Post on 16-Jan-2016
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Chapter 15: Lipids
Chem 20
El Camino College
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Lipids
Lipids are a family of biomolecules that are not soluble in water but can be extracted by organic solvents of low polarity like ether or chloroform.
There are two kinds of lipids, one with fatty acids and one without fatty acids called steroids.
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Lipids
Lipids that have fatty acids are waxes, triacylglycerols, glycerophospholipids and prostaglandins
Steroids are characterized by the steroid nucleus of four fused carbon rings.
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Fatty Acids
With only few exceptions, the fatty acids are all straight-chain compounds containing an even number of carbons ( the molecules are built up two carbons at a time from acetate units).
They consist of long carbon chains attached to a carboxylic acid group
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Fatty Acids
OH
O
lauric acid, found in coconut, is a saturated fatty acid
O
OH
oleic acid, found in olives, is a monounsaturated fatty acid
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Fatty Acids
Saturated fatty acids only contain single bonds on the carbon chain
Mono-unsaturated fatty acids contain one double bond on the carbon chain
Poly-unsaturated fatty acids contain two or more double bonds on the carbon chain
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Saturated Fatty Acids
Lauric acid (12 C) CH3 -(CH2 )10 –COOH
Myristic acid (14 C) CH3 -(CH2 )12 –COOH
Palmitic acid (16 C) CH3 -(CH2 )14 –COOH
Stearic acid (18 C) CH3 -(CH2 )16 –COOH
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Unsaturated Fatty Acids
Oleic acid (18 C) CH3 -(CH2 )7 - CH =CH-(CH2 )7 -COOH
Linoleic Acid (18 C) CH3 -(CH2)4 - CH =CH-CH2 -CH =CH-(CH2 )7 –
COOH
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Unsaturated Fatty Acids
Linolenic Acid (18 C) CH3 -CH2 – (CH =CH-CH2 )3-(CH2 )6 –COOH
Arachidonic Acid (20 C) CH3 –(CH2 )3 – (CH2 -CH =CH)4-(CH2 )3 –COOH
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Unsaturated Fatty Acids
configuration is almost invariably cis. (cis isomer has lower melting point than trans,
making the cell membrane semiliquid).
Linoleic acid, linolenic acid and arachidonic acid are called essential fatty acids, because they must be obtained from the diet.
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Fatty Acids
Draw stearic acid, a saturated fatty acid found in animal fat containing 18 carbons
OH
O
CH3(CH2)16COOH
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Triacylglycerols: Fats & Oils
Fatty acids are stored in the body as triacylglycerols (also called triglycerides)
These molecules have three ester groups connecting fatty acids to glycerol
HC
H2C O
H2C O
O
O
O
O
triglyceride
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Triacylglycerols: Fats & Oils In an esterification reaction, an alcohol reacts
with a carboxylic acid This is the same reaction that makes
triglycerides
HC
H2C OH
H2C OH
OH
glycerol
HO
O
(CH2)16CH3
+ 3
3 fatty acids
HC
H2C O
H2C O
O
O
O
O
(CH2)16CH3
(CH2)16CH3
(CH2)16CH3
triglyceride
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Triacylglycerols: Fats & Oils Some triglycerides contain 3 different fatty acids Draw a triglyceride containing the following fatty
acids in any order
HO
O
(CH2)10CH3
HO
O
(CH2)7CH=CH(CH2)5CH3
HO
O
(CH2)12CH3
lauric acid
palmitoleic acid
myristic acid
O
O
(CH2)10CH3
O
O
(CH2)7CH=CH(CH2)5CH3
O
O
(CH2)12CH3
H2C
CH
H2C
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Hydrogenation of oil Although unsaturated fats are healthier, they are liquid By doing a hydrogenation reaction to unsaturated fats, some of the C=C double bonds
become single bonds This makes a creamier, appealing-looking product that clogs arteries Margarine, vegetable shortening, and the “creme” fillings in cakes and cookies have been
partially hydrogenated.
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Making Saturated Fats
O
O
(CH2)7CH=CH(CH2)7CH3
O
O
(CH2)7CH=CH(CH2)7CH3
O
O
(CH2)7CH=CH(CH2)7CH3
H2C
CH
H2C
PtHC
H2C O
H2C O
O
O
O
O
(CH2)16CH3
(CH2)16CH3
(CH2)16CH3
+ 3 H2
Glyceryl trioleate can react with hydrogen in the presence of a Pt catalyst to form glyceryl tristearate
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Hydrolysis of Fats
Triacylglycerols are hydrolyzed in the presence of strong acids or enzymes to produce glycerol and fatty acids.
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Soap
Soap is made by heating fat with a strong base such as NaOH, KOH.
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Micelles
Micelles are spherical clusters containing hundreds of soap molecules. The non-polar hydrocarbon chains ‘dissolve” in each other. The polar –COO- groups dissolve in water. Similarly charged micelles repel each other..
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Micelles
The cleansing property of soap is due to the dissolution of non-polar ends of soap molecules in oil droplets forming a stable emulsion of oil and water.
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Wax Wax is an ester of a saturated fatty acid and a long-chain alcohol.
Beeswax CH3 -(CH2 )14 –CO-O-(CH2 )29 -CH3
Carnaubawax CH3 -(CH2 )24 –CO-O-(CH2 )29 -CH3
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Prostaglandins
Prostaglandins are hormone-like substances, produced from arachidonic acid.
They have 5 carbon ring and 20 carbon atoms including a 5 carbon ring.
They have important functions in body.
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Glycerophospholipids Phospholipids are similar to triglycerides, except except that one –OH of glycerol is replaced
by the ester of phosphoric acid and an amino alcohol bonded though a phosphodiester bond. They are an important component of cell membranes They make up much of the fatty coating of nerve cells (called the myelin sheath) They combine with triglycerides and cholesterol to make them more soluble.
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Glycerophospholipids Note the phosphate gp and amine gps have charges Given a drawing, you should be able to identify a phospholipid
HC
H2C O
H2C O
O
O
O
O
(CH2)16CH3
(CH2)16CH3
O P
O
O
O CH2CH2NH3
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Glycerophospholipids
The amino alcohols are most often ethanolamine, choline.
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Steroids
Steriods contain a steroid nucleus Memorize the structure of the steroid nucleus
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Steroids Biologically important steroids include cholesterol, testosterone, estrogen, progesterone, and cortisone Some steroids are hormones, or chemical messenger molecules in the body Derivatives of testosterone are called anabolic steroids. These illegal substances increase muscle mass, but they also are toxic to
the body.
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Steroids
HO
cholesterol
HO
OH
O
OH
estrogen testosterone
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Cell Membranes
Phospholipids are a major component of cell membranes They have polar “heads” and nonpolar “tails” In a membrane, the polar heads face the outside, and the nonpolar tails face the inside
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Cell Membranes
HC
CH2
H2C O
O
O
O
O
(CH2)16CH3
(CH2)16CH3
OP
O
O
OH3NH2CH2C
O
polar head
nonpolar tails
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