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Chapter Fourteen Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.14–2 Biochemistry: An Overview
Copyright © Houghton Mifflin Company. All rights reserved.14–3 Biochemistry is the study of the chemical substances found in living systems and the chemical interactions of these substances with each other. A biochemical substance is a chemical substance found within a living organism. They are divided into two groups: bioorganic and bioinorganic substances. Bioinorganic substances include water and inorganic salts (substances not containing carbon). Bioorganic substances include carbohydrates, lipids, proteins, and nucleic acids.
Copyright © Houghton Mifflin Company. All rights reserved.14–4 Mass composition data for the human body in terms of major types of biochemical substances. Data for the human body
Copyright © Houghton Mifflin Company. All rights reserved.14–5 THERE ARE FOUR MAIN CLASSES OF BIOMOLECULES Carbohydrates Lipids Proteins Nucleic Acids
Copyright © Houghton Mifflin Company. All rights reserved.14–6 Occurrence and Functions of Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.14–7 Carbohydrates are the most abundant class of bioorganic molecules overall. They constitute about 75% by mass of dried plant materials. Recall that proteins are the most abundant bioorganic molecules in the human body. In green (chlorophyll-containing) plants carbohydrates are produced via photosynthesis. CO 2 + H 2 O + solar energy chlorophyll carbohydrates + O 2 --In the form of cellulose, carbohydrates serve as structural elements. --In the form of starch, carbohydrates provide energy reserves for plants.
Copyright © Houghton Mifflin Company. All rights reserved.14–8 In humans carbohydrates have the following functions: Oxidation of carbohydrates provides energy. Storage of carbohydrates as glycogen provides a Short-term energy reserve. Carbohydrates provide carbon atoms for the synthesis of proteins, lipids, and nucleic acids. Carbohydrates form part of the structural framework of DNA and RNA biomolecules. Carbohydrate markers on cell surfaces play key roles in cell-cell recognition processes.
Copyright © Houghton Mifflin Company. All rights reserved.14–9 General Types of Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.14–10 A carbohydrate is a polyhydroxy aldehyde, a poly- hydroxy ketone, or a compound that yields such a substance upon hydrolysis. The carbohydrate, glucose is a polyhydroxy aldehyde and the carbohydrate, fructose is a polyhydroxy ketone. glucose fructose aldehyde ketone
Copyright © Houghton Mifflin Company. All rights reserved.14–11 CLASSES OF CARBOHYDRATES Monosaccharides Disaccarides Polysaccarides
Copyright © Houghton Mifflin Company. All rights reserved.14–12 A monosaccharide is a carbohydrate that contains a single polyhydroxy aldehyde or polyhydroxy ketone unit such as glucose or fructose. glucose fructose aldehyde ketone
Copyright © Houghton Mifflin Company. All rights reserved.14–13 An oligosaccharide is a carbohydrate that contains from two to ten monosaccharide units covalently bonded to each other. e.g. glucose-glucose-glucose-glucose-glucose The most common type of oligosaccharides are disaccharides. A disaccharide is a carbohydrate that contains two monosaccharide units covalently bonded to each other. Sucrose and lactose are disaccharides. Sucrose = Glucose + Fructose
Copyright © Houghton Mifflin Company. All rights reserved.14–14 A polysaccharide is a carbohydrate that contains many monosaccharide units covalently bonded to each other. They can consist of tens of thousands of monosaccharide units. For example, cellulose and starch are poly- saccharides. Paper, cotton, and wood are primarily cellulose. Starch is a component of bread, pasta, rice, corn, beans, etc.
Copyright © Houghton Mifflin Company. All rights reserved.14–15 Classification of Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–16 Introduction to Carbohydrates Carbohydrates are a large class of naturally occurring polyhydroxy aldehydes and ketones. Monosaccharides also known as simple sugars, are the simplest carbohydrates containing 3-7 carbon atoms. A sugar containing an aldehyde is known as an aldose. A sugar containing a ketone is known as a ketose.
