This is “Carbohydrates”, chapter 16 from the bookIntroduction to ... · 2. Identify the differences among monosaccharides, disaccharides, and polysaccharides. 4.The simplest carbohydrate

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890

Chapter 16

Carbohydrates

Chapter 16 Carbohydrates

891

Opening Essay

In the United States, 17.9 million people have been diagnosed with diabetes, and experts estimate that at leastanother 5.7 million people have the disease but have not been diagnosed. In 2006, diabetes was the seventhleading cause of death, listed on 72,507 death certificates. Moreover, it was a contributing factor in over 200,000deaths in which the cause was listed as something else, such as heart or kidney disease.

People with diabetes are impaired in their ability to metabolize glucose, a sugar needed by the body for energy;as a result, excessive quantities of glucose accumulate in the blood and the urine. The characteristic symptomsof diabetes are weight loss, constant hunger, extreme thirst, and frequent urination (the kidneys excrete largeamounts of water in an attempt to remove the excess sugar from the blood).

An important diagnostic test for diabetes is the oral glucose tolerance test, which measures the level of glucose inblood plasma. A first measurement is made after a fast of at least 8 h, followed by another measurement 2 h afterthe person drinks a flavored solution of 75 g of glucose dissolved in water. At the second measurement, theglucose plasma level should be no higher than 139 mg/dL. Individuals with a value between 140 and 199 mg/dLare diagnosed with prediabetes, while those with a value of 200 mg/dL or above are diagnosed with diabetes.Following a diagnosis of diabetes a person will need to monitor his or her blood glucose levels daily (or moreoften) using a glucose meter.

Figure 16.1Using a Glucose Meter to Test Blood Glucose Level

© Thinkstock


892

Glucose is one of the carbohydrates you will learn about in this chapter as we beginthe study of biochemistry1—the chemistry of molecules found in living organisms.Later we will study the other three major types of macromolecules found in livingorganisms: lipids, proteins, and nucleic acids.

1. The chemistry of moleculesfound in living organisms.


893

16.1 Carbohydrates

LEARNING OBJECTIVE

1. Recognize carbohydrates and classify them as mono-, di-, orpolysaccharides.

All carbohydrates2 consist of carbon, hydrogen, and oxygen atoms and arepolyhydroxy aldehydes or ketones or are compounds that can be broken down toform such compounds. Examples of carbohydrates include starch, fiber, the sweet-tasting compounds called sugars, and structural materials such as cellulose. Theterm carbohydrate had its origin in a misinterpretation of the molecular formulas ofmany of these substances. For example, because its formula is C6H12O6, glucose was

once thought to be a “carbon hydrate” with the structure C6·6H2O.

2. A compound composed ofcarbon, hydrogen, and oxygenatoms that is a polyhydroxyaldehyde or ketone or acompound that can be brokendown to form such acompound. It is one of thethree main components of thehuman diet.


894

EXAMPLE 1

Which compounds would be classified as carbohydrates?

1.

2.

3.

4.

Solution

1. This is a carbohydrate because the molecule contains an aldehydefunctional group with OH groups on the other two carbon atoms.

2. This is not a carbohydrate because the molecule does not contain analdehyde or a ketone functional group.

3. This is a carbohydrate because the molecule contains a ketonefunctional group with OH groups on the other two carbon atoms.

4. This is not a carbohydrate; although it has a ketone functional group,one of the other carbons atoms does not have an OH group attached.


16.1 Carbohydrates 895

SKILL-BUILDING EXERCISE

Which compounds would be classified as carbohydrates?

1.

2.

3.

4.

Green plants are capable of synthesizing glucose (C6H12O6) from carbon dioxide

(CO2) and water (H2O) by using solar energy in the process known as

photosynthesis3:

6CO2 + 6H2O + 686 kcal → C6H12O6 + 6O2

(The 686 kcal come from solar energy.) Plants can use the glucose for energy orconvert it to larger carbohydrates, such as starch or cellulose. Starch providesenergy for later use, perhaps as nourishment for a plant’s seeds, while cellulose isthe structural material of plants. We can gather and eat the parts of a plant that

3. The process by which plantsuse solar energy to convertcarbon dioxide and water toglucose.



store energy—seeds, roots, tubers, and fruits—and use some of that energyourselves. Carbohydrates are also needed for the synthesis of nucleic acids andmany proteins and lipids.

Animals, including humans, cannot synthesize carbohydrates from carbon dioxideand water and are therefore dependent on the plant kingdom to provide these vitalcompounds. We use carbohydrates not only for food (about 60%–65% by mass of theaverage diet) but also for clothing (cotton, linen, rayon), shelter (wood), fuel(wood), and paper (wood).

The simplest carbohydrates—those that cannot be hydrolyzed to produce evensmaller carbohydrates—are called monosaccharides4. Two or moremonosaccharides can link together to form chains that contain from two to severalhundred or thousand monosaccharide units. Prefixes are used to indicate thenumber of such units in the chains. Disaccharide5 molecules have twomonosaccharide units, trisaccharide molecules have three units, and so on. Chainswith many monosaccharide units joined together are called polysaccharides6. Allthese so-called higher saccharides can be hydrolyzed back to their constituentmonosaccharides.

Note

Compounds that cannot be hydrolyzed will not react with water to form two ormore smaller compounds.

CONCEPT REVIEW EXERCISES

1. Why is photosynthesis important?

2. Identify the differences among monosaccharides, disaccharides, andpolysaccharides.

4. The simplest carbohydrate thatcannot be hydrolyzed toproduce smaller carbohydratemolecules.

5. A carbohydrate containing twomonosaccharide units.

6. A carbohydrate containingmany monosaccharide units.



ANSWERS

1. Photosynthesis is the process by which solar energy is used to reduce carbondioxide to carbohydrates, which are needed for energy by plants and otherliving organisms that eat plants.

2. A monosaccharide is the simplest carbohydrate and cannot be hydrolyzed toproduce a smaller carbohydrate; a disaccharide is composed of twomonosaccharide units; and a polysaccharide contains many saccharide units.

KEY TAKEAWAYS

• Carbohydrates are an important group of biological molecules thatincludes sugars and starches.

• Photosynthesis is the process by which plants use energy from sunlightto synthesize carbohydrates.

• A monosaccharide is the simplest carbohydrate and cannot behydrolyzed to produce a smaller carbohydrate molecule. Disaccharidescontain two monosaccharide units, and polysaccharides contain manymonosaccharide units.

EXERCISES

1. When an aqueous solution of trehalose is heated, two molecules of glucose areproduced for each molecule of trehalose. Is trehalose a monosaccharide, adisaccharide, or a polysaccharide?

2. When an aqueous solution of arabinose is heated, no other molecules areproduced. Is arabinose a monosaccharide, a disaccharide, or a polysaccharide?

ANSWER

1. Trehalose is a disaccharide because it is hydrolyzed into two molecules ofglucose (a monosaccharide).



16.2 Classes of Monosaccharides

LEARNING OBJECTIVES

1. Classify monosaccharides as aldoses or ketoses and as trioses, tetroses,pentoses, or hexoses.

2. Distinguish between a D sugar and an L sugar.

The naturally occurring monosaccharides contain three to seven carbon atoms permolecule. Monosaccharides of specific sizes may be indicated by names composedof a stem denoting the number of carbon atoms and the suffix -ose. For example, theterms triose, tetrose, pentose, and hexose signify monosaccharides with, respectively,three, four, five, and six carbon atoms. Monosaccharides are also classified asaldoses or ketoses. Those monosaccharides that contain an aldehyde functionalgroup are called aldoses7; those containing a ketone functional group on the secondcarbon atom are ketoses8. Combining these classification systems gives generalnames that indicate both the type of carbonyl group and the number of carbonatoms in a molecule. Thus, monosaccharides are described as aldotetroses,aldopentoses, ketopentoses, ketoheptoses, and so forth. Glucose and fructose arespecific examples of an aldohexose and a ketohexose, respectively.

