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Chapter 13 Carbohydrates 1 Carbohydrates Carbohydrates are • a major source of energy from our diet • made from elements carbon, hydrogen, and oxygen • also called saccharides, which means “sugars” Carbohydrates • are produced by photosynthesis in plants • such as glucose are synthesized in plants from CO2, H2O, and energy from the sun • are oxidized in living cells to produce CO2, H2O, and energy 2 Types of Carbohydrates The types of carbohydrates are • monosaccharides, the simplest carbohydrates • disaccharides, which consist of two monosaccharides • polysaccharides, which contain many monosaccharides 3 Monosaccharides Monosaccharides • consist of three to eight carbon chains with one carbon in a carbonyl group • are further classified as an aldose, which contains an aldehyde group, or a ketose, which contains a ketone carbonyl group • have several hydroxyl groups 4 Types of Monosaccharides Monosaccharides are also classified by the number of carbon atoms present. triose (3 C atoms) tetrose (4 C atoms) pentose (5 C atoms) hexose (6 C atoms) An aldopentose indicates a five-carbon saccharide with an aldehyde group. 5 Aldoses Aldoses are monosaccharides with • an aldehyde group • many hydroxyl (OH) groups O ║ C─H aldose HCOH HCOH CH2OH Erythose, an aldotetrose (4 carbons, aldehyde group) 6 Ketoses Ketoses are monosaccharides with • a ketone group • many hydroxyl (OH) groups CH2OH C=O HCOH HCOH HCOH Fructose, a ketohexose (6 carbons, ketone group) CH2OH ketose Fischer Projections A Fischer projection • is used to represent carbohydrates • places the most oxidized group at the top • shows chiral carbons as the intersection of vertical and horizontal lines 8 D and L Notations In a Fischer projection, the OH group on the • chiral carbon farthest from the carbonyl group determines an L or D isomer. • left is assigned the letter L for the L form • right is assigned the letter D for the D form 9 Examples of D and L Isomers of Monosaccharides 10 Learning Check Identify each as the D or L isomer. A. B. C. CH2OH O C H HO H HO H H HO H HO CH2OH __-Ribose O O C H HO H OH H OH H H OH CH2OH __-Threose CH2OH __-Fructose 11 Solution Identify each as the D or L isomer. A. B. C. CH2OH O C H HO H HO H H HO H HO CH2OH L-Ribose O O C H HO H OH H OH H H OH CH2OH L-Threose CH2OH D-Fructose 12 Glucose and Fructose D-glucose is • found in fruits, corn syrup, and honey • an aldohexose with the formula C6H12O6 • known as dextrose and blood sugar in the body D-fructose is • obtained from lactose • an aldohexose with the formula C6H12O6 • important in cell membranes of the brain and nerve cells The sweet taste of honey comes from the monosaccharides D-glucose and D-fructose. Blood Glucose Level In the body, • glucose has a normal blood level of 70–90 mg/dL • a glucose tolerance test measures blood glucose for several hours after ingesting glucose D-Galactose D-Galactose • • • • is an aldohexose, C6H12O6 is not found free in nature is obtained from lactose, a disaccharide has a similar structure to glucose except for the OH on carbon 4 In a condition called galactosemia, an enzyme needed to convert galactose to glucose is missing. O C H H OH HO H HO H H OH CH2OH D-Galactose 15 Cyclic Structures Cyclic structures • are the prevalent form of monosaccharides with five or six carbon atoms O O • form when the hydroxyl group on carbon 5 reacts with the aldehyde group or ketone group Guide for Drawing Hayworth Structures 17 Drawing the Cyclic Structure for Glucose Step 1 Number the carbon chain and turn clockwise to form a linear open chain. 18 Drawing the Cyclic Structure for Glucose Step 2 Fold clockwise to make a hexagon. • Bond the carbon 5 –O– to carbon 1. • Place the carbon 6 group above the ring. • Write the –OH groups on carbon 2 and 4 below the ring. • Write the –OH group on carbon 3 above the ring. • Write a new –OH on carbon 1. 19 Drawing the Cyclic Structure for Glucose Step 3 Write the new –OH on carbon 1 down for the form and up for the form. 20 Mutarotation of -D-Glucose and β-D-Glucose in Solution When placed in solution, • cyclic structures open and close • -D-glucose converts to β-D-glucose and vice versa • at any time, only a small amount of open chain forms C H 2O H C H 2O H O OH OH OH OH -D-Glucose (36%) H O OH O O C OH OH D-Glucose (open) (trace) C H2O H H OH OH β-D-Glucose OH (64%) OH Cyclic Structure of Fructose Fructose • is a ketohexose • forms a cyclic structure • reacts the —OH on carbon 5 with the C=O on carbon 2 C H 2O H C O HO C H H C OH H C OH C H 2O H C H 2O H O C H 2O H OH OH O OH OH OH C H 2O H OH C H 2O H D-Fructose α-D-Fructose -D-Fructose 22 Learning Check Write the cyclic forms of D-galactose. O C H H OH HO H HO H H OH CH2OH D-Galactose 23 Solution Step 1 Number the carbon chain and turn clockwise to form a O linear open chain. 