2. Lipids
• Lipids (Greek: lipos, means fat) - are heterogeneous class of naturally
occurring organic substances which are insoluble in water and highly
soluble in organic solvents like ether, chloroform, benzene and acetone.
• Chemically LIPIDS are various types of esters of different alcohols.
• In addition to alcohol and fatty acids, some of the lipids may contain
phosphoric acid, nitrogenous base and carbohydrates.
• Bloor’s Criteria
• According to Bloor, lipids are compounds having the following
characteristics:
• They are insoluble in water BUT soluble in organic solvents, such as ether,
chloroform, benzene, acetone, etc.
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3. Lipids
• Lipids include fats, oils, waxes and related compounds.
• An oil is a lipid which is liquid at ordinary temperature.
• Distinction between fats and oils is a purely physical one.
• Chemically they are all esters of glycerol with higher fatty acids.
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4. Classification of lipids
1. Simple Lipids
• Esters of fatty acids with various alcohols. They include:
(a) Neutral fats (Triacylglycerol, TG): These are triesters of fatty acids with glycerol.
(b) Waxes are esters of fatty acids with higher alcohols.
2. Compound Lipids
• Esters of fatty acids containing groups, other than in addition, to an alcohol and fatty
acids.
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5. Classification of lipids
• (a) Phospholipids: They are fats containing in addition to fatty acid
and glycerol, a phosphoric acid residue, a nitrogenous base and other
substituents. Examples: phosphatidyl choline (Lecithin), phosphatidyl
ethanolamine (Cephalin) etc.
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6. Classification of lipids
• (b) Glycolipids: Lipids containing carbohydrate moiety are called
glycolipids. These are of two types: Cerebrosides, Gangliosides
• (c) Sulpholipids: Lipids characterised by possessing sulphate groups.
• (d) Aminolipids (Proteolipids)
• (e) Lipoproteins: Lipids as prosthetic group to proteins.
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7. Classification of lipids
3. Derived Lipids
• These are obtained by hydrolysis of those given in group I and II, which still
possess the general characteristics of lipids.
• (a) Fatty acids may be saturated, unsaturated or cyclic.
• (b) Monoglycerides (Monoacylglycerol) and Diglycerides (Diacylglycerol).
• (c) Alcohols: Cholesterol and other steroids including Vit D.
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8. Fatty acids
• Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to
36 carbons long (C4 to C36).
• Saturated FA: Those which contain no double bonds.
• Unsaturated FA: Those which contain one or more double bonds.
• (a) Saturated FA: Their general formula is CnH2n+1 COOH Examples:
• Acetic acid CH3COOH, Propionic acid C2H5COOH and Butyric acid
C3H7COOH
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9. Fatty acids
• (b) Unsaturated FA: They are classified further according to degree of
unsaturation.
• (1) Mono unsaturated fatty acids: They contain one double bond.
Their general formula is Cn H2n–1 COOH
• Example: Oleic acid C17H33 COOH is found in nearly all fats (formula
18 : 1; 9).
• (2) Polyunsaturated fatty acids: There are three polyunsaturated
fatty acids of biological importance. Linoleic acid , Linolenic acid,
Arachidonic acid
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12. Nomenclature of FA
• According to standard nomenclature, Carbon atoms are numbered from the –
COOH carbon (carbon No. 1).
• The carbon adjacent to –COOH gr. i.e. carbon number 2 is known as α-carbon,
carbon atom 3 is β-carbon and the end –CH3 carbon is known as the ω-carbon
(‘Omega’ carbon).
• A widely used convention is to express the fatty acids by a formula to indicate the
no. of carbon atoms, no. of double bonds and position of double bond.
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13. Nomenclature of FA
• The (Δ) sign is used for double bond, e.g. Δ9 denotes double bond
between 9 and 10 C.
• For example oleic Acid (mol. formula C17H33 COOH) has one double
bond between C9 and C10, thus: 18: 1 Δ9
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14. Nomenclature of FA
• For polyunsaturated fatty acids (PUFAs), an alternative convention is
used.
• The carbon of the methyl group is called the (omega) carbon and is given
the number 1.
• In this convention, PUFAs with a double bond between C-3 and C-4 are
called omega-3 fatty acids, and those with a double bond between C-6 and
C-7 are omega-6 fatty acids.
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16. Isomerism
• Two types of isomers can occur in an unsaturated fatty acid.
(a) Geometric Isomers: They depend on the orientation of the radicals
around the axis of the double bonds. If the H are on the same side of the
bond, it is called as ‘cis’ form. If the H are on the opposite side, a ‘trans’ form
is produced. ‘Cis’ form is comparatively unstable and is more reactive.