7. A monosaccharide thatcontains an aldehydefunctional group.

8. A monosaccharide thatcontains a ketone functionalgroup on the second carbonatom.


899

EXAMPLE 2

Draw an example of each type of compound.

1. a ketopentose2. an aldotetrose

Solution

1. The structure must have five carbon atoms with the secondcarbon atom being a carbonyl group and the other four carbonatoms each having an OH group attached. Several structures arepossible, but one example is shown.

2. The structure must have four carbon atoms with the first carbonatom part of the aldehyde functional group. The other threecarbon atoms each have an OH group attached. Severalstructures are possible, but one example is shown.


16.2 Classes of Monosaccharides 900


Draw an example of each type of compound.

1. an aldohexose

2. a ketotetrose

The simplest sugars are the trioses. The possible trioses are shown in part (a) ofFigure 16.2 "Structures of the Trioses"; glyceraldehyde is an aldotriose, whiledihydroxyacetone is a ketotriose. Notice that two structures are shown forglyceraldehyde. These structures are stereoisomers9, isomers having the samestructural formula but differing in the arrangement of atoms or groups of atoms inthree-dimensional space. If you make models of the two stereoisomers ofglyceraldehyde, you will find that you cannot place one model on top of the otherand have each functional group point in the same direction. However, if you placeone of the models in front of a mirror, the image in the mirror will be identical tothe second stereoisomer in part (b) of Figure 16.2 "Structures of the Trioses".Molecules that are nonsuperimposable (nonidentical) mirror images of each otherare a type of stereoisomer called enantiomers10 (Greek enantios, meaning“opposite”).

Note

Cis-trans (geometric) isomers were discussed in Chapter 13 "Unsaturated andAromatic Hydrocarbons", Section 13.2 "Cis-Trans Isomers (GeometricIsomers)". These are another type of stereoisomers.

9. An isomer that has the samestructural formula but differsin the arrangement of atoms orgroups of atoms in three-dimensional space.

10. Stereoisomers that arenonsuperimposable mirrorimages of each other.



Figure 16.2 Structures of the Trioses

(a) D- and L-glyceraldehyde are mirror images of each other and represent a pair of enantiomers. (b) A ball-and-stick model of D-glyceraldehyde is reflected in a mirror. Note that the reflection has the same structure as L-glyceraldehyde.

A key characteristic of enantiomers is that they have a carbon atom to which fourdifferent groups are attached. Note, for example, the four different groups attachedto the central carbon atom of glyceraldehyde (part (a) of Figure 16.2 "Structures ofthe Trioses"). A carbon atom that has four different groups attached is a chiralcarbon11. If a molecule contains one or more chiral carbons, it is likely to exist astwo or more stereoisomers. Dihydroxyacetone does not contain a chiral carbon andthus does not exist as a pair of stereoisomers. Glyceraldehyde, however, has a chiralcarbon and exists as a pair of enantiomers. Except for the direction in which eachenantiomer rotates plane-polarized light, these two molecules have identicalphysical properties. One enantiomer has a specific rotation of +8.7°, while the otherhas a specific rotation of −8.7°.

H. Emil Fischer, a German chemist, developed the convention commonly used forwriting two-dimensional representations of the monosaccharides, such as those inpart (a) of Figure 16.2 "Structures of the Trioses". In these structural formulas, the

11. A carbon atom that has fourdifferent groups attached to it.



aldehyde group is written at the top, and the hydrogen atoms and OH groups thatare attached to each chiral carbon are written to the right or left. (If themonosaccharide is a ketose, the ketone functional group is the second carbonatom.) Vertical lines represent bonds pointing away from you, while horizontallines represent bonds coming toward you. The formulas of chiral moleculesrepresented in this manner are referred to as Fischer projections.

The two enantiomers of glyceraldehyde are especially important becausemonosaccharides with more than three carbon atoms can be considered as beingderived from them. Thus, D- and L-glyceraldehyde provide reference points fordesignating and drawing all other monosaccharides. Sugars whose Fischerprojections terminate in the same configuration as D-glyceraldehyde are designatedas D sugars12; those derived from L-glyceraldehyde are designated as L sugars13.

Note

By convention, the penultimate (next-to-last) carbon atom has been chosen asthe carbon atom that determines if a sugar is D or L. It is the chiral carbonfarthest from the aldehyde or ketone functional group.

12. A sugar whose Fischerprojection terminates in thesame configuration as D-glyceraldehyde.

13. A sugar whose Fischerprojection terminates in thesame configuration as L-glyceraldehyde.



Looking Closer: Polarized Light

A beam of ordinary light can be pictured as a bundle of waves; some move upand down, some sideways, and others at all other conceivable angles. When abeam of light has been polarized, however, the waves in the bundle all vibratein a single plane. Light altered in this way is called plane-polarized light. Much ofwhat chemists know about stereoisomers comes from studying the effects theyhave on plane-polarized light. In this illustration, the light on the left is notpolarized, while that on the right is polarized.

Sunlight, in general, is not polarized; light from an ordinary light bulb or anordinary flashlight is not polarized. One way to polarize ordinary light is to passit through Polaroid sheets, special plastic sheets containing carefully orientedorganic compounds that permit only light vibrating in a single plane to passthrough. To the eye, polarized light doesn’t “look” any different fromnonpolarized light. We can detect polarized light, however, by using a secondsheet of polarizing material, as shown here.



In the photo on the left, two Polaroid sheets are aligned in the same direction;plane-polarized light from the first Polaroid sheet can pass through the secondsheet. In the photo on the right, the top Polaroid sheet has been rotated 90° andnow blocks the plane-polarized light that comes through the first Polaroidsheet.

Certain substances act on polarized light by rotating the plane of vibration.Such substances are said to be optically active. The extent of optical activity ismeasured by a polarimeter, an instrument that contains two polarizing lensesseparated by a sample tube, as shown in the accompanying figure. With thesample tube empty, maximum light reaches the observer’s eye when the twolenses are aligned so that both pass light vibrating in the same plane. When anoptically active substance is placed in the sample tube, that substance rotatesthe plane of polarization of the light passing through it, so that the polarizedlight emerging from the sample tube is vibrating in a different direction thanwhen it entered the tube. To see the maximum amount of light when thesample is in place, the observer must rotate one lens to accommodate thechange in the plane of polarization.



Figure 16.3Diagram of a Polarimeter

Some optically active substances rotate the plane of polarized light to the right(clockwise) from the observer’s point of view. These compounds are said to bedextrorotatory; substances that rotate light to the left (counterclockwise) arelevorotatory. To denote the direction of rotation, a positive sign (+) is given todextrorotatory substances, and a negative sign (−) is given to levorotatorysubstances.


1. What is a chiral carbon?

2. Describe how enantiomers differ.



ANSWERS

1. A chiral carbon is a carbon atom with four different groups attached to it.

2. Enantiomers are mirror images of each other; they differ in the arrangementsof atoms around a chiral carbon.

KEY TAKEAWAYS

• Monosaccharides can be classified by the number of carbon atoms in thestructure and/or the type of carbonyl group they contain (aldose orketose).

• Most monosaccharides contain at least one chiral carbon and can formstereoisomers.

• Enantiomers are a specific type of stereoisomers that are mirror imagesof each other.



EXERCISES

1. Identify each sugar as an aldose or a ketose and then as a triose, tetrose,pentose, or hexose.

a. D-glucose

b. L-ribulose

c. D-glyceraldehyde



2. Identify each sugar as an aldose or a ketose and then as a triose, tetrose,pentose, or hexose.

a. dihydroxyacetone

b. D-ribose

c. D-galactose



3. Identify each sugar as an aldose or a ketose and then as a D sugar or an L sugar.

a.



b.