1 1C H HO HO H 22 3 4 5 H OH H H H OH OH H C HOH2C 6 O 5 4 OH H 33 H 2 OH 1 H OH 6 CH2OH D-Galactose 24 Solution Step 2 Fold clockwise to make a hexagon. • Bond the carbon 5 O to carbon 1 and change the =O to OH on carbon 1. • Place the carbon 6 group above the ring. CH2OH • Write the OH groups on carbon 2 and 6 carbon 4 below the ring and the OH group on carbon 3 above the ring. OH 4 H 5 OH H OH H 3 O C 1 H 2 H HO 25 Solution Step 3 Write the new –OH on carbon 1 down for the form or up for the form. H2C OH O OH OH H2C OH O OH H OH OH H OH -D-Galactose OH OH -D-Galactose Reducing Sugars Reducing sugars • are monosaccharides that can be oxidized to give a carboxylic acid • contain an aldehyde group in the open chain form • undergo reaction in the Benedict’s test • include the monosaccharides glucose, galactose, and fructose 27 Oxidation of D-Glucose 28 Conversion of Fructose to Glucose • Fructose, a ketohexose, is also a reducing sugar. • In Benedict’s solution, which is basic, a rearrangement occurs between the ketone group on carbon 2 and the hydroxyl group on carbon 1. • This produces an aldehyde group with an adjacent hydroxyl, thereby converting fructose to glucose. 29 Reduction of Monosaccharides The reduction of monosaccharides • produces sugar alcohols, or alditols • involves the reduction of the carbonyl group to an alcohol • such as D-glucose gives D-glucitol, also called sorbitol 30 Learning Check Write the products of the oxidation and reduction of D-mannose. O C H HO H HO H H OH H OH CH2OH D-Mannose 31 Solution Write the products of the oxidation and reduction of D-mannose. CH2OH HO H HO H Reduction O O C H C OH HO H HO H Oxidation HO H HO H H OH H OH H OH H OH H OH H OH CH2OH D-Mannitol CH2OH D-Mannose CH2OH D-Mannonic acid 32 Important Disaccharides A disaccharide consists of two monosaccharides. Monosaccharides 33 Disaccharide glucose + glucose maltose + H2O glucose + galactose lactose + H2O glucose + fructose sucrose + H2O Maltose Maltose is • a disaccharide also known as malt sugar • composed of two D-glucose molecules • obtained from the hydrolysis of starch • used in cereals, candies, and brewing • found in both the and β forms Maltose is linked by an -1,4-glycosidic bond formed from the OH on carbon 1 of the first glucose and OH on carbon 4 of the second glucose. 34 Lactose Lactose • is found in milk and milk products • is a disaccharide of β-D-galactose and α- or β-D-glucose • contains a β-1,4-glycosidic bond 35 Sucrose Sucrose, or table sugar, • is obtained from sugar cane and sugar beets • consists of α-D-glucose and β-D-fructose • has an α,β-1,2glycosidic bond 36 Learning Check For the following disaccharide, list the two monosaccharide units and the type of glycosidic bond present. 37 Solution For the following disaccharide, provide the reaction for two monosaccharide units and the type of glycosidic bond present. galactose + glucose melibiose + H2O The sugar melibiose is formed from two monosaccharides, galactose and glucose, which are joined by a -1,6-glycosidic bond. 38 Sweetness of Sweeteners Sugars and artificial sweeteners • differ in sweetness • are compared to sucrose (table sugar), which is assigned a value of 100 39 Artificial Sweeteners - Sucralose Sucrolose is made from sucrose by replacing some of the hydroxyl groups with chlorine atoms. Sucralose is the sweetening agent in Splenda. Artificial Sweeteners - Aspartame Aspartame is • marketed as Nutra-Sweet • a noncarbohydrate sweetener made from aspartic acid and a methyl ester of the amino acid phenylalanine 41 Polysaccharides Polysaccharides • are polymers of D-glucose • include amylose and amylopectin, starches made of α-D-glucose • include glycogen (animal starch in muscle), which is made of α-D-glucose • include cellulose (plants and wood), which is made of β-D-glucose CH2OH O OH OH OH OH α-D-Glucose 42 Polysaccharide: Amylose Amylose is • a straight-chain polysaccharide made of glucose molecules • linked by -1,4 glycosidic bonds Polysaccharide: Amylopectin Amylopectin, found in plants, • is a polymer of α-D-glucose molecules • is a branched-chain polysaccharide • has α-1,4-glycosidic bonds between the glucose units • has α-1,6 bonds to branches 44 Glycogen Glycogen, or animal starch, • is the polysaccharide that stores α-D-glucose in muscle • is similar to amylopectin, but is more highly branched • the glucose units are joined by α-1,4-glycosidic bonds, and branches occurring about every 10–15 glucose units are attached by α-1,6-glycosidic bonds 45 Cellulose Cellulose, the major structural unit of wood, • is a polysaccharide of glucose units in unbranched chains • has β-1,4-glycosidic bonds • cannot be digested by humans because humans cannot break down β-1,4-glycosidic bonds 46 Concept Map for Carbohydrates 47