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11 Focus on Health & Medicine: Prostaglandins and Leukotrienes

11 Focus on Health & Medicine: Prostaglandins and Leukotrienes

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600



LIPIDS



O



O

CH2CH



CH(CH2)3COOH



(CH2)6COOCH3

CH3



CH

HO



CHCH(CH2)4CH3



CH

HO



OH

PGE2

(dinoprostone)



CHCH2C(CH2)3CH3



OH

misoprostol



More recently, it has been discovered that two different cyclooxygenase enzymes, called COX-1

and COX-2, are responsible for prostaglandin synthesis. COX-1 is involved with the usual production of prostaglandins, but COX-2 is responsible for the synthesis of additional prostaglandins

in inflammatory diseases like arthritis. NSAIDs—nonsteroidal anti-inflammatory drugs—like

aspirin and ibuprofen, inactivate both the COX-1 and COX-2 enzymes. This activity also results

in an increase in gastric secretions, making an individual more susceptible to ulcer formation.

A group of anti-inflammatory drugs that block only the COX-2 enzyme was developed in the 1990s.

These drugs—rofecoxib, valdecoxib, and celecoxib—do not cause an increase in gastric secretions,

and thus were touted as an especially effective treatment for patients with arthritis, who need daily

doses of these medications. Unfortunately, both rofecoxib and valdecoxib have been removed from

the market, since their use has been associated with an increased risk of heart attack and stroke.

Celecoxib, on the other hand, has fewer side effects, so it is still available and widely used.

H2NSO2



H2NSO2

CH3



O

O



N



O



N

CF3



N

CH3SO2



CH3



Generic name: rofecoxib

Trade name: Vioxx



PROBLEM 19.31



Generic name: celecoxib

Trade name: Celebrex



Generic name: valdecoxib

Trade name: Bextra



(a) What functional groups are present in PGE2? (b) Draw a skeletal structure for PGE2. The

double bond closest to the COOH group is cis and the remaining double bond is trans.



19.11B



ASTHMA AND LEUKOTRIENES



Asthma is an obstructive lung disease that affects millions of Americans. Because it involves episodic constriction of small airways, bronchodilators such as albuterol (Section 15.3) are used to

treat symptoms by widening airways. Since asthma is also characterized by chronic inflammation, inhaled steroids that reduce inflammation are also commonly used.

The leukotrienes are molecules that contribute to the asthmatic response by constricting smooth

muscles, especially in the lungs. Leukotrienes, such as leukotriene C4 (LTC4), are synthesized in

several steps from arachidonic acid. The first step involves an oxidation reaction catalyzed by an

enzyme called a lipoxygenase.

H

C



H

C

CH3(CH2)4(CH



CHCH2)4(CH2)2COOH



lipoxygenase



several

steps



H



CH2

C



arachidonic acid

H



H



C



C



C



C



H



H

C5H11



CHCH(OH)(CH2)3COOH

S



CHCONHCH2COOH



C



NHCOCH2CH2CHCOOH



H

LTC4



smi26573_ch19.indd 600



CH2



NH2



12/16/08 11:26:23 AM



CHAPTER HIGHLIGHTS



601



New asthma drugs act by blocking the synthesis of leukotriene C4 from arachidonic acid. For

example, zileuton (trade name: Zyflo) inhibits the lipoxygenase enzyme needed in the first step.

By blocking the synthesis of leukotriene C4, a compound responsible for the disease, zileuton

treats the cause of asthma, not just its symptoms.

HO

CH

S



N



CONH2



CH3



Generic name: zileuton

Trade name: Zyflo



PROBLEM 19.32



(a) A sulfur atom between two alkyl groups (RSR) is called a sulfide. Identify all other

functional groups in LTC4. (b) Label the double bonds in LTC4 as cis or trans.