4. Identify each sugar as an aldose or a ketose and then as a D sugar or an L sugar.

a.

b.



ANSWERS

1. a. aldose; hexoseb. ketose; pentosec. aldose; triose

3. a. aldose; D sugarb. ketose; L sugar



16.3 Important Hexoses

LEARNING OBJECTIVE

1. Identify the structures of D-glucose, D-galactose, and D-fructose anddescribe how they differ from each other.

Although a variety of monosaccharides are found in living organisms, three hexosesare particularly abundant: D-glucose, D-galactose, and D-fructose (Figure 16.4"Structures of Three Important Hexoses"). Glucose and galactose are bothaldohexoses, while fructose is a ketohexose.

Figure 16.4 Structures of Three Important Hexoses

Each hexose is pictured with a food source in which it is commonly found.

Source: Photos © Thinkstock.


913

Glucose

D-Glucose, generally referred to as simply glucose, is the most abundant sugarfound in nature; most of the carbohydrates we eat are eventually converted to it ina series of biochemical reactions that produce energy for our cells. It is also knownby three other names: dextrose, from the fact that it rotates plane-polarized light ina clockwise (dextrorotatory) direction; corn sugar because in the United Statescornstarch is used in the commercial process that produces glucose from thehydrolysis of starch; and blood sugar because it is the carbohydrate found in thecirculatory system of animals. Normal blood sugar values range from 70 to 105 mgglucose/dL plasma, and normal urine may contain anywhere from a trace to 20 mgglucose/dL urine.

The Fischer projection of D-glucose is given in Figure 16.5 "Cyclization of D-Glucose". Glucose is a D sugar because the OH group on the fifth carbon atom (thechiral center farthest from the carbonyl group) is on the right. In fact, all the OHgroups except the one on the third carbon atom are to the right.

Galactose

D-Galactose does not occur in nature in the uncombined state. It is released whenlactose, a disaccharide found in milk, is hydrolyzed. The galactose needed by thehuman body for the synthesis of lactose is obtained by the metabolic conversion ofD-glucose to D-galactose. Galactose is also an important constituent of theglycolipids that occur in the brain and the myelin sheath of nerve cells. (For moreinformation about glycolipids, see Chapter 17 "Lipids", Section 17.3 "Membranesand Membrane Lipids".) For this reason it is also known as brain sugar. The structureof D-galactose is shown in Figure 16.4 "Structures of Three Important Hexoses".Notice that the configuration differs from that of glucose only at the fourth carbonatom.

Fructose

D-Fructose, also shown in Figure 16.4 "Structures of Three Important Hexoses", isthe most abundant ketohexose. Note that from the third through the sixth carbonatoms, its structure is the same as that of glucose. It occurs, along with glucose andsucrose, in honey (which is 40% fructose) and sweet fruits. Fructose (from the Latinfructus, meaning “fruit”) is also referred to as levulose because it has a specificrotation that is strongly levorotatory (−92.4°). It is the sweetest sugar, being 1.7times sweeter than sucrose, although many nonsugars are several hundred orseveral thousand times as sweet (Table 16.1 "The Relative Sweetness of SomeCompounds (Sucrose = 100)").


16.3 Important Hexoses 914

Table 16.1 The Relative Sweetness of Some Compounds (Sucrose = 100)

Compound Relative Sweetness

lactose 16

maltose 32

glucose 74

sucrose 100

fructose 173

aspartame 18,000

acesulfame K 20,000

saccharin 30,000

sucralose 60,000



Looking Closer: Artificial Sweeteners

Although sweetness is commonly associated with mono- and disaccharides, it isnot a property found only in sugars. Several other kinds of organic compoundshave been synthesized that are far superior as sweetening agents. These so-called high-intensity or artificial sweeteners are useful for people with diabetesor other medical conditions that require them to control their carbohydrateintake. The synthetic compounds are noncaloric or used in such smallquantities that they do not add significantly to the caloric value of food.

The first artificial sweetener—saccharin—was discovered by accident in 1879. Itis 300 times sweeter than sucrose, but it passes through the body unchangedand thus adds no calories to the diet. After its discovery, saccharin was useduntil it was banned in the early 1900s. However, during the sugar-short years ofWorld War I, the ban was lifted and was not reinstated at the war’s end. Onedrawback to the use of saccharin is its bitter, metallic aftertaste. The initialsolution to this problem was to combine saccharin with cyclamate, a secondartificial sweetener discovered in 1937.

In the 1960s and 1970s, several clinical tests with laboratory animals implicatedboth cyclamate and saccharin as carcinogenic (cancer-causing) substances. Theresults from the cyclamate tests were completed first, and cyclamate wasbanned in the United States in 1969. Then a major study was released in Canadain 1977 indicating that saccharin increased the incidence of bladder cancer inrats. The US Food and Drug Administration (FDA) proposed a ban on saccharinthat raised immediate public opposition because saccharin was the onlyartificial sweetener still available. In response, Congress passed the SaccharinStudy and Labeling Act in 1977, permitting the use of saccharin as long as anyproduct containing it was labeled with a consumer warning regarding thepossible elevation of the risk of bladder cancer. Today this warning is no longerrequired; moreover, the FDA is currently reviewing the ban on cyclamate, as 75additional studies and years of usage in other countries, such as Canada, havefailed to show that it has any carcinogenic effect.

A third artificial sweetener, aspartame, was discovered in 1965. This whitecrystalline compound is about 180 times sweeter than sucrose and has noaftertaste. It was approved for use in 1981 and is used to sweeten a wide varietyof foods because it blends well with other food flavors. Aspartame is not used inbaked goods, however, because it is not heat stable.



In the body (or when heated), aspartame is initially hydrolyzed to threemolecules: the amino acids aspartic acid and phenylalanine and an alcoholmethanol. Repeated controversy regarding the safety of aspartame arises partlyfrom the fact that the body metabolizes the released methanol toformaldehyde. It should be noted, though, that a glass of tomato juice has sixtimes as much methanol as a similar amount of a diet soda containingaspartame. The only documented risk connected to aspartame use is forindividuals with the genetic disease phenylketonuria (PKU); these individualslack the enzyme needed to metabolize the phenylalanine released whenaspartame is broken down by the body. Because of the danger to people withPKU, all products containing aspartame must carry a warning label.

Acesulfame K, discovered just two years after aspartame (1967), was approvedfor use in the United States in 1988. It is 200 times sweeter than sugar and,unlike aspartame, is heat stable. It has no lingering aftertaste.

One of the newest artificial sweeteners to gain FDA approval (April 1998) for usein the United States is sucralose, a white crystalline solid approximately 600times sweeter than sucrose. Sucralose is synthesized from sucrose and hasthree chlorine atoms substituted for three OH groups. It is noncaloric because itpasses through the body unchanged. It can be used in baking because it is heatstable.

All of the extensive clinical studies completed to date have indicated that theseartificial sweeteners approved for use in the United States are safe forconsumption by healthy individuals in moderate amounts.




1. Describe the similarities and differences in the structures of D-glucose and D-galactose.

2. Describe similarities and differences in the structures of D-glucose and D-fructose.

ANSWERS

1. Both monosaccharides are aldohexoses. The two monosaccharides differ in theconfiguration around the fourth carbon atom.

2. Both monosaccharides are hexoses. D-glucose is an aldohexose, while D-fructose is a ketohexose.



KEY TAKEAWAY

• Three abundant hexoses in living organisms are the aldohexoses D-glucose and D-galactose and the ketohexose D-fructose.

