Chapter 27 Carbohydrates

Transcription

1 Chapter 27 Carbohydrates Green plants turn 2 O, CO 2, and sunlight into carbohydrates. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris ein, Scott Pattison, and Susan Arena

2 Course Outline 27.1 Carbohydrates: A First Class of Biochemicals 27.2 Classification of Carbohydrates 27.3 Importance of Carbohydrates for Life 27.4 Monosaccharides 27.5 Structure of Glucose and Other Aldoses 27.6 Cyclic Structure of Glucose; Mutarotation 27.7 emiacetals and Acetals 27.8 Structures of Galactose and Fructose 2

3 Course Outline 27.9 Pentoses Disaccharides Structures and Properties of Disaccharides Sweeteners and Diet Redox Reactions of Monosaccharides Polysaccharides Derived from Glucose Complex Polysaccharides Chapter 27 Summary 3

4 Carbohydrates: A First Class of Biochemicals Carbohydrates are a principal class of energyyielding nutrients that are in great abundance. The other two classes are fats and proteins. Carbohydrates are polyhydroxy aldehydes or ketones (aldehydes and ketones with many hydroxyl groups). The simplest carbohydrates are glyceraldehyde and dihydroxyacetone. 4

5 Carbohydrates: A First Class of Biochemicals Carbohydrates are important in society because they provide: (1) basic diets in the form of starch and sugar and (2) clothing and shelter. Many of the chemical properties of carbohydrates are determined by the chemistry of the hydroxyl and carbonyl functional groups 5

6 Classification of Carbohydrates The four major types of carbohydrates are: 1) Monosaccharides 2) Disaccharides 3) Oligosaccharides 4) Polysaccharides These classifications are based on the units (monosaccharides) in the molecules. 6

7 Classification of Carbohydrates A monosaccharide is the smallest unit of a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate unit. It is the basic carbohydrate unit of cellular metabolism. Glucose is a monosaccharide. Monosaccharides like glucose are important sources of cellular energy. 7

8 Classification of Carbohydrates A disaccharide yields two monosaccharides either the same or different when hydrolyzed. Disaccharides are often used by plants and animals to transport monosaccharides from one cell to another. 8

9 Classification of Carbohydrates The monosaccharides and disaccharides generally have names ending in ose like glucose, sucrose, and lactose. Monosaccharides and disaccharides are water-soluble carbohydrates, have a characteristically sweet taste, and are often called sugars. 9

10 Classification of Carbohydrates An oligosaccharide is a carbohydrate with at least two but not more than six monosaccharide units linked together. A polysaccharide is a macromolecular substance that can be hydrolyzed to yield many monosaccharide units. 10

11 Classification of Carbohydrates Polysaccharides are important structural materials in plants and animals. These carbohydrates also serve as a storage depot for monosaccharides which cells use for energy. 11

12 Classification of Carbohydrates Carbohydrates can also be classified by the: a) Number of carbon atoms in the molecule b) Functional group present in the molecule c) Spatial orientation of the molecule d) Optical activity of the molecule 12

13 Classification of Carbohydrates The monosaccharides shown below are classified based on the number of carbon atoms in the molecules. Monosaccharides commonly have three to seven carbon atoms. 13

14 Classification of Carbohydrates Mononosaccharides can also be classified based on whether they have the aldehyde or ketone functional group. Monosaccharides with a CO (aldehyde) group are known as aldoses while those with a C=O (ketone) group are known as ketoses. The ketone group is usually on carbon #2. 14

15 Classification of Carbohydrates Monosaccharides can be classified based on their spatial orientation (stereochemistry). A monosaccharide can be classified as a D or L isomer, depending on the spatial orientation of the and groups attached to the carbon atom adjacent to the terminal primary alcohol group. The D isomer is represented when the is written to the right of this carbon in the Fischer projection formula. The L isomer is represented when this is 15 written to the left.

16 Classification of Carbohydrates 16

17 Classification of Carbohydrates The letters D and L do not refer to the direction of optical rotation of a carbohydrate. Monosaccharides that rotate plane-polarized light to the right are known as (+) isomers while those that rotate light to the left are ( ) isomers. 17

18 Classification of Carbohydrates The D and L forms of any specific compound are enantiomers. D-glucose and L-glucose are enantiomers. Notice that orientation of the hydroxyl groups on the carbon atoms adjacent to the terminal primary alcohol groups. 18

19 Your Turn! Write the projection formula for a D-aldopentose. 19

20 Your Turn! This is one example of a D-aldopentose. This molecule is a D-isomer because of the orientation of the hydroxyl group (see arrow). The molecule is an aldose because it is an aldehyde and a pentose because it contains five carbon atoms. C O C O C C D-configuration C 2 20

21 Your Turn! Classify the following as D- or L-monosaccharides and as aldoses or ketoses. CO C 2 C 2 C C O C O C C O C C O C O C C O C C C 2 C 2 C 2 21

22 Your Turn! Classify the following as D- or L-monosaccharides and as aldoses or ketoses. CO C 2 C 2 C C O C O C C O C C O C O C C O C C C 2 C 2 C 2 D-aldose L-ketose D-ketose 22

