chapter 5 lecture general, organic, and biological chemistry: an integrated approach laura frost,...

Chapter 5 Lecture

General, Organic, and Biological Chemistry: An Integrated Approach

Laura Frost, Todd Deal and Karen Timberlakeby Richard Triplett

© 2011 Pearson Education, Inc.

Carbohydrates: Life’s Sweet Molecules

Chapter 5

Chapter 5 2© 2011 Pearson Education, Inc.

Chapter Outline

5.1 Classes of Carbohydrates

5.2 Monosaccharides

5.3 Oxidation and Reduction Reactions

5.4 Ring Formation—The Truth about

Monosaccharide Structure

5.5 Disaccharides

5.6 Polysaccharides

5.7 Carbohydrates and Blood


Introduction to Carbohydrates

• Carbohydrates are sugars and provide energy when consumed.

• Our bodies break down carbohydrates to extract energy. Carbon dioxide and water are released in the process.

• Glucose is the primary carbohydrate our bodies use to produce energy.

• Carbohydrates are classified as biomolecules.


Introduction to Carbohydrates, Continued

• Simple carbohydrates are referred to as simple sugars and are often sweet to the taste.

• Consumption of more sugar than is needed for energy results in conversion of these sugars to fat.

• Complex carbohydrates include starches and the plant and wood fibers known as cellulose.


Introduction to Carbohydrates, Continued

• Carbohydrates are found on the surface of cells where they act as “road signs” allowing molecules to distinguish one cell from another.

• ABO blood markers found on red blood cells are made up of carbohydrates. They allow us to distinguish our body’s blood type from a foreign blood type.

• Carbohydrates in our body prevent blood clots. They are also found in our genetic material.



• Monosaccharides are the simplest carbohydrates. They cannot be broken down to smaller carbohydrates.

• Disaccharides consist of two monosaccharide units joined together; they can be split into two monosaccharides. Sucrose, table sugar, can be broken down into glucose and fructose.

• Oligosaccharides contain anywhere from three to nine monosaccharide units. ABO blood groups are oligosaccharides.


5.1 Classes of Carbohydrates, Continued

Polysaccharides are large molecules containing 10 or more monosaccharide units. Carbohydrate units are connected in one continuous chain or the chain can be branched.


5.2 Monosaccharides

• Monosaccharides contain the elements carbon, hydrogen, and oxygen, and have the general formula Cn(H2O)n, where n is a whole number 3 or greater.

• Monosaccharides contain several functional groups. They contain the hydroxyl group represented as –OH. They also contain a carbonyl group, which is an oxygen double bonded to a carbon atom. The carbonyl group may be an aldehyde or a ketone.


5.2 Monosaccharides, Continued

The functional groups of glucose are shown in the figure below.



Functional Groups in Monosaccharides—Alcohols, Aldehydes, and Ketones

Alcohols• Alcohol is an organic compound containing the

–OH group.• Ethanol is one of the simplest alcohols and is

prepared from the fermentation of simple sugars in grains and fruits. Ethanol is present in beer and liquors, and is used as an alternative fuel blend, such as gasohol and E85 (85% ethanol and 15% gasoline).



Alcohols• Alcohols are classified by the number of alkyl

groups attached to the carbon atom containing the hydroxyl group. The number of alkyl groups impacts the reactivity of the alcohol.

• Primary (1o) alcohols have one alkyl group attached to the alcoholic carbon.

• Secondary (2o) alcohols have two alkyl groups attached to the alcoholic carbon.

• Tertiary (3o) alcohols have three alkyl groups attached to the alcoholic carbon.



Alcohols• Monosaccharides contain both primary and

secondary alcohols.



Aldehydes• An aldehyde is an organic compound containing

the carbonyl group.• Benzaldehyde, a compound responsible for the

aroma of almonds and cherries, is one example.



Aldehydes• Members of this family always

contain a carbonyl group with a hydrogen atom bonded to one side and an alkyl or aromatic bonded to the other. An exception is formaldehyde (a preservative), which has two hydrogens bonded to the carbonyl group.



