The lipids are a heterogeneous group of compounds that include fats, oils, steroids, waxes, and related compounds. They share the properties of being relatively insoluble in water but soluble in nonpolar solvents. Lipids serve important functions such as energy storage, structural components of cell membranes, and insulation. They can also be classified based on their structure into simple lipids like triglycerides and complex lipids like phospholipids and glycolipids. Phospholipids and glycolipids are amphipathic molecules that form lipid bilayers, which are the basic structure of biological membranes.

1. Lipids
The lipids are a heterogeneous group of compounds,
including fats, oils, steroids, waxes, and related
compounds, that are related more by their physical than by
their chemical properties.
They have the common property of being
(1) relatively insoluble in water and
(2) soluble in nonpolar solvents such as ether and
chloroform.

2. Functions of lipids
 Storage form of energy
 Important dietary components because of their high
energy value and also because of the fat-soluble vitamins
and the essential fatty acids contained in the fat of natural
foods.
 Structural components of biomembranes
 Serve as thermal insulators in the subcutaneous tissues
and around certain organs
 Nonpolar lipids act as electrical insulators, allowing rapid
propagation of depolarization waves along myelinated
nerves

3. Functions of lipids(Contd.)
 Provide shape and contour to the body
 Act as metabolic regulators
 Combinations of lipid and protein (lipoproteins) are
important cellular constituents, occurring both in the cell
membrane and in the mitochondria, and serving also as
the means of transporting lipids in the blood.

4. Structure of fatty acid
• Fatty Acids are hydrocarbon derivatives,
• Fatty acids are carboxylic acids with hydrocarbon chains
ranging from 4 to 36 carbons long (C4 to C36).
• In some fatty acids, this chain is unbranched and fully
saturated (contains no double bonds);
• In others the chain contains one or more double bonds.
• The double bonds of polyunsaturated fatty acids are
separated by a methylene group (-CH=CH-CH2-CH=CH-)
• Double bond causes a bend or kink in the hydrocarbon tail.

5. Structure of fatty acid
• In nearly all naturally
occurring unsaturated fatty
acids, the double bonds are
in the cis configuration.
• Trans fatty acids are
produced by fermentation in
the rumen of dairy animals,
are obtained from dairy
products, meat and by
conversion of oil into ghee.
• Diets high in trans fatty acids
correlate with increased
blood levels of LDL (bad
cholesterol) and decreased
HDL (good cholesterol).

6. Nomenclature of fatty acid
• A simplified nomenclature for these compounds specifies
the chain length and number of double bonds, separated
by a colon;
e.g., the 16-carbon saturated palmitic acid is abbreviated
16:0,
• The 18-carbon oleic acid, with one double bond, is 18:1
• The positions of any double bonds are specified by
superscript numbers following
• A 20-carbon fatty acid with one double bond between C-9
and C-10 (C-1 being the carboxyl carbon) and another
between C-12 and C-13 is designated

7. Physical properties
• The physical properties of the fatty acids are largely
determined by the length and degree of unsaturation of the
hydrocarbon chain.
• The nonpolar hydrocarbon chain accounts for the poor
solubility of fatty acids in water.
• At room temperature (25°C), the saturated fatty acids have
a waxy consistency, whereas Unsaturated fatty acids are
oily liquids.
• Because of the kinks in the hydrocarbon tails, unsaturated
fats can’t pack as closely together, making them liquid at
room temperature.