23 Importance of Carbohydrates Carbohydrates are the most abundant organic chemical in nature. Why are they so important in biochemistry? They are used by essentially all cells as an energy source. They are easily transported between and within cells. They provide essential structural support for both plant and animal cells. 23

24 Monosaccharides The most important monosaccharides are pentoses and hexoses as seen on the next slide... 24

25 Monosaccharides 25

26 Monosaccharides Glucose is the most important of the monosaccharides. It is an aldohexose and is found in the free state in plant and animal tissue. Glucose is also known as dextrose and grape sugar. Glucose is a component of the disaccharides sucrose, maltose, and lactose and is the monomer in the polysaccharides amylose, amylopectin, cellulose, and glycogen. Glucose is the key sugar of the body and is carried by the 26 bloodstream to all body parts.

27 Monosaccharides Galactose is an aldohexose like glucose and occurs, along with glucose, in lactose and in many oligo- and polysaccharides. Galactose is synthesized in the mammary glands to make the disaccharide lactose (milk sugar). It is also a constituent of glycolipids and glycoproteins in many cell membranes. 27

28 Monosaccharides Fructose, also known as levulose, it is a ketohexose that occurs in fruit juices, honey, and, along with glucose, is a constituent of the disaccharide sucrose. Fructose is the major constituent of the polysaccharide inulin, a starchlike substance present in many plants such as dahlia tubers, chicory roots, and Jerusalem artichokes. Fructose is the sweetest of all the common sugars, being about twice as sweet as glucose. This accounts for the sweetness of high-fructose corn syrup and honey. 28

29 Monosaccharides The Fischer projections of D-glucose, D-galactose and D- fructose are shown here. 29

30 Monosaccharides The Fischer projections of the pentoses D-ribose and D-2- deoxyribose are shown here. 30

31 Structure of Glucose and Other Aldoses The structures of monosaccharides are frequently drawn as Fischer projections. These drawings have the following characteristics. The keto or aldehyde group is placed at the top of the projection. Each interior carbon atom is generally shown as an intersection point between two lines. The atom and group attached to interior carbons are written to left or right of the projection. 31

32 Structure of Glucose and Other Aldoses The simplest carbohydrate is glyceraldehyde. The Fischer projections of D-glyceraldehyde is shown here. 32

33 Structure of Glucose and Other Aldoses This is the Fischer projections of L-glyceraldehyde. 33

34 Structure of Glucose and Other Aldoses The enantiomers of glyceraldehyde are also known as epimers. Epimers are stereoisomers that differ only in the configuration at a single chiral carbon atom. In the case of glyceraldehyde the chiral carbon atom is #2. 34

35 Structure of Glucose and Other Aldoses The enantiomers of glucose can also be represented by Fischer projection formulas. 35

36 Structure of Glucose and Other Aldoses Glucose is an aldohexose with four chiral carbon atoms. According to the 2 n formula glucose should have 16 optical isomers. Eight have the D-configuration and eight have the L- configuration. Remember that D- and L- isomers are enantiomers. The D-isomers of the aldoses (with 3-6 carbon atoms) are shown on Figure 27.1 on the following slide... 36

37 37

38 Your Turn! Draw the enantiomers of D-allose and D-talose using Figure 27.1 on the previous slide. 38

39 Your Turn! Draw the enantiomers of D-allose and D-talose using Figure 27.1 on the previous slide. CO The enantiomer of D- allose is L-allose. These molecules are mirror images. O O O CO O C 2 C 2 D-allose 39 L-allose

40 Your Turn! Draw the enantiomers of D-allose and D-talose using Figure 27.1 on the previous slide. The enantiomer of D- talose is L-talose. These molecules are mirror images. O O O CO CO O C 2 C 2 D-talose 40 L-talose

41 Structure of Glucose and Other Aldoses The 16 aldohexoses have all been synthesized, but only D- glucose, D-mannose, and D-galactose appear to be of considerable biological importance. The laboratory conversion of one aldose into another aldose containing one more carbon atom is known as the Kiliani Fischer synthesis. 41

42 Structure of Glucose and Other Aldoses The Kiliani Fischer synthesis involves: the addition of CN to form a cyanohydrin hydrolysis of the CN group to CO; and reduction with sodium amalgam, Na(g), to form the aldehyde. The synthesis of two aldotetroses from an aldotriose is shown on the following slide... 42

43 Structure of Glucose and Other Aldoses 43

44 Your Turn! Show the conversion of D-threose (a tetrose) to two D- pentoses using the Kiliani Fischer synthesis. O C O C 2 D-threose 44

45 Your Turn! CN CO C O O C O C 2 D-threose O O C 2 CN O O C 2 CO O O C 2 D-xylose C O O O O C 2 C 2 C 2 D-lyxose 45

46 Cyclic Structure of Glucose; Mutatoration Straight open-chain D-glucose is so reactive that almost all molecules quickly rearrange their bonds to form two new structures. These structures are six-membered rings called pyranose sugars. 46

47 Cyclic Structure of Glucose; Mutatoration An interesting phenomenon occurs when the α- and β- forms of glucose are put into separate solutions and allowed to stand for several hours. The phenomena that occurs is called mutarotation. 47

