Lipids are diverse organic compounds that are insoluble in water but soluble in non-polar solvents, playing essential roles in nutrition, energy storage, and cellular structure. Key types include triglycerides, phospholipids, and cholesterol, each with unique functions ranging from energy provision to membrane formation. Understanding lipid chemistry is crucial for recognizing their impact on health and diseases like obesity and cardiovascular issues.

1. Chemistry of lipid
Dr Ifat Ara Begum
Associate Professor
Department of Biochemistry
Dhaka Medical College, Dhaka

2. What is lipid?:
Lipids are heterogeneous group of
organic compounds, having a
common property of being
relatively insoluble in water &
soluble in non-polar organic
solvents like ether, chloroform,
benzene, alcohol .

3.  Lipids are not polymeric substances like proteins, polysaccharides &
nucleic acids
 Building block of most of the lipids: Fatty acid
 Lipids lack fatty acid: Cholesterol

4. Why you should know about chemistry of lipids?
 Essential roles of lipids in nutrition and health
 Understanding of many important biomedical
conditions, including obesity, DM &
atherosclerosis.

5. Biological importance / functions of Lipid
Includes biological importance / functions of:
 Neutral fat / Triglyceride (TG) / Triacylglycerol (TAG)
 Phospholipid
 Glycolipid
 Cholesterol

6.  Energy:
Source of energy
 Important storage form of energy
(as fat)
 Constitute biological membrane
 Provides :
Essential fatty acids
 Arachidonic acid for synthesis of
eicosanoids
 2nd messenger for hormones
 Helps in:
Absorption of fat soluble vitamins
 Coagulation

7.  Acts as:
 Subcutaneous thermal insulator
 Perineural electrical insulator
 Mechanical cushion around internal organs to protect them
from mechanical injury
 Precursor of steroid hormone, bile acid and vitamin D
 Lipotropic factor
 Surfactant to prevent collapsing tendency of alveoli

8. Classification of
Lipid

9. Classification based on composition
Simple Lipids Complex / Compound Lipids Derived Lipids
Esters of fatty acids
with alcohol
Esters of fatty acids with alcohol
along with other non lipid
substances
Derivatives obtained by hydrolysis of simple &
complex lipids which still possess the general
characteristics of lipids
Simple lipid = Fatty
acid + Alcohol
(Do not contain any
non-lipid substances)
Complex lipid = Fatty acid +
Alcohol + Other non lipid
substances
Hydrocarbon ring + Hydrocarbon side chain
Examples:
Neutral fat, Wax
Examples:
Phospholipid, Glycolipid &
Lipoprotein
Examples:
Fatty acid, Glycerol , Alcohol, Steroids,
Cholesterol, Prostaglandins, Ketone bodies,
Fat soluble vitamins etc

10. Classification based on polarity
Non polar lipid Polar Lipid / Amphipathic Lipid
Hydrophobic Lipids having both polar (hydrophilic) & non polar
(hydrophobic) groups
Water insoluble Show limited degree of water solubility due to their
hydrophilic groups
Example:
TAG
 Different esters (cholesterol ester,
retinol ester, vitamin D ester) etc
Example:
Fatty acid (-COOH group is polar group)
Cholesterol (-OH group)
Phospholipid (PO4 , Nitrogen base) etc

12. Structural versus depot lipid
Structural lipid / Tissue Lipid Depot Lipid
Constitute biological membrane Constitute storage lipid in adipose tissue
Chemically, they are Glycolipid, Phospholipid
& Cholesterol
Chemically, they are TAG
Contain mostly UFA Contain mostly SFA
Turnover rate is slow Turnover rate is rapid
Half life is long Half life is short (˂ 2 days)
Precursor of cellular signaling molecules Source of energy