Aldehydes• Monosaccharides can

contain an aldehyde group on one end of the molecule in addition to multiple hydroxyl groups.



Ketones• A ketone also contains the carbonyl group, but

has an alkyl or aromatic group on both sides of the carbonyl group.

• Acetone is the simplest ketone. It is the main component of fingernail polish remover.



Ketones• A wide variety of biologically important compounds

contain a ketone group.• Pyruvate is a ketone-containing compound formed

during the breakdown of glucose. • Butanedione, the flavor of butter, contains two ketone

groups.



• Monosaccharides that contain an aldehyde group are referred to as an aldose. Those that contain a ketone group are referred to as a ketose.

• Monosaccharides are classified according to the number of carbon atoms. Most common monosaccharides have three to six carbon atoms.– Triose contains three carbons.– Tetrose contains four carbons.– Pentose contains five carbons.– Hexose contains six carbons.



• Carbohydrates are further classified on whether they contain an aldehyde or ketone group.

• For example, glucose, the most abundant monosaccharide found is nature, contains six carbons and an aldehyde group. It is classified as an aldohexose.

• Fructose, known as fruit sugar, contains six carbons and a ketone group. It is classified as a ketohexose.



Aldohexose and ketopentose differ in the number of carbon atoms and in the type of carbonyl group they contain.



Stereochemistry in Monosaccharides

Multiple chiral centers• Recall that a chiral center is a carbon atom that

has four different atoms or groups of atoms attached to it.

• Glucose, a ketohexose, contains four different chiral centers, each with a tetrahedral geometry.

Objectives for Today

Use molecular models and Fischer projections to distinguish between stereoisomers, enantiomers, diastereomers, and epimers in carbohydrates

Recognize common chemical reactions of carbohydrates




Multiple chiral centers• Carbons 2 through 5 of

glucose are tetrahedral and have four different atoms or groups of atoms attached. Carbons 1 and 6 are not chiral centers. Why?



Multiple chiral centers• Groups bonded to each chiral center have two

different arrangements or mirror images, which result in stereoisomers.

• The number of stereoisomers for a molecule increases with the number of chiral centers in the molecule.

• The general formula for determining the number of stereoisomers is 2n, where n is the number of chiral centers present in the molecule.

• Glucose has 4 chiral centers, so there are 16 stereoisomers, 24 = 16.



Representing stereoisomers—the Fischer projection

• Fischer projection is a simple way of indicating chiral molecules by showing their three-dimensional structure in two dimensions, without showing all the wedges and dashes on all the chiral centers.

• In the Fischer projection, horizontal lines on a chiral center represent wedges, and vertical lines on a chiral center represent dashes.



• Representing stereoisomers—the Fischer projection– In the Fischer projection, a chiral carbon is not

shown, but is implied at the intersection of lines.

– Consider the Fischer projection of glyceraldehyde, the simplest aldose, shown on the next slide.

Another View, Fischer Projections


• Fischer projections depict three-dimensional shapes for chiral molecules, with the chiral carbon represented by the intersection of two lines.




• D and L designations of sugars are based on the Fischer projection positioning in glyceraldehyde.

• All D-sugars have the –OH on the chiral carbon farthest from the carbonyl group on the right side of the molecule.

• All L-sugars have the –OH on the chiral carbon farthest from the carbonyl group on the left side of the molecule.

• Most sugars in nature have the D designation.




• Enantiomers are written as if there is a mirror placed between the two molecules.

• Enantiomers of D- and L-glucose are:



Stereoisomers that are not enantiomers• How are all stereoisomers of D-glucose related

since only one mirror image exists for any stereoisomer?

• Stereoisomers that are not enantiomers are called diastereomers.

• Diastereomers are stereoisomers that are not exact mirror images.

Table of Aldoses




Some Important Monosaccharides

• Glucose is the most abundant monosaccharide found in nature.

• Glucose is also known as dextrose, blood sugar, and grape sugar.

• Glucose is broken down in cells to produce energy.