CHAPTER HIGHLIGHTS

KEY TERMS

Active transport (19.7)

Adrenal cortical steroid (19.9)

Anabolic steroid (19.9)

Androgen (19.9)

Cell membrane (19.7)

Cephalin (19.6)

Eicosanoid (19.11)

Estrogen (19.9)

Facilitated transport (19.7)

Fat (19.4)

Fat-soluble vitamin (19.10)

Fatty acid (19.2)

High-density lipoprotein (19.8)

Hormone (19.9)



Hydrolyzable lipid (19.1)

Hydrophilic (19.2)

Hydrophobic (19.2)

Lecithin (19.6)

Leukotriene (19.11)

Lipid (19.1)

Lipid bilayer (19.7)

Lipoprotein (19.8)

Low-density lipoprotein (19.8)

Nonhydrolyzable lipid (19.1)

Oil (19.4)

Omega-n acid (19.2)

Phosphatidylcholine (19.6)

Phosphatidylethanolamine (19.6)



Phosphoacylglycerol (19.6)

Phosphodiester (19.6)

Phospholipid (19.6)

Progestin (19.9)

Prostaglandin (19.11)

Saponification (19.5)

Saturated fatty acid (19.2)

Soap (19.5)

Sphingomyelin (19.6)

Steroid (19.8)

Triacylglycerol (19.4)

Unsaturated fatty acid (19.2)

Wax (19.3)



KEY REACTIONS

[1]



Hydrolysis of waxes (19.3)

O



O

+



C

R



H 2O



H2SO4



OR'

wax



C

R

OH

fatty acid



+



H



OR'



alcohol



[2] Hydrolysis of triacylglycerols in the presence of acid or enzymes (19.5)

O

CH2



O



C

O



R



CH



O



C

O



R'



CH2



O



C



R''



triacylglycerol



smi26573_ch19.indd 601



+



3 H2O



H2SO4

or

lipase



CH2



OH



CH



OH



CH2



OH



O

+



HO



C



O

R



+



HO



C



O

R'



+



HO



C



R''



three fatty acids



glycerol



12/16/08 11:26:24 AM



602



LIPIDS



[3] Hydrolysis of triacylglycerols in the presence of base—Saponification (19.5)

O

CH2



O



C

O



R

CH2



CH



O



C

O



R'



CH2



O



C



R''



triacylglycerol



+ 3 NaOH



H2O



OH



CH



OH



CH2



OH



O

+



Na



+−O



C



O

R



+



Na+−O



C



O

R'



+



Na+−O



C



R''



soaps



glycerol



KEY CONCEPTS

❶ What are the general characteristics of lipids? (19.1)

• Lipids are biomolecules that contain many nonpolar C C

and C H bonds, making them soluble in organic solvents

and insoluble in water.

• Hydrolyzable lipids, including waxes, triacylglycerols, and

phospholipids, can be converted to smaller molecules on

reaction with water.

• Nonhydrolyzable lipids, including steroids, fat-soluble

vitamins, and eicosanoids, cannot be cleaved into smaller

units by hydrolysis.

❷ How are fatty acids classified and what is the relationship

between their melting points and the number of double

bonds they contain? (19.2)

• Fatty acids are saturated if they contain no carbon–carbon

double bonds and unsaturated if they contain one or more

double bonds. Unsaturated fatty acids generally contain

cis double bonds.

• As the number of double bonds in the fatty acid increases,

its melting point decreases.

❸ What are waxes? (19.3)

• A wax is an ester (RCOOR') formed from a fatty acid

(RCOOH) and a high molecular weight alcohol (R'OH).

Since waxes contain many nonpolar C C and C H bonds,

they are hydrophobic.

• Waxes (RCOOR') are hydrolyzed to fatty acids (RCOOH)

and alcohols (R'OH).

❹ What are triacylglycerols, and how do the triacylglycerols

in a fat and oil differ? (19.4)

• Triacylglycerols, or triglycerides, are triesters formed

from glycerol and three molecules of fatty acids.

A monounsaturated triacylglycerol contains one

carbon–carbon double bond, whereas polyunsaturated

triacylglycerols have more than one carbon–carbon

double bond.

• Fats are triacylglycerols derived from fatty acids having

few double bonds, making them solids at room temperature.

Fats are generally obtained from animal sources.