13. Dietary lipids:
TAG : 90%
 Cholesterol
 Phospholipid
Neutral lipids:
 They carry no charge at normal body pH
 Examples:
 TAG
 Cholesterol & cholesterol esters
Monoglyceride, diglyceride
Lipids present in neural tissues:
 Phospholipids:
Lecithin, Cephalin, Sphingomyelin / Sphingophospholipid
 Glycolipids:
Cerebroside, Ganglioside, Sulfocerebroside

14. A short detail on some
important types of
lipids

15. 1. Neutral fat
(TG / TAG)

16. G
L
Y
C
E
R
O
L
Fatty Acid 1
Fatty Acid 2
Fatty Acid 3

17.  Triester of fatty acids with glycerol
 Regarded as Neutral, because, they don’t have any charge at normal body pH
 If 3 fatty acids are of same type: Simple TAG
 If 3 fatty acids are of different types: Mixed TAG & it predominates in nature

18.  Source of neutral fat:
Plant source : Contains more UFA.
e.g. vegetable oil
 Animal source: Contains more
SFA. e.g. ghee, egg etc
 Fatty acids found mostly in neutral fat:
Palmitic acid, Stearic acid, Oleic acid

19. Fat Oil
Triester of FA with
glycerol that remains
Solid at room
temperature (25 degree
C)
Triester of FA with
glycerol that remains
liquid at room temperature
(In other words, fat in
liquid state)
Contains more
saturated fatty acid
Contains more unsaturated
fatty acid
This difference is purely physical. Chemically both
are same (triacyl glycerol)

20. Physical properties of fat:
 Colorless, tasteless, odorless
 Neutral
 Water insoluble but soluble in fat solvents
 Specific gravity: ˂ 1.0
 Makes emulsion with water

21. Chemical properties of fat:
Hydrolysis: On hydrolysis, fat produces FA
and glycerol
Saponification: Hydrolysis of fat by alkali to
produce glycerol & soap
Rancidity: Auto oxidation of fat by
atmospheric oxygen to peroxides, epoxides &
aldehydes leading to unpleasant taste & odor
with change of color
Peroxidation: Auto oxidation in vivo to
produce lipid peroxides and free radicals
which causes tissue damage & initiate the
process of malignancy, aging &
atherosclerosis
Hydrogenation & hardening: UFA of fat by
absorbing hydrogen turns into SFA which
elevates melting point of fat & makes it solid
at room temperature

22. Biological importance / functions of Fat / TAG
 Source of energy
 Storage form of energy
 Add taste & palatability to food
 Provides EFA
 Helps in absorption of fat soluble
vitamins
 Acts as:
 Subcutaneous thermal insulator
 Perineural electrical insulator
 Mechanical cushion around internal organs
to protect them from mechanical injury
 Cosmetic effect:
Gives shape & contour to body
 High plasma TAG level increases the risk of:
 Coronary artery disease
 Fatty liver disease
 Acute pancreatitis

23. 2. WAX

24.  Esters of fatty acids with long chain monohydric alcohol
 Examples:
Cholesterol ester (Cholesterol + FA)
 Vitamin D ester (Vitamin D + FA)
 Retinol ester (Retinol +FA)

25. 3. Phospholipid

26.  Esters of fatty acid with alcohol attached to
phosphoric acid, with / without nitrogen base
[i.e. Phospholipid = Fatty acid + Alcohol +
Phosphoric acid ± Nitrogen base]
 Complex / Compound lipid
 In most of the cases, nitrogen base is present
 H3PO4 is essential component
 Has amphipathic nature
 Based on nature of alcohol present, two types of
PL – Glycerophospholipid &
Sphingophospholipid (Sphingomyelin)

27. A) Glycerophospholipid:
Here the alcohol is glycerol
FA + Glycerol + Phosphoric acid ±
Nitrogen base
 Fatty acids are attached with 1st &
2nd carbons and phosphate is
attached with 3rd carbon of glycerol
 At least one UFA is present usually
at 2nd carbon of glycerol

