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 C 2, 2, and energy from the sun are oxidized in living cells to produce C 2, 2, 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 ( ) groups C C C aldose C 2 Erythose, an aldotetrose (4 carbons, aldehyde group) 6
Ketoses Ketoses are monosaccharides with a ketone group many hydroxyl ( ) groups Fructose, a ketohexose (6 carbons, ketone group) C 2 C= C C C C 2 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 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. C C 2 C C 2 -Ribose -Threose -Fructose C 2 C 2 11
Solution Identify each as the D or L isomer. A. B. C. C C 2 C C 2 C 2 C 2 L-Ribose L-Threose D-Fructose 12
D-glucose is Glucose and Fructose found in fruits, corn syrup, and honey an aldohexose with the formula C 6 12 6 known as dextrose and blood sugar in the body D-fructose is obtained from lactose an aldohexose with the formula C 6 12 6 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, C 6 12 6 is not found free in nature is obtained from lactose, a disaccharide has a similar structure to glucose except for the on carbon 4 In a condition called galactosemia, an enzyme needed to convert galactose to glucose is missing. C C 2 D-Galactose 15
Cyclic Structures Cyclic structures are the prevalent form of monosaccharides with five or six carbon atoms form when the hydroxyl group on carbon 5 reacts with the aldehyde group or ketone group
Guide for Drawing ayworth 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 to carbon 1. Place the carbon 6 group above the ring. Write the groups on carbon 2 and 4 below the ring. Write the group on carbon 3 above the ring. Write a new on carbon 1. 19
Drawing the Cyclic Structure for Glucose Step 3 Write the new 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 2 C 2 C C 2 -D-Glucose D-Glucose (open) β-d-glucose (36%) (trace) (64%)
Cyclic Structure of Fructose Fructose is a ketohexose forms a cyclic structure reacts the on carbon 5 with the C= on carbon 2 C 2 C C 2 C 2 C 2 C C C 2 C C 2 D-Fructose α-d-fructose -D-Fructose 22
Learning Check Write the cyclic forms of D-galactose. C C 2 D-Galactose 23
Solution Step 1 Number the carbon chain and turn clockwise to form a linear open chain. 1 2 3 4 5 6 C C 2 D-Galactose 2 C C 6 5 4 3 2 1 24
Solution Step 2 Fold clockwise to make a hexagon. Bond the carbon 5 to carbon 1 and change the = to on carbon 1. Place the carbon 6 group above the ring. Write the groups on carbon 2 and carbon 4 below the ring and the group on carbon 3 above the ring. 4 6 C 2 5 3 2 C 1 25
Solution Step 3 Write the new on carbon 1 down for the form or up for the form. 2 C 2 C -D-Galactose -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
xidation 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. C C 2 D-Mannose 31
Solution Write the products of the oxidation and reduction of D-mannose. C 2 Reduction C 2 C C 2 xidation C C 2 D-Mannitol D-Mannose D-Mannonic acid 32
Important Disaccharides A disaccharide consists of two monosaccharides. Monosaccharides glucose + glucose glucose + galactose glucose + fructose Disaccharide maltose + 2 lactose + 2 sucrose + 2 33
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 on carbon 1 of the first glucose and 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-1,2- glycosidic 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 + 2 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 C 2 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
47 Concept Map for Carbohydrates