Lipids are non-polar (hydrophobic) organic compounds, insoluble
in water, soluble in organic solvents (ether, acetone,
carbontetrachloride). They contain carbon, hydrogen, and oxygen;
sometimes nitrogen and phosphorus. In most cases they yield fatty
acids on hydrolysis. They take place in lipid and plant metabolism.
What are Lipids?
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Fatty acids Fatty acids (FAs) consist of hydrocarbon chain with
a carboxylic acid at one end straight-chain organic acids Most
naturally occurring fatty acids have an even number of carbon atoms
They can be saturated and unsaturated. Unsaturated fatty acids have
lower melting points than saturated fatty acid.
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Fatty acids Monounsaturated FAs one double bond Polyunsaturated
FAs many double bonds Eicosanoids (include prostaglandins,
leukotriens, prostacyclins, thromboxanes) Double bonds in fatty
acids are usually in the cis configuration.
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Linoleic acid is a nutritionally essential fatty acid. that
must be ingested by humans and other animals because the body
requires it but cannot synthesize it. It is found in large conc. in
corn, peanut, soyabean oils but not in olive oil. Absence of
linoleic acid by infants weight loss and ezema. Linoleic acid
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Classification of lipids according to their structures
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Simple lipids esters of FAs (FAT, OIL, WAX) [One or two
chemical identities) Hydrolysis of a simple lipid Simple lipid + H
2 O fatty acid + alcohol If the alcohol is glycerol (FAT or OIL) If
the alcohol is a monohydric alcohol WAX Hydrolysis of complex
lipids more fatty acids + alcohol +other compound Phospholipid + H
2 O (hydrolysis) FA + alcohol + phosphorus + nitrogen compound.
Phospholipids phosphoglyceride or phosphosphingoside Glycolipid + H
2 O (hydrolysis) FA +a carbohydrate + sphingosine
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Precursor lipids compounds resulting from hydrolysis of simple
or complex lipids (FA, sphingosine) Derived lipids lipids which are
formed due to the transformation of fatty acids (Prostaglandins,
Fat-soluble vitamins) Classification of lipids according to their
structures
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Fats and oils Fats with a melting point below room temperature
are called oils.
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Iodine number The iodine number of a fat or an oil is the umber
of grams of iodine that will react with the double bond present in
100 g of fat or oil Higher iodine number higher degree of
unsaturation Generally iodine number of animal fats < iodine
number of vegetable oils Iodine number of fats 70
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Some uses of lipids in the body Fats serve as fuel. Fats serve
as reserve supply of food and energy. Fats are stored in special
adipose tissues and serve as a protector for vital organs. Fats act
as heat insulators. Some lipids allow rapid propagation of
electrical signals.
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Physical properties White or yellowish solid or liquids Pure
fats and oils are odorless and tasteless. Over a period of time
they become rancid and develop an unpleasant odor and tatse.
Lighter than water. When shaken with water temporarily emulsion.
Emulsion can made permanent by addition of emulsifying agent such
soap. Fats and oils must be emulsified before they can be
digested.
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Chemical reactions Hydrolysis 3. Enzyme
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Saponification
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Hydrogenation In Practice not all double bonds are
hydrogenated. Hydrogenation lowers the iodine number.
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Acrolein test: Test for fats or oils which contain glycerol
Glycerol (KHSO 4 ) (heat) Acrolein (strong odor) Rancidity:
unpleasant odor or taste developed when fats stand at room
temperature for a short period of time. Rancidity is due to
hydrolysis and oxidation reactions. Oxidation of double bonds short
chain aldehydes and acids bad odor and taste (Antioxidant Vit. E
and C.) Fats + Water in butter (in presence of microorganisms)
hydrolysis of fats Butyric acid disagreeable odor. Fats and foods
containing fats have to be covered and stored in the
refrigerator
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Cleansing action of soaps CH 3 -(CH 2 ) 16 -COONa (Sodium
stearate) Non-Polar Polar Hydrophopic Hydrophilic Micelle
Mechanical Washing causes the oil to break down into small drops
Soap emulsifies then the oil and prevents it from coalescing. Soap
acts also as surfactant lowers surface tension
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Detergents Synthetic compounds used as cleansing agents. Soaps
do not work in hard water insoluble Ca and Mg salts Detergents work
also in hard water Soaps (alkaline); Detergents (neutral)
Detergents can be used on silks and wool; Sops not. Detergents:
Sodium salts if long chain alcohol sulfates Example: Sodium
laurylsulfate C 12 H 23 OH + H 2 SO 4 C 11 H 23 CH 2 OSO 3 H + H 2
O C 11 H 23 CH 2 OSO 3 H + NaOH C 11 H 23 CH 2 OSO 3 Na + H 2
O
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Complex lipids and cell Membranes: an overview Some functions
of membranes (40-50% lipids; 50-60% Proteins) 1.Mechanical support.
2.Seperate contents of the cells from the environment 3.Structural
support for proteins (pumps; receptors)
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Biochemistry II Self aggregation of lipids Introduction to
lipids Polar end (hydrophilic end) non-polar end hydrophobic
end)
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Glycerophospholipids Glycerophospholipids (phosphoglycerides),
are common constituents of cellular membranes. They have a glycerol
backbone. Hydroxyls at C1 & C2 are esterified to fatty acids.
An ester forms when a hydroxyl reacts with a carboxylic acid, with
loss of H 2 O.
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Phosphatidate In phosphatidate: fatty acids are esterified to
hydroxyls on C1 & C2 the C3 hydroxyl is esterified to P i.
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In most glycerophospholipids (phosphoglycerides), P i is in
turn esterified to OH of a polar head group ( X ): e.g., serine,
choline, ethanolamine, glycerol, or inositol. The 2 fatty acids
tend to be non-identical. They may differ in length and/or the
presence/absence of double bonds.
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Phosphatidylinositol, with inositol as polar head group, is one
glycerophospholipid. In addition to being a membrane lipid,
phosphatidylinositol has roles in cell signaling.
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Phosphatidylcholine, with choline as polar head group, is
another glycerophospholipid. It is a common membrane lipid.
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Each glycerophospholipid includes a polar region: glycerol,
carbonyl O of fatty acids, P i, & the polar head group ( X )
non-polar hydrocarbon tails of fatty acids (R 1, R 2 ).
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Structure of phospholipids
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Sphingosine may be reversibly phosphorylated to produce the
signal molecule sphingosine-1-phosphate. Other derivatives of
sphingosine are commonly found as constituents of biological
membranes. Sphingolipids are derivatives of the lipid sphingosine,
which has a long hydrocarbon tail, and a polar domain that includes
an amino group.