Copyright © Houghton Mifflin Company. All rights reserved.14–17 The family name ending -ose indicates a carbohydrate. A sugar containing an aldehyde and five carbons is called an aldopentose. A sugar containing an aldehyde and six carbons is called an aldohexose. Simple sugars are known by common names such as glucose, ribose, fructose, etc. The term sugar is also associated with sweetness and many mono and disaccharides have a sweet taste. CARBOHYDRATE TERMINOLOGY
Copyright © Houghton Mifflin Company. All rights reserved.14–18 The number of carbon atoms in an aldose or ketose may be specified by tri, tetr, pent, hex, or hept. For example, glucose is an aldohexose and fructose is a ketohexose. Monosaccharides react with each other to form disaccharides and polysaccharides (splitting out water – a condensation reaction). Monosaccharides are chiral molecules and exist mainly in cyclic forms rather than the straight chain.
Copyright © Houghton Mifflin Company. All rights reserved.14–19 An Historical Aside: Most sugars have the suffix -ose Albert Szent-Györgyi (1893-1986) was an Hungarian-American chemist who won the 1937 Nobel Prize in Physiology or Medicine for his isolation of vitamin C (ascorbic acid). He thought it might be a sugar. He isolated the compound, but didn’t know its structure. “Originally I called ascorbic acid “ignose,” not knowing what it was (ignosco meaning, I don’t know). The editor of the Biochemistry Journal reprimanded me for making jokes about science. My proposition “Godnose” was no more successful.” How he came to identify vitamin C: He had tried to isolate it from a number of plant sources “One night my wife gave me paprika for supper and I had no desire to eat it, but had no courage to say no. So I told my wife that instead of eating it I would take it to the laboratory. By midnight I knew that it was a treasure trove of ascorbic acid.”
Copyright © Houghton Mifflin Company. All rights reserved.14–20 Handedness (“Chirality”) in Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–21 Most monosaccharides exist in two forms; i.e. a left-handed and a right-handed form. They are related in the same way that your left hand and right hand are related to each other; i.e. as mirror images. The hands are similar, but not identical because they are not “superimposable.” All objects have mirror images, but in some cases the mirror images are superimposable on the object, and in other cases the mirror image is not superimposable on the object. For example, a flat dinner plate has a superimposable mirror image, but your hands do not.
Copyright © Houghton Mifflin Company. All rights reserved.14–22 The mirror image of the right hand is the left hand. Conversely, the mirror image of the left hand is the right hand. Hands are not superimposable.
Copyright © Houghton Mifflin Company. All rights reserved.14–23 A person’s left and right hands are not superimposable upon each other. The fingers and the thumbs do not match up, and therefore your hands are mirror images of each other, but they are not superimposable.
Copyright © Houghton Mifflin Company. All rights reserved.14–24 A simple example of a molecule that possesses handedness (also called chirality) is the trisubstituted methane molecule, bromochloroiodomethane Br Cl-C-H I Notice that this molecule, just like your hands, is not superimposable upon its mirror image.
Copyright © Houghton Mifflin Company. All rights reserved.14–25 Examples of simple molecules that have nonsuperimposable mirror images. (a) The molecule bromochloroiodomethane. (b) The molecule glyceraldehyde. models
Copyright © Houghton Mifflin Company. All rights reserved.14–26 A carbon atom containing four different groups, as in bromochloroiodomethane and glyceraldehyde, is called a chiral center. A chiral center is a carbon atom in a molecule that has four different groups attached to it. A molecule that contains a chiral center is said to be chiral. A chiral molecule is a molecule whose mirror image is not superimposable upon itself. Most chiral molecules contain carbon atoms that have four different groups attached to them. An achiral molecule is a molecule whose mirror image is superimposable upon itself. Achiral molecules generally do not possess carbon atoms containing four different groups. Each of the two nonsuperimposable forms (the left handed and the right handed forms) are called enantiomers.