• Oils are triacylglycerols derived from fatty acids having a

larger number of double bonds, making them liquid at room

temperature. Oils are generally obtained from plant sources.



smi26573_ch19.indd 602



❺ What hydrolysis products are formed from a

triacylglycerol? (19.5)

• Triacylglycerols are hydrolyzed in acid or with enzymes (in

biological systems) to form glycerol and three molecules

of fatty acids. Base hydrolysis of a triacylglycerol forms

glycerol and sodium salts of fatty acids—soaps.

❻ What are the major types of phospholipids? (19.6)

• All phospholipids contain a phosphorus atom, and

have a polar (ionic) head and two nonpolar tails.

Phosphoacylglycerols are derived from glycerol, two

molecules of fatty acids, phosphate, and an alcohol (either

ethanolamine or choline). Sphingomyelins are derived from

sphingosine, a fatty acid (that forms an amide), a phosphate,

and an alcohol (either ethanolamine or choline).

❼ Describe the structure of the cell membrane. How do

molecules and ions cross the cell membrane? (19.7)

• The main component of the cell membrane is

phospholipids, arranged in a lipid bilayer with the ionic

heads oriented towards the outside of the bilayer, and the

nonpolar tails on the interior.

• Small molecules like O2 and CO2 diffuse through the

membrane from the side of higher concentration to the side

of lower concentration. Larger polar molecules and some

ions travel through channels created by integral membrane

proteins (facilitated diffusion). Some cations (Na+, K+,

and Ca2+) must travel against the concentration gradient, a

process called active transport, which requires energy input.

❽ What are the main structural features of steroids? What

is the relationship between the steroid cholesterol and

cardiovascular disease? (19.8)

• Steroids are tetracyclic lipids that contain three six-membered

rings and one five-membered ring. Because cholesterol is

insoluble in the aqueous medium of the blood, it is transported

through the bloodstream in water-soluble particles called

lipoproteins.

• Low-density lipoprotein particles (LDLs) transport cholesterol

from the liver to the tissues. If the blood cholesterol level

is high, it forms plaque on the walls of arteries, increasing

the risk of heart attack and stroke. High-density lipoprotein

particles (HDLs) transport cholesterol from the tissues to the

liver, where it is metabolized or eliminated.



12/16/08 11:26:24 AM



PROBLEMS



❾ What is a hormone? Give examples of steroid hormones.

(19.9)

• A hormone is a molecule that is synthesized in one part

of an organism, and elicits a response at a different site.

Steroid hormones include estrogens and progestins

(female sex hormones), androgens (male sex hormones),

and adrenal cortical steroids such as cortisone, which are

synthesized in the adrenal gland.

❿ Which vitamins are fat soluble? (19.10)

• Fat-soluble vitamins are lipids required in small quantities

for normal cell function, and which cannot be synthesized

in the body. Vitamins A, D, E, and K are fat soluble.



603



⓫ What are the general characteristics of prostaglandins and

leukotrienes? (19.11)

• Prostaglandins are a group of carboxylic acids that contain

a five-membered ring and are derived from arachidonic

acid. Prostaglandins cause inflammation, decrease gastric

secretions, inhibit blood platelet aggregation, stimulate

uterine contractions, and relax the smooth muscle of the

uterus.

• Leukotrienes, acyclic molecules derived from arachidonic

acid, contribute to the asthmatic response by constricting

smooth muscles in the lungs.



PROBLEMS

Selected in-chapter and end-of-chapter problems have brief answers provided in Appendix B.



General Characteristics of Lipids

19.33



19.34



19.35

19.36



Label each compound as a hydrolyzable or

nonhydrolyzable lipid.

a. prostaglandin

e. phosphoacylglycerol

b. triacylglycerol

f. lecithin

c. leukotriene

g. cholesterol

d. vitamin A

Label each compound as a hydrolyzable or

nonhydrolyzable lipid.

a. eicosanoid

e. wax

b. oleic acid

f. estrogen

c. phospholipid

g. PGE1

d. cephalin

In which solvents might a wax be soluble: (a) H2O;

(b) CH2Cl2; (c) CH3CH2OCH2CH3?

In which solvents or solutions might a steroid be soluble:

(a) blood plasma; (b) CCl4; (c) 5% NaCl solution?