EXERCISES

1. Identify each sugar by its common chemical name.

a. blood sugarb. levulose


a. dextroseb. brain sugar

3. Identify each sugar as an aldohexose or a ketohexose.

a. glucoseb. galactosec. fructose

4. What hexose would you expect to be most abundant in each food?

a. honeyb. milkc. cornstarch

ANSWERS

1. a. D-glucoseb. D-fructose

3. a. aldohexoseb. aldohexosec. ketohexose



16.4 Cyclic Structures of Monosaccharides

LEARNING OBJECTIVES

1. Define what is meant by anomers and describe how they are formed.2. Explain what is meant by mutarotation.

So far we have represented monosaccharides as linear molecules, but many of themalso adopt cyclic structures. This conversion occurs because of the ability ofaldehydes and ketones to react with alcohols:

In some cases, OH and carbonyl groups on the same molecule are able to react withone another in an intramolecular reaction. Thus, monosaccharides larger thantetroses exist mainly as cyclic compounds (Figure 16.5 "Cyclization of D-Glucose").You might wonder why the aldehyde reacts with the OH group on the fifth carbonatom rather than the OH group on the second carbon atom next to it. Recall fromChapter 12 "Organic Chemistry: Alkanes and Halogenated Hydrocarbons", Section12.9 "Cycloalkanes", that cyclic alkanes containing five or six carbon atoms in thering are the most stable. The same is true for monosaccharides that form cyclicstructures: rings consisting of five or six carbon atoms are the most stable.


920

Figure 16.5 Cyclization of D-Glucose

D-Glucose can be represented with a Fischer projection (a) or three dimensionally (b). By reacting the OH group onthe fifth carbon atom with the aldehyde group, the cyclic monosaccharide (c) is produced.

When a straight-chain monosaccharide, such as any of the structures shown inFigure 16.4 "Structures of Three Important Hexoses", forms a cyclic structure, thecarbonyl oxygen atom may be pushed either up or down, giving rise to twostereoisomers, as shown in Figure 16.6 "Monosaccharides". The structure shown onthe left side of Figure 16.6 "Monosaccharides", with the OH group on the firstcarbon atom projected downward, represent what is called the alpha (α) form. Thestructures on the right side, with the OH group on the first carbon atom pointedupward, is the beta (β) form. These two stereoisomers of a cyclic monosaccharide areknown as anomers14; they differ in structure around the anomeric carbon15—thatis, the carbon atom that was the carbonyl carbon atom in the straight-chain form.

14. Stereoisomers that differ instructure around what was thecarbonyl carbon atom in thestraight-chain form of amonosaccharide.

15. The carbon atom that was thecarbonyl carbon atom in thestraight-chain form of amonosaccharide.


16.4 Cyclic Structures of Monosaccharides 921

Figure 16.6 Monosaccharides

In an aqueous solution, monosaccharides exist as an equilibrium mixture of three forms. The interconversionbetween the forms is known as mutarotation, which is shown for D-glucose (a) and D-fructose (b).

It is possible to obtain a sample of crystalline glucose in which all the moleculeshave the α structure or all have the β structure. The α form melts at 146°C and has aspecific rotation of +112°, while the β form melts at 150°C and has a specific rotationof +18.7°. When the sample is dissolved in water, however, a mixture is soonproduced containing both anomers as well as the straight-chain form, in dynamicequilibrium (part (a) of Figure 16.6 "Monosaccharides"). You can start with a purecrystalline sample of glucose consisting entirely of either anomer, but as soon asthe molecules dissolve in water, they open to form the carbonyl group and thenreclose to form either the α or the β anomer. The opening and closing repeatscontinuously in an ongoing interconversion between anomeric forms and isreferred to as mutarotation16 (Latin mutare, meaning “to change”). At equilibrium,the mixture consists of about 36% α-D-glucose, 64% β-D-glucose, and less than0.02% of the open-chain aldehyde form. The observed rotation of this solution is+52.7°.

Even though only a small percentage of the molecules are in the open-chainaldehyde form at any time, the solution will nevertheless exhibit the characteristicreactions of an aldehyde. As the small amount of free aldehyde is used up in areaction, there is a shift in the equilibrium to yield more aldehyde. Thus, all the

16. The ongoing interconversionbetween anomeric forms of amonosaccharide to form anequilibrium mixture.



molecules may eventually react, even though very little free aldehyde is present ata time.

In Figure 16.5 "Cyclization of D-Glucose" and Figure 16.6 "Monosaccharides", andelsewhere in this book, the cyclic forms of sugars are depicted using a conventionfirst suggested by Walter N. Haworth, an English chemist. The molecules are drawnas planar hexagons with a darkened edge representing the side facing toward theviewer. The structure is simplified to show only the functional groups attached tothe carbon atoms. Any group written to the right in a Fischer projection appearsbelow the plane of the ring in a Haworth projection, and any group written to theleft in a Fischer projection appears above the plane in a Haworth projection.

The difference between the α and the β forms of sugars may seem trivial, but suchstructural differences are often crucial in biochemical reactions. This explains whywe can get energy from the starch in potatoes and other plants but not fromcellulose, even though both starch and cellulose are polysaccharides composed ofglucose molecules linked together. We will examine the effects of these differencesmore closely in Section 16.7 "Polysaccharides" and when we discuss enzymespecificity in Chapter 18 "Amino Acids, Proteins, and Enzymes", Section 18.5"Enzymes".


1. Define each term.

a. mutarotationb. anomerc. anomeric carbon

2. How can you prove that a solution of α-D-glucose exhibits mutarotation?



ANSWERS

1. a. the ongoing interconversion between anomers of a particularcarbohydrate to form an equilibrium mixture

b. a stereoisomer that differs in structure around what was the carbonylcarbon atom in the straight-chain form of a monosaccharide

c. the carbon atom that was the carbonyl carbon atom in the straight-chainform of a monosaccharide

2. Place a sample of pure α-D-glucose in a polarimeter and measure its observedrotation. This value will change as mutarotation occurs.

KEY TAKEAWAYS

• Monosaccharides that contain five or more carbons atoms form cyclicstructures in aqueous solution.

• Two cyclic stereoisomers can form from each straight-chainmonosaccharide; these are known as anomers.

• In an aqueous solution, an equilibrium mixture forms between the twoanomers and the straight-chain structure of a monosaccharide in aprocess known as mutarotation.

EXERCISES

1. Draw the cyclic structure for β-D-glucose. Identify the anomeric carbon.

2. Draw the cyclic structure for α-D-fructose. Identify the anomeric carbon.

3. Given that the aldohexose D-mannose differs from D-glucose only in theconfiguration at the second carbon atom, draw the cyclic structure for α-D-mannose.

4. Given that the aldohexose D-allose differs from D-glucose only in theconfiguration at the third carbon atom, draw the cyclic structure for β-D-allose.



ANSWERS

1.

3.



16.5 Properties of Monosaccharides

LEARNING OBJECTIVE

1. Identify the physical and chemical properties of monosaccharides.

Monosaccharides such as glucose and fructose are crystalline solids at roomtemperature, but they are quite soluble in water, each molecule having several OHgroups that readily engage in hydrogen bonding. The chemical behavior of thesemonosaccharides is likewise determined by their functional groups.

An important reaction of monosaccharides is the oxidation of the aldehyde group,one of the most easily oxidized organic functional groups. Aldehyde oxidation canbe accomplished with any mild oxidizing agent, such as Tollens’ reagent orBenedict’s reagent. (For more information about aldehyde oxidation, see Chapter 14"Organic Compounds of Oxygen", Section 14.5 "Reactions of Alcohols".) With thelatter, complexed copper(II) ions are reduced to copper(I) ions that form a brick-redprecipitate [copper(I) oxide; Figure 16.7 "Benedict’s Test"].

