Lipids are a diverse group of compounds, including fats, oils, and steroids, characterized by their insolubility in water and solubility in nonpolar solvents. They serve various functions such as energy storage, insulation, and forming cellular components, and include classifications like simple lipids, compound lipids, and derived lipids. Fatty acids, a component of lipids, are categorized based on carbon chain length and saturation, while triglycerides (neutral fats) store fat in animals and plants and are important in metabolic processes.

1. lipids
• The lipids are a heterogeneous group of
compounds, including fats, oils, steroids,
waxes, and related compounds, which 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. E.g. fat and oils.

2. Functions of lipids
• lipids are storage form of energy, Fat is stored in adipose tissue.
• It also serves as a thermal insulator in the subcutaneous tissues
and around certain organs.
• Nonpolar lipids act as electrical insulators, allowing rapid
propagation of depolarization waves along myelinated nerves.
• 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.
• Lipids are also required for solubility of fat soluble vitamins.
• Lipids are major source of metabolic water.
• Lipids synthesize the hormones(steroid hormones, prostaglandins)

3. Classification of lipids
• 1. Simple lipids: Esters of fatty acids with
various alcohols.
• a. Fats: Esters of fatty acids with glycerol. Oils
are fats in the liquid state.
• b. Waxes: Esters of fatty acids with higher
molecular weight monohydric alcohols.
• c. cholesterol ester, ester of vitamin A& D

4. 2. Compound lipids
Esters of fatty acids containing groups in addition to an
alcohol and a fatty acid.
• a. Phospholipids: Lipids containing, in addition to fatty acids
and an alcohol, a phosphoric acid residue. They frequently
have nitrogen containing bases and other substituents, eg, in
glycerophospholipids the alcohol is glycerol and in
sphingophospholipids the alcohol is sphingosine.
• b. Glycolipids (glycosphingolipids): Lipids containing a fatty
acid, sphingosine, and carbohydrate .
• C. Other complex lipids: Lipids such as sulfolipids and
aminolipids. Lipoproteins may also be placed in this category.

5. 3. derived lipids:
• hydrolysed products of simple and compound
lipids are derived lipids.These include fatty
acids, glycerol, cholesterol, other alcohol, fatty
aldehydes, and ketone bodies .
• 4. miscellaneous lipids: these include
heterogenous group of compounds possessing
characteristics of lipids. E.g. carotenoids,
squalene, hydrocarbons, terpenes, vit E and
vit K.

6. Fatty acids
• Fatty acids are carboxylic acids with
hydrocarbon chains ranging from 4 to 36
carbons long (C4 to C36). The general
molecular formula is R-COOH.

7. Classification of fatty acid
• 1. Classification based on total no of carbon
• Even chain fatty acids (naturally occuring f.a)
• Odd chain fatty acids (found in micro-organism and milk)
• 2. Classification based on length of hydrocarbon
• Short chain f.a (C2-C6)
• Medium chain f.a (C8-C14)
• Long chain f.a (C16-C36)
• 3. Classification based on nature of hydrocarbon
• i. Saturated fatty acids
• ii. Unsaturated fatty acids which may be sub classified into Mono-unsaturated and
Polyunsaturated.
• iii. Branched chain fatty acids
• Hydroxy fatty acids

8. Nomenclature of fatty acids

9. Essential fatty acids

10. NEUTRAL FATS
• Neutral fats are also called as triacylglycerols
(TAG) or triglycerides (TG). These are esters of the
trihydric alcohol, glycerol with fatty acids.
Naturally occurring fats and oils are mixtures of
triglycerides. If all the three hydroxyl groups of
the glycerol are esterified to the same fatty acid, a
simple triacyl glycerol is formed, e.g. Tripalmitin,
Triolein, etc. A mixed triglyceride is formed, when
different fatty acids are esterified to the hydroxyl
groups of glycerol.

11. TAG

12. Properties of TAG
• i. hydrophobic and insoluble in water.they can be store in
large amounts in body.in animals adipose tissue store fat &
in plants TAG is stored in seeds.
• ii. Oils are liquids at 20oC; they are triglycerides which
contain a higher proportion of unsaturated fatty acids or
short chain triglycerides. Oils are generally of plant origin.
• iii. Fats are solids at room temperature and contain mainly
saturated long chain fatty acids.
• Fats are mainly of animal origin.
• iv. When the constituent fatty acids have a higher chain
length and are predominantly saturated,
• ‘hard fat' is formed, e.g. pig fat.

13. Saponification
• When triglycerides are hydrolysed by alkali, the
process is known as saponification. The products
are glycerol and soaps
• Saponification number is defined as the number
of milligrams of NAOH/KOH required to saponify
one gram of fat. It is an indication of the length of
fatty acid in that fat, and is inversely proportional
to it.Human fat has a saponification number of
194-198, butter has 210-230 and coconut oil has
253-262.
•

14. Iodine number:
• iodine number of a fat is defined as the
number of grams of iodine taken up by 100
grams of fat. It is an index of the degree of
unsaturation and is directly proportional to
the content of unsaturated fatty acids. Higher
the iodine number, higher is the degree of
unsaturation, e.g. iodine number of butter is
28, and that of sunflower oil is 130.