19.40



O

OH

O

OH



19.41

19.42

19.43



19.44



Fatty Acids, Waxes, and Triacylglycerols

19.37



19.38



19.39



smi26573_ch19.indd 603



Rank the fatty acids in each group in order of increasing

melting point.

a. CH3(CH2)14COOH, CH3(CH2)3CH=CH(CH2)7COOH,

CH3(CH2)12COOH

b. CH3(CH2)16COOH, CH3(CH2)7CH=CH(CH2)7COOH,

CH3(CH2)5CH=CH(CH2)7COOH

(a) What is the difference between a saturated,

monounsaturated, and polyunsaturated fatty acid?

(b) Give an example of each having the same number

of carbons. (c) Rank these compounds in order of

increasing melting point.

How does each of the following affect the melting point

of a fatty acid: (a) increasing the number of carbon

atoms; (b) increasing the number of double bonds?



How would you expect the melting points of the

following fatty acids to compare? Explain your choice.



19.45



19.46



Is a fatty acid a hydrolyzable lipid? Explain your choice.

Why are soaps water soluble, but the fatty acids from

which they are derived, water insoluble?

Draw the structure of a wax formed from palmitic acid

[CH3(CH2)14COOH] and each alcohol.

a. CH3(CH2)21OH

b. CH3(CH2)11OH

c. CH3(CH2)9OH

Draw the structure of a wax formed from a 30-carbon

straight chain alcohol and each carboxylic acid.

a. lauric acid

b. myristic acid

c. CH3(CH2)22COOH

What hydrolysis products are formed when each wax is

treated with aqueous sulfuric acid?

a. CH3(CH2)16COO(CH2)17CH3

b. CH3(CH2)12COO(CH2)25CH3

c. CH3(CH2)14COO(CH2)27CH3

d. CH3(CH2)22COO(CH2)13CH3

What hydrolysis products are formed when each wax is

treated with aqueous sulfuric acid?

a. CH3(CH2)18COO(CH2)29CH3

b. CH3(CH2)24COO(CH2)23CH3

c. CH3(CH2)14COO(CH2)17CH3

d. CH3(CH2)12COO(CH2)15CH3



12/16/08 11:26:24 AM



604



19.47



19.48



19.49



19.50



19.51



LIPIDS



Draw a triacylglycerol that fits each description:

a. a triacylglycerol formed from lauric, myristic, and

linoleic acids

b. an unsaturated triacylglycerol that contains two cis

double bonds in one fatty acid side chain

c. a saturated triacylglycerol formed from three

14-carbon fatty acids

d. a monounsaturated triacylglycerol

Draw a triacylglycerol that fits each description:

a. a triacylglycerol formed from two molecules of lauric

acid and one molecule of palmitic acid

b. a polyunsaturated triacylglycerol formed from three

molecules of linoleic acid

c. a trans triacylglycerol that contains two trans double

bonds

d. an unsaturated triacylglycerol formed from linolenic

acid

Consider the following four types of compounds:

[1] fatty acids; [2] soaps; [3] waxes; [4] triacylglycerols.

For each type of compound: (a) give the general

structure; (b) draw the structure of a specific example;

(c) label the compound as water soluble or water

insoluble; (d) label the compound as soluble or insoluble

in the organic solvent hexane [CH3(CH2)4CH3].

How do fats and oils compare with respect to each of the

following features?

a. identity and number of functional groups present

b. number of carbon–carbon double bonds present

c. melting point

d. natural source

Answer the following questions about the given

triacylglycerol.

O

O



C

O



(CH2)18CH3



CH



O



C

O



(CH2)16CH3



CH2



O



C



(CH2)10CH3



Answer the following questions about the given

triacylglycerol.

O



19.53



CH2



O



C

O



(CH2)14CH3



CH



O



C

O



(CH2)7(CH



CH2



O



C



(CH2)7CH



CHCH2)2(CH2)3CH3

CH(CH2)5CH3



a. What fatty acids are used to form this triacylglycerol?

b. Would you expect this triacylglycerol to be a solid or a

liquid at room temperature?

c. What regions are hydrophobic?

d. What regions are hydrophilic?

e. What hydrolysis products are formed when the

triacylglycerol is treated with aqueous sulfuric acid?