Copyright © Houghton Mifflin Company. All rights reserved.14–27 Handedness in Carbohydrates Carbohydrates are chiral molecules since they have carbon atoms carrying four different groups. The simplest three-carbon, naturally occurring carbohydrate, glyceraldehyde, has a chiral carbon atom and exists as a pair of enantiomers – a “right-handed” D form and a “left-handed” L form.
Copyright © Houghton Mifflin Company. All rights reserved.14–28 By convention the D-enantiomer has the OH group on the right hand side of the chiral carbon atom. And therefore by convention the L-enantiomer has the OH group on the left hand side of the chiral carbon atom. D derives from the Latin, dextro = right and L derives from the Latin, levo = left.
Copyright © Houghton Mifflin Company. All rights reserved.14–29 Two forms of glyceraldehyde (D and L) have the same physical properties except they behave differently in the presence of a polarized light. The two forms of glyceraldehyde rotate the plane of a polarized light in opposite directions by the same amount. An instrument known as a polarimeter can be used to measure the degree of rotation of the plane of a polarized light.
Copyright © Houghton Mifflin Company. All rights reserved.14–30 Principles of a polarimeter, used to determine optical activity. A solution of an optically active isomer rotates the plane of the polarized light by a characteristic amount.
Copyright © Houghton Mifflin Company. All rights reserved.14–31 As the number of carbon atoms in a monosaccharide increases, the number of chiral centers present in the molecule increases. For example, glucose has four chiral centers. In general, compounds with n chiral carbon atoms have a maximum of 2 n possible stereoisomers and half that many pairs of enantiomers. Glucose, has four chiral carbon atoms and a total of 2 4 = 16 possible stereoisomers (8 pairs of enantiomers).
Copyright © Houghton Mifflin Company. All rights reserved.14–32 In a multi-chiral-center molecule such as glucose, handedness is determined by the chiral center most distant from the carbonyl group. D-glucose L-glucose DL
Copyright © Houghton Mifflin Company. All rights reserved.14–33 Two pairs of enantiomers: the four isomeric aldotetroses, 2,3,4-trihydroxybutanals. DLDL
Copyright © Houghton Mifflin Company. All rights reserved.14–34 All naturally-occurring carbohydrate molecules are made up of right-handed (D) enantiomers. Many drugs are active only in one of their two chiral forms: Lipitor, the cholesterol-lowering drug, is only active in its left-handed form. Why this is Important: Right-handed and left-handed molecules elicit different biological responses. For example, D-epinephrine (adrenaline) is 20 times more potent in its biological effect than is its L-isomer.
Copyright © Houghton Mifflin Company. All rights reserved.14–35 Thalidomide - A Case in Point This drug comes in two mirror-image forms. It was used in Europe in the 1950s to relieve morning sickness in pregnant women, but was found to cause severe birth defects (deformed arms and legs) in some cases. It was not distributed in the U.S. because an alert inspector, Dr. Francis Kelsey, at the U.S. FDA delayed the drug’s release. Originally it was believed that one enantiomer was effective and the other caused birth defects. Later studies have shown that the body rapidly converts one form into the other (racimizes the drug) so that the given form doesn’t matter. Ironic twist: Recently thalidomide has been found to reduce the immune system’s inflammatory response in a host of illnesses, including arthritis, lupus, cancer, leprosy, and AIDs. Note: A racimic mixture contains both forms of a chiral compound. (The mixture is sometimes called a “racemate.”)
Copyright © Houghton Mifflin Company. All rights reserved.14–36 Historical Note Louis Pasteur first separated two optical isomers (enatiomers) of the sodium ammonium salt of tartaric acid in 1848. He mechanically separated crystals of the d- and l- forms of the compound by hand. Tartaric acid, H 2 C 4 H 4 O 6, is also called 2,3-dihydroxybutanedioic acid: HOOC--CHOH-CHOH-COOH Pasteur was lucky! --Many, if not most, such salts crystallize as racemic crystals, not as separate d- and l- crystals --He worked in the cool Paris climate; above 26° C the salt he worked with itself crystallizes as a racemate. Ref. Kauffman & Myers, J. Chem. Educ. 52, 777-781 (1975)
Copyright © Houghton Mifflin Company. All rights reserved.14–37 The D and L Families of Sugars: Drawing Sugar Molecules Fisher Projection: represent three- dimensional structures of stereoisomers on a flat page. A chiral carbon atom is represented in the Fisher projection as the intersection of two crossed lines. Bonds that point up out of the page are shown as horizontal lines, and bonds that point behind the page are shown as vertical lines, see the following scheme.
