipid bilayer forms the basic structure of all cell membranes. (Refer Cell Membrane)
Micelles
Surface
Aqueous medium
(Hydropholic)
Hydrophobic core
Micelle
Micelles are spherical aggregates of amphipathic molecules with their polar heads on the
surface interacting with surrounding aqueous medium and their hydrophobic tails interacting
with each other making a hydrophobic core.
Bile salts and lecithin (both amphipathic molecules) present in bile form micelles in the
intestine and help in the absorption of digested products of dietary lipids.
Emulsions :
Emulsions are much larger particles, formed by amphipathic lipids, which forms a surface layer
separating the main bulk of neutral lipids (triacylglycerol, cholesteryl ester, etc) from the
aqueous phase.
Aqueous medium
Hydrophobic core containing neutral lipids
Bile salts present in the bile act as emulsifying agent and help in digestion of dietary fat. (Refer
to digestion of fat).
Detergent action of soaps is due to emulsification of oils.
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Liposome
Liposomes are artificially produced lipid vesicle and are clinically used for drug delivery to
specific tissues14.
Aqueous Phase
Non-polar Phase
Liposome (unilamellar)
They are also used as carrier of enzymes, genes, etc, to specific organs. This has important
application in cancer therapy, vaccines, etc.
Lipoproteins
These are molecular complexes (aggregates) of lipids with proteins.
They are transport vehicles for TAG and cholesterol in the body.
Lipoprotein consists of a neutral lipid core with TAG and cholesteryl ester surrounded by a coat
shell of phospholipids, cholesterol and proteins (apolipoproteins).
Phospholipids and apolipoproteins, being amphipathic, help to solubilize the otherwise
insoluble lipids, triacylglycerol and cholesteryl ester.
The polar heads of phospholipids are oriented towards the surface and interact with aqueous
medium. The non-polar tails interact with the central hydrophobic core containing the neutral
lipids, namely, triacylglycerol and cholesteryl ester.
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Apoplipoprotein
Aqueous medium (Hydrophilic)
Phospholipids
Hydrophobic core containing
TAG and Cholesteryl ester
Free Cholesterol
Representation of a lipoprotein
Types of Lipoproteins:
There are 4 types of lipoproteins depending upon their density (separation in the
ultracentrifuge). In the increasing order of density they are chylomicrons, very low density
lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL).
The density of lipoprotein increases from chylomicron to HDL with the increase in protein
concentration and decrease in lipid component.
Plasma lipoproteins can also be separated by electrophoresis and named according to the
extent of migration – -lipoproteins (HDL), which moves farthest, pre--lipoproteins (VLDL), lipoproteins (LDL) and chylomicrons.
Electrophretic separation of lipoproteins
Chylomicron LDL
(+)
VLDL HDL
(–)
Origin
-lipoprotein pre--lipoprotein -lipoprotein
Functions of Lipoproteins
Type
Chylomicrons
Transport Function
Transport of dietary TAG from intestine to extra
hepatic tissues and dietary cholesterol from
intestine to liver.
Very low density lipoproteins Transport (export) of TAG and cholesterol from
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(VLDL)
the liver and delivery of triacylglycerol to the
extra hepatic tissues.
Low density lipoproteins (LDL)
Also called Lethally dangerous lipoprotein.
Transport of cholesterol from the liver to extra
hepatic tissues. Hence referred to as “bad
cholesterol”. (Refer to metabolism).
(VLDL delivers TAG to extra hepatic
tissues and gets transformed to LDL)
High density lipoproteins (HDL)
Also called “highly desirable lipoprotein” or “good
cholesterol”. Transport of cholesterol from extra
hepatic tissues to liver.
REFERENCES
Reference 1. Examples of organic solvents : alcohol, ether, benzene, chloroform, etc.
Reference 2. :
Length
Examples for fatty acids
Name
No of C
Atoms
Saturate /Unsaturated
Source
Acetic acid
(CH3COOH)
C2
Saturated
Vinegar
Butyric acid
Butanoic acid
C4
Saturated
Butter
Caproic acid
n-hexanoic
C6
Saturated
C5
Saturated
Short chain fatty acids :
Water
soluble
Medium chain fatty acids :
Caprylic
n-octanoic
Capric acid
C10
“
26
n-decanoic
Lauric acid
n-dodecanoic
Long chain fatty acids :
C12
“
Coconut
SATURATED LONG CHAIN FATTY ACIDS:
Myristic acid
Tetradecanoic acid
C14
C13H27COOH
Nutmeg
Palmitic acid
Hexadecanoic acid
C16
C15H31COOH
“
Stearic acid
Octadecan oic acid
C18
C17H35COOH
“
“
Arachidic acid
Eicosanoic acid
C20
C19H39COOH
“
Peanut
“
Animal fat
UNSATURATED LONG CHAIN FATTY ACIDS :
Name
Oleic acid
No. of C atoms
C18
C17H33COOH
No. & position
of double bonds
Mono unsaturated
(C18 : 1: Δ9, -9)
POLYUNSATURATED FATTY ACIDS (PUFA) :
Linoleic acid
C18
C17H31COOH
Dienoic
(C18 : 2: Δ9,12, -6)
Linolenic acid,
C18
C17H29COOH
Dienoic
(C18 : 3: Δ9,12,15, -6)
Arachidonic acid
C20
Trienoic
(C20 : 4 : Δ5, 8,11,14, -6)
Lignoceric acid
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Tetraeicosanoic acid
Reference 3. : Saponification number : No. of mg of alkali required to saponify 1 gm of fat /oil.
Importance : Saponification Number of fat or oil is inversely proportional to the average
molecular weight of the fatty acid residues.
Reference 4 :
Structure of ethanolamine and cephalin
Reference 5 :
Structure of choline and lecithin
Reference 6 :
Structure of serine and phosphatidyl serine
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Reference 7:
Structure of inositol and phosphatidyl inositol
Reference 8:
Structure of cardiolipin. The structure in the box is phosphatidic acid.
Reference 9:
Structure of sphingosine.
Reference 10:
Structure of ceramide.
Fatty acid
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Reference 11:
Structure of sphingomyelin.
Reference 12 :
Structure of Isoprene unit
Reference 13 :
Structure of Cyclo-pentano-perhydro-phenanthrene ring
Phenanthrene Ring
Cyclopentane Ring
.
Reference 14 :
Liposome : A liposome is a large, stable, spherical or ellipsoidal vesicle
enclosing a hollow water-filled central core and covered by either a single closed bilayer of
amphipathic molecules by a process called sonication, (unilamellar liposome) or several
concentric closed bilayers forming a multilayer membrane (multilamellar liposome).
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