Draw the products formed when each triacylglycerol

is hydrolyzed under each of the following conditions:

[1] water and H2SO4; [2] water and NaOH.

O



a.



CH2



O



C

O



(CH2)14CH3



CH



O



C

O



(CH2)14CH3



CH2



O



C



(CH2)16CH3



O



b.



19.54



CH2



CH2



O



C

O



(CH2)14CH3



CH



O



C

O



(CH2)7CH



CH(CH2)7CH3



CH2



O



C



(CH2)7CH



CH(CH2)5CH3



Draw the products formed when each triacylglycerol

is hydrolyzed under each of the following conditions:

[1] water and H2SO4; [2] water and NaOH.

O



a.



a. What fatty acids are used to form this triacylglycerol?

b. Would you expect this triacylglycerol to be a solid or a

liquid at room temperature?

c. What regions are hydrophobic?

d. What regions are hydrophilic?

e. What hydrolysis products are formed when the

triacylglycerol is treated with aqueous sulfuric acid?



smi26573_ch19.indd 604



19.52



CH2



O



C

O



(CH2)10CH3



CH



O



C

O



(CH2)14CH3



CH2



O



C



(CH2)16CH3



O



b.



CH2



O



C

O



(CH2)7CH



CH



O



C

O



(CH2)16CH3



CH2



O



C



(CH2)7CH



CH(CH2)7CH3



CH(CH2)5CH3



12/16/08 11:26:25 AM



PROBLEMS



Phospholipids and Cell Membranes

19.55

19.56

19.57



19.58



19.59



19.60



19.61

19.62



19.63



19.64



What distinguishes a cephalin from a lecithin? Draw a

general structure of each type of compound.

In what ways are phosphoacylglycerols and

sphingomyelins similar? In what ways are they different?

Which of the following are phospholipids:

(a) triacylglycerols; (b) leukotrienes; (c) sphingomyelins;

(d) fatty acids?

Which of the following are phospholipids:

(a) prostaglandins; (b) cephalins; (c) lecithins;

(d) steroids?

Draw a phospholipid that fits each description.

a. a cephalin formed from two molecules of palmitoleic

acid

b. a phosphatidylcholine formed from two molecules of

lauric acid

c. a sphingomyelin formed from stearic acid and

ethanolamine

Draw a phospholipid that fits each description.

a. a lecithin formed from two molecules of oleic acid

b. a phosphatidylethanolamine formed from two

molecules of myristic acid

c. a sphingomyelin formed from palmitic acid and

choline

Why don’t triacylglycerols form lipid bilayers?

How would a cell membrane having phospholipids that

contain a high percentage of oleic acid differ from a

cell membrane having phospholipids that contain a high

percentage of stearic acid?

In transporting molecules or ions across a cell membrane,

what is the difference between diffusion and facilitated

transport? Give an example of a molecule or ion that

crosses the membrane by each method.

In transporting molecules or ions across a cell membrane,

what is the difference between facilitated transport and

active transport? Give an example of a molecule or ion

that crosses the membrane by each method.



605



19.68

19.69



19.70

19.71



19.72



Prostaglandins and Leukotrienes

19.73

19.74

19.75

19.76

19.77

19.78



19.66



19.67



smi26573_ch19.indd 605



Draw the structure of the anabolic steroid 4-androstene3,17-dione, also called “andro,” from the following

description. Andro contains the tetracyclic steroid

skeleton with carbonyl groups at C3 and C17, a double

bond between C4 and C5, and methyl groups bonded to

C10 and C13.

Draw the structure of the anabolic steroid methenolone

from the following description. Methenolone contains the

tetracyclic steroid skeleton with a carbonyl group at C3, a

hydroxyl at C17, a double bond between C1 and C2, and

methyl groups bonded to C1, C10, and C13.

Why must cholesterol be transported through the

bloodstream in lipoprotein particles?



What are the similarities and differences between

prostaglandins and leukotrienes?

Why aren’t prostaglandins classified as hormones?

What two structural features characterize all

prostaglandins?

List three biological functions of prostaglandins in the

body.

Explain why aspirin and celecoxib differ in how they act

as anti-inflammatory agents.

