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Chapter 17 Lipids

17.4 Chemical Properties of Triacylglycerols

17.5 Glycerophospholipids

17.6 Sphingolipids

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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The chemical reactions of triacylglycerols are similar

to those of alkenes and esters.

In hydrogenation, double bonds in unsaturated fatty acids react with H2 in the presence of a Ni or Pt catalyst.

In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme.

Chemical Properties of Triacylglycerols

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Hydrogenation of Oils

The hydrogenation of oils Adds hydrogen (H2) to the carbon atoms of double bonds. Converts double bonds to single bonds. Increases the melting point. Produces solids such as margarine and shortening.

Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings

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Hydrogenation

Ni + 3H2

glyceryl tripalmitoleate

(tripalmitolean)

glyceryl tripalmitate

(tripalmitin)

O

(CH2)14CH3C

O

(CH2)14CH3C

O

(CH2)14CH3C

O

O

OCH2

CH2

CH

CH(CH2)7CH3(CH2)5CH

O

C

CH(CH2)7CH3(CH2)5CH

O

C

CH(CH2)7CH3(CH2)5CH

O

C

O

O

OCH2

CH2

CH

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What product(s) is obtained from the complete

hydrogenation of glyceryl trioleate?

1. Glycerol and 3 oleic acids

2. Glyceryltristearate

3. Glycerol and 3 stearic acids

Learning Check

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What product(s) is obtained from the complete

hydrogenation of glyceryl trioleate?

2. Glyceryltristearate

Solution

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Olestra, A Fat Substitute

Olestra is Used in foods as an artificial fat. Sucrose linked by ester bonds to several long-chain

fatty chains. Not broken down in the intestinal tract.


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Cis and Trans Fatty Acids

Unsaturated fatty acids can be Cis with bulky groups on same side of C=C. CH3─ (CH2)5 (CH2)7─ COOH cis

C=C H H

Trans have bulky groups on opposite sides of C=C. CH3─ (CH2)5 H

C=C trans

H (CH2)7─ COOH

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Trans Fatty Acids and Hydrogenation Trans fatty acids

Are formed during hydrogenation when cis double bonds are converted to trans double bonds.

In the body behave like saturated fatty acids.

Are estimated to make up 2-4% of our total Calories.

Are reported in several studies, to raise LDL-cholesterol and lower HDL-cholesterol.

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Trans fatty acids (commonly termed trans fats) are a type of unsaturated fat (and may be monounsaturated or polyunsaturated).

Trans fats occur naturally, in small quantities, in meat and dairy products from ruminants. Most trans fats consumed today, however, are industrially created as a side effect of partial hydrogenation of plant oils — a process developed in the early 1900s and first commercialized as Crisco in 1911. Partial hydrogenation changes a fat's molecular structure (raising its melting point and reducing rancidity), but this process also results in a portion of the changed fat becoming trans fat.

Unlike other fats, trans fats are neither required nor beneficial for health.[1] Eating trans fat increases the risk of coronary heart disease.[2] For these reasons, health authorities worldwide recommend that consumption of trans fat be reduced to trace amounts. Trans fats from partially hydrogenated oils are generally considered to be more of a health risk than those occurring naturally.[3]

Trans fats are tightly regulated in a few countries, are mandatory on product labels in many others, and are the central issue in several ongoing lawsuits (particularly against fast food outlets). Many companies are voluntarily removing trans fats from their products, or establishing trans-free product lines.

Chemically, trans fats are made of the same building blocks as non-trans fats, but have a different shape. In trans fat molecules, the double bonds between carbon atoms (characteristic of all unsaturated fats) are in the trans rather than the cis configuration, resulting in a straighter, rather than a kinked shape. As a result, trans fats are less fluid and have a higher melting point than the corresponding cis fats.

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Learning Check

(1) True or (2) False

A. There are more unsaturated fats in vegetable oils.

B. Vegetable oils have higher melting points than fats.

C. Hydrogenation of oils converts some cis-double bonds to trans- double bonds.

D. Animal fats have more saturated fats.

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Solution

(1) True or (2) False

A. T There are more unsaturated fats in vegetable oils.

B. F Vegetable oils have higher melting points than fats.

C. T Hydrogenation of oils converts some cis-double

bonds to trans- double bonds.