• Diabetics have difficulty getting glucose in their cells, which is why they must monitor their blood glucose levels regularly.

• Glucose is one of the monosaccharides of sucrose (table sugar) and lactose (milk sugar) as well as the polysaccharides glycogen, starch, and cellulose.



• Galactose is found combined with glucose in the disaccharide lactose, which is present in milk and other dairy products.

• A single chiral center (carbon 4) in galactose is arranged opposite that of glucose, which makes it a diastereomer of glucose.

• Diastereomers that differ by one chiral center are called epimers.



• Mannose, a monosaccharide, is found in some fruits and vegetables.

• Cranberries contain high amounts of mannose, which has been shown to be effective in urinary tract infections.

• Mannose is an epimer of glucose.

Table of Aldoses



• Fructose, a ketose, is commonly referred to as fruit sugar or levulose.

• Fructose is combined with glucose to give sucrose, or table sugar.

• Fructose is the sweetest monosaccharide and is

found in fruits, vegetables, and honey.

• Fructose is not an epimer of glucose, but it can be broken down for energy in the body.


40

D-Fructose

D-Fructose• is a ketohexose,

C6H12O6.

• is the sweetest carbohydrate.

• is found in fruit juices and honey.

• converts to glucose in the body.

Copyright © 2009 by Pearson Education, Inc.

CH2OH

HO H

H OH

H OH

CH2OH

D-Fructose

C O



• Pentoses are five-carbon sugars and include ribose and 2-deoxyribose, which are parts of nucleic acids that make up genetic material.

• Ribonucleic acid (RNA) contains ribose, and deoxyribonucleic acid (DNA) contains 2-deoxyribose.

• The difference between these two pentoses is the absence of an oxygen atom on carbon 2 of deoxyribose.

• Ribose is also found in the vitamin riboflavin and other biologically important molecules.

D-Ribose




Oxidation and Reduction

• Oxidation and reduction reactions are commonly called redox reactions.

• Oxidation is a loss of electrons.

• Reduction is a gain of electrons.

• The mnemonic “OIL RIG” helps remember redox reactions. Oxidation Is Loss, Reduction Is Gain.


5.3 Oxidation and Reduction Reactions, Continued

• When copper metal (shiny orange metal) is exposed to oxygen, an ionic compound, copper(II) oxide, is produced. This compound is greenish in color. This reaction is shown as:

• The copper atoms in the reactant lose electrons to form the Cu2+ ions in the product. The copper has undergone oxidation.



• The electrons lost by copper atoms are transferred to the oxygen atom, which then becomes O2-. Oxygen has undergone reduction.

• While the copper was being oxidized, oxygen was being reduced.

• Copper then becomes the reducing agent (causing oxygen to be reduced) and oxygen is the oxidizing agent (causing copper to be oxidized).



• Organic molecules are oxidized if they gain oxygen or lose hydrogen, and they are reduced if they lose oxygen or gain hydrogen.

• Some biological reactions undergo oxidation and reduction. A summary of these characteristics are as follows:



Monosaccharides and Redox

• An aldehyde functional group can undergo oxidation by gaining oxygen or it can undergo reduction by gaining hydrogen.

• During oxidation, aldehydes form carboxylic acids, and during reduction, they form alcohols.

• In monosaccharides, oxidation produces a sugar acid, and reduction produces a sugar alcohol.



• Benedict’s test is a useful test to determine the presence of an oxidation reaction that occurs with sugars.

• Aldose sugars are oxidized by Cu2+ ion, while the Cu2+ ion is reduced to Cu+ ion.



The product of this reaction, copper(I) oxide (Cu2O), is not soluble and forms a brick red precipitate in solution.



• Aldoses are easily oxidized. They serve as reducing agents and are referred to as reducing sugars.

• Fructose and other ketoses are also reducing sugars, even though they do not contain an aldehyde group.

• The oxidizing agents can cause a rearrangement of the ketose to an aldose.



This rearrangement can be shown as:



• Benedict’s test can be used in urine dipsticks to determine the level of glucose in urine. Excess glucose in urine suggests high levels of glucose in blood, which is an indicator of diabetes.