28. Primary glycerophospholipid: Phosphatidic
acid
 Others are derivatives of phosphatidic acid
 Major FA found in glycerophospholipid:
- PUFA (EFA)
- MUFA (Oleic acid, palmitoleic acid)
- SFA (Palmitic acid, stearic acid)

29. Common Glycerophospholipid Composition Nitrogen base
Phosphatidic acid (PA) FA + Glycerol + H3PO4 Absent
Lecithin (Phosphatidylcholine) PA + Choline Choline
Cephalin (Phosphatidylethanol amine) PA + Ethanol amine Ethanol amine
Phosphatidylserine PA + Serine Serine
Phosphatidylinositol (Lipositol) PA + Inositol Absent
Phosphatidylglycerol PA + Glycerol Absent
Diphosphatidylglycerol (Cardiolipin) 2 (PA + Glycerol) Absent
Lyso phospholipids Glycerophospholipid with one FA
removed from C2 or C1 of
glycerol
Mostly present

30. B) Sphingophospholipid /
Sphingomyelin:
 Constitute biological membrane & myelin sheath
 Here the alcohol is sphingol (sphingosine)
 FA + Sphingol + Phosphoric acid + Choline
 As (FA + Sphingol) together make Ceramide, so
we can write
Sphingomyelin = Ceramide + H3PO4 + Choline
 FA : 16-26 C SFA (palmitic acid, stearic acid)
 Choline: Nitrogenous substance
 H3PO4 & Choline : Non lipid substances

31. Biological importance / functions of Phospholipid (esp. glyceroPL)
 Constitutes biological membrane.
 Membrane PL provides arachidonic acid to synthesize
eicosanoids
 Helps in:
 Coagulation : Cephalin
 Lipoprotein formation
 Acts as:
 Source of 2nd messenger for hormones: Phosphatidyl inositol
 Lipotropic factor to prevent fatty liver: Lecithin
 Surfactant in the lung to prevent collapsing tendency of
alveoli : Dipalmitoyl lecithin
 In bile, they
solubilize cholesterol
&
prevent gall stone
formation

32. 4. Glycolipid /
Glycosphingolipid

33.  Lipids containing carbohydrate
 FA + Sphingol (Sphingosine) + Carbohydrate
[As (FA + Sphingol) together make Ceramide, so
we can write
Glycolipid = Ceramide + Carbohydrate]
 Lipid part : FA with sphingol
 Non lipid part: Carbohydrate
 Carbohydrates present here: Monosaccharide,
amino sugar, sialic acid
 FA present here: Mostly cerebronic acid, nervonic
acid, lignoceric acid etc

34. Types of Glycolipids based on nature of carbohydrate
component
Cerebrosides Gangliosides
Carbohydrate component:
Monosaccharide
Example:
Glucocerebroside,
Galactocerebroside
Carbohydrate component:
Monosaccharide
+
Complex carbohydrate (like
amino sugar, sialic acid)

37. Sulfolipid :
 Synonym: Sulfatide / Sulfoglycolipid
 Glycolipid with SO4 group
 Example:
Sulfocerebrosides like glucocerebroside SO4 , galactocerebroside SO4

38. Biological importance / functions of Glycolipids
 Constitutes cell membrane.
 Regulates:
Cellular interaction
Inter cellular communication
Cellular growth and development
 Acts as:
 Cell surface receptor
 Blood group substance
 Tumour antigen

39. 5. Lipoproteins

40. Lipid
Protein
Lipoprotein
 Lipoproteins are globular
macromolecular complex consisting
of lipids and specific protein
(apoprotein / apolipoprotein)
 Purpose / Importance of LP
formation:
To solubilize lipids in plasma
water to facilitate their transport in
biological system
To provide efficient mechanism
for lipid delivery to the tissues &
lipid removal from the tissues

