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Biochemistry - Introduction to lipids - Simplified
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It contains brief explanation to the biochemistry of lipids.
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CHEMISTRY OF LIPIDS (UNIT I)The greasy material which may be extracted from animal and plant tissues by solvents such as hot
alcohol or ether, generally represents a complex mixture. This mixture, depending on the tissue extracted,
usually contains representatives of several or all of the classes of organic compounds known as fats, waxes,
phospholipids, glycolipids and sterols. These substances, though often very different in chemical structure and
properties, are commonly called as lipids. Fats and their derivatives collectively constitute the class, lipids. (In
Greek, Lipos means fat). Lipids are relatively insoluble in water but freely soluble in non-polar organic solvents
like benzene, chloroform, ether, acetone etc. The most commonly used solvent for extraction of lipid is a
combination of chloroform and methanol in the ratio 2:1.
Functions of lipid
Storage form of energy: There are two significant advantages to using triacylglycerols (TAGs) as stored
fuels, rather than polysaccharides such as glycogen and starch. First, because the carbon atoms of fatty
acids are more reduced than those of sugars, oxidation of TAGs yield more than twice as much energy as
the oxidation of carbohydrates. Second, because TAGs are hydrophobic, the organism that carries fat as
fuels does not have to carry the extra weight of water of hydration that is associated with the stored
polysaccharides.
Structural component of biological membrane. E.g. Phospholipids
Metabolic regulators. E.g. Steroid hormones, prostaglandins
Act as surfactants, detergents and emulsifying agents. E.g. Bile salts
Helps in absorption of fat soluble vitamins.
Protect internal organs by providing a cushioning effect
The fatty acid, arachidonic acid is the precursor for prostaglandins, leukotrienes and thromboxanes.
The corticosteroids, sex hormones and Vitamin D are synthesized from cholesterol.
Provide insulation against changes in external temperature
In preparation of soap, detergents, grease and various oils of paint industry
Classification of lipids
Based on chemical nature, lipids may be classified as
1. Simple lipids: These are esters of fatty acids with glycerol or other higher alcohols. E.g. Wax,
triacylglycerol or neutral fat
2. Compound lipids: These are esters of fatty acids, containing group in addition to an alcohol and fatty acid.
Based on additional groups they are further classified as-
Phospholipids- containing phosphoric acid
, Non-phosphorylated lipids
3. Derived lipids: Compounds which are derived from simple and compound lipid. E.g. Fatty acid, steroid,
prostaglandin
4. Lipids complexed to other compounds. E.g. proteolipid, lipoproteins
Fatty Acids
The building blocks of lipids are fatty acids. Fatty acids are aliphatic carboxylic acids which contains
carboxyl group and non-polar hydrocarbon group with the general formula R-COOH. Hydrocarbon chain
length ranges from 4 to 36 carbons long (C4 to C36). The hydrocarbon chain is hydrophobic and the
carboxylate group is hydrophilic. Fatty acids occur primarily as esters of glycerol. Oleic acid is by far the
most abundant fatty acid in nature. Palmitic acid ranks next to oleic acid in its widespread occurrence.
Classification of Fatty Acids
1. Based on the number of carbon atom
a. Even chain fatty acids- fatty acids with even number of carbon atoms. (4-30 C atoms). In natural
fats, fatty acids usually occur as straight chains with even numbered carbon atoms.
b. Odd chain fatty acids- fatty acids with odd number of carbon atoms. (3-25 C atoms)
2. Based on the length of carbon chain
a. Short chain with 2 to 6 carbon atoms
b. Medium chain with 8 to 14 carbon atoms
c. Long chain with 16 to 22 carbon atoms
3. Based on nature of hydrocarbon chain
a. Saturated fatty acid.
b. Unsaturated fatty acid can be sub-classified into
i. Monounsaturated fatty acid- single double bond
ii. Polyunsaturated fatty acid- 2 or more double bonds
c. Branched chain fatty acid: Mainly found in waxes. Can be classified in to 2 series.
i. Iso series, where branching occurs on the carbon adjacent to the terminal methyl group.
E.g. Isobutyric acid
ii. Anti iso series, where branching occurs 2 carbons away from the terminal methyl group.
d. Cyclic fatty acid. Eg:- Hydnocarpic acid and chaulmoogric acid from chaulmoogra oil.
Antimicrobial activity of Hydnocarpic acid was utilized by traditional Indian and Chinese
medicines for the treatment of leprosy in humans.
e. Hydroxy fatty acid. Eg:- cerebronic acid, the hydroxyl group is attached on the α- carbon atom.
Ricinoleic acid, the main fatty acid in castor oil.
, Nomenclature of fatty acids
In the naming of fatty acids, saturated acids end in - anoic acid e.g. octanoic acid and the unsaturated
fatty acids with double bonds end in – enoic acids, e.g. octadecenoic acid (oleic acid).
Carbon atoms are numbered from the carboxyl carbon (carbon No.1). The carbon atom adjacent to the
carboxyl carbon (No.2) is also known as the α - carbon. Carbon atom No.3 is the β - carbon and the
distal methyl carbon is known as the ω - carbon or n- carbon atom.
The fatty acids are generally designated as follows (Δ, delta system of numbering). For example, oleic
acid is written as 18: 1(Δ9). The number 18 indicates the number of carbon atoms, 1 indicates the
number of double bond and the superscript 9 indicates the position of the double bonds i.e. the double
bond is between carbon atoms 9 and 10 of the fatty acid. Sometimes the Δ is omitted and oleic acid is
indicated as (18: 1; 9.)
Alternatively, the position of a double bond can be denoted by counting from the distal end, with the
carbon atom, (the methyl carbon) as number (ω-system). For example ω -3 fatty acid, linolenic acid (has
a double bond between ω-3 and ω-4 C atom), ω-6 fatty acid, linoleic and arachidonic (has a double bond
between ω-6 and ω-7 C atom) and the ω 9 fatty acid, oleic acid (has a double bond between the C atoms
ω-9 and ω-10 of the fatty acid.
6 5 4 3 2 1
CH3-CH2- CH2- CH2- CH2-COOH
ω1 ω2 ω3 ω4 ω5 ω6
Saturated Fatty Acids
They have the general formula CH3-(CH2) n-COOH or CnH2nO2. Most common member of even carbon
saturated fatty acid is Palmitic acid followed by stearic acid. Even numbered saturated fatty acids having less
than 10 carbon atoms are liquid at room temperature. Short chain fatty acids (acetic, propionic and butyric acid)
are collectively called Volatile fatty acids.
They are named by adding the suffix ‘anoic’ after the hydrocarbon.eg: butanoic, hexanoic
Common Saturated fatty acids
Formula No:of Systematic Common Occurrence
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