• Aldoses or ketoses can be reduced by hydrogen under the correct conditions, producing sugar alcohols.

• Sugar alcohols are produced commercially as artificial sweeteners and found in sugar-free foods.



Reduction of glucose produces the sugar alcohol, sorbitol, which is an artificial sweetener.



• When glucose levels are high in the blood stream, sorbitol can be produced by an enzyme called aldose reductase.

• High levels of sorbitol can contribute to cataracts, which is a clouding of the lens in the eye.

• Cataracts are commonly seen in diabetics.


Use molecular models and Fischer projections to distinguish between stereoisomers, enantiomers, diastereomers, and epimers in carbohydrates

Recognize common chemical reactions of carbohydrates



Transform Fischer projections of monosaccharides into ring structures

Examine important disaccharides and polysaccharides



5.4 Ring Formation—The Truth about Monosaccharide Structure

• Carbonyl groups can also react with a hydroxyl functional group (–OH).

• When this happens, a hemiacetal functional group is formed as shown:


5.4 Ring Formation—The Truth about Monosaccharide Structure, Continued

A hemiacetal can form within a monosaccharide since it contains both a carbonyl and several hydroxyl functional groups.



• The carbonyl carbon that reacts to form the hemiacetal is referred to as the anomeric carbon.

• Two ring arrangements can be produced. These are termed anomers, and are referred to as the alpha () and beta (β) anomer.

• The position of the –OH group on the anomeric carbon relative to the position of the carbon outside the ring determines the type of anomer present.



• In the six-member ring (five carbons and an oxygen) form of D-isomers, called a pyranose, carbon 6 is always drawn on the top side of the ring.

• In the anomer, the –OH on the anomeric carbon is trans to the carbon outside the ring.

• In the β anomer, the –OH on the anomeric carbon is cis to the carbon outside the ring.



• D-Fructose contains both a ketone group and several hydroxyl groups.

• The ring structure of D-fructose contains four carbons and an oxygen to form a five-membered ring called a furanose.

• In a furanose, carbons 1 and 6 remain outside the ring.



• In a five-membered and six-membered ring, the anomers are distinguished similarly.

• In the alpha anomer, the –OH on the anomeric carbon is trans to the carbon outside the ring.

• In the beta anomer, the –OH on the anomeric carbon is cis to the carbon outside the ring.


5.5 Disaccharides

Condensation and Hydrolysis—Forming and Breaking Glycosidic Bonds

• The –OH group that is most reactive in a monosaccharide is the one on the anomeric carbon.

• When this hydroxyl group reacts with another hydroxyl group on another monosaccharide a glycosidic bond is formed.


5.5 Disaccharides, Continued

Formation of glycosides is an example of another type of organic reaction. During this reaction, a molecule of water is eliminated as two molecules join.



• Condensation reaction is a type of reaction that occurs when two molecules are joined and a water molecule is produced. This type of reaction is referred to as a dehydration reaction.

• Hydrolysis reaction is the reverse of a condensation reaction. A larger molecule forms two smaller molecules and water is consumed as a reactant.



Condensation reactions occur between different types of functional groups that contain an –H in a polar bond, like O–H or N–H, and an –OH group that can be removed to form water.



Naming Glycosidic Bonds

• In the slide showing the formation of maltose, we observed that the glycosidic bond was in the alpha position. If that bond had been in the beta position, a different molecule would have been formed with a different three-dimensional structure.

• In naming glycosidic bonds, it is necessary to name the configuration as well as the carbons involved in the bond formation.



• In the case of maltose, the glycosidic bond is specified as α(1→4) and is simply stated as alpha-one-four.

• If the –OH group had been in the beta configuration when the glycosidic bond was formed, the bond would be in the β(1→4) configuration. The molecule formed would be named cellobiose and would have a different two-dimensional and three-dimensional shape than maltose.



Three Important Disaccharides—Maltose, Lactose, and Sucrose

The formation of these three common disaccharides is outlined below.



