Digestion of lipids

Dr. Samah KotbLecturer of Biochemistry

2015

Cellular Biochemistry and metabolism 1

CLS 331

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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.

Definition

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

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1) 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

Properties of saturated fatty acids

Closely packed.

Strong attractions between

chains.

High melting points.

Solids at room

temperature.

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Examples of Saturated fatty acids

LAURIC ACID

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2) unsaturated fatty acids

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.

1. Simple Lipids

2. Complex Lipids

3.Precursor & Derived Lipids

Lipids

Classification of lipids

Simple lipids

Esters of fatty acids and alcohol. (trihydric or monohydric alcohol).

CH 2R

Fatty alcoholOH C R

Fatty acidHO

O

+

H2O

CH 2R O C R

O

Esterase (lipase) 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

Esters of fatty acids with various alcohols along with an additional group.

a. Phospholipids: Glycerophospholipids , Sphingophospholipids.

b. Glycolipids

c. Other Complex Lipids: Lipoproteins ,aminolipids and

sulfolipids.

Simple lipid + Additional group = Complex Lipids

Complex lipids

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DIGESTION

OF

LIPIDS

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The major lipids in the diet are triacylglycerols.

Hydrolysis of triacylglycerols is initiated by gastric lipases that attack the ester bond, forming 1,2-diacylglycerols and free fatty acids, aiding emulsification.

Digestion & Absorption of lipids

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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.

Digestion & Absorption of lipids

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Triacylglycerols

gastric lipases

1,2-diacylglycerols + free fatty acids

2-monoacylglycerols + free fatty acids

Pancreatic lipase small intestine

Stomach

glycerol

free fatty acids+

hydrolysis

Portal vein

Secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct .

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Monoacylglycerols are hydrolyzed with difficulty to glycerol and free fatty acids.

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.

Digestion & Absorption of lipids

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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.

Digestion & Absorption of lipids

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The bile salts pass on to the ileum, where most are absorbed into the enterohepatic circulation.

Within the intestinal epithelium, 1-monoacylglycerols are hydrolyzed to fatty acids and glycerol and 2-monoacylglycerols are re-acylated to triacylglycerols via the monoacylglycerol pathway.

Digestion & Absorption of lipids

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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.

Digestion & Absorption of lipids

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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 .

Digestion & Absorption of lipids

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