Any carbohydrate capable of reducing either Tollens’ or Benedict’s reagentswithout first undergoing hydrolysis is said to be a reducing sugar17. Because boththe Tollens’ and Benedict’s reagents are basic solutions, ketoses (such as fructose)

17. Any carbohydrate capable ofreducing a mild oxidizingagent, such as Tollens’ orBenedict’s reagents, withoutfirst undergoing hydrolysis.


926

also give positive tests due to an equilibrium that exists between ketoses andaldoses in a reaction known as tautomerism.

Figure 16.7 Benedict’s Test

Benedict’s test was performed on three carbohydrates, depicted from left to right: fructose, glucose, and sucrose. Thesolution containing sucrose remains blue because sucrose is a nonreducing sugar.

These reactions have been used as simple and rapid diagnostic tests for thepresence of glucose in blood or urine. For example, Clinitest tablets, which are usedto test for sugar in the urine, contain copper(II) ions and are based on Benedict’stest. A green color indicates very little sugar, whereas a brick-red color indicatessugar in excess of 2 g/100 mL of urine.


1. Why are monosaccharides soluble in water?

2. What is a reducing sugar?


16.5 Properties of Monosaccharides 927

ANSWERS

1. Monosaccharides are quite soluble in water because of the numerous OHgroups that readily engage in hydrogen bonding with water.

2. any carbohydrate capable of reducing a mild oxidizing agent, such as Tollens’or Benedict’s reagents, without first undergoing hydrolysis

KEY TAKEAWAYS

• Monosaccharides are crystalline solids at room temperature and quitesoluble in water.

• Monosaccharides are reducing sugars; they reduce mild oxidizingagents, such as Tollens’ or Benedict’s reagents.

EXERCISES

1. Which gives a positive Benedict’s test—L-galactose, levulose, or D-glucose?

2. Which gives a positive Benedict’s test—D-glyceraldehyde, corn sugar, or L-fructose?

3. D-Galactose can be oxidized at the sixth carbon atom to yield D-galacturonicacid and at both the first and sixth carbon atoms to yield D-galactaric acid.Draw the Fischer projection for each oxidation product.

4. D-Glucose can be oxidized at the first carbon atom to form D-gluconic acid, atthe sixth carbon atom to yield D-glucuronic acid, and at both the first and sixthcarbon atoms to yield D-glucaric acid. Draw the Fischer projection for eachoxidation product.



ANSWERS

1. All three will give a positive Benedict’s test because they are allmonosaccharides.

3.



16.6 Disaccharides

LEARNING OBJECTIVES

1. Identify the structures of sucrose, lactose, and maltose.2. Identify the monosaccharides that are needed to form sucrose, lactose,

and maltose.

In Section 16.4 "Cyclic Structures of Monosaccharides", you learned thatmonosaccharides can form cyclic structures by the reaction of the carbonyl groupwith an OH group. These cyclic molecules can in turn react with another alcohol.Disaccharides (C12H22O11) are sugars composed of two monosaccharide units that

are joined by a carbon–oxygen-carbon linkage known as a glycosidic linkage18.This linkage is formed from the reaction of the anomeric carbon of one cyclicmonosaccharide with the OH group of a second monosaccharide.

18. The carbon–oxygen-carbonlinkage betweenmonosaccharide units in morecomplex carbohydrates, suchas disaccharides orpolysaccharides.


930

The disaccharides differ from one another in their monosaccharide constituentsand in the specific type of glycosidic linkage connecting them. There are threecommon disaccharides: maltose, lactose, and sucrose. All three are white crystallinesolids at room temperature and are soluble in water. We’ll consider each sugar inmore detail.

Maltose

Maltose occurs to a limited extent in sprouting grain. It is formed most often by thepartial hydrolysis of starch and glycogen. In the manufacture of beer, maltose isliberated by the action of malt (germinating barley) on starch; for this reason, it isoften referred to as malt sugar. Maltose is about 30% as sweet as sucrose. The humanbody is unable to metabolize maltose or any other disaccharide directly from thediet because the molecules are too large to pass through the cell membranes of theintestinal wall. Therefore, an ingested disaccharide must first be broken down byhydrolysis into its two constituent monosaccharide units. In the body, suchhydrolysis reactions are catalyzed by enzymes such as maltase. The same reactionscan be carried out in the laboratory with dilute acid as a catalyst, although in thatcase the rate is much slower, and high temperatures are required. Whether itoccurs in the body or a glass beaker, the hydrolysis of maltose produces twomolecules of D-glucose.

Maltose is a reducing sugar. Thus, its two glucose molecules must be linked in sucha way as to leave one anomeric carbon that can open to form an aldehyde group.The glucose units in maltose are joined in a head-to-tail fashion through an α-linkage from the first carbon atom of one glucose molecule to the fourth carbonatom of the second glucose molecule (that is, an α-1,4-glycosidic linkage; see Figure16.8 "An Equilibrium Mixture of Maltose Isomers"). The bond from the anomericcarbon of the first monosaccharide unit is directed downward, which is why this isknown as an α-glycosidic linkage. The OH group on the anomeric carbon of thesecond glucose can be in either the α or the β position, as shown in Figure 16.8 "AnEquilibrium Mixture of Maltose Isomers".

maltose ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯H+ or maltase 2 D-glucose


16.6 Disaccharides 931

Figure 16.8 An Equilibrium Mixture of Maltose Isomers

Lactose

Lactose is known as milk sugar because it occurs in the milk of humans, cows, andother mammals. In fact, the natural synthesis of lactose occurs only in mammarytissue, whereas most other carbohydrates are plant products. Human milk containsabout 7.5% lactose, and cow’s milk contains about 4.5%. This sugar is one of thelowest ranking in terms of sweetness, being about one-sixth as sweet as sucrose (seeTable 16.1 "The Relative Sweetness of Some Compounds (Sucrose = 100)" in Section16.3 "Important Hexoses"). Lactose is produced commercially from whey, a by-product in the manufacture of cheese. It is important as an infant food and in theproduction of penicillin.

Lactose is a reducing sugar composed of one molecule of D-galactose and onemolecule of D-glucose joined by a β-1,4-glycosidic bond (the bond from theanomeric carbon of the first monosaccharide unit being directed upward). The twomonosaccharides are obtained from lactose by acid hydrolysis or the catalyticaction of the enzyme lactase:



Many adults and some children suffer from a deficiency of lactase. Theseindividuals are said to be lactose intolerant19 because they cannot digest thelactose found in milk. A more serious problem is the genetic diseasegalactosemia20, which results from the absence of an enzyme needed to convertgalactose to glucose. Certain bacteria can metabolize lactose, forming lactic acid asone of the products. This reaction is responsible for the “souring” of milk.

19. The inability to digest thelactose found in milk or dairyproducts due to a deficiency ofthe enzyme lactase.

20. A genetic disease caused by theabsence of one of the enzymesneeded to convert galactose toglucose.



EXAMPLE 3

For this trisaccharide, indicate whether each glycosidic linkage is α or β.

Solution

The glycosidic linkage between sugars 1 and 2 is β because the bond isdirected up from the anomeric carbon. The glycosidic linkage betweensugars 2 and 3 is α because the bond is directed down from the anomericcarbon.


1. For this trisaccharide, indicate whether each glycosidic linkage is α or β.



To Your Health: Lactose Intolerance and Galactosemia

Lactose makes up about 40% of an infant’s diet during the first year of life.Infants and small children have one form of the enzyme lactase in their smallintestines and can digest the sugar easily; however, adults usually have a lessactive form of the enzyme, and about 70% of the world’s adult population hassome deficiency in its production. As a result, many adults experience areduction in the ability to hydrolyze lactose to galactose and glucose in theirsmall intestine. For some people the inability to synthesize sufficient enzymeincreases with age. Up to 20% of the US population suffers some degree oflactose intolerance.