Copyright © Houghton Mifflin Company. All rights reserved.14–38
Copyright © Houghton Mifflin Company. All rights reserved.14–39 In a Fisher projection, the aldehyde or ketone carbonyl group of a monosaccharide is always placed at the top.
Copyright © Houghton Mifflin Company. All rights reserved.14–40 Note There is no correlation between the D and L designation and the direction of rotation of the plane of polarized light. The D and L terms relate only to the position of the –OH group on the bottom carbon in a Fisher projection.
Copyright © Houghton Mifflin Company. All rights reserved.14–41 Elephant Pheromones
Copyright © Houghton Mifflin Company. All rights reserved.14–42 Names and Structures of Biochemically Important Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–43 Structure of Glucose and Other Monosaccharides D-Glucose, sometimes called dextrose or blood sugar, is the most widely occurring of all monosaccharides. In nearly all living organisms, D-glucose serves as a source of energy for all biochemical reactions. D-glucose is stored in polymeric form as starch in plants and as glycogen in animals.
Copyright © Houghton Mifflin Company. All rights reserved.14–44 D-glucose a.k.a. dextrose Most dietary carbohydrates are polymers of D-glucose. Upon digestion the D-glucose units are released, which become the major energy source to fuel daily activities.
Copyright © Houghton Mifflin Company. All rights reserved.14–45 D-galactoseD-glucose D-galactose differs from D-glucose in the configuration of only one carbon atom. The chiral carbon atom at C-4 in D-galactose has the opposite chirality from that in D-glucose. D-galactose and D-glucose are isomers because these two aldohexoses have the same molecular formula, C 6 H 12 O 6. This very small difference in chemical structure is enough to give these two molecules very different biological properties.
Copyright © Houghton Mifflin Company. All rights reserved.14–46 D-fructose D-glucose D-galactose D-fructose is also a hexose, but it is not an aldohexose. It is a ketohexose. The only difference in chemical structure between D-glucose and D-fructose is at carbons 1 and 2. All three of these monosaccharides are structural isomers because they have the same molecular formula, C 6 H 12 O 6.
Copyright © Houghton Mifflin Company. All rights reserved.14–47 Chemical Portraits: Three Important Isomeric Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–48 D-glucose D-ribose 2-deoxy-D-ribose D-glucose is an aldohexose, but D-ribose is an aldopentose. D-ribose is a component of RNA. 2- Deoxy-D-ribose is also an important monosaccharide found in DNA. The prefix deoxy indicates that it is the same as D-ribose except that it is missing an oxygen atom at the 2 position.
Copyright © Houghton Mifflin Company. All rights reserved.14–49 Cyclic Forms of Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–50 Up until now the structures of monosaccharides have been depicted as open-chain polyhydroxy aldehydes and ketones. Experimental studies indicate however, that in solution Monosaccharides containing five or more carbons exist predominantly as cyclic structures rather than open-chain structures. The cyclic forms of monosaccharides result from the tendency of the carbonyl group to react with a neighboring hydroxyl group to form a hemiacetal (aldoses) or a hemiketal (ketoses). Open-chain formCyclic hemiacetal form
Copyright © Houghton Mifflin Company. All rights reserved.14–51 Figure 14.7 The cyclic hemiacetal forms of d- glucose result from the intramolecular reaction between the carbonyl group and the hydroxyl group on carbon 5. The cyclic structures can exist in two forms called anomers. alpha = OH and CH 2 OH opposite sides, beta = same side
Copyright © Houghton Mifflin Company. All rights reserved.14–52 Ordinarily, crystalline glucose is entirely in the -form. When dissolved in water, an equilibrium becomes established between the open chain and the two cyclic forms of glucose. The optical rotation of a fresh solution of (or glucose gradually changes from its original value until it reaches a value representing the equilibrium mixture. The equilibrium solution has 37% alpha and 63% beta, with only about 0.01% of the straight chain. Monosaccharides form white crystalline water-soluble solids. Most monosaccharides are sweet-tasting, digestible, and non-toxic
Copyright © Houghton Mifflin Company. All rights reserved.14–53 The structure of D-galactose: Just like D- glucose it can also exist as an open chain hydroxy aldehyde or as a pair of cyclic hemiacetals.