• Example: Oleic acid and elaidic acid both have same molecular formula –
C17H33COOH.
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19. Isomerism
(b) Positional Isomers: A variation in the location of the double bonds
along the unsaturated fatty acids chain produces isomer of that
compound.
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21. Physical Properties of FA
• Melting point: MP of saturated fatty acids increases with increase in the no. of
carbon atoms in the chain. For example the MP of butyric acid is 8C, Palmitic acid
is 62C and that of Stearic acid it is 70C.
• Higher the degree of saturation and higher the chain length, higher will be the
MP.
• Solubility: Solubility of a fatty acid increases with increase in the double bond.In
contrast as the chain length increses, the solubility decreses due to increase in
no. of Methylene groups.
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22. Chemical properties of Fatty acids
1. Formation of salts: Fatty acids form salts with alkali and alkaline
earth metals. Salts of Na, K, Ca and Mg are soaps. Sodium and
potassium salts are soluble while calcium and magnesium salts are
insoluble in water.
2. Formation of detergents: Reduction of COOH group of fatty acids
produces alkyl alcohols which can be sulfated to form alkyl sulfates
which act as detergents.
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23. Chemical properties of Fatty acids
3. Formation of esters: Fatty acids in combination with alcohol form
esters. Mono, di and tri glycerides are examples.
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24. Chemical properties of Fatty acids
4. Formation of eicosanoids: Prostaglandins and related compounds are
called eicosanoids.
• All eicosanoids are derived from arachidonic acid (20:4(Δ 5,8,11,14)), the
20-carbon polyunsaturated fatty acid from which they take their general
name (Greek eikosi, “twenty”).
• There are three classes of eicosanoids: prostaglandins, thromboxanes, and
leukotrienes.
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25. Chemical properties of Fatty acids
• The eicosanoids are considered paracrine or "local hormones."
They have specific effects on target cells close to their site of formation.
They are rapidly degraded, so they are not transported to distal sites within the body.
• Prostaglandins (PG) contain a five-carbon ring originating from the chain of
arachidonic acid.
• Their name derives from the prostate gland, the tissue from which they
were first isolated by Bengt Samuelsson and Sune Bergström.
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27. Biological Processes Regulated by Eicosanoids
1. Blood clotting
• Thromboxane A2 stimulates constriction of blood vessels and platelet
aggregation
• Prostacyclin dilates blood vessels and inhibits platelet aggregation
2. Inflammatory response
• Prostaglandins mediate aspects of inflammatory response
3. Reproductive system
• Stimulation of smooth muscle by PGE2
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28. Biological Processes Regulated by Eicosanoids
4. Gastrointestinal tract
• Prostaglandins inhibit gastric secretion
• Prostaglandins increase secretion of protective mucus
• Inhibition of hormone-sensitive lipases
5. Kidneys
• Prostaglandins dilate renal blood vessels
• Results in increased water and electrolyte excretion
6. Respiratory tract
• Leukotrienes promote the constriction of bronchi
• Prostaglandins promote bronchodilation
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29. Chemical properties
• Hydrogenation: It is the addition of hydrogen to unsaturated double
bonds present in oils.
• Thus liquid fats( oils) are converted to solid fats(ghee). This process is
called as hardening of oils.
• Halogenation: Halogens( F, I, Br) are readily added to the double
bonds of unsaturated fatty acids.
• This property is used as an index of degree of unsaturation of an oil or
fat..(iodine number).
• Iodine Number: The no. of grams of iodine required to the double
bonds in 100 grams of fat.
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30. Chemical properties
• Oxidation: The double bonds in unsaturated fatty acids are oxidizable.
• The spontaneous oxidation forms thin, transparent coating on the
surfaces to which oils is applied. This is called drying of oils and is
used in the manufacturing of paints and varnishes.
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31. Chemical Properties
4. Rancidity: When lipid-rich foods are exposed too long to the oxygen in air,
they may spoil and become rancid.
• The unpleasant taste and smell associated with rancidity result from the
oxidative cleavage of double bonds in unsaturated fatty acids, which
produces aldehydes and carboxylic acids of shorter chain length and
therefore higher volatility; these compounds pass readily through the air to
your nose.
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32. Chemical Properties
• To improve the shelf life of vegetable oils used in cooking, and to increase
their stability at the high temperatures used in deep frying, commercial
vegetable oils are prepared by partial hydrogenation.
• This process converts many of the cis double bonds in the fatty acids to
single bonds and increases the melting temperature of the oils so that they
are more nearly solid at room temperature (margarine is produced from
vegetable oil in this way).