In people with lactose intolerance, some of the unhydrolyzed lactose passesinto the colon, where it tends to draw water from the interstitial fluid into theintestinal lumen by osmosis. At the same time, intestinal bacteria may act onthe lactose to produce organic acids and gases. The buildup of water andbacterial decay products leads to abdominal distention, cramps, and diarrhea,which are symptoms of the condition.

The symptoms disappear if milk or other sources of lactose are excluded fromthe diet or consumed only sparingly. Alternatively, many food stores now carryspecial brands of milk that have been pretreated with lactase to hydrolyze thelactose. Cooking or fermenting milk causes at least partial hydrolysis of thelactose, so some people with lactose intolerance are still able to enjoy cheese,yogurt, or cooked foods containing milk. The most common treatment forlactose intolerance, however, is the use of lactase preparations (e.g., Lactaid),which are available in liquid and tablet form at drugstores and grocery stores.These are taken orally with dairy foods—or may be added to them directly—toassist in their digestion.

Galactosemia is a condition in which one of the enzymes needed to convertgalactose to glucose is missing. Consequently, the blood galactose level ismarkedly elevated, and galactose is found in the urine. An infant withgalactosemia experiences a lack of appetite, weight loss, diarrhea, and jaundice.The disease may result in impaired liver function, cataracts, mentalretardation, and even death. If galactosemia is recognized in early infancy, itseffects can be prevented by the exclusion of milk and all other sources ofgalactose from the diet. As a child with galactosemia grows older, he or sheusually develops an alternate pathway for metabolizing galactose, so the need



to restrict milk is not permanent. The incidence of galactosemia in the UnitedStates is 1 in every 65,000 newborn babies.

Sucrose

Sucrose, probably the largest-selling pure organic compound in the world, is knownas beet sugar, cane sugar, table sugar, or simply sugar. Most of the sucrose soldcommercially is obtained from sugar cane and sugar beets (whose juices are14%–20% sucrose) by evaporation of the water and recrystallization. The darkbrown liquid that remains after the recrystallization of sugar is sold as molasses.

The sucrose molecule is unique among the common disaccharides in having anα-1,β-2-glycosidic (head-to-head) linkage. Because this glycosidic linkage is formedby the OH group on the anomeric carbon of α-D-glucose and the OH group on theanomeric carbon of β-D-fructose, it ties up the anomeric carbons of both glucoseand fructose.

This linkage gives sucrose certain properties that are quite different from those ofmaltose and lactose. As long as the sucrose molecule remains intact, neithermonosaccharide “uncyclizes” to form an open-chain structure. Thus, sucrose isincapable of mutarotation and exists in only one form both in the solid state and in



solution. In addition, sucrose does not undergo reactions that are typical ofaldehydes and ketones. Therefore, sucrose is a nonreducing sugar.

The hydrolysis of sucrose in dilute acid or through the action of the enzyme sucrase(also known as invertase) gives an equimolar mixture of glucose and fructose. This1:1 mixture is referred to as invert sugar because it rotates plane-polarized light inthe opposite direction than sucrose. The hydrolysis reaction has several practicalapplications. Sucrose readily recrystallizes from a solution, but invert sugar has amuch greater tendency to remain in solution. In the manufacture of jelly and candyand in the canning of fruit, the recrystallization of sugar is undesirable. Therefore,conditions leading to the hydrolysis of sucrose are employed in these processes.Moreover, because fructose is sweeter than sucrose, the hydrolysis adds to thesweetening effect. Bees carry out this reaction when they make honey.

The average American consumes more than 100 lb of sucrose every year. Abouttwo-thirds of this amount is ingested in soft drinks, presweetened cereals, andother highly processed foods. The widespread use of sucrose is a contributing factorto obesity and tooth decay. Carbohydrates such as sucrose, are converted to fatwhen the caloric intake exceeds the body’s requirements, and sucrose causes toothdecay by promoting the formation of plaque that sticks to teeth.

CONCEPT REVIEW EXERCISE

1. What monosaccharides are obtained by the hydrolysis of each disaccharide?

a. sucroseb. maltosec. lactose

ANSWER

1. a. D-glucose and D-fructoseb. two molecules of D-glucosec. D-glucose and D-galactose



KEY TAKEAWAYS

• Maltose is composed of two molecules of glucose joined by anα-1,4-glycosidic linkage. It is a reducing sugar that is found in sproutinggrain.

• Lactose is composed of a molecule of galactose joined to a molecule ofglucose by a β-1,4-glycosidic linkage. It is a reducing sugar that is foundin milk.

• Sucrose is composed of a molecule of glucose joined to a molecule offructose by an α-1,β-2-glycosidic linkage. It is a nonreducing sugar thatis found in sugar cane and sugar beets.



EXERCISES


a. milk sugarb. table sugar


a. cane sugarb. malt sugar

3. For each disaccharide, indicate whether the glycosidic linkage is α or β.

a.

b.

4. For each disaccharide, indicate whether the glycosidic linkage is α or β.

a.

b.



5. Identify each disaccharide in Exercise 3 as a reducing or nonreducing sugar. Ifit is a reducing sugar, draw its structure and circle the anomeric carbon. Stateif the OH group at the anomeric carbon is in the α or the β position.

6. Identify each disaccharide in Exercise 4 as a reducing or nonreducing sugar. Ifit is a reducing sugar, draw its structure and circle the anomeric carbon. Stateif the OH group at the anomeric carbon is in the α or β position.

7. Melibiose is a disaccharide that occurs in some plant juices. Its structure is asfollows:

a. What monosaccharide units are incorporated into melibiose?b. What type of linkage (α or β) joins the two monosaccharide units of

melibiose?c. Melibiose has a free anomeric carbon and is thus a reducing sugar. Circle

the anomeric carbon and indicate whether the OH group is α or β.

8. Cellobiose is a disaccharide composed of two glucose units joined by aβ-1,4-glycosidic linkage.

a. Draw the structure of cellobiose.b. Is cellobiose a reducing or nonreducing sugar? Justify your answer.



ANSWERS

1. a. lactoseb. sucrose

3.

a.

b.

5. 3a: nonreducing; 3b: reducing

7. a. galactose and glucoseb. α-glycosidic linkage

c.



16.7 Polysaccharides

LEARNING OBJECTIVE

1. Compare and contrast the structures and uses of starch, glycogen, andcellulose.

The polysaccharides are the most abundant carbohydrates in nature and serve avariety of functions, such as energy storage or as components of plant cell walls.Polysaccharides are very large polymers composed of tens to thousands ofmonosaccharides joined together by glycosidic linkages. The three most abundantpolysaccharides are starch, glycogen, and cellulose. These three are referred to ashomopolymers because each yields only one type of monosaccharide (glucose) aftercomplete hydrolysis. Heteropolymers may contain sugar acids, amino sugars, ornoncarbohydrate substances in addition to monosaccharides. Heteropolymers arecommon in nature (gums, pectins, and other substances) but will not be discussedfurther in this textbook. The polysaccharides are nonreducing carbohydrates, arenot sweet tasting, and do not undergo mutarotation.

Starch

Starch is the most important source of carbohydrates in the human diet andaccounts for more than 50% of our carbohydrate intake. It occurs in plants in theform of granules, and these are particularly abundant in seeds (especially the cerealgrains) and tubers, where they serve as a storage form of carbohydrates. Thebreakdown of starch to glucose nourishes the plant during periods of reducedphotosynthetic activity. We often think of potatoes as a “starchy” food, yet otherplants contain a much greater percentage of starch (potatoes 15%, wheat 55%, corn65%, and rice 75%). Commercial starch is a white powder.

