1 Carbohydrates Chapter 27 Hein * Best * Pattison * Arena Colleen Kelley Chemistry Department Pima...

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Carbohydrates Chapter 27

Hein * Best * Pattison * Arena

Colleen KelleyChemistry DepartmentPima Community College

Version 1.0

Chapter Outline27.1 Carbohydrates: A First Class of Biochemicals

27.2 Classification of Carbohydrates

27.3 Importance of Carbohydrates

27.4 Monosaccharides

27.5 Structure of Glucose and Other Aldoses

Chapter Outline (continued)

27.6 Cyclic Structure of Glucose; Mutarotation

27.7 Hemiacetals and Acetals

27.8 Structures of Galactose and Fructose

27. 9 Pentoses

27.10 Disaccharides

27.11 Structures and Properties of Disaccharides

27.12 Sweeteners and Diet

27.13 Redox Reactions of Monosaccharides

27.14 Polysaccharides Derived from Glucose

Carbohydrates: Carbohydrates: A First Class of A First Class of BiochemicalsBiochemicals

• Carbohydrates are generally defined as polyhydroxy aldehydes or ketones or substances that yield these compounds when hydrolyzed.

glyceraldehyde

dihydroxyacetone

Classification Classification of of

CarbohydratesCarbohydrates

A carbohydrate can be classified as:

1.monosaccharide

2.disaccharide

3.oligosaccharide

4.polysaccharide

Monosaccharides

• A monosaccharide is a carbohydrate that cannot be hydrolyzed to simpler carbohydrate units.

• The monosaccharide is the basic carbohydrate unit of cellular metabolism.

Disaccharides• A disaccharide yields two

monosaccharides – either alike or different – when hydrolyzed:

disaccharide + water 2 monosaccharidesH+ or

enzymes

Monosaccharides & Disaccharides

• Disaccharides are often used by plants or animals to transport monosaccharides from one cell to another.

• The monosaccharides and disaccharides generally have the ending –ose – for example, glucose, sucrose, and lactose.

• These are water-soluble carbohydrates, which have a characteristically sweet taste and are called sugars.

Oligosaccharides

• An oligosaccharide has two to six monosaccharide units linked together.

Polysaccharides• A polysaccharide is a macromolecular

substance that can be hydrolyzed to yield many monosaccharide units:

polysaccharide + water monosaccharidesH+ or

enzymes

• Polysaccharides are important structural supports, particularly in plants, and also serve as a storage depot for monosaccharides, which cells use for energy.

Other Ways to Classify Carbohydrates• As a triose, tetrose, pentose, hexose, or

heptose• As an aldose or ketose• As a D or L isomer• As a (+) or (-) isomer• As a furanose or a pyranose• As having an alpha () or beta ()

configuration

Importance of Importance of CarbohydratesCarbohydrates

Importance of Carbohydrates

1. Carbohydrates are very effective energy-yielding nutrients.

2. Carbohydrates can serve as very effective building materials.

3. Carbohydrates are important water-soluble molecules.

MonosaccharidesMonosaccharidesMonosaccharidesMonosaccharides

Monosaccharides

• The hexose monosaccharides are the most important carbohydrate sources of cellular energy.

• Three hexoses – glucose, galactose, and fructose – are of major significance in nutrition.– All three have the same formula, C6H12O6, and thus

deliver the same amount of cellular energy.– They differ in structure, but are biologically

interconvertible.

• Glucose (dextrose) is the most important of the monosaccharides.

• It is an aldohexose and is found in the free state in plant and animal tissue.

• Galactose is also an aldohexose and occurs, along with glucose, in lactose and in many oligo- and polysaccharides such as pectin and gums.

• Fructose, also know as levulose, is a ketohexose that occurs in fruit juices, honey, and along with glucose, as a constituent of sucrose.

Structures of Glucose Structures of Glucose and Other Aldosesand Other Aldoses

Epimers• Any two monosaccharides that differ

only in the configuration around a single carbon atom are called epimers.

• D- and L-glyceraldehyde are epimers.