8. Storage Lipids (simple lipids)
Neutral fats or Triacyl Glycerides
• Triglycerides are composed of three fatty acids each in
ester linkage with a single glycerol containing the same
kind of fatty acid
e.g., tristearin, tripalmitin, and triolein
• The polar hydroxyls of glycerol and carboxylates of the
fatty acids are bound in ester linkages, triacylglycerols are
nonpolar, hydrophobic molecules, essentially insoluble in
water.
• Lipids have lower specific gravities than water, which
explains why mixtures of oil and water have two phases:
oil, with the lower specific gravity, floats on the aqueous
phase.
• Triglycerides are the storage form of energy in adipose
tissue

9. Significance of storage lipids
1. Oxidation of triacylglycerols yields more than twice as
much energy as the oxidation of carbohydrates.
2. Triacylglycerols deposits could meet energy needs for
months by drawing on their fat stores,compared to one
day’s energy supply in the form of glycogen.
3. Under the skin storage serve as insulation against low
temperatures. Seals, walruses, penguins, and other warm-
blooded polar animals.
4. In hibernating animals, it serves the dual purposes of
insulation and energy storage.
5. In whales, a store of triacylglycerols and waxes allows the
animals to match the buoyancy of their bodies to that of
their surrounding during deep dives in cold water.

10. Many Foods Contain Triacylglycerols
• Vegetable oils such as corn (maize) and olive oil are
composed largely of triacylglycerols with unsaturated fatty
acids and thus are liquids at room temperature
• They are converted industrially into solid by hydrogenation
which converts double bonds into single or into trans form.
• Foods containing only saturated fatty acids, such as
tristearin, the major component of beef fat, are white,
greasy solids at room temperature.
• fatty foods have tandency to become rancid i.e.,
appearance of unpleasant taste and smell of fats due to
oxidation.

11. Waxes
• They are esters of long-chain (C14 to C36) saturated and
unsaturated fatty acids with long-chain (C16 to C30) alcohols
• In plankton, waxes are the chief storage form of metabolic
fuel.
• Their firm consistency and water-repellent property enable
to serve diverse functions like protection, lubrication and
prevent evaporation.
• Used as bases for the preparation of cosmetics, ointments,
lotions, polishes, lubricants and candles.

12. Structural lipids (Compound Lipids)
1. Phospholipids
 Contain in addition to fatty
acids and glycerol/or other
alcohol, a phosphoric acid
residue, nitrogen containing
base and other substituents.
 Phospholipids may be
regarded as derivatives of
phosphatidic acid , in which
the phosphate is esterified
with the —OH of a suitable
alcohol.
 They are amphipathic
molecules containing a polar
head and a hydrophobic
portion

13. Classification of phospholipids
Based on nature of alcohol-
1) Glycerophospholipids- Glycerol is the alcohol group.
Examples-
o Phosphatidyl choline
o Phosphatidyl ethanolamine
o Phosphatidyl serine
o Phosphatidyl inositol
o Phosphatidic acid
o Cardiolipin
o Plasmalogen
o Platelet activating factor
o Phosphatidyl Glycerol
2) Sphingophospholipids- Sphingol is the alcohol group
Example- Sphingomyelin

14. 1) Glycerophospholipids
Phosphatidylcholines (Lecithins)
 Phosphoacylglycerols containing choline are the most
abundant phospholipids of the cell membrane
 Are present a large proportion of the body's store of
choline.
 Choline is important in nervous transmission, as
acetylcholine, and as a store of labile methyl groups.

15. 1) Glycerophospholipids
 Brain and nervous tissue are rich in Phosphatidyl
ethanolamine.
 Phosphatidyl inositol is an important constituent of cell
membrane phospholipids; upon stimulation by a suitable
hormone agonist, it is cleaved into diacylglycerol and
inositol trisphosphate, both of which act as internal signals
or second messengers.
 Cardiolipin is abundantly found in mitochondrial
membrane.This is the only phospholipid with antigenic
properties.
 Plasmalogens constitute as much as 10% of the
phospholipids of brain and muscle.

16. 1) Glycerophospholipids
Platelet activating factor (PAF)
 Ether glycerophospholipid
 Contains an unsaturated alkyl group in an ether link to
carbon -1
 An acetyl residue at carbon 2 of the glycerol backbone.
 PAF activates inflammatory cells and mediates
hypersensitivity, acute inflammatory and anaphylactic
reactions.
 Causes platelets to aggregate and degranulate and
neutrophils and alveolar macrophages to generate
superoxide radicals.

