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Notes on Chemistry of Lipids

Lipids may be defned as compounds which are relatively insoluble in water, but freely soluble in non-polar organic solvents, such as benzene, chloroform, ether, hot alcohol, acetone, etc.

Classification of Lipids

I Simple lipids
a. Triacylglycerol or Triglycerides or neutral fat
b. Waxes

II. Compound lipids
A. Phospholipids, containing phosphoric acid

1. Nitrogen containing glycerophosphatides:
i. Lecithin (phosphatidylcholine)
ii. Cephalin (phosphatidylethanolamine)
iii. Phosphatidylserine

2. Non-nitrogen glycerophosphatides
i. Phosphatidylinositol
ii. Phosphatidylglycerol
iii. Diphosphatidylglycerol (cardiolipin)

3. Plasmalogens, having long chain alcohol
i. Choline plasmalogen
ii. Ethanolamine plasmalogen

4. Phospho sphingosides, with sphingosine

B. Non-phosphorylated lipids
1. Glycosphingolipids (carbohydrate)
i. Cerebrosides (ceramide monohexosides)
ii. Globosides (ceramide oligosaccharides)
iii. Gangliosides (ceramide + oligosaccharides + N-acetyl neuraminic acid)

2. Sulfolipids or sulfatides
i. Sulfated cerebrosides
ii. Sulfated globosides
iii. Sulfated gangliosides

III. Derived lipids
Fatty acids, steroids, prostaglandins , leukotrienes, terpenes, dolichols, etc.

IV. Lipids complexed to other compounds
Proteolipids and lipoproteins.

Functions of Lipids

1.Storage form of energy (triacylglycerol)

2. Structural components of biomembranes (phospholipids and cholesterol)

3. Metabolic regulators (steroid hormones and prostaglandins)

4. Act as surfactants, detergents and emulsifying agents (amphipathic lipids)

5. Act as electric insulators in neurons

6. Provide insulation against changes in external temperature (subcutaneous fat)

7. Give shape and contour to the body

8. Protect internal organs by providing a cushioning effect (pads of fat)

9. Help in absorption of fat soluble vitamins (A, D, E and K)

10. Improve taste and palatability of food


Fatty acids, are included in the group of derived lipids They are generally found in ester linkage in different classes of lipids. In the human body, free fatty acids are formed only during metabolism.

Classification of Fatty Acid

1.Depending on total number of carbon atoms:
a. Even chain:
They have carbon atoms 2,4,6 and similar series. Most of the naturally occurring lipids contain even chain fatty acids.
b. Odd chain:
They have carbon atoms 3, 5, 7, etc. Odd numbered fatty acids
are seen in microbial cell walls. They are also present in milk.

2. Depending on length of hydrocarbon chain:
a. Short chain with 2 to 6 carbon atoms
b. Medium chain with 8 to 14 carbon atoms
c. Long chain with 16 and above, usually up to 24 carbon atoms
d. Very long chain fatty acids (more than 24 carbon).

3. Depending on nature of hydrocarbon chain:
a. Saturated fatty acids
b. Unsaturated fatty acids which may be subclassifed into
Monounsaturated (monoenoic) having single double bond or Polyunsaturated (polyenoic) with 2 or more double bonds.
c. Branched chain fatty acids
d. Hydroxy fatty acids


They have the general formula CH3-(CH2)n-COOH

1.They are named by adding the suffx ‘anoic’ after the hydrocarbon.
2. The two carbon acetic acid and 4 carbon butyric acid are important metabolic intermediates

Clinical application of saturated fatty acid

1.Excessive fat deposits cause obesity. Truncal obesity is a risk factor for heart attack. Abnormality in cholesterol and lipoprotein metabolism leads to atherosclerosis and cardiovascular diseases

2. In diabetes mellitus, the metabolisms of fatty acids and lipoproteins are deranged, leading to ketosis


They are similar to saturated fatty acids in the reaction of the carboxylic group but also show properties due to presence of the double bond. The polyunsaturated fatty acids (PUFA) exist in cis confguration in naturally occurring lipids.

Clinical Significance of PUFA

1.Linoleic and linolenic acids are polyunsaturated fatty acids.

2. They are called essential fatty acids, because they cannot be synthesized by the body and have to be supplied in the diet.

3. Unsaturated fatty acids are also designated as:-
w3 family — Linolenic acids
w6 family — Linoleic and Arachidonic acids
w9 family — Oleic acid

4. Arachidonic acid is the precursor of prostaglandins. Arachidonic acid can be synthesized in the body, if the essential fatty acids are supplied in the diet.

5. The penta-enoic acid present in fsh oils is of great nutritional importance ( w3 unsaturated fatty acid).

6. Eicosanoids (eicosa = twenty) are derived from 20 C arachidonic acid. They are poly-enoic fatty acids. They are prostanoids (prostaglandins, prostacyclins, thromboxanes) and leukotrienes.


They are present in dairy products and in hydrogenated edible oils Trans fatty acids adversely affects multiple risk factors for chronic diseases, including composition of blood lipids and lipoproteins, systemic inflammation, endothelial dysfunction, insulin resistance, diabetes and adiposity.

NEUTRAL FATS:- Neutral fats are also called as triacylglycerols (TAG) or triglycerides (TG)

 Mixed Triglycerides

1.Naturally occurring fats and oils are mixtures of triglycerides.

