1. Lipids
Risha Shiny

2. LIPIDS - Introduction
 Lipids are the heterogeneous group of compounds related to
fatty acids and include fats, oils, waxes and other related
substances. Chemically, they are esters of fatty acids.
 They are oily or greasy substances, insoluble in water, soluble in
organic compounds like acetone, ether, chloroform and
benzene, hydrophobic in nature. Variously called as lipins or
lipoids.
 The term ‘lipid’ was first used by the German biochemist Bloor
in 1943 for major class of tissue components and foodstuffs.

3. Bloor’s Classification

4. Simple Lipids
Simple lipids are the esters of fatty acids with various alcohols
They are classified into
 Fats and oils (Triglycerides):
Esters of fatty acids with glycerol, fat is solid at room
temperature whereas oil is liquid.
 Waxes:
Esters of fatty acids with alcohols other than glycerol

5. Simple Lipids – Fats and oils (Triglycerides)
 Fats and oils, also known as triglycerides are the esters of
glycerol with 3 fatty acid molecules.

6. Hydrolysis
Fats are hydrolysed by the enzyme lipases to yield fatty acids and glycerol.
The fats first split to produce diglycerides, part of these are then split to monoglycerides.
Finally, part of the monoglycerides split to yield fatty acid and glycerol.

7. Fatty Acids
 long-chain organic acids having 4 to 30 c atoms
 They have a single carboxyl group and a long, nonpolar hydrocarbon ‘tail’.
 hydrocarbon ‘tail’ gives most lipids their hydrophobic & oily or greasy
nature.
 Fatty acids do not occur in free state in cells or tissues but are present in
covalently bound form in different classes of lipids.
 Fatty acids which occur in natural fats are usually monocarboxylic &
contain an even number of C atoms.

8. Structure of fatty acid

9. Fatty Acid Classification
On the basis of length of Carbon chain, they are classified into:
 Short chain fatty acids – contains less than 6-C atoms. Example: Butyric
acid (4-C)
 Medium chain fatty acids – contains 8-14 C atoms. Example: Lauric acid
(12-C)
 Long chain fatty acids – contains 16-24 C atoms. Example: stearic acid (18-
C)

10. Fatty acid classification
Based on presence or absence of double bonds fatty acids grouped into 2 types:
 Saturated fatty acids
 Unsaturated fatty acids

11. Saturated straight chain fatty acids

12. Branched saturated fatty acids - examples

13. Unsaturated fatty acids
Monoethenoid acids are commonly known as monounsaturated fatty acids (MUFA)
and the remaining ones are known as polyunsaturated fatty acids (PUFA)
Unsaturated fatty acids contain double bonds. Based on the degree of unsaturation, they are
divided into several groups:

14. Properties
 State
Fats containing saturated fatty acids are solid at room temperature.
The animal fats belong to this category. Most plant fats possess
unsaturated fatty acids and are henceforth, liquid at room temperature.
 Colour, odour, taste
When pure, the fats are colourless, odourless and possess an extremely
bland taste. They are capable of absorbing a variety of odours and
hence flavour during storage.

15. Properties
 Solubility
Fats are sparingly soluble in water (hydrophobic), but freely soluble in
organic solvents like acetone, ether, choloroform and benzene.
 Melting point
Melting point of fats depends on the chain length of the fatty acid and
the degree of unsaturation.
Short chain length and unsaturation enhance the fluidity of fatty acids
and of their derivatives

16. Properties
 Specific gravity
Specific gravity of the fats is less than 1 (about 0.86) and therefore, they
float on water surface. Solid fats are lighter than the liquid fats. Oils
spread on water to form thin monomolecular layers.
In general, either unsaturation of the fatty acid chain increase or increase
in chain length of fatty acid residues tend to increase the specific gravity.
 Insulation
The fats possess high insulating power, they are bad conductor of heat.
The layer of fat below the skin provides a sort of blanket for warm-
blooded animals. This is especially important for whales and seals which
have to maintain high temperature in cold waters.

17. Geometric isomerism
The presence of double bond in the unsaturated fatty acid part of the
fat molecule produces geometric (cis-trans) isomerism.
Example:

18. Properties
 Surface tension
The force in which the surface molecules are held together is called surface
tension. When liquid fat is poured on water, it spreads uniformly over the
surface of water in the form of a unimolecular layer and thus reduces the
surface tension of water.
 Emulsification
It is a process by which a lipid mass is converted into a number of small lipid
droplets.
When water and oil are mixed together and vigorously shaken, a dispersion
of oil droplets in water and vice-versa is formed. When shaking stops, the
phases start to separate. However, when an emulsifier is added to the
system, the droplets remain dispersed and stable emulsion is obtained.

19. Functions / Biological role of lipids
 Food material: Lipids provide food, highly rich in calorific value. One gram
lipid produces 9.3 kilocalories of heat.
 Food reserve: Lipids insoluble in aqueous solutions and hence can be
stored readily in the body as a food reserve.
 Structural component: Lipids are an important constituent of the cell
membrane.
 Heat insulation: The fats are characterized for their high insulating
capacity. Great quantities of fat are deposited in the subcutaneous layers
in aquatic mammals such as whale and in animals living in cold climates.

