Lipids are a heterogeneous group of organic compounds that are insoluble in water but soluble in organic solvents. They serve many important functions in the body including as structural components of cell membranes, storage of metabolic energy, transport of fat-soluble vitamins and hormones, and protection and insulation. Lipids are classified based on the presence or absence of glycerol and other components. Major classes of lipids include fatty acids, triglycerides, phospholipids, sphingolipids, sterols such as cholesterol and vitamin D, and other compounds like prostaglandins.

1. Chemistry of Lipids
Lecture No. 3

2. What are Lipids?

3. Lipids or Fats
• Lipids are heterogenous group of
compounds related either actually or
potentially to the fatty acids.
• They are water-insoluble organic
substances found in cells, which are
soluble in organic, non-polar solvents
such as chloroform, ether, acetone,
benzene, n-hexane, TCA, CCl, etc.
• They are widely distributed in nature.

4. What are the functions of lipids?
• The importance of lipids can be seen from their varied
functions. They are:
• 1. As membrane structural components
• 2. As intracellular storage depots of metabolic fuel
• As transport form of metabolic fuel
• As regulatory substances (some hormones)
• As protective form of the cell walls of many bacteria, of
the leaves of higher plants, of the exoskeleton of insects
and the skin of vertebrates
• As enzymes cofactors (some vitamins)
• As transport form of some neurotransmitters
(acetylcholine)
• As receptors in nerve ending membranes
• As determinants of immunological specificity
• As part of the outer coating between the body of the
organism and the environment to prevent infection and

5. Classification of Lipids
A. Classification by White – based on the presence or
absence of glycerol
1. Fatty acids
2. Lipids containing glycerol
a) Neutral fats (triacylglycerols)
b) Phosphoglycerides or phospholipids
c) Cardiolipin
d) Plasmalogens
3. Lipids not containing glycerol
a) Sphingolipids
b) Alipathic alcohol and waxes
c) Terpenes
d) Steroids
e) Prostaglandins
4. Lipids combined with other classes of compounds
a) lipoproteins and proteolipids
b) Glycolipids

6. Classification by Bloors
1.Simple Lipids
a) Fats and Oils
b) Waxes
2. Compound Lipids
a) Phospholipids
b) Glycolipids
3. Derived Lipids
a) Fatty acids
b) Alcohol
c0 Sterols

12. Fatty Acids
• Fatty acids are the most abundant occurring
lipids. They are the building blocks of the
several classes of lipids and neutral fats,
phosphoglycerides, glycolipids, cholesterol
esters and some waxes. They are the
products of fat hydrolysis
• They are mainly straight chain, alipathic,
monocarboxylic acids, ranging in chain
lengths from 4 -24 C atoms
• Type formula: R-COOH
• Fatty acids may be saturated or unsaturated

13. Fatty Acids

14. Saturated fatty acids
• Saturated fatty acids belong to acetic acid
series with general formula CnH2nO2 -. They
contain up to 8 C and are liquid at room
temperature. Longer chains of saturated FA
are solids. The systematic name ends in
“anoic”
• Examples: Lauric or duodecanoic
Myristic or tetradecanoic
Palmitic or hexadecanoic
Stearic or octadecanoic
Arachidic or didecanoic

18. Triacylglycerols (TAG)
• Storage of fatty acids in organisms is
largely in the form of TAG or fats.
• These substances are triesters of fatty
acids and glycerol.
• The long chains of fatty acids are
extraordinarily for energy storage,
because they contain C in a fully reduced
form and will therefore yield a maximum
amount of energy on oxidation.

19. Unsaturated fatty acids
• Unsaturated fatty acids contain double bond
and their systematic names end in “enoic.”
They are unstable and reactive due to the
presence of the double bond – therefore they
are capable of taking up molecules of H2O, H,
I2, O2, B2 at each double bond
• Double bonds in naturally occurring fatty
acids are always in a cis- form as opposed to
trans- configuration.
• The presence of the double bond gives rise to
the possibility of geometric isomerism

21. Oleic acid
Oleic acid is
CH3(CH2)7CH=CH(CH2)7COOH
Oleic acid is the most abundant fatty acid in
nature forming about 50% of the total fatty
acids in many fats and is found in all
naturally existing fats

23. Indispensable fatty acids
• The body has the ability to desaturate
fatty acids but more unsaturated fatty
acids which are necessary for nutrition
and growth can be formed in the body.
• They have to be supplied from the diet.
• They become indispensable items in
the diet.
• These are linoleic acid, linolenic acid
and arachidonic acid.