17. 2) Sphingophospholipids
Sphingomyelin
 Backbone is sphingosine (aliphatic amino alcohol).
 A long chain fatty acid is attached to amino group of
sphingosine to form Ceramide.
 The alcohol group at carbon-1 of sphingosine is esterified
to phosphoryl choline, producing sphingomyelin.
 Sphingomyelin is an important component of myelin of
nerve fibers.

18. Functions of Phospholipids
 Components of cell membrane, mitochondrial membrane
and lipoproteins
 Participate in lipid absorption and transportation from
intestine
 Play important role in blood coagulation
 Required for enzyme action- especially in mitochondrial
electron transport chain
 Choline acts as a lipotropic agent
 Act as reservoir of second messenger- Phosphatidyl
Inositol
 Act as cofactor for the activity of Lipoprotein lipase
 Phospholipids of myelin sheath provide insulation around
the nerve fibers

19. 2) Glycolipids (Glycosphingolipids)
 Glycolipids differ from sphingomyelins in that they do not
contain phosphoric acid and the polar head function is
provided by monosaccharide or oligosaccharide attached
directly to ceramide by an O- glycosidic linkage.
 The number and type of carbohydrate moieties present,
determine the type of glycosphingolipid. There are two
types of Glycolipids-
A) Neutral glycosphingolipids
B) Acidic glycosphingolipids

20. a) Neutral Glycosphingolipids
Cerebrosides
 These are ceramide monosaccharides, that contain either
a molecule of galactose (Galactocerebroside) or glucose
(Glucocerebroside).
 Found predominantly in the brain and nervous tissue with
high concentration in myelin sheath.
 Ceramide oligosaccharides (Globosides) are produced by
attaching additional monosaccharides to Glucocerebroside.
 Lactosyl ceramide contains lactose.

21. b) Acidic Glycosphingolipids (Gangliosides)
 They are negatively charged at physiological pH
 The negative charge is imparted by N- acetyl Neuraminic
acid (Sialic acid)
 Brain gangliosides may contain up to four Sialic acid
residues and based on that they are -GM, GD, GT and GQ,
containing 1,2,3 or 4 Sialic acid residues
 Four important types of Gm series are-
 GM1, GM2 and GM3
 GM1 is complex of all

22. Functions of Glycosphingolipids
 They occur particularly in the outer leaflet of the plasma
membrane, where they contribute to cell surface
carbohydrates.
 They act as cell surface receptors for various hormones,
and growth factors.
 Play important role in cellular interactions, growth and
development.
 They are source of blood group antigens and various
embryonic antigens.
 GM1 acts as a receptor for cholera toxin in human
intestine.

23. Cholesterol
• Sterols have four fused rings and a hydroxyl group.
Cholesterol, the major sterol in animals, is both a structural
component of membranes and precursor to a wide variety
of steroids.
• Steroid hormones are potent biological signals that
regulate gene expression.
• Bile acids are polar derivatives of cholesterol that act as
detergents in the intestine.
Cholesterol

24. Amphipathic lipids
• Fatty acids, phospholipids, sphingolipids, bile salts, and, to a
lesser extent, cholesterol contain polar groups.
• Amphipathic molecules have a nonpolar region (hydrophobic)
and a polar region (hydrophilic).
• They become oriented at oil:water interfaces with the polar group
in the water phase and the nonpolar group in the oil phase.
• A bilayer of such amphipathic lipids is the basic structure in
biological membranes.

25. Amphipathic lipids
• Liposomes - Liposomes may be formed by sonicating an
amphipathic lipid in an aqueous medium.
• They consist of spheres of lipid bilayers that enclose part of
the aqueous medium.
• Liposomes are carriers of drugs in the circulation, targeted
to specific organs, e.g., in cancer therapy.
• Micelles are spherical structures that contain anywhere
from a few dozen to a few thousand amphipathic
molecules.
• TAGs and cholestrol are carried through the bloodstream in
protein packages called lipoproteins, made up of lipid on
the inside and protein on the outside.