Carbohydrates: Life’s Sweet Molecules Chapter 5
Chapter 5 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
Chapter 5 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.
Chapter 5 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.
Chapter 5 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.
Chapter 5 5.1 Classes of Carbohydrates • Monosaccharidesare the simplest carbohydrates. They cannot be broken down to smaller carbohydrates. • Disaccharidesconsist 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.
Chapter 5 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.
Chapter 5 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.
Chapter 5 5.2 Monosaccharides, Continued The functional groups of glucose are shown in the figure below.
Chapter 5 5.2 Monosaccharides, Continued 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).
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued Alcohols • Monosaccharides contain both primary and secondary alcohols.
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued Aldehydes • Monosaccharides can contain an aldehyde group on one end of the molecule in addition to multiple hydroxyl groups.
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued • 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. • Triosecontains three carbons. • Tetrosecontains four carbons. • Pentosecontains five carbons. • Hexosecontains six carbons.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.2 Monosaccharides, Continued Aldohexose and ketopentose differ in the number of carbon atoms and in the type of carbonyl group they contain.
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued 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?
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.2 Monosaccharides, Continued Representing stereoisomers—the Fischer projection
Chapter 5 5.2 Monosaccharides, Continued Representing stereoisomers—the Fischer projection • 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.
Chapter 5 5.2 Monosaccharides, Continued Representing stereoisomers—the Fischer projection • Enantiomers are written as if there is a mirror placed between the two molecules. • Enantiomers of D- and L-glucose are:
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued
Chapter 5 5.2 Monosaccharides, Continued 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.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.2 Monosaccharides, Continued • 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.
Chapter 5 5.3 Oxidation and Reduction Reactions 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.
Chapter 5 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.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued • 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).
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued • 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:
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued 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.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued • 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.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued The product of this reaction, copper(I) oxide (Cu2O), is not soluble and forms a brick red precipitate in solution.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued • 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.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued This rearrangement can be shown as:
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued • 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.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued Reduction of glucose produces the sugar alcohol, sorbitol, which is an artificial sweetener.
Chapter 5 5.3 Oxidation and Reduction Reactions, Continued • 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.
Chapter 5 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:
Chapter 5 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.