24. Indispensable fatty acids
• Linoleic acid – cis-9 octadecadienoic
acid
18:2,9,12
• Linolenic acid – 1,5 octadecatrienoic
acid
18:3,9,12,15
• Arachidonic acid – cis-5,8,11,14
eicosatetraenoic acid
20:4,5,8,11,14

27. Glycerophospholipids or
Phosphoglycerides
• They are the major class of naturally
occurring phospholipids, lipids with
phosphate-containing head groups.
• These compounds make up a significant
fraction of the membrane lipids throughout
the bacterial, plant and animal kingdoms.
• Examples are: phosphotidyl choline,
phosphotidylethanolamine,
phosphotidylserine, phosphotidylinositol

30. Cardiolipin & Plasmalogens
• Cardiolipin consists of 2 molecules of
phosphatidic acid connected by a glycerol bridge
• It is of immunological importance
• Plasmalogens make up 10% of the
phospholipids of the brain and muscles.
• They are phospholipids where alkyl ether is
attached at alpha position of glycerol.
• Three principal classes of plasmalogens are:
1. phosphotidalcholine
2. phosphotidalserine
3. phosphotidal ethanolamine

31. Plasmalogens

33. Sphingolipids
• They are comlex lipids containing sphingosine or a
related base, as their backbone
• All sphingolipids contain 3 characteristics:
1. 1 molecule of fatty acid
2. 1 molecule of sphingosine or 1 of its
derivatives
3. a polar head group
• The greatest concentration of sphingolipids is found
in the CNS, particularly in the white matter, although
all human tissues contain some.
• Kinds of sphingolipids are:
1. Shingomyelin
2. Glycosphingolipid
a) Ganglioside
b) Cerebroside

34. Sphingomyelin

35. Ceramide

37. Chemical Reaction of Lipids

42. Steroids
• Steroids are found in association with fats
and separated from the fats after
saponification in the “unsaponifiable
residue.”
• They all possess similar cyclic nucleus
resembling phenanthrene (rings A, B, and C),
to which a cyclopentane ring is attached
• Steroid nucleus is
cyclopentanoperhydrophenanthrene
• There is an oxygenated substituent on C3
which is shared by almost all naturally
occurring steroids.

43. Biologically Important Steroids
• Sterols – Cholesterol, ergosterol,
lanosterol
• Bile Acids – cholic acid, glycholic acid,
• Adrenocortical Hormones –
costicosterone, cortisol, aldosterone
• Sex hormones – androgen, estrogen
• Vitamins – Vitamin D, K

44. Lanosterol and Ergosterol

45. Cholesterol
• Cholesterol is widely distributed in
cells particularly in the nervous tissue
• It is the major component of the
plasma membrane.
• It is the parent compound of all
steroids synthesized in the body.
• It occurs in animal fats, but not in plant
fats.

46. Cholesterol

48. Estrogen
• Estrogen differs from all steroids in that
ring A is aromatic
• Hydroxyl group at position 3 is phenolic
rather than alcoholic
• Because of this fact, estrogens behave
like weak acids and are extractable from
benzene solution with aqueous alkali

50. Androgen or Testosterone
• Androgen or male sex hormone is C19
steroid and belongs to the class of
steroids devoid of C side chain at C17
• Synthesized from the testis
• Has a OH in C17

51. Testosterone
It is also called
androgen.
It is C19 steroid.
This is a steroid
without a carbon
side chain at
position 17.

52. Progesterone
• It is also an
adrenal cortex
hormone and a
C21 steroid
• But it has CH3
instead of the
CH2OH of
corticosterone

53. Cortisosterone
It is a C21 steroid
Contains 2
carbon atoms in
the side chain at
position 17

58. Bile Acids
• Bile acids or salts are steroids in nature.
Four different salts occur in human bile:
Glycocholic acid, deoxycholic acid, cholic
acid and lithocholic acid
• They are strong emulsifying agent that
help disperse fatty materials
• They stimulate intestinal motility

59. Bile Acids
• Cholic acid Deoxycholic acid

60. Terpenes

61. Terpenes

62. Examples of Terpenes
Monoterpene -
limonene
Diterpene – phytol
Triterpene – squalene
Tetraterpene – α,ß,γ-
carotene
Polyterpene –
natural rubber,
gutta perchae

63. Vitamin D
• Vitamin D2 Vitamin D3

64. Vitamin K

65. Vitamin K2

67. Prostaglandins
• They are family of fatty acids which have
hormonal or regulatory nature.
• The best known prostaglandins are PGE1
PGF1a PGF2a
• The prostaglandins differ from each other
with respect to their biological activity,
although all show at least some activity in
lowering blood pressure and inducing
muscles to contract

74. Thromboxane A2

76. Leukotrienes

79. Micelle

80. Micelles
• When fatty acids are placed in water, they
appear to dissolve, they are not, in true
solution, but present as micelles.
• Micelles or microscopic fat cells have the
properties of bisolubility.
• Bile acids are strong emulsifying agents
that help dispense fatty acids into
micelles, which may be absorbed or more
rapidly broken down by enzymes.

81. Lipids in Cell Membranes

87. Chemical tests for Lipids
• Liebermann-Burchard test - for cholesterol
• Acrolein test – for glycerol
• Saponication – fats react with alkali to produce
metallic salt of fatty acid or soap
• Halogenation – when halogens are added to the
double bond of unsaturated FA, the reaction
proceeds spontaneously in a given suitable
solvent.
• Hydrogenation – when unsaturated FA are
hydrogenated in the presence of catalyst like Pt,
Pa, or Ni

88. Biological Significance of Lipids
• The current biological significance of
lipids can be seen in the following:
• 1. Obesity
• 2. Arteriosclerosis
• Role of polyunsaturated fatty acids
(PUFA)in nutrition and health