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33. Chemical Properties
• Partial hydrogenation has another, undesirable, effect: some cis
double bonds are converted to trans double bonds.
• Dietary trans fatty acids raise the level of triacylglycerols and of LDL
(“bad”) cholesterol in the blood, and lower the level of HDL (“good”)
cholesterol, and these changes alone are enough to increase the risk
of coronary heart disease.
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34. Essential fatty acids
• Three polyunsaturated fatty acids, linoleic acid, linolenic acid and
arachidonic acid are called “essential fatty acids” (EFA).
• They cannot be synthesized in the body and must be provided in the
diet.
• Lack of EFA in the diet can produce growth retardation and other
deficiency manifestation symptoms.
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35. Docosahexaenoic acid (DHA)
• Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid which is
synthesized from α-linolenic or obtained directly from dietary fish oil.
• This fatty acid is present in high concentrations in retina, cerebral
cortex, testes and sperms.
• DHA is particularly needed for development of the brain and retina
and is supplied via the placenta and milk.
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36. Alcohols
• Alcohols contained in the lipid molecule includes glycerol, cholesterol
and the higher alcohols, e.g. cetylalcohol, C16H33COOH (usually
found in waxes).
• 1. Glycerol: Glycerol is commonly called as “glycerin” it is the simplest
trihydric alcohol as it contains three hydroxyl groups in the molecule.
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37. Steroids and sterols
• Steroids are synthesized from the five carbon
isoprene unit.
• Part of a diverse collection of lipids called
isoprenoids / terpenes
• Contain the steroid carbon skeleton
• A collection of 4 fused carbon rings
38. Cholesterol
• Cholesterol is the most important sterol in human body. Its molecular
formula is C27H45OH.
• It possesses “cyclopentanoperhydrophenanthrene nucleus”.
• It has an –OH group at C3.
• Properties: The name cholesterol is derived from the Greek word
meaning solid bile.
• Its melting point is 147 to 150o C.
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39. Steroids and sterols
• Cholesterol
• Cell membranes, precursor to bile salts, male and female sex hormones,
vitamin D, and the adrenocortical hormones
• Have been linked to atherosclerosis
• Bile salts
• Important in the lipid digestion
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41. Other sterols of biological importance
• 7-Dehydrocholesterol
• It is an important sterol present in the skin.
• This differs from cholesterol only in having a second double bond, between C7 and C8.
• Source: In man, 7-dehydrocholesterol may be obtained partly by synthesis from
cholesterol in skin and/or intestinal wall.
• In the epidermis of skin, UV rays of sunshine change 7-dehydrocholesterol (pre-
cholecalciferol) to cholecalciferol (vitamin D3).
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42. Other sterols of biological importance
• Ergosterol: It is a plant sterol, first isolated from ergot, a fungus of rye
and later from yeast and certain mushrooms. Ergosterol
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43. Vit D
• Vitamin D3, also called cholecalciferol, is normally formed in the skin from 7-
dehydrocholesterol by the action of sunlight (UV component of sunlight).
• Vitamin D3 is not itself biologically active, but it is converted by enzymes in the
liver and kidney to 1,25-dihydroxyvitamin D3 (calcitriol), a hormone that
regulates calcium uptake in the intestine and calcium levels in kidney and bone.
• Deficiency of vitamin D leads to defective bone formation and the disease rickets
and osteomalacia.
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44. Vit D
• Vitamin D2 (ergocalciferol) is a commercial product formed by UV
irradiation of the ergosterol of yeast.
• Both have the same biological effects, and D2 is commonly added to milk
and butter as a dietary supplement.
• Like steroid hormones, the product of vitamin D metabolism, 1,25-
dihydroxyvitamin D3, regulates gene expression by interacting with specific
nuclear receptor proteins.
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45. Vitamin D3 production and metabolism
• Cholecalciferol (vitamin D3) is produced in the skin by UV irradiation of 7-
dehydrocholesterol, which breaks the bond shaded light red.
• In the liver, a hydroxyl group is added at C-25; in the kidney, a second
hydroxylation at C-1 produces the active hormone, 1,25-dihydroxyvitamin
D3.
• This hormone regulates the metabolism of Ca in kidney, intestine, and
bone.