Copyright © Houghton Mifflin Company. All rights reserved.14–54 Anomers: Cyclic sugars that differs only in positions of substituents at the hemiacetal carbon; the -form has the –OH group on the opposite side from the –CH 2 OH; the -form has the –OH group on the same side as the –CH 2 OH group.
Copyright © Houghton Mifflin Company. All rights reserved.14–55 D-fructose D-fructose is a ketohexose and it too can exist in cyclic forms, but it has mostly five membered rings instead of six membered rings.
Copyright © Houghton Mifflin Company. All rights reserved.14–56 D-ribose is an aldopentose and it too cyclizes into a cyclic five membered ring structure. D-ribosebeta-D-ribose
Copyright © Houghton Mifflin Company. All rights reserved.14–57 14.7 Some Important Monosaccharides Monosaccharides are generally high-melting, white, crystalline solids that are soluble in water and insoluble in nonpolar solvents. Most monosaccharides are sweet tasting, digestible, and nontoxic.
Copyright © Houghton Mifflin Company. All rights reserved.14–58 Corrections to the text – Question 12.59, parts (c) and (d) The answers given in the back of the book (Appendix 5) for these questions label the compounds as “chlorides.” They should be called “bromides.” Thanks to M. Crabtrey. Question 14.17. Skip this question, it is not well defined. Thanks to C. Hager Page 399: The compound on the left in the middle of the page is galactose, not glucose. Thanks to G. Bradds Question 15.5. The answers to (a) and (d) in the back of the book are incorrect. They should read “saturated.” Thanks to G. Bradds Question 13.17. The answer in the back of the book omits a –CH 2 - group. Thanks to J. Scarlett.
Copyright © Houghton Mifflin Company. All rights reserved.14–59 14.8 Reactions of Monosaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–60 14.8 Reactions of Monosaccharides Reactions with Oxidizing Agents: Reducing Sugars When an open chain aldehyde form of an aldose monosaccharide, is oxidized, its equilibrium with the cyclic form is displaced. The aldehyde group of the monosaccharide is ultimately oxidized to a carboxylic acid group.
Copyright © Houghton Mifflin Company. All rights reserved.14–61 D-glucoseD- gluconic acid Since the sugar reduces the oxidizing agent, the sugar is called a reducing sugar. A reducing sugar is a carbohydrate that gives a positive test with Tollens reagent (Ag + ) and Benedict’s reagent (Cu ++ ). Tollens and Benedict’s reagents can be used to check for D-glucose in the urine (a symptom of diabetes).
Copyright © Houghton Mifflin Company. All rights reserved.14–62 With Benedict’s reagent the solution remains blue if the urine is normal (contains no D-glucose) and it gives a red precipitate if it contains D-glucose. This is such a common laboratory test that it has been simplified by the use of plastic dip strips. Benedict’s: Cu ++ Cu + red precipitate Tollens: Ag + Ag silver mirror
Copyright © Houghton Mifflin Company. All rights reserved.14–63 The glucose content of urine can be determined by dipping a plastic strip treated with oxidizing agents into the urine sample and comparing the color change of the strip to a color chart that indicates glucose concentration.