How does zileuton treat the cause of asthma, not just

relieve its symptoms?



Vitamins

19.79

19.80

19.81



Steroids

19.65



Why are LDLs soluble in the blood?

Describe the role of HDLs and LDLs in cholesterol

transport in the blood. What is the relationship of HDL

and LDL levels to cardiovascular disease?

What are anabolic steroids? Give an example. What

adverse effects arise from using anabolic steroids?

(a) Draw the structure of an estrogen and an androgen.

(b) What structural features are similar in the two

steroids? (c) What structural features are different?

(d) Describe the biological activity of each steroid.

(a) Draw the structure of an androgen and a progestin.

(b) What structural features are similar in the two

steroids? (c) What structural features are different?

(d) Describe the biological activity of each steroid.



19.82



What are vitamins and why must they be present in the

diet?

Why is vitamin D technically not a vitamin?

Answer each question with regards to vitamins A and D.

a. How many tetrahedral carbons does the vitamin

contain?

b. How many trigonal planar carbons does the vitamin

contain?

c. Identify the functional groups.

d. Label all polar bonds.

e. What function does the vitamin serve in the body?

f. What problems result when there is a deficiency of the

vitamin?

g. Give a dietary source.

Answer each question in Problem 19.81 for vitamins E

and K.



General Questions

19.83



Give an example of each type of lipid.

a. a monounsaturated fatty acid

b. a wax that contains a total of 30 carbons

c. a saturated triacylglycerol

d. a sphingomyelin derived from ethanolamine



12/16/08 11:26:25 AM



606



19.84



19.85



LIPIDS



Give an example of each type of lipid.

a. a polyunsaturated fatty acid

b. a wax derived from a 12-carbon fatty acid

c. a polyunsaturated triacylglycerol

d. a cephalin

Block diagrams representing the general structures of

three types of lipids are drawn. Which terms describe

each diagram: (a) phospholipid; (b) triacylglycerol;

(c) hydrolyzable lipid; (d) sphingomyelin;

(e) phosphoacylglycerol? More than one term may

apply to a diagram.



19.89



CH3CH2CH



glycerol



phosphate



19.90

alcohol



A



CHCH2CH



CHCH2CH



CHCH2CH



CHCH2CH



CH(CH2)5COOH



7,10,13,16,19-docosapentaenoic acid



fatty acid



fatty acid



Some fish oils contain triacylglycerols formed

from the polyunsaturated fatty acid, 7,10,13,16,19docosapentaenoic acid.



a. Draw a skeletal structure showing the cis arrangement

at each double bond.

b. Label the hydrophobic and hydrophilic portions of the

fatty acid.

c. How does the melting point of this fatty acid compare

to its all trans isomer?

d. Would you expect this fatty acid to be a solid or a

liquid at room temperature?

e. What type of omega-n acid is this fatty acid?

Some marine plankton contain triacylglycerols formed

from the polyunsaturated fatty acid, 3,6,9,12,15octadecapentaenoic acid.



CH3CH2CH



CHCH2CH



CHCH2CH



CHCH2CH



CHCH2CH



CHCH2COOH



sphingosine



3,6,9,12,15-octadecapentaenoic acid



a. Draw a skeletal structure showing the cis arrangement

at each double bond.

b. Label the hydrophobic and hydrophilic portions of the

fatty acid.

c. How does the melting point of this fatty acid compare

to the melting point of oleic acid?

d. Would you expect this fatty acid to be a solid or a

liquid at room temperature?

e. What type of omega-n acid is this fatty acid?



fatty acid



phosphate



alcohol



B



glycerol



fatty acid



Applications



fatty acid



19.91

fatty acid

C



19.86



Which “cartoon” represents a soap and which represents

a phosphoacylglycerol? What structural features are

present in the polar head and nonpolar tails of each

compound?



A



19.87

19.88



B



Why are phosphoacylglycerols more water soluble than

triacylglycerols?

How are soaps and phosphoacylglycerols similar in

structure? How do they differ?



smi26573_ch19.indd 606



19.92



19.93

19.94



The main fatty acid component of the triacylglycerols in

coconut oil is lauric acid, CH3(CH2)10COOH. Explain

why coconut oil is a liquid at room temperature despite

the fact that it contains a large fraction of this saturated

fatty acid.