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48. Simple lipids(glycerides)
• Glycerides are lipid esters
• Alcohol group of glycerol form an ester with a fatty acid
• Esterification may occur at one, two, or all three alcohol positions producing:
• Monoglyceride
• Diglyceride
• Triglyceride
• A neutral triacylglycerol or a triglyceride
• Triglycerides are nonionic and nonpolar
• Triglycerides serve as energy storage in adipose cells
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49. Triglycerides
• Glycerides are lipid esters
• A triglyceride places fatty acid chains at each alcohol group of the glycerol
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50. Lipids as pigments in plants and bird feathers
• Compounds with long conjugated systems absorb light in the visible
region of the spectrum.
• Birds acquire the pigments that color their feathers red or yellow by
eating plant materials that contain carotenoid pigments, such as
canthaxanthin and zeaxanthin.
• The differences in pigmentation between male and female birds are
the result of differences in intestinal uptake and processing of
carotenoids.
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52. Compound lipids
Compound(conjugated) lipoids are lipids conjugated with other substances.
They include:
1. Phospholipids formed of lipid, phosphoric acid and nitrogenous base
2. Glycolipids formed of lipid and carbohydrate
3. Sulpholipids, lipids containing sulphate.
4. Lipoproteins formed of lipid part and protein part.
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53. Phosphoglycerides
• Phospholipid is a more general
term
• Any lipid containing phosphorus
• Phosphoglycerides contain:
• Glycerol
• Fatty acid
• Phosphoric acid with an amino
alcohol
• Replace an end fatty acid of a
triglyceride with a phosphoric
acid linked to an amino alcohol
G
l
y
c
e
r
o
l
Fatty Acid
Fatty Acid
Phosphoric Acid
Alcohol
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54. Phosphoglycerides
• Have hydrophobic and hydrophilic domains
• Structural components of membranes
• Emulsifying agents
• Suspended in water, they spontaneously rearrange into ordered structures
• Hydrophobic group to center
• Hydrophilic group to water
• Basis of membrane structure
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55. Phosphoglycerides- types
• The phospho-amino-alcohol is highly hydrophilic
• They are used in:
• Cell membranes
• Emulsifying
• Micelle-forming agents in the blood
• Two types
• Ones made with choline are called lecithin
• Those made with either ethanolamine or serine are called cephalins
• Other Phosphoglycerides include phosphatidic acid, lecithins, cephalins, phosphatidyl inositol, plasmalogen
and cardiolipin.
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56. Non-glyceride Lipids
Sphingolipids
• These lipids are based on sphingosine
• Long-chain
• Nitrogen-containing
• Alcohol
• Amphipathic, like phospholipids
• Polar head group
• Two nonpolar fatty acid tail
• Structural component of cellular membranes
• Major categories
• Sphingomyelins
• Glycosphingolipids
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57. Types of Sphingolipids
• Sphingomyelins
• Structural lipid of nerve cell
membranes
• Myelin sheath feature
• Glycosphingolipids
• Built on a ceramide
• Cerebrosides have a single
monosaccharide head group
• Glucocerebroside
• Galactocerebroside
59. Lipoproteins
• A lipoprotein is a biochemical assembly that contains both proteins and lipids.
• The lipids or their derivatives may be covalently or non-covalently bound to the
proteins.
• Many enzymes, transporters, structural proteins, antigens, adhesins and toxins
are lipoproteins.
• Examples include the high density and low density lipoproteins of the blood, the
transmembrane proteins of the mitochondrion and the chloroplast, and bacterial
lipoproteins.
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60. Classification
• General categories of lipoproteins, listed in order from larger and less dense (more fat
than protein) to smaller and more dense (more protein, less fat):
• Chylomicrons - carry triacylglycerol (fat) from the intestines to the liver, skeletal muscle,
and to adipose tissue.
• Very low density lipoproteins (VLDL) - carry (newly synthesised) triacylglycerol from the
liver to adipose tissue.
• Intermediate density lipoproteins (IDL) - are intermediate between VLDL and LDL. They
are not usually detectable in the blood.
• Low density lipoproteins (LDL) - carry cholesterol from the liver to cells of the body.
Sometimes referred to as the "bad cholesterol" lipoprotein.
• High density lipoproteins (HDL) - collects cholesterol from the body's tissues, and brings
it back to the liver. Sometimes referred to as the "good cholesterol" lipoprotein.
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61. Biological Functions of Lipids
• As an energy source, lipids provide 9 kcal of energy per gram
• Triglycerides provide energy storage in adipocytes
• Phosphoglycerides, sphingolipids, and steroids are structural components
of cell membranes
• Steroid hormones are critical intercellular messengers
• Lipid-soluble vitamins (A, E, D, K)
• Dietary fat acts as a carrier of lipid-soluble vitamins into cells of small
intestine
• Provide shock absorption and insulation
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