Starch is a mixture of two polymers: amylose21 and amylopectin22. Naturalstarches consist of about 10%–30% amylase and 70%–90% amylopectin. Amylose is alinear polysaccharide composed entirely of D-glucose units joined by theα-1,4-glycosidic linkages we saw in maltose (part (a) of Figure 16.9 "Amylose").Experimental evidence indicates that amylose is not a straight chain of glucoseunits but instead is coiled like a spring, with six glucose monomers per turn (part(b) of Figure 16.9 "Amylose"). When coiled in this fashion, amylose has just enoughroom in its core to accommodate an iodine molecule. The characteristic blue-violetcolor that appears when starch is treated with iodine is due to the formation of the

21. A linear polymer of glucoseunits found in starch.

22. A branched polymer of glucoseunits found in starch.


942

amylose-iodine complex. This color test is sensitive enough to detect even minuteamounts of starch in solution.

Figure 16.9 Amylose

(a) Amylose is a linear chain of α-D-glucose units joined together by α-1,4-glycosidic bonds. (b) Because of hydrogenbonding, amylose acquires a spiral structure that contains six glucose units per turn.

Amylopectin is a branched-chain polysaccharide composed of glucose units linkedprimarily by α-1,4-glycosidic bonds but with occasional α-1,6-glycosidic bonds,which are responsible for the branching. A molecule of amylopectin may containmany thousands of glucose units with branch points occurring about every 25–30units (Figure 16.10 "Representation of the Branching in Amylopectin andGlycogen"). The helical structure of amylopectin is disrupted by the branching ofthe chain, so instead of the deep blue-violet color amylose gives with iodine,amylopectin produces a less intense reddish brown.


16.7 Polysaccharides 943

Figure 16.10 Representation of the Branching in Amylopectin and Glycogen

Both amylopectin and glycogen contain branch points that are linked through α-1,6-linkages. These branch pointsoccur more often in glycogen.

Dextrins are glucose polysaccharides of intermediate size. The shine and stiffnessimparted to clothing by starch are due to the presence of dextrins formed whenclothing is ironed. Because of their characteristic stickiness with wetting, dextrinsare used as adhesives on stamps, envelopes, and labels; as binders to hold pills andtablets together; and as pastes. Dextrins are more easily digested than starch andare therefore used extensively in the commercial preparation of infant foods.

The complete hydrolysis of starch yields, in successive stages, glucose:

starch → dextrins → maltose → glucose

In the human body, several enzymes known collectively as amylases degrade starchsequentially into usable glucose units.

Glycogen

Glycogen is the energy reserve carbohydrate of animals. Practically all mammaliancells contain some stored carbohydrates in the form of glycogen, but it is especiallyabundant in the liver (4%–8% by weight of tissue) and in skeletal muscle cells(0.5%–1.0%). Like starch in plants, glycogen is found as granules in liver and muscle



cells. When fasting, animals draw on these glycogen reserves during the first daywithout food to obtain the glucose needed to maintain metabolic balance.

Note

About 70% of the total glycogen in the body is stored in muscle cells. Althoughthe percentage of glycogen (by weight) is higher in the liver, the much greatermass of skeletal muscle stores a greater total amount of glycogen.

Glycogen is structurally quite similar to amylopectin, although glycogen is morehighly branched (8–12 glucose units between branches) and the branches areshorter. When treated with iodine, glycogen gives a reddish brown color. Glycogencan be broken down into its D-glucose subunits by acid hydrolysis or by the sameenzymes that catalyze the breakdown of starch. In animals, the enzymephosphorylase catalyzes the breakdown of glycogen to phosphate esters of glucose.

Cellulose

Cellulose, a fibrous carbohydrate found in all plants, is the structural component ofplant cell walls. Because the earth is covered with vegetation, cellulose is the mostabundant of all carbohydrates, accounting for over 50% of all the carbon found inthe vegetable kingdom. Cotton fibrils and filter paper are almost entirely cellulose(about 95%), wood is about 50% cellulose, and the dry weight of leaves is about10%–20% cellulose. The largest use of cellulose is in the manufacture of paper andpaper products. Although the use of noncellulose synthetic fibers is increasing,rayon (made from cellulose) and cotton still account for over 70% of textileproduction.

Like amylose, cellulose is a linear polymer of glucose. It differs, however, in that theglucose units are joined by β-1,4-glycosidic linkages, producing a more extendedstructure than amylose (part (a) of Figure 16.11 "Cellulose"). This extreme linearityallows a great deal of hydrogen bonding between OH groups on adjacent chains,causing them to pack closely into fibers (part (b) of Figure 16.11 "Cellulose"). As aresult, cellulose exhibits little interaction with water or any other solvent. Cottonand wood, for example, are completely insoluble in water and have considerablemechanical strength. Because cellulose does not have a helical structure, it does notbind to iodine to form a colored product.



Figure 16.11 Cellulose

(a) There is extensive hydrogen bonding in the structure of cellulose. (b) In this electron micrograph of the cell wallof an alga, the wall consists of successive layers of cellulose fibers in parallel arrangement.

Cellulose yields D-glucose after complete acid hydrolysis, yet humans are unable tometabolize cellulose as a source of glucose. Our digestive juices lack enzymes thatcan hydrolyze the β-glycosidic linkages found in cellulose, so although we can eatpotatoes, we cannot eat grass. However, certain microorganisms can digestcellulose because they make the enzyme cellulase, which catalyzes the hydrolysis ofcellulose. The presence of these microorganisms in the digestive tracts ofherbivorous animals (such as cows, horses, and sheep) allows these animals todegrade the cellulose from plant material into glucose for energy. Termites alsocontain cellulase-secreting microorganisms and thus can subsist on a wood diet.This example once again demonstrates the extreme stereospecificity of biochemicalprocesses.



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© Thinkstock


1. What purposes do starch and cellulose serve in plants?

2. What purpose does glycogen serve in animals?

ANSWERS

1. Starch is the storage form of glucose (energy) in plants, while cellulose is astructural component of the plant cell wall.

2. Glycogen is the storage form of glucose (energy) in animals.



KEY TAKEAWAYS

• Starch is a storage form of energy in plants. It contains two polymerscomposed of glucose units: amylose (linear) and amylopectin(branched).

• Glycogen is a storage form of energy in animals. It is a branchedpolymer composed of glucose units. It is more highly branched thanamylopectin.

• Cellulose is a structural polymer of glucose units found in plants. It is alinear polymer with the glucose units linked through β-1,4-glycosidicbonds.

EXERCISES

1. What monosaccharide is obtained from the hydrolysis of each carbohydrate?

a. starchb. cellulosec. glycogen

2. For each carbohydrate listed in Exercise 1, indicate whether it is found inplants or mammals.

3. Describe the similarities and differences between amylose and cellulose.

4. Describe the similarities and differences between amylopectin and glycogen.

ANSWERS

1. a. glucoseb. glucosec. glucose

3. Amylose and cellulose are both linear polymers of glucose units, but theglycosidic linkages between the glucose units differ. The linkages in amyloseare α-1,4-glycosidic linkages, while the linkages in cellulose they areβ-1,4-glycosidic linkages.



16.8 End-of-Chapter Material


949

Chapter Summary

To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in thefollowing summary and ask yourself how they relate to the topics in the chapter.

Carbohydrates, a large group of biological compounds containing carbon, hydrogen, and oxygen atoms, includesugars, starch, glycogen, and cellulose. All carbohydrates contain alcohol functional groups, and either analdehyde or a ketone group (or a functional group that can be converted to an aldehyde or ketone). The simplestcarbohydrates are monosaccharides. Those with two monosaccharide units are disaccharides, and those withmany monosaccharide units are polysaccharides. Most sugars are either monosaccharides or disaccharides.Cellulose, glycogen, and starch are polysaccharides.

