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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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