Maltose• Maltose is known as malt sugar.• It is formed by the breakdown of starch.• Malted barley, a key ingredient in beer,

contains high levels of maltose.• During germination of barley seeds, the starch

goes through hydrolysis to form maltose. This process is halted by drying and roasting barley seeds prior to their germination.

• One of the anomeric carbons is free, so maltose is a reducing sugar.



Maltose, Continued• The glycosidic bond is α(1→4).



Lactose• Lactose is known as milk sugar.• It is found in milk and milk products.• An intolerance to lactose can occur in people

who inherit or lose the ability to produce the enzyme lactase that hydrolyzes lactose into its monosaccharide units.

• The glycosidic bond is (1→4).• One of the anomeric carbons is free, so

lactose is a reducing sugar.



Sucrose• Sucrose is known as table sugar.• It is the most abundant disaccharide found in

nature.• Sucrose is found in sugar cane and sugar

beets.• The glycosidic bond is (1→2).• Both anomeric carbons of the

monosaccharides in sucrose are bonded, therefore, sucrose is not a reducing sugar. It will not react with Benedict’s reagent.


Transform Fischer projections of monosaccharides into Haworth ring structures

Examine important disaccharides



Examine important polysaccharides Relationship of carbohydrates to blood

type



5.6 Polysaccharides

Polysaccharides

Polysaccharides are large molecules of monosaccharides that are connected to each other through their anomeric carbons. There are two types of polysaccharides:

1. Storage polysaccharides contain only -glucose units. Three important ones are starch, glycogen, and amylopectin.

2. Structural polysaccharides contain only -glucose units. Two important ones are cellulose and chitin. Chitin contains a modified -glucose unit.


5.6 Polysaccharides, Continued

Storage and structural polysaccharides are made up of glucose units, but they are structurally and functionally different because of their glycosidic bonds and difference in branching.



Storage Polysaccharides

Amylose and amylopectin—starch• Starch is a mixture of amylose and amylopectin

and is found in plant foods.• Amylose makes up 20% of plant starch and is

made up of 250–4000 D-glucose units bonded α(1→4) in a continuous chain.

• Long chains of amylose tend to coil.• Amylopectin makes up 80% of plant starch and

is made up of D-glucose units connected by α(1→4) glycosidic bonds.



Amylose and amylopectin—starch• About every 25 glucose units of amylopectin, a

branch of glucose units are connected to the glucose by an α(1→6) glycosidic bond.

• During fruit ripening, starch undergoes hydrolysis of the α(1→4) bonds to produce glucose and maltose, which are sweet.

• When we consume starch, our digestive system breaks it down into glucose units for use by our bodies.



Glycogen• Glycogen is a storage polysaccharide found in

animals.• Glycogen is stored in the liver and muscles.• Its structure is identical to amylopectin, except

that α(1→6) branching occurs about every 12 glucose units.

• When glucose is needed, glycogen is hydrolyzed in the liver to glucose.



Structural Polysaccharides

Cellulose• Cellulose contains glucose units bonded

(1→4).• This glycosidic bond configuration changes the

three-dimensional shape of cellulose compared with that of amylose.

• The chain of glucose units is straight. This allows chains to align next to each other to form a strong rigid structure.



Cellulose• Cellulose is an insoluble fiber in our diet

because we lack the enzyme cellulase to hydrolyze the (1→4) glycosidic bond.

• Whole grains are a good source of cellulose.• Cellulose is important in our diet because it

assists with digestive movement in the small and large intestine.

• Some animals and insects can digest cellulose because they contain bacteria that produce cellulase.



Chitin• Chitin makes up the exoskeleton of insects and

crustaceans and cell walls of some fungi.• It is made up of N-acetylglucosamine containing

(1→4) glycosidic bonds.• It is structurally strong.• Chitin is used as surgical thread that

biodegrades as a wound heals.• It serves as a protection from water in insects.• Chitin is also used to waterproof paper, and in

cosmetics and lotions to retain moisture.



ABO Blood Types

• ABO blood types refer to carbohydrates on red blood cells.