D-glyceraldehyde

L-glyceraldehyde

Figure 27. 1 Configurations of the D-family of aldoses. The hydroxyl group on the new chiral carbon atom, added in going from triose to tetrose to pentose to hexose, is shown in red.

Figure 27.2 An example of the Kilani-Fischer synthesis in which two aldotetrose molecules are formed from an aldotriose molecule.

Cyclic Structure of Cyclic Structure of Glucose; MutarotationGlucose; Mutarotation

Figure 27.3 Mutarotation of D-glucose

Anomers

• When two cyclic isomers differ only in their stereo arrangement about the carbon involved in mutarotation, they are called anomers.

• Mutarotation is the process by which anomers are interconverted.

Figure 27. 4 Three-dimensional representations of the chair form of -D-glucopyranose

Hemiacetals Hemiacetals and Acetalsand AcetalsHemiacetals Hemiacetals and Acetalsand Acetals

• Cyclic structures of monosaccharides are intramolecular hemiacetals.

• Five- or six-membered rings are especially stable.

hemiacetal structure in -D-glucopyranose

Glycoside• When a monosaccharide hemiacetal reacts with

an alcohol, the product is an acetal.

• In carbohydrate terminology, this acetal structure is called a glycoside.

acetal structure

glycosidic linkage

Structures of Structures of Galactose and Galactose and

FructoseFructose

Galactose

D-galactose -D-galactopyranose -D-galactopyranose

FructoseCH2OH

DisaccharidesDisaccharidesDisaccharidesDisaccharides

Disaccharides• Disaccharides are carbohydrates composed

of two monosaccharide residues united by a glycosidic linkage.

• sucrose + water glucose + fructose

• lactose + water galactose + glucose

• maltose + water glucose + glucose

H+ or sucrase

lactase

maltase

Structures and Structures and Properties of Properties of DisaccharidesDisaccharides

• Disaccharides contain an acetal structure (glycosidic linkage), and some also contain a hemiacetal structure.

• Maltose: O

-1,4-glycosidic linkage

Sweeteners Sweeteners and Dietand Diet

Redox Reactions of Redox Reactions of MonosaccharidesMonosaccharides

Oxidation• The aldehyde groups in monosaccharides can

be oxidized to monocarboxylic acids by mild oxidizing agents such as bromine water.

+ Br2 + H2O

+ 2HBr

Reduction• Monosaccharides can be reduced to their

corresponding polyhydroxy alcohols by reducing agents such as H2/Pt or sodium amalgam, Na(Hg).

+ H2 / Pt

Redox Test for Carbohydrates

• Under prescribed conditions, some sugars reduce silver ions to free silver, and copper (II) ions to copper (I) ions.

• Such sugars are called reducing sugars.

Polysaccharides Polysaccharides Derived from Derived from

GlucoseGlucose

Polysaccharides Polysaccharides Derived from Derived from

GlucoseGlucose

Starch• Starch is found in plants, mainly in the seeds,

roots, or tubers.

• Corn, wheat potatoes, rice and cassava are the chief sources of dietary starch.

• The two main components of starch are amylose and amylopectin. – Amylose molecules are unbranched chains

composed of about 25-1300 -D-glucose units joined by -1,4-glycosidic linkages.

Figure 27.7 Representation of amylose.

Glycogen• Glycogen is the energy-storage carbohydrate of

the animal kingdom.

• It is formed by the polymerization of glucose and is stored in the liver and in muscle tissues.

• Structurally, it is very similar to the amylopectin fraction of starch, except that it is more highly branched.

• The -1,6-glycosidic linkages occur on one of every 12-18 glucose units.

Cellulose

• Cellulose is the most abundant organic substance found in nature.

• It is the chief structural component of plants and wood.

Figure 27.9 Two representations of cellulose. In the three-dimensional drawing, note the hydrogen bonding that links the extended cellulose polymers to form cellulose fibers.

1 Carbohydrates Chapter 27 Hein * Best * Pattison * Arena Colleen Kelley Chemistry Department Pima...

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