Many carbohydrates exist as stereoisomers, in which the three-dimensional spatial arrangement of the atomsin space is the only difference between the isomers. These particular stereoisomers contain at least one chiralcarbon, a carbon atom that has four different groups bonded to it. A molecule containing a chiral carbon isnonsuperimposable on its mirror image, and two molecules that are nonsuperimposable mirror images of eachother are a special type of stereoisomer called enantiomers. Enantiomers have the same physical properties,such as melting point, but differ in the direction they rotate polarized light.

A sugar is designated as being a D sugar or an L sugar according to how, in a Fischer projection of the molecule,the hydrogen atom and OH group are attached to the penultimate carbon atom, which is the carbon atomimmediately before the terminal alcohol carbon atom. If the structure at this carbon atom is the same as that ofD-glyceraldehyde (OH to the right), the sugar is a D sugar; if the configuration is the same as that of L-glyceraldehyde (OH to the left), the sugar is an L sugar.

Monosaccharides of five or more carbons atoms readily form cyclic structures when the carbonyl carbon atomreacts with an OH group on a carbon atom three or four carbon atoms distant. Consequently, glucose in solutionexists as an equilibrium mixture of three forms, two of them cyclic (α- and β-) and one open chain. In Haworthprojections, the alpha form is drawn with the OH group on the “former” carbonyl carbon atom (anomericcarbon) pointing downward; the beta form, with the OH group pointing upward; these two compounds arestereoisomers and are given the more specific term of anomers. Any solid sugar can be all alpha or all beta.Once the sample is dissolved in water, however, the ring opens up into the open-chain structure and then closesto form either the α- or the β-anomer. These interconversions occur back and forth until a dynamic equilibriummixture is achieved in a process called mutarotation.

The carbonyl group present in monosaccharides is easily oxidized by Tollens’ or Benedict’s reagents (as well asothers). Any mono- or disaccharide containing a free anomeric carbon is a reducing sugar. The disaccharidemaltose contains two glucose units joined in an α-1,4-glycosidic linkage. The disaccharide lactose contains a


16.8 End-of-Chapter Material 950

galactose unit and a glucose unit joined by a β-1,4-glycosidic linkage. Both maltose and lactose contain a freeanomeric carbon that can convert to an aldehyde functional group, so they are reducing sugars; they alsoundergo mutarotation. Many adults, and some children, have a deficiency of the enzyme lactase (which isneeded to break down lactose) and are said to be lactose intolerant. A more serious problem is the geneticdisease galactosemia, which results from the absence of an enzyme needed to convert galactose to glucose.

The disaccharide sucrose (table sugar) consists of a glucose unit and a fructose unit joined by a glycosidic linkage.The linkage is designated as an α-1,β-2-glycosidic linkage because it involves the OH group on the first carbonatom of glucose and the OH group on the second carbon atom of fructose. Sucrose is not a reducing sugarbecause it has no anomeric carbon that can reform a carbonyl group, and it cannot undergo mutarotationbecause of the restrictions imposed by this linkage.

Starch, the principal carbohydrate of plants, is composed of the polysaccharides amylose (10%–30%) andamylopectin (70%–90%). When ingested by humans and other animals, starch is hydrolyzed to glucose andbecomes the body’s energy source. Glycogen is the polysaccharide animals use to store excess carbohydratesfrom their diets. Similar in structure to amylopectin, glycogen is hydrolyzed to glucose whenever an animalneeds energy for a metabolic process. The polysaccharide cellulose provides structure for plant cells. It is a linearpolymer of glucose units joined by β-1,4-glycosidic linkages. It is indigestible in the human body but digestibleby many microorganisms, including microorganisms found in the digestive tracts of many herbivores.



ADDITIONAL EXERCISES

1. Draw the Fischer projections for D-glucose and D-ribose. Identify all thefunctional groups in each structure.

2. Draw the Fischer projections for D-galactose and D-fructose. Identify all thefunctional groups in each structure.

3. L-Fucose is an aldohexose that is often incorporated into oligosaccharidesattached to cell membranes. It is also known as 6-deoxy-L-galactose. Draw thestructure of L-fucose.

4. D-glucitol, also known as sorbitol, is added to shredded coconut to keep it softand to pharmaceutical products to increase the absorption of nutrients. It isprepared industrially by the reduction of D-glucose. Propose a structure for D-glucitol.

5. Which would give a positive Benedict’s test—lactose, amylopectin, D-ribose,sucrose, D-glyceraldehyde, or amylose?

6. Which enzyme hydrolyzes each carbohydrate?

a. maltoseb. lactosec. cellulosed. sucrose

7. What structural characteristics are necessary if a disaccharide is to be areducing sugar? Draw the structure of a hypothetical reducing disaccharidecomposed of two aldohexoses.

8. Raffinose, a trisaccharide found in beans and sugar beets, contains D-galactose,D-glucose, and D-fructose. The enzyme α-galactase catalyzes the hydrolysis ofraffinose to galactose and sucrose. Draw the structure of raffinose. (Thelinkage from galactose to the glucose unit is α-1,6).

9. What reagent(s) could be used to carry out each conversion?

a.



b.

10. What reagents are necessary to carry out each conversion?

a.

b.

11. The structure of lactulose is shown here. What monosaccharide units composethis disaccharide?

12. N-acetylglucosamine is synthesized from D-glucosamine, which in turn isobtained from D-glucose. What reagents are needed for the conversion of D-glucosamine to N-acetylglucosamine?



13. Hyaluronic acid is a heteropolymer that acts as a lubricating agent in the fluidsof joints and the eyes. Its structure consists of repeating disaccharide unitscontaining glucuronic acid and N-acetylglucosamine connected by aβ-1,3-linkage. Draw the structure of the disaccharide unit found in hyaluronicacid.

14. Several artificial sweeteners are discussed in this chapter.

a. Which are currently approved for use in the United States?b. Which has (or have) a bitter, metallic aftertaste?c. Which was (or were) most recently approved for use in the United States?d. Which contain(s) potassium?

15. If 3.0 mmol (3.0 × 10−3 mol) samples of saccharin, cyclamate, aspartame, andacesulfame K were each dissolved in separate beakers containing 500 mL ofpure water, which solution would have the sweetest taste? Which solutionwould have the least sweet taste? Justify your answers.

16. Identify two functional groups found in aspartame, acesulfame K, andsucralose.

17. Why does a deficiency of lactase lead to cramps and diarrhea?

18. How does galactosemia differ from lactose intolerance in terms of the cause ofthe disease and its symptoms and severity?



ANSWERS

1.

3.

5. Lactose, D-ribose, and D-glyceraldehyde would give a positive Benedict’s test.

7. To be a reducing sugar, a disaccharide must contain an anomeric carbon atomthat can open up to form an aldehyde functional group, as shown in thisdisaccharide (answers will vary).

9. a. The carbohydrate is being oxidized; Tollens’ or Benedict’s reagent could beused.

b. To form the compound shown, an aldehyde must react with methanol(CH3OH) and an acid catalyst.

11. galactose and fructose



13.

15. Sucralose would be expected to have the sweetest taste because its relativesweetness is the highest. Lactose would have the least sweet taste because ithas the lowest relative sweetness.

17. Intestinal bacteria can act on the lactose present in the intestine to produceorganic acids and gases. The buildup of water and bacterial decay productsleads to cramps and diarrhea.



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This is “Carbohydrates”, chapter 16 from the bookIntroduction to ... · 2. Identify the differences among monosaccharides, disaccharides, and polysaccharides. 4.The simplest carbohydrate