2. If all the three hydroxyl groups of the glycerol are esterifed to the same fatty acid, a simple triacylglycerol is formed, e.g. Tripalmitin, Triolein.

3.A mixed triglycerol is formed, when different fatty acids are esterifed to the hydroxyl groups of glycerol

Physical Properties of Triacylglycerols

1.They are hydrophobic and insoluble in water

2.Oils are liquids at 20oC; they are triacylglycerols . Oils are generally of plant origin.

3.Fats are solids at room temperature and contain mainly saturated long chain fatty acids.

4.When the constituent fatty acids have a higher chain length and are predominantly saturated, ‘hard fat’ is formed, e.g. pig fat. v. Fats containing medium chain triacylglycerols or unsaturated fatty acids are soft fats, e.g. butter, coconut oil.

Storage of Energy as Fat
The triacylglycerols are the storage form of lipids in the adipose tissue.

Hydrolysis of Triacylglycerols

Triacylglycerols in the body are hydrolyzed by enzymes, lipases which are hydrolases Triacylglycerol is sequentially hydrolyzed to diacylglycerol and monoacylglycerol and fnally glycerol plus 3 fatty acids.


When triacylglycerols are hydrolyzed by alkali, the process is known as saponifcation. Saponifcation number is defned as the number of milligrams of potassium hydroxide required to saponify one gram of fat.

Iodine Number

Iodine number of a fat is defned as the number of grams of iodine taken up by 100 grams of fat 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.

Rancidity of Fat

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 triacylglycerol molecules due to traces of hydrolytic enzymes present in naturally occurring fats and oils.


They are esters of higher fatty acids with higher monohydroxy aliphatic alcohols They are used as the base for the preparation of cosmetics, ointments, polishes, lubricants and candles.


1.These are derivatives of phosphatidic acid, which is the simplest phospholipid

2.Phosphatidic acid is made up of one glycerol to which two fatty acid residues are esterifed to carbon atoms 1 and 2. The 3rd hydroxyl group is esterifed to a phosphoric acid.

Micellar Formation

chemistry of lipids
Fig.1:- Phospholipids form micelles and liposomes


When phospholipids are distributed in water, their hydrophobic parts keep away from water, forming molecular aggregates called micelle.


A lipid bilayer will close on itself under appropriate conditions to form liposomes. Drugs, proteins, enzymes, genes, etc. may be encapsulated by the liposomes which could act as carriers for these substances to target organs. Liposomes have important applications in cancer chemotherapy, antimicrobial therapy, gene therapy, vaccines and diagnostic imaging.


They are one of the most recently developed delivery systems that are making a niche as the peptide/protein carriers. The delivery system has been successfully utilized for the delivery of insulin, hemoglobin and various antigens. Oral delivery of enzymes like serratiopeptidase has also been achieved.

 Lung Surfactants

Normal lung function depends on a constant supply of lung surfactants. It is produced by epithelial cells. It decreases surface tension of the aqueous layer of lung and prevents collapse of lung alveoli. Constituents of surfactants are dipalmitoyl lecithin, phosphatidyl glycerol, cholesterol and surfactant proteins A, B and C. During fetal life, the lung synthesizes sphingomyelin before 28th week of gestation. But as fetus matures, more lecithin is synthesized. The lecithin sphingomyelin (LS) ratio of amniotic fluid is an index of fetal maturity. A ratio of 2 indicates full lung maturity. Low surfactant level can lead to respiratory distress syndrome (RDS), which is a common cause of neonatal morbidity.

 Respiratory Distress Syndrome (RDS)

It is due to a defect in the biosynthesis of dipalmitoyl lecithin (DPL), the main pulmonary surfactant. Premature infants have a higher incidence of RDS because the immature lungs do not synthesize enough DPL.

Phosphatidylethanolamine or Cephalin

Cephalin differs from lecithin in that the nitrogen base ethanolamine is present instead of choline.


This compound plays a vital role in the mediation of hormone action on biomembranes and acts as a second messenger.


It is formed by esterifcation of phosphatidic acid to glycerol. Decreased cardiolipin level leads to mitochondrial dysfunction, and is accounted for heart failure, hypothyroidism and some types of myopathies.


Sphingomyelins are the only sphingolipid that contain phosphate and have no sugar moiety. Sphingomyelin combined with fatty acid is called ceramide, which is a component of glycosphingolipids.

 Antiphospholipid antibody(APL) syndrome

Antiphospholipid antibodies have been found to be present in blood in patients with autoimmune diseases and in patients with thrombotic episodes. In pregnancy, the presence of these antibodies can cause complications like miscarriage, eclampsia
and preterm labor. Here the antibodies are developed against membrane phospholipid, cardiolipin.

 Non-phosphorylated Lipids

Glycosphingolipids (Glycolipids)
They are seen widely in nervous tissues. This group of lipids do not contain phosphoric acid; instead they contain carbohydrates and ceramide.

Globosides (Ceramide Oligosaccharides)
They contain two or more hexoses or hexosamines, attached to a ceramide molecule.

They are formed when ceramide oligosaccharides have at least one molecule of NANA (N-acetyl neuraminic acid) (sialic acid) attached to them.

Sulfolipids or Sulfatides
These are formed when sulfate groups are attached to ceramide oligosaccharides. Failure of degradation of these compounds results in accumulation of these complex lipids in CNS. This group of inborn errors is known as lipid storage diseases.

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