• These chemical markers are oligosaccharides that contain either three or four sugar units.

• Sugar units are D-galactose, L-fucose, N-acetylglucosamine, and N-acetylgalactosamine.


5.7 Carbohydrates and Blood, Continued

The following shows the carbohydrates and their attachments in type O, type A, and type B blood. Type AB blood has both type A and type B sets on their blood cells.



• Type O blood is considered the universal donor while type AB blood is considered the universal acceptor.

• The following table shows the compatibility of blood groups.



Heparin

• Heparin is a medically important polysaccharide because it prevents clotting in the bloodstream.

• It is a highly ionic polysaccharide of repeating disaccharide units of an oxidized monosaccharide and D-glucosamine. Heparin also contains sulfate groups that are negatively charged.

• It belongs to a group of polysaccharides called glycosaminoglycans.


Examine important polysaccharides Relationship of carbohydrates to blood

type



Chapter Summary


Carbohydrates are classified as monosaccharides (simple sugars), disaccharides (two monosaccharide units), oligosaccharides (three to nine monosaccharide units), and polysaccharides (many monosaccharide units).


Chapter Summary, Continued

5.2 Monosaccharides

• A monosaccharide has a molecular formula of Cn(H2O)n, where n = 3–6.

• Fischer projections that highlight chiral centers are used to represent monosaccharides.

• Most monosaccharides in nature are D-isomers.




• Multiple chiral centers lead to enantiomers or diastereomers.

• Important monosaccharides are glucose, galactose, fructose, mannose, ribose, and deoxyribose.




• Oxidation is a loss of electrons and reduction is a gain of electrons.

• These are common types of reactions in organic molecules.

• For organic molecules, oxidation is the addition of oxygen and reduction is the addition of hydrogen.



5.3 Oxidation and Reduction Reactions,

Continued

• The anomeric carbon of carbohydrates is highly reactive and can be oxidized to a carboxylic acid or reduced to an alcohol.

• Monosaccharides are considered reducing sugars because their anomeric carbon can react.



5.4 Ring Formation—The Truth about

Monosaccharide Structure

• A hydroxyl group and the carbonyl group can react to enclose the hydroxyl’s oxygen in a ring.

• Because the carbonyl group is planar, two possible ring arrangements about the anomeric carbon occur when the ring forms. These are termed the α and anomers.



5.5 Disaccharides

• Condensation and hydrolysis are common reactions that occur in biomolecules.

• Condensation reactions produce a water molecule while bonding two molecules together.

• Hydrolysis reactions consume a molecule of water while a molecule is broken into two smaller molecules.




• Carbohydrates form glycosides when an anomeric carbon reacts with a hydroxyl group on a second molecule. The bond formed is called a glycosidic bond.

• Glycosidic bonds are named by designating the anomer of the reacting monosaccharide and the carbons that are bonded, for example, α(1→4).



5.6 Polysaccharides

• A polysaccharide consists of many monosaccharide units bonded together through glycosidic bonds.

• Glucose is stored as glycogen in animals and starch in plants.

• Starch consists of amylose, a linear chain of glucose, and amylopectin, a branched chain of glucose.




• Glycogen contains many more branches in its structure than amylopectin.

• Two important polysaccharides are cellulose in plants and chitin in arthropods and fungi.

• Cellulose consists of (1→4) and is the structural component of plants. It has a linear structure.




• Chitin is linear. It contains N-acetylglucosamine.

• Cellulose and chitin form strong, water-resistant materials when the linear chains are aligned to each other.




• The ABO blood groups are oligosaccharides on the surface of red blood cells.

• The O blood group is considered the universal donor.

• Heparin, a polysaccharide, functions in the blood as an anticoagulant and is found as a coating on medical tubing and syringes during blood transfusions.

chapter 5 lecture general, organic, and biological chemistry: an integrated approach laura frost,...

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chapter outline5

simplest carbohydrates

continuedsimple carbohydrates

complex carbohydrates

smaller carbohydrates

monosaccharide units

abo blood groups