41. Organization of LP:
Constituents of LP (TAG, CE,
PL, FC, Apoprotein) are
organized in to a globular
structure with -
 a central core of non polar
hydrophobic lipid (TAG & CE)
 surrounded by a middle layer
of amphipathic lipids (PL &
FC)
 with an external interrupted
layer of apoproteins

42. Types
of
Lipoprotein
Chylomicron
Very low density lipoproteins - VLDL
Low density lipoprotein – LDL
High density lipoprotein - HDL
Others: CMR, VLDLR (IDL)

43. • C-I,II, III:
Liver
• Liver
• Macrophage
• B-48: Intestine
• B-100: Liver
• A-I: Liver & Intestine
• A-II:Liver
• A-IV: Intestine
Apo
A
Apo
B
Apo
C
Apo
E

44. Protein component of LP that provides
hydrophilic character to LP particles
to facilitate their transport in aqueous
plasma
Act as :
- Cofactor (activator) for enzymes of
LP metabolism (e.g. Apo C-II for LPL)
- Inhibitor of enzymes of LP
metabolism (e.g. Apo A-II for LPL)
Acts As Ligand to recognize LP receptors
on cell surface for uptake of
LP.
e.g. Apo A is ligand for HDL receptor
- Maintains structural stability of LP
- Determines metabolic fates of LP and
facilitate exchange of lipids between LPs
Functions / Importance of
Apoproteins in LP

45. Types of
LP
(origin)
Size Density Apoprotein
(%)
Lipid
(%)
Main lipid apoprotein
Inherent Acquired
CM
(intestine)
2 98 Dietary TAG (90% of
lipid core)
B48 & A C & E
(from HDL)
VLDL
(liver)
8 92 Endogenous TAG
(80% of lipid core)
B100 C & E
(from HDL)
LDL
(from
VLDL in
blood)
20 80 Cholesterol (65% of
lipid core),
PL
B100 -
HDL
(Liver)
50 50 Cholesterol
PL
A, C, E -

46. Chylomicron:
- Supports exogenous (dietary) lipid
transport (esp. TAG & Cholesterol)
- Carries dietary lipids from intestine to
liver & other peripheral tissues
VLDL:
- Supports endogenous lipids transport
- Carries cholesterol (via LDL) & TAG from
liver to peripheral tissues
- Precursor of LDL
LDL:
- Supports endogenous lipid transport
- Receives cholesterol from VLDL &
HDL and then deposit cholesterol to
peripheral tissues
HDL:
- Supports endogenous lipid transport
- Acts as a reservoir of apoprotein
- Helps in CM & VLDL metabolism
- Through RCT, acts as cholesterol scavengers in
peripheral tissues and removes cholesterol from
peripheral tissues back to liver
Functions of diff. LP

47. Remember
 93% of total body cholesterol resides in intracellular
compartment
 Only 7% is found in plasma where cholesterol is carried by :
LDL (60-70%)
HDL (20-30%)
 VLDL (10-15%)
 Atherogenic LPs : VLDL, CMR, IDL, LDL
 Antiatherogenic LP: HDL

48. 6. Fatty Acid

49.  Carboxyl group containing organic acid
 Weak acid
 There is an aliphatic hydrocarbon chain with one carboxyl group at the end
of the chain
 Amphipathic character (hydrophilic carboxylate anion COO- & hydrophobic
hydrocarbon chain)
 General formula: CH3 - (CH2)n – COOH

50.  Short chain FA is water soluble because of their short
hydrophobic hydrocarbon chain
 Long chain FA is highly water insoluble because of their long /
predominant hydrophobic hydrocarbon portion, so they are
transported in circulation with plasma albumin
Two forms in body:
i. As esters in neutral fat & oils
ii. In unesterified form as FFA (a transport form in plasma)

51.  Distribution of plasma FA:
 45% found with TAG
 35% with PL
 15% with CE
 5% as FFA
 >90% of FA in human body contains even no. of carbon atoms (mostly14 to
24 C)
Of these most abundant are 16C FA (Palmitic acid) or 18C FA (Stearic acid)
 <5% of FA in human body contains odd no. of carbon atoms