Unlike many fats and oils, the cocoa butter used to make

chocolate is remarkably uniform in composition. All

triacylglycerols contain oleic acid bonded to the 2° OH

group of glycerol, and either palmitic acid or stearic acid

esterified to the 1° OH groups. Draw the structures of two

possible triacylglycerols that compose cocoa butter.

What is the difference between vegetable oil from plants

and motor oil, obtained from crude oil (Section 12.6)?

Why do laboratory blood results now report the levels

of cholesterol, HDLs, and LDLs? What are the current

recommendations for these values?



12/16/08 11:26:25 AM



PROBLEMS



19.95

19.96

19.97

19.98



Can an individual survive on a completely fat-free diet?

Can an individual survive on a cholesterol-free diet?

Why should saturated fats in the diet be avoided?

Why is it recommended that polyunsaturated oils be

substituted for saturated fats in the diet?



607



19.99



Why do animals that live in cold climates have

triacylglycerols that contain a higher percentage of

unsaturated fatty acid side chains?

19.100 If the serum cholesterol level in an adult is 167 mg/dL,

how much cholesterol is contained in 5.0 L of blood?

(Recall from Chapter 1 that 1 L = 10 dL.)



CHALLENGE QUESTIONS

19.101 Sometimes it is possible to convert one lipid molecule to



another by a simple organic reaction that you learned in

previous chapters. What type of reaction converts each

starting material to the given product?

a. cortisone → cortisol (Section 19.9)

b. estradiol → estrone (Section 19.9)

c. PGE2 → PGE1 (Section 19.11)



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19.102 How many triacylglycerols can be prepared from three



different fatty acids? Draw all possible structures,

excluding stereoisomers.



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20

CHAPTER OUTLINE

20.1



Introduction



20.2



Monosaccharides



20.3



The Cyclic Forms of

Monosaccharides



20.4



Reduction and Oxidation of

Monosaccharides



20.5



Disaccharides



20.6



Polysaccharides



20.7



FOCUS ON THE HUMAN BODY:

Useful Carbohydrate Derivatives



20.8



FOCUS ON THE HUMAN BODY:

Blood Type



CHAPTER GOALS

In this chapter you will learn how to:

❶ Identify the three major types of

carbohydrates

❷ Recognize the major structural

features of monosaccharides

❸ Draw the cyclic forms of

monosaccharides and classify them as

𝛂 or 𝛃 anomers

❹ Draw reduction and oxidation

products of monosaccharides

❺ Recognize the major structural

features of disaccharides

❻ Describe the characteristics of

cellulose, starch, and glycogen

❼ Give examples of some carbohydrate

derivatives that contain amino groups,

amides, or carboxylate anions

❽ Describe the role that carbohydrates

play in determining blood type



Milk contains lactose, a carbohydrate formed from two simple sugars, glucose and galactose.



CARBOHYDRATES

IN Chapter 20, we turn our attention to carbohydrates, the largest group of organic

molecules in nature, comprising approximately 50% of the earth’s biomass. Carbohydrates can be simple or complex, having as few as three or as many as thousands of carbon atoms. The glucose metabolized for energy in cells, the sucrose

of table sugar, and the cellulose of plant stems and tree trunks are all examples of

carbohydrates. Carbohydrates on cell surfaces determine blood type, and carbohydrates form the backbone of DNA, the carrier of all genetic information in the cell.

Unlike the lipids of Chapter 19, which are composed of mainly carbon–carbon and

carbon–hydrogen bonds and few functional groups, carbohydrates have many polar

functional groups, whose structure and properties can be understood by applying

the basic principles of organic chemistry.



608



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INTRODUCTION



609



20.1 INTRODUCTION

Carbohydrates, commonly referred to as sugars and starches, are polyhydroxy aldehydes

and ketones, or compounds that can be hydrolyzed to them. Historically, the word carbohydrate was given to this group of compounds because the molecular formula of simple carbohydrates could be written as Cn(H2O)n, making them hydrates of carbon.

