Digestion of lipids
Cellular Biochemistry and
metabolism 1
CLS 331
Dr. Samah Kotb
Lecturer of Biochemistry
2015
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Definition
 Lipids
are complex molecules composed of carbon
, hydrogen , and oxygen.
 Lipids are insoluble in water and soluble in nonpolar
solvents.
 Most lipids are non-polar and are hydrophobic because
they contain hydrocarbon chains.
Functions of lipids
A. The Functions of Fat in the Body:
1. Energy source.
2. Serves as an energy reserve.
3. Structural component of cell membrane.
Cell Membrane
4. Thermal Insulator :
Subcutaneous layer of fat
reduces heat loss in cold
weather (helps maintain body
temperature).
CLASSIFICATIONS OF FATTY ACID
• Length of the Carbon Chain
– Long-chain, medium-chain, short-chain
• Degree of Unsaturation
– Saturated, Unsaturated (Mono, Poly)
• Location of Double Bonds
– Omega-3 Fatty Acid, Omega-6 Fatty Acid
1) Saturated fatty acids
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Properties of saturated fatty acids
 General Formula: CH3 (CH2)n COOH.
 Each C being “saturated” with H.
 Contain only single C–C bonds.
 long, straight chain.
Properties of saturated fatty acids
Closely packed.
Strong attractions
between chains.
High melting points.
Solids at room
temperature.
Examples of Saturated fatty
acids
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LAURIC ACID
2) unsaturated fatty acids
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Properties of unsaturated fatty acids
 Contain one or more double C=C
bonds.
The links made by double bonded C
prevent the molecules from packing
tightly together.
 Few interactions between chains.
Classification of lipids
Lipids
1. Simple
Lipids
2. Complex
Lipids
3.Precursor &
Derived
Lipids
Simple lipids
 Esters of fatty acids and alcohol.
(trihydric or monohydric alcohol).
H2O
O
R CH2 OH
+
Fatty alcohol
HO C
R
Fatty acid Esterase (lipase)
O
R CH2 O C
R
ester (lipid)
Triglycerides
 Triglycerides are the main dietary fats in
human body.
 It is a naturally occurring ester of three
fatty acids and glycerol that is the chief
constituent of fats and oils.
 Glycerol forms the “backbone” of the fat.
FAT vs. OIL
 Fat:
• is solid at room temperature.
• is prevalent in meats, whole
milk, butter, and cheese.
 Oil:
• Contain high proportion of
unsaturated fatty acids and saturated
fatty acids.
• is liquid at room temperature.
• is prevalent in plants such as olive
and safflower.
corn oil contains 86%
unsaturated fatty acids
and
14% saturated
fatty acids
Complex lipids
Esters of fatty acids with various alcohols along
with an additional group.
Simple lipid + Additional group = Complex Lipids
a. Phospholipids: Glycerophospholipids , Sphingophospholipids.
b. Glycolipids
c. Other Complex Lipids: Lipoproteins ,aminolipids and
sulfolipids.
DIGESTION
OF
LIPIDS
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Digestion & Absorption of lipids
 The
major lipids in the diet are triacylglycerols.
 Hydrolysis of triacylglycerols is initiated by gastric
lipases that attack the ester bond, forming 1,2diacylglycerols and free fatty acids, aiding
emulsification.
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Digestion & Absorption of lipids
 Pancreatic lipase is secreted into the small
intestine and requires a further pancreatic protein,
colipase, for activity.
 It is specific for the primary ester links in
triacylglycerols resulting in 2-monoacylglycerols
and free fatty acids as the major end-products of
luminal triacylglycerol digestion.
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Triacylglycerols
gastric lipases
Stomach
1,2-diacylglycerols + free fatty acids
Pancreatic lipase
small intestine
2-monoacylglycerols + free fatty acids
hydrolysis
free fatty acids
+
glycerol
Secreted as chylomicrons into
the lymphatics, entering the
blood stream via the thoracic
duct .
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Portal vein
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Digestion & Absorption of lipids
 Monoacylglycerols
are
hydrolyzed
difficulty to glycerol and free fatty acids.
with
 Bile salts, formed in the liver and secreted in
the bile, enable emulsification of the products of
lipid digestion into micelles and liposomes
together with phospholipids and cholesterol from
the bile.
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Digestion & Absorption of lipids
Because the micelles are soluble, they allow the
products of digestion, including the fat soluble
vitamins, to be transported through the aqueous
environment of the intestinal lumen and permit
close contact with the brush border of the mucosal
cells, allowing uptake into the epithelium, mainly of
the jejunum.
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Digestion & Absorption of lipids
 The bile salts pass on to the ileum, where most
are absorbed into the enterohepatic circulation.
 Within
the
intestinal
epithelium,
1monoacylglycerols are hydrolyzed to fatty acids and
glycerol and 2-monoacylglycerols are re-acylated to
triacylglycerols via the monoacylglycerol pathway.
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Digestion & Absorption of lipids
 Glycerol released in the intestinal lumen is not
reutilized but passes into the portal vein; glycerol
released within the epithelium is reutilized for
triacylglycerol synthesis via the normal phosphatidic
acid pathway.
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Digestion & Absorption of lipids
 All long-chain fatty acids absorbed are converted
to triacylglycerol in the mucosal cells and, together
with the other products of lipid digestion, secreted
as chylomicrons into the lymphatics, entering the
blood stream via the thoracic duct .
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