Copyright © Houghton Mifflin Company. All rights reserved.14–64 The carbonyl group in monosaccharides can be reduced to an hydroxy group. D-glucoseD- glucitol
Copyright © Houghton Mifflin Company. All rights reserved.14–65 D-glucitol is also known by its common name of sorbitol. It is used in chewing gum because bacteria that cause tooth decay do not use sorbitol as a food source. It is also used in other foods and it is used in cosmetics as a moisturizing agent due to its high affinity for water. Sorbitol accumulation in the eye is a major factor in the formation of cataracts due to diabetes. D-glucose D- glucitol
Copyright © Houghton Mifflin Company. All rights reserved.14–66 --Momosaccharides can be linked to each other through condensation reactions, i.e., reactions that split out water molecules and join parts. --In disaccharides and polysaccharides, monosaccharides are connected to each other through glycosidic bonds. --Hydrolysis is the reverse of a condensation reaction; digestion of carbohydrates involves hydrolysis. CONDENSATION REACTIONS
Copyright © Houghton Mifflin Company. All rights reserved.14–67 --The –OH group of a sugar can add a –PO 3 H 2 group to form phosphate esters. --Phosphate esters of monosaccharides appear as reactants and products throughout the metabolism of carbohydrates. alpha-D-glucose-1-phosphatealpha-D-glucose-6-phosphate PHOSPHATE ESTERS
Copyright © Houghton Mifflin Company. All rights reserved.14–68 Disaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–69 Disaccharides are made up of two mono- saccharides. For example, sucrose, table sugar, is a disaccharide made up of one glucose and one fructose. Sucrose = D-glucose-D-fructose Lactose = D-galactose-D-glucose Maltose = D-glucose-D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.14–70 Most fruits and fresh vegetables contain mono and disaccharides. The glycosidic link is formed as a condensation reaction between two –OH groups: R-OH + HO-R’ R-O-R’ + H 2 O
Copyright © Houghton Mifflin Company. All rights reserved.14–71 Two monosaccharides can be coupled to form a disaccharide. They are connected by a glycosidic linkage. The glycosidic linkage is the carbon- oxygen-carbon bond that joins the two components of a glycoside together. + H 2 O
Copyright © Houghton Mifflin Company. All rights reserved.14–72 The three naturally occurring common disaccharides are: --Maltose: Two a-glucoses are joined by an a-1,4-link. maltose = a disaccharide maltose = D-glucose-D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.14–73 The three forms of maltose present in aqueous solution.
Copyright © Houghton Mifflin Company. All rights reserved.14–74 Lactose, also known as milk sugar: It is the major carbohydrate found in mammalian milk. Two monosaccharides are joined by a beta-1,4-link. The enzyme lactase can hydrolyze lactose to form an equal mixture of D-galactose and D-glucose. Some adults lack lactase and are “lactose intolerant.” lactose = a disaccharide Lactose = D-galactose-D-glucose
Copyright © Houghton Mifflin Company. All rights reserved.14–75 Sucrose, table sugar: Sugar beets and sugar cane are the most common sources of sucrose. One molecule of D-fructose and one molecule of D-glucose are joined together by a 1,2-link between the anomeric carbons. sucrose = a non-reducing sugar sucrose = D-glucose-D-fructose The enzyme sucrase is present in the human body to break down sucrose into equal amounts of glucose and fructose.
Copyright © Houghton Mifflin Company. All rights reserved.14–76 Chemical Portraits: Biologically Imported Disaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–77 Polysaccharides
Copyright © Houghton Mifflin Company. All rights reserved.14–78 Some Important Polysaccharides Polysaccharides are polymers of many monosaccharides linked together through glycosidic bonds. The three most important polysaccharides are cellulose, starch, and glycogen. Cellulose is a fibrous substance that provides structure in plants. It consists entirely of several thousand β-units joined together in a long straight chain by β -1,4- links (it is unbranched).