52. Sources of FA:
i. Endogenous sources: Synthesis of NEFA within the body
ii. Exogenous / Dietary sources:
• For saturated FA: Animal fat, milk, butter, ghee, coconut oil, palm
oil, Vanashpati (hydrogenated fat) etc
• For Unsaturated FA: vegetable oil, fish oil, cod liver oil etc

53. Numbering & designation of carbons in Fatty acid
CH3-CH2…… CH2 - CH2 - CH2- COOH
1234
αβγω

54. Classification of fatty acid
Three types of classification:
A. Based on total number of carbon (chain length)
B. Based on saturation of carbon
C. Nutritional classification

55. Based on chain length :
i) Short chain Fatty acid:
 No. of C: <10
Mostly found in milk
e.g. Acetic acid (2C), Propionic acid
(3C) etc
ii) Long chain Fatty acid:
 No. of C: >10
e.g. Palmitic acid (16C),
Stearic acid (18C) etc
Fatty Acid
(Based on saturation of
carbon)
Unsaturated
MUFA
(Oleic acid)
PUFA
(EFA)
Saturated

57. Fatty Acid
(Nutritional Classification)
NEFA
Palmitic acid, Stearic
acid
EFA
Linoleic acid, Linolenic
acid

58. Delta numbering system for UFA:
 C atoms are numbered from –COOH
group
 Total C number, total no. of double
bond & the position of the double
bond are indicated numerically
 Linoleic acid: 18 : 2 ; 9 ,12
[ 18C fatty acid with 2 double bonds,
which are placed between carbon 9 – 10
and carbon 12 – 13]
Omega (ω) numbering system for
UFA:
 C atoms are numbered starting from ω
carbon end (methyl end) of chain
 Position / distance of first double bond from
ω carbon is mentioned
 ω3- fatty acid / Linolenic acid:
[The first double bond closest to ω-carbon
end begins at 3rd carbon counted from this end]

60. Cis Isomeric Form of FA Trans Isomeric Form of FA
Double bonds (C = C) of UFA are
arranged in cis isomeric form where
both hydrogen atoms are present on the
same side of double bonds
Double bonds (C = C) of UFA are arranged in trans
isomeric form where both hydrogen atoms are present on
the opposite side of double bonds
It makes UFA more fluid It makes UFA less fluid and trans FA behave like SFA
These are found in nature These are not commonly found in nature. They are
produced as a byproduct during hydrogenation treatment
on PUFA of natural oil to make it harden fat. e.g. solid
margarine (hydrogenated vegetable oil)
Trans FA containing TAG is called Trans fat

61. Importance of Trans FA / Fat:
Consumption of trans FA / trans fat elevates LDL and lowers HDL, so, more risk of
coronary artery disease & DM

62. Properties of fatty acids
Physical properties:
 Water solubility:
 Melting point
 Isomerism
Chemical properties:
Soap formation
Ester formation
 Oxidation
Hydrogenation
Halogenation

63. Biological importance / functions of fatty acids
 Metabolic fuel : When oxidized , they form ATP
 Participates in synthesis of
 PL
 GL
 Cholesterol ester
 FA derivatives act as intracellular messenger. e.g. PG
 Risk of atherosclerosis & CAD:
 SFA & trans FA increase LDL & decrease HDL, so the risk of CAD increases.
 UFA decreases LDL, so the risk of CAD decreases

64. 1. Linoleic acid : 18 C ω6 fatty
acid with 2 double bonds 2. Linolenic acid : 18 C ω3 fatty acid
with 3 double bonds
Arachidonic acid : 20 C ω6
fatty acid with 4 double bonds
[Semi essential as can be
synthesized from Linoleic acid]
Functions:
- Component of biological membrane & maintain
its fluidity
- Precursor of Eicosanoids
- Helps in Gonadal function, reproduction, Vision,
Growth
- Reduce plasma cholesterol by increasing its
excretion via bile & its oxidation to bile acid
Essential Fatty Acids
(PUFA)