15. Rancidity
• Fats and oils have a tendency to become rancid.
The term rancidity refers to the appearance of an
unpleasant smell and taste for fats and oils.
• Hydrolytic rancidity is due to partial hydrolysis of
the triacyl glycerol molecules due to traces of
hydrolytic enzymes present in naturally occurring
fats and oils.
• Oxidative rancidity is the result of partial
oxidation of unsaturated fatty acids with resultant
formation of epoxide and peroxidesof fatty acids.
antioxidants

16. PHOSPHOLIPIDS
• 1.Glycerophospholipid- backbone is glycerol.

17. Types of glycerophospholipid

18. Ether linked phospholipid

19. Sphingophospholipid
• backbone is sphingosine. sphingomyelin is only
sphingophospholipid found in myelin sheath.
•

20. • Functions of phospholipid
• 1. In association with proteins, phospholipids form the structural
components of membranes and regulate membrane permeability.
• 2. Phospholipids (lecithin, cephalin and cardiolipin) in the
mitochondria are responsible for maintaining the conformation of
electron transport chain components, and thus cellular
respiration.
• 3. Phospholipids participate in the absorption of fat from the
intestine.
• 4. Phospholipids are essential for the synthesis of different
lipoproteins, and thus participate in the transport of lipids.
• 5. Accumulation of fat in liver (fatty liver) can be prevented by
phospholipids, hence they are
• regarded as lipotropic factors.

21. • 6. Arachidonic acid, an unsaturated fatty acid liberated from
phospholipids, serves as a precursor for the synthesis of
eicosanoids (prostaglandins, prostacyclins, thromboxanes etc.).
• 7. Phospholipids participate in the reverse cholesterol transport
and thus help in the removal of cholesterol from the body.
• 8. Phospholipids act as surfactants (agents lowering surface
tension). For instance Dipalmitoyl phosphatidyl choline is an
important lung surfactant. Respiratory distress syndrome in
infants is associated with insufficient production of this surfactant.
• 9. Cephalin, an important group of phospholipids participate in
blood clotting.
• 10. Phospholipids ( phosphatidylinositol ) are involved in signal
transmission across membranes. They act as secondary messenger
for hormones.
• 11. cardiolipin has antigenic property.

22. Glycolipids
• They are seen widely in nervous tissues. This
group of lipids do not contain phosphoric acid;
instead they contain carbohydrates and ceramide.
• Cerebrosides- they contain one or more sugars.
They are found in white mater of brain. E.g.
glucocerebrosides, galactocerebrosides e.t.c.
• Ganglosides- they contain oligosaccharides with
at least one N-acetyl neuraminic acid (sialic acid).
They are found in gray mater of brain and
ganglions.

23. Sterols
• Sterols are membrane lipids found in most of eukaryotic cell
membrane. They have a fused ring system i.e.
cyclopentanoperhydrophenanthrene ring system.

24. • Cholesterol
• Cholesterol, exclusively found in animals, is the most abundant
animal sterol. It is widely distributed in all cells and is a major
component of cell membranes and lipoproteins. Cholesterol
• (Creek: chole-bile, steros- solid, ol- alcohol) was first isolated from
bile. Cholesterol literally means 'solid alcohol from bile. The
molecular formula of cholesterol is (C27H45OH). Due to the
presence of an -OH group, cholesterol is weakly amphipathic. The
occurrence of cholesterol is much higher in the membranes of
sub-cellular organelles. Cholesterol is found in association with
fatty acids to- form cholestervl esters (esterification occurs at the
OH group of C3).Cholesterol is an yellowish crystalline solid.
Cholesterol is insoluble in water and soluble in organic solvents
such as chloroform, benzene, ether etc.

25. Functions and significance
• The level of cholesterol in blood is related to the development of
atherosclerosis and myocardial infarction. Abnormality of cholesterol
metabolism may lead to cardiovascular accidents and heart attacks.
Normal level of cholesterol in human blood is 150-250mg/dl.
• Cell membranes: Cholesterol is a component of membranes and has a
modulating effect on the fluid state of the membrane.
• Nerve conduction: Cholesterol has an insulating effect on nerve fibers.
• Bile acids and bile salts are derived from cholesterol. Bile salts are
important for fat absorption.
• Steroid hormones: Glucocorticoids, androgens and estrogens are from
cholesterol.
• 6. Vitamin D3 is derived from 7-dehydro-cholesterol.
• 7. Esterification: The OH group of cholesterol is esterified to fatty acids to
form cholesterol esters. This esterification occurs in the body by transfer
of a PUFA moiety by lecithin cholesterol acyl transferase.

26. Plant sterol

27. Terpenes and terpenoids
The lipids which carbon skeleton have structural relationship
to isoprene i.e. (2-methylbutadiene) are called terpenes.
These terpenes include essential oils such as citral, camphor,
rubber & variety of plant pigments eg carotenes, lycopenes,
squalene e.t.c. cholesterol and other sterols/ steroids are also
examples of terpenoids.

28. • Terpenoids are compounds related to
terpenes, which may include some oxygen
functionality or some rearrangement, however
the two terms are often used interchangeably.
• Monoterpenoid eg camphor
• Diterpenoid eg retinol
• Tri terpenoids eg cholesterol
• Tetra terpenoid eg beta carotene
• Polyterpenoid eg rubber