Copyright © Houghton Mifflin Company. All rights reserved.14–79 IT’S IN THE BONDS! Cotton is almost pure cellulose and wood is about 50% cellulose. Even though cellulose is a polymer of glucose units attached in a long chain, it is not nutritional for humans because we cannot hydrolyze the beta (1 4) linkage. We lack the proper enzymes. Horses, cows, and sheep contain bacteria in their digestive systems that have special enzymes (cellulases) and therefore can digest cellulose. In this way termites can use wood as a source of glucose for nutritional needs.
Copyright © Houghton Mifflin Company. All rights reserved.14–80 The structures of cellulose (a) and chitin (b). In both substances, all glycosidic linkages are of the (1 4) type. These polymers are made of long linear, unbranched chains.
Copyright © Houghton Mifflin Company. All rights reserved.14–81 Starch --Starch, like cellulose, is a polymer of glucose. --Starch is fully digestible and is an essential part of the human diet. --In starch, the glucose units are joined by a-1,4-links. (We can digest these.)
Copyright © Houghton Mifflin Company. All rights reserved.14–82 --Two different glucose polysaccharides can be isolated from most starches: amylose and amylopectin. --Amylose is a straight chain glucose polymer and usually accounts for 15-20% of the starch. --Amylopectin is a highly branched glucose polymer and accounts for about 80-85% of the starch. --The glycosidic linkages in amylose are alpha 1 4, but in amylopectin they are alpha 1 4 and alpha 1 6. --These alpha linkages can be broken down in the human digestive tract by the enzyme amylase. Starches contained in potatoes, rice, corn, wheat, etc. are major food sources. Amylose and Amylopectin
Copyright © Houghton Mifflin Company. All rights reserved.14–83 Some polysaccharides have a linear chain structure (a); others have a branched chain structure (b). amylose amylopectin
Copyright © Houghton Mifflin Company. All rights reserved.14–84 Two perspectives on the structure of the polysaccharide amylopectin. (a) Molecular structure of amylopectin. (b) An overview of the branching that occurs in the amylopectin structure. Each circle is a glucose unit.
Copyright © Houghton Mifflin Company. All rights reserved.14–85 --Glycogen, also called animal starch, serves as the energy storage role that starch serves in plants. Some of the glucose from starches we eat is used immediately as fuel, and some is stored as glycogen for later use. It is stored in muscle cells and in liver cells. --Glycogen has a structure similar to amylopectin in that it contains both alpha (1 4) and alpha (1 6) glycosidic linkages. Glycogen is about three times more highly branched than amylopectin and it has a much larger molecular weight. --When excess glucose is present in the blood from eating too much starch, it is converted to glycogen and stored for future use. When needed, it is converted back to glucose. GLYCOGEN
Copyright © Houghton Mifflin Company. All rights reserved.14–86 Variations on the Carbohydrate Theme --Monosaccharides with modified functional groups are components of a wide variety of biomolecules. --Short chains of monosaccharides, known as oligosaccharides, enhance the function of proteins and lipids to which they are bonded. --In some cases, oligosaccharides form recognition sites on cell surfaces.
Copyright © Houghton Mifflin Company. All rights reserved.14–87 EXAMPLES --Chitin: the hard shells of lobsters, beetles, and spiders. --Heparin: an agent that prevents or retards the clotting of blood. --Glycoproteins: perform important function at the cell surface. They can function as receptors for molecular messengers or drugs. They are also responsible for the familiar A, B, O system of typing blood.
Copyright © Houghton Mifflin Company. All rights reserved.14–88 Chemistry at a Glance: Biochemically Important Carbohydrates
Copyright © Houghton Mifflin Company. All rights reserved.14–89 Chapter Summary Monosaccharides are compounds with 3 to 7 carbon atoms, an aldehyde group on carbon 1 or a ketone on carbon 2, and hydroxyl group on all other carbon atom. Monosaccharides are chiral molecules. A monosaccharide with n chiral carbon atom may have 2 n stereoisomers, half that number of pair of enantiomers. Fisher projection formula represent open-chain structures of monosaccharides.