Carbohydrates are classified into three groups:

• Monosaccharides (Sections 20.2–20.4)

• Disaccharides (Section 20.5)

• Polysaccharides (Sections 20.6–20.7)



Monosaccharides or simple sugars are the simplest carbohydrates. Glucose and fructose, the

two major constituents of honey, are monosaccharides. Glucose contains an aldehyde at one end

of a six-carbon chain, and fructose contains a ketone. Every other carbon atom has a hydroxyl

group bonded to it. Monosaccharides cannot be converted to simpler compounds by hydrolysis.

CHO



CH2OH



H



C



OH



C



O



HO



C



H



HO



C



H



H



C



OH



H



C



OH



H



C



OH



H



C



OH



aldehyde



+



CH2OH



ketone



CH2OH



fructose

glucose

common monosaccharides

honey



Disaccharides are composed of two monosaccharides joined together. Lactose, the principal

carbohydrate in milk, is a disaccharide. Disaccharides contain at least one acetal carbon—a

carbon atom singly bonded to two OR (alkoxy) groups. Although disaccharides contain no carbonyl groups, they are hydrolyzed to simple monosaccharides that contain an aldehyde or ketone,

as we will learn in Section 20.5.

acetal carbon

CH2OH



CH2OH



O



O



HO



O



OH



OH

OH HO

lactose

a common disaccharide



HO



Polysaccharides have three or more monosaccharides joined together. Starch, the main carbohydrate found in the seeds and roots of plants, is a polysaccharide composed of hundreds

of glucose molecules joined together. Like disaccharides, polysaccharides contain acetals but

no carbonyl groups, and they are hydrolyzed to simple monosaccharides that contain carbonyl

groups. Pasta, bread, rice, and potatoes are foods that contain a great deal of starch.



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610



CARBOHYDRATES



CH2OH



CH2OH

O



O

*

HO



CH2OH



OH



O

*



O



CH2OH

O

*



O



O



HO



OH HO

OH HO

starch

a polysaccharide

[Acetal carbons are labeled with *.]



*



O



OH



Carbohydrates are storehouses of chemical energy. Carbohydrates are synthesized in green plants

and algae through photosynthesis, a process that uses the energy from the sun to convert carbon

dioxide and water into glucose and oxygen. Almost all of the oxygen in the atmosphere results

from photosynthesis. Plants store glucose in the form of polysaccharides like starch and cellulose

(Section 20.6).

Energy is stored in photosynthesis.



+



6 CO2



6 H2O



sunlight

chlorophyll



C6H12O6

glucose



+



6 O2



Energy is released in metabolism.



Chlorophyll in green leaves converts

CO2 and H2O to glucose and O2

during photosynthesis.



The energy stored in glucose bonds is released when glucose is metabolized. The oxidation of

glucose is a multistep process that forms carbon dioxide, water, and a great deal of energy. Although

the metabolism of lipids provides more energy per gram than the metabolism of carbohydrates,

glucose is the preferred source when a burst of energy is needed during exercise. Glucose is water

soluble, so it can be quickly and easily transported through the bloodstream to tissues.



PROBLEM 20.1



Draw a Lewis structure for glucose that clearly shows the aldehyde carbonyl group and all lone

pairs on the oxygen atoms.



PROBLEM 20.2



Label the hemiacetal carbon—the carbon bonded to one OH and one OR group—in lactose

(Section 16.8). Label all other hydroxyls as 1°, 2°, or 3°.



20.2 MONOSACCHARIDES

Monosaccharides, the simplest carbohydrates, generally have three to six carbon atoms in a

chain, with a carbonyl group at either the terminal carbon, numbered C1, or the carbon adjacent

to it, numbered C2. In most carbohydrates, each of the remaining carbon atoms has a hydroxyl

group. Monosaccharides are drawn vertically, with the carbonyl group at (or near) the top.

C1

or

C2



C



carbonyl at C1



aldehyde



aldose



carbonyl at C2



ketone



ketose



O



3–6 C’s

C3



OH on all (or most) other C’s



C4

monosaccharide



• Monosaccharides with a carbonyl group at C1 are aldehydes called aldoses.

• Monosaccharides with a carbonyl group at C2 are ketones called ketoses.



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