65. Deficiency of EFA:
 Retarded growth
 Impaired gonadal function & reproductive
failure
 Dermatitis
 Degenerative changes in arterial wall
 Fatty liver
 Poor wound healing & hair loss
 Faulty vision
Sources of EFA:
 Vegetable oil (Except coconut oil
and palm oil)
 Fish oil (Linolenic acid rich)
 Cod liver oil (Linoleic acid rich)

66. 7. Cholesterol

67.  A 27 C Amphipathic lipid containing steroid
nucleus
 Steroid nucleus :
 Cyclopentano perhydro phenatherene nucleus
 A 19 C compound made of 4 fused non
aromatic hydrocarbon rings (A, B, C, D)
 Two methyl groups denoted as C18 and C19
 Steroid nucleus is attached with
- One OH group at C3
- A branched chain of 8 carbons at C17
 A double bond between C5 and C6

68. Functions of cholesterol:
 Constituent of biological membrane
 Precursor of:
Steroid hormones
 Bile acid , bile salts
 Vitamin D
 High plasma level of cholesterol is associated
with atherosclerotic disorders like
- Stroke
- Coronary artery disease etc
Sources of Cholesterol:
 Endogenous synthesis
 Exogenous / Dietary: Only
from animal foods . e.g. egg
yolk, liver, mutton, prawn, beef
etc

69. “All sterols are steroids but all steroids are not sterols”
Steroid substances:
Steroid nucleus + one oxygen atom
at C-3 position of nucleus.
e.g. Cortisol, Aldosterone,
Testosterone etc
Sterol compounds:
Steroid nucleus + one hydroxyl
group at C-3 position of nucleus.
e.g. Cholesterol, Bile acid, Vitamin
D etc

71. 8. Eicosanoids

72.  Signaling molecules derived from made by oxidation of 20 C PUFA
(eicosanoic acid )
Or
Prostaglandins and related compounds

74. Name of the eicosanoids
Eicosanoid includes:
 Prostaglandins (PG)
 Prostacyclin (PG-I2)
 Thromboxane (TX)
 Leukotrienes (LT)
 Lipoxins (LX)
Prostanoid

75. Justify, “Prostaglandins are local hormones”
 They act on target cells close to their site of formation to
produce specific effects
 They are rapidly degraded,
so they are not transported to distal sites within the body

76. Justify, “Prostaglandins are not true hormones”
Points PG True hormones
Origin Almost all tissues Specialized glands
Site of action Locally Distant sites
Transport via blood Not transported Transported
Action on parent cells Yes No
Synthesis & storage Synthesis as per need & not stored
in tissues
Not such

77. Clinical use of Prostaglandins
 Control of :
 Inflammation by suppression of PG
synthesis
 Hypertension (PG E2 & PG I2)
 Relief of asthma and nasal congestion:
PG E2
 To prevent :
 Peptic ulcers by decreasing HCl
secretion (PG E2) and treatment as
well
 Thrombotic events (PG I2)
 Control of hypertension:
PG E2 & PG I2

78. 9. Ketone Body

79.  Water-soluble derived lipid
 Metabolic products that are produced in excess during excessive
breakdown of fatty acid
 Freely transportable form of acetyl units (don’t need to be with LP
/albumin for transportation in blood)
 Acetoacetate, Beta hydroxy butyrate, and acetone are often referred to
as ”Ketone bodies”
 “Acetoacetate” is the primary ketone body.

81. Interrelationship of
ketone bodies

82. Ketone
bodies are
synthesized
only in liver

83. Alternate sources to
glucose for energy
Production of ketone bodies under
conditions of cellular energy deprivation
Utilization of ketone
bodies by the brain