Copyright © Houghton Mifflin Company. All rights reserved.14–90 In solution, open-chain monosaccharides with five or six carbons establish equilibria with cyclic forms that are hemiacetals. The hemiacetal carbon is refereed to as the anomeric carbon. Oxidation of a monosaccharide can results in a carboxyl group on the carbon 1. Reaction with an alcohol converts the –OH group on the anomeric carbon to a –OR group through a bond known as glycosidic bond. Chapter Summary Continued
Copyright © Houghton Mifflin Company. All rights reserved.14–91 Disaccharides result from glycosidic bond formation between two monosaccharides. Polysaccharides result from glycosidic bond formation between many monosaccharides. Chitin is a hard structural polysaccharide found in the shells of lobster and insects. Heparin is a polysaccharide that plays a role in blood clotting. Glycoproteins have short carbohydrate chains bonded to proteins; the carbohydrate segments function as receptors at cell surfaces. Chapter Summary Continued
Copyright © Houghton Mifflin Company. All rights reserved.14–92 Cellulose is a fibrous substance that provides structure in plants. They consist entirely of several thousand b-units joined together in a long straight chain by b-1,4-links. Starch, like cellulose, is a polymer of glucose. Starch is fully digestible and is an essential part of the human diet. In starch, glucose units are joined by a-1,4-links. Glycogen, also called animal starch, serves as the energy storage role as starch serves in plants. Chapter Summary Continued
Copyright © Houghton Mifflin Company. All rights reserved.14–93 What you absolutely must know from this chapter: Carbohydrates are the most abundant bioorganic compounds in the living world. They are formed by photosynthesis: 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 (light) You should know what roles carbohydrates play in plants and animals. There are five important monosaccarides (glucose, galactose, fructose, ribose, and deoxyribose). Know whether each is an aldose or a ketose, and how many carbon atoms each has. Know what “chirality” and “optical activity” refer to, and how these terms apply to sugars. Know what a “chiral center” is. Understand why chirality is important in biochemistry and for the chemistry of life in general.
Copyright © Houghton Mifflin Company. All rights reserved.14–94 Understand that in water monosaccarides exist predominantly in cyclic forms, rather than the linear forms pictured early in the chapter. Appreciate that aldose sugars can be oxidized to carboxylic acids (in the same way that aldehydes react in general - see chapter 13). Understand that monosaccharides can be joined together by condensation reactions between their OH groups, splitting out water: R-OH + HO-R’ R-O-R’ +H 2 O The links are called or glycosidic bonds, depending on their geometry. Know the three most important disaccharides: sucrose = (glucose)-(fructose) lactose = (galactose)-(glucose) maltose = (glucose)-(glucose)
Copyright © Houghton Mifflin Company. All rights reserved.14–95 Understand that the glycosidic bonds between these units can be broken by hydrolysis--addition of a water molecule. Note that this is the reverse of a condensation reaction. Note the following: --Sucrose is common table sugar. --Fructose is the most common sweetening agent in colas, etc., because it is sweeter than sucrose and cheaper. --Many people become “lactose intolerant” as they age because they lose the ability to produce the enzyme lactase, which digests lactose. --Blood glucose levels are regulated by the hormone insulin. Appreciate that polysaccharides are polymers composed of chains (possibly branched) of monosaccharides (their monomer units). Know the distinctive characteristics of the principal polysaccharides: cellulose, starch, glycogen, and chitin, and where they are found.
Copyright © Houghton Mifflin Company. All rights reserved.14–96 Understand why we can’t digest cellulose (or chitin, for that matter) and how grazing animals can digest it by using bacteria in their guts. Understand the differences between amylose and amylopectin, the two forms of polysaccharides found in starch.
Copyright © Houghton Mifflin Company. All rights reserved.14–97 To Do List Read chapter 14!! Do additional problems Review Lecture notes for Chapter Fourteen Do practice test T/F Do practice test MC
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