D. T Animal fats have more saturated fats.

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Hydrolysis

In hydrolysis, Triacylglycerols split into glycerol and three fatty acids. An acid or enzyme catalyst is required.

OCH2

OCH

OCH2

OHCH2

OHCH

OHCH2

O

(CH2)14CH3CHO

H2OO

(CH2)14CH3C

O

(CH2)14CH3C

O

(CH2)14CH3C

H+

+3

+ 3

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Saponification and Soap

Saponification Is the reaction of a fat with a strong base. Splits triacylglycerols into glycerol and the salts of

fatty acids. Is the process of forming “soaps” (salts of fatty

acids). With KOH gives softer soaps.

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Saponification

O

O

C (CH2)14CH3

CH O

O

C (CH2)14CH3

CH2 O

O

C (CH2)14CH3

CH2

+ 3NaOH

Na+ -O

O

C (CH2)14CH33

OH

CH OH

CH2 OH

CH2

+

“soap”

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What products are obtained from the complete hydrolysis of glyceryl trioleate?


2. Glyceryl tristearate

3. Glycerol and 3 stearic acids

Learning Check

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What products are obtained from the complete hydrolysis of glyceryl trioleate?


Solution

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Learning Check

Write the product of the following reaction.

OCH2

O

O

CH2

CH

CH(CH2)7CH3(CH2)5CH

O

CO

CH(CH2)7CH3(CH2)5CHCO

C CH(CH2)7CH3(CH2)5CH

H2

Ni+ 3

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Solution

O

O

CH2

CH

OCH2

(CH2)14CH3

O

C

O

(CH2)14CH3C

(CH2)14CH3

O

C

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Chapter 17 Lipids

17.5 Glycerophospholipids

Glycerol

PO4Amino alcohol

Fatty acid

Fatty acid

Copyright © 2007 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Glycerophospholipids are The most abundant lipids in cell membranes. Composed of glycerol, two fatty acids, phosphate

and an amino alcohol.

Glycerol

PO4Amino alcohol

Glycerophospholipids

Fatty acid

Fatty acid

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Glycerophospholipids Are Polar

A glycerophospholipid has Two nonpolar fatty acid chains. A phosphate group and a polar amino alcohol. CH3

│+ +

HO−CH2−CH2−N−CH3 HO−CH2−CH2−NH3

│ choline CH3 ethanolamine +

NH3

│ Amino alcohols HO−CH2−CH−COO−

serine

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Structure and Polarity of A Glycerophospholipid


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Lecithin and cephalin are glycerophospholipids Abundant in brain and nerve tissues. Found in egg yolk, wheat germ, and yeast.

Lecithin and Cephalin

Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings

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Identify the following lipidsas a Wax, Triacylglycerols, Glycerophospholipid or Steroid.

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Identify the following lipidsas a Wax, Triacylglycerols, Glycerophospholipid or Steroid.

Wax

Triacylglycerols

Glycerophospholipid

Steroid

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Sphingolipids

Sphingolipids Are similar to phospholipids. Contain sphingosine (a long-chain amino alcohol), a

fatty acid, phosphate, and a small amino alcohol. Have polar and nonpolar regions.

sphingosine

PO4amino alcohol

fatty acid

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Sphingosine

Sphingosine is a long-chain unsaturated amino alcohol.

CH3−(CH2)12 −CH=CH−CH−OH

│

CH−NH2

│

CH2−OH

sphingosine

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In sphingomyelin, a sphingolipid found in nerve cells There is an amide bond between a fatty acid and

sphingosine, an 18-carbon alcohol.

Sphingolipids


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Glycosphingolipids

Glycosphingolipids contain monosaccharides bonded to the –OH of sphingosine by a glycosidic bond.


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Glycosphingolipids and CerebrosidesGlycosphingolipids Are sphingolipids that contain

monosaccharides. Can be a cerebroside with galactose.


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Gangliosides

Gangliosides Are similar to cerebrosides, but contain two or

more monosaccharides. Are important in neurons. Act as receptor for hormones and viruses. Can accumulate and cause genetic diseases.

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Gangliosides

GM2 is a ganglioside

That accumulates in Tay-Sachs disease.


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Lipid Diseases (Lipidoses)

In many lipid diseases, the deficiency of an enzyme causes the accumulation of glycolipids.


TABLE 17.3

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