20.  Fatty acid absorption: Phospholipids play an important role in the
absorption and transportation of fatty acids.
 Hormone synthesis: The sex hormones, adrenocorticoids, cholic acids &
also vitamin D are all synthesized from cholesterol, a steroidal lipid.
 Vitamin carriers: Lipids act as carriers of natural fat-soluble vitamins such
as vitamin A, D & E.
 Antibiotic agent: Squalamine, a steroid from the blood of sharks, has
been shown to be an antibiotic and antifungal agent of intense activity.
This seems to explain why sharks rarely contract infections and almost
never get cancer.
 Blood cholesterol lowering effect: stearic acid & oleic acid are known to
lower the blood cholesterol levels

21. Simple triglycerides
 A fat molecule contains 3 moles of fatty acids which may be similar or dissimilar.
 Simple/symmetric triglycerides contains single kind of fatty acid in all the 3
positions (α, β, α’).
Example: Tripalmitin, tristearin, triolein
These are infrequent in natural fats, however, can be synthesized in laboratory

22. Mixed triglycerides
 A fat molecule contains 3 moles of fatty acids which may be similar or dissimilar.
 Mixed/asymmetrical triglycerides contains two or three different kind of fatty acid units in
the molecule. Example: Oleodipalmitin, Oleopalmitostearin
In general, fatty acid attached to C-1 is saturated, that attached to C-2 is unsaturated, while
that on C-3 can be either.
They are named according to the placement of acyl radical on glycerol

23. Waxes
 Waxes are the esters of fatty acids with alcohols other than glycerol.
 Some examples of waxes are
Spermwhale wax (spermaceti) is rich in cetyl palmitate
Beeswax is rich in myricyl palmitate
Carnauba wax, the hardest known wax

24. Compound Lipids
Compound lipids are the esters of fatty acids with alcohol and also possess additional groups
like phosphorous, nitrogen, etc.,
They are classified into
 Phospholipids (phosphatids)
Compounds that contain a phosphoric acid, nitrogen bases and other substituents in
addition to fatty acids and glycerol.
 Glycolipids (cerebrosides)
Compounds of fatty acids with carbohydrates and contain nitrogen but no phosphoric acid.
The glycolipids also include certain structurally related compounds comprising the groups
gangliosides, sulfolipids and sulfatids.

25. Phospholipids
Phospholipids
Phosphosphingosides
Phosphoinositols
Phosphoglycerides
Cephalin
Plasmologens
Lecithin

26. Phosphoglyceride - Lecithin
 Lecithin is known as phosphotidyl cholines. In addition to glycerol and 2 moles of fatty acids, the
lecithins also contain phosphoric acid and a nitrogen base choline at either the end or middle carbon
atom of glycerol unit. Accordingly, two forms of lecithins, α and β are recognized.
 On complete hydrolysis, lecithin yields choline, phosphoric acid, glycerol and 2 moles of fatty acids.
 But partial hydrolysis of lecithins by lecithinases (active principles found in snake venoms) causes
removal of only one fatty acid to yield substances called lysolecithins. These, therefore, contain only
one acyl radical. When subjected into the blood stream by sting as a result of snake bite or by needle,
the lysolecithins cause rapid rupture (hemolysis) of the red blood corpuscles.
Lecithin Lysolecithin

27. Phosphoglyceride - Cephalin
 Similar in structure to the lecithins except that the choline is replaced by either ethanolamine or
serine. Serine is the biochemical precursor of ethanolamine.
 Accordingly, two types of cephalins are recognized, phosphatidyl ethanolamine and phosphatidyl
serine. Like lecithins, the cephalins also exist in 2 forms, α and β, depending upon the relative
positions of the two substituent fatty acids.
 Venoms containing lecithinases also split off fatty acids from cephalins, leaving hemolytic
lysocephalins.

28. Phosphoglyceride - Plasmologens
 Structurally, these resemble lecithins and cephalins but have one of the fatty acids replaced
by an unsaturated ether.
 Since the nitrogen base can be choline, ethanolamine or serine, three types of plasmalogens
are accordingly distinguished : phosphatidal choline, phosphatidal ethanolamine and
phosphatidal serine.

29. Phosphoinositols / Phosphotidyl inositols
 In phospholipids, a cyclic hexahydroxy alcohol called inositol replaces base.
 On hydrolysis, the phosphoinositides yield 1 mole of glycerol, two moles of fatty acid, 1 mole
of inositol and 1, 2, or 3 moles of phosphoric acid. Accordingly, mono-, di- or
triphosphoinositides are found.
 Phosphoinositides are also classified as glycolipids, in as much as they contain carbohydrate
residue.

30. Phosphosphingosides / Sphingomyelins
 These compounds are commonly found in nerve tissue esp., in the myelin sheath of the nerve
(hence their name, sphingomyelins) and apparently lack in plants and the microorganisms.
 On hydrolysis, the phosphosphingosides yield equimolar amounts of fatty acid, phosphoric
acid, choline and sphingosine or dihydrosphingosine but no glycerol.
 These differ from other phospholipids in their lack of glycerol and the presence of another
nitrogenous base sphingosine or a closely related dihydrosphingosine, besides choline, in
place of glycerol.