48 Cyclic Structure of Glucose; Mutatoration During mutarotation the two cylic forms convert into each other through the open-chain form. The resulting equilibrium solution contains about 36% α and 64% β molecules with a trace of open-chain molecules. 48

49 Cyclic Structure of Glucose; Mutatoration The cyclic forms only differ in the stereo arrangement of the carbon atom involved in the mutarotation (carbon #1). This carbon atom is called the anomeric carbon. 49

50 Cyclic Structure of Glucose; Mutatoration Two cyclic isomers that differ only in their stereo arrangement about the carbon involved in mutarotation are called anomers. The α- and β- forms are anomers. 50

51 Cyclic Structure of Glucose; Mutatoration In α-anomers the hydroxyl group on the anomeric carbon is below the plane of the molecule. In the β-anomers the hydroxyl group on the anomeric carbon is above the plane. 51

52 Cyclic Structure of Glucose; Mutatoration The cyclic forms of D-glucose may be represented by either Fischer projection formulas or by aworth perspective formulas... 52

53 Carbohydrates aworth Structures of Monosaccharides C 2 O 53

54 Cyclic Structures Cyclic structures are the prevalent form of monosaccharides with 5 or 6 carbon atoms. O O form when the hydroxyl group on carbon 5 reacts with the aldehyde group or ketone group. 54

55 Drawing the Cyclic Structure for Glucose STEP 1: Number the carbon chain and turn clockwise to form a linear open chain. O C C C C C O C 2 OC 2 C C C C C O 55

56 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 groups on carbon 2 and carbon 4 below the ring. Write the group on carbon 3 above the ring. Write a new on carbon 1. C O

57 Cyclic Structure for Glucose (continued) STEP 3: Write the new on carbon 1 down for the form. up for the form. C 2 O C 2 O -D-Glucose -D-Glucose 57

58 Summary of the Formation of Cyclic Glucose 58

59 Cyclic Structure of Glucose; Mutatoration In the cyclic Fischer projection formulas of the D-aldoses, the α-form has the on the anomeric carbon written on the right; in the β-form, the on the anomeric carbon is on the left. 59

60 Cyclic Structure of Glucose; Mutatoration The aworth formula represents the molecule as a flat hexagon with the and groups above and below the plane of the hexagon. 60

61 Your Turn! Write the abbreviated pyranose aworth perspective formulas and names for the two anomers of D-galactose. CO O O C 2 D-galactose 61

62 Your Turn! Number the open-chain, draw the aworth pyranose ring, and add the groups to all the ring carbons except the anomeric carbon.. 1 CO O O C 2 D-galactose 4 6 C 2 5 O C 2 5 O

63 Your Turn! Add the groups to the anomeric carbon atoms and name the anomers. Notice that the hydroxyl group on the anomeric carbon is above the ring in the β-isomer and below the ring in the α-isomer. 6 C 2 5 O C 2 5 O D-galactopyranose + 6 C 2 5 O D-galactopyranose 63

64 emiacetals and Acetals The reactions of aldehydes (and ketones) to form hemiacetals and acetals has already been covered. The hemiacetal structure consists of an ether linkage and an alcohol linkage on the same carbon atom. 64

65 emiacetals and Acetals The acetal structure has two ether linkages on the same carbon atom. 65

66 emiacetals and Acetals The cyclic structures of monosaccharides are intramolecular hemiacetals. Five and six - membered ring hemiacetals are stable... 66

67 emiacetals and Acetals In an aqueous solution, the ring often opens and the hemiacetal momentarily reverts to the open chain aldehyde. When the open chain closes, it forms either the α- or the β-anomer during mutarotation as discussed previously. 67

68 emiacetals and Acetals Cyclic acetals are known as glycosides which are derivatives of hemiacetals. 68

69 emiacetals and Acetals Glycosides like the methyl isomers shown below are less reactive than the corresponding monosaccharides. The methyl isomers shown below will not undergo mutarotation. 69

70 Structures of Galactose and Fructose Galactose has the same structure as glucose except the configuration at carbon four is reversed as shown here. 70

71 Structures of Galactose and Fructose Galactose is an aldohexose like glucose, and like glucose it also exists in the α- and β-cyclic pyranose forms. 71

72 Structures of Galactose and Fructose Fructose is a ketohexose and like glucose it also exists in the open-chain and cyclic forms as shown here. Notice that fructose is a furanose (a five-membered ring). The five-membered ring results because of the position of the keto group in the open chain form. 72

73 Fructose Cyclic Structure of Fructose is a ketohexose. forms a cyclic structure. reacts the on carbon 5 with the C=O on carbon 2. C 2 O C O O C C O C O C 2 O D-Fructose C 2 O O C 2 O O O O α-d-fructose C 2 O O O O C 2 O O -D-Fructose 73

74 Pentoses D-Ribose and its derivative D- 2-deoxyribose are the most important pentoses because they are found in the nucleic acids RNA and DNA. The 2-deoxy in D-2- deoxyribose means an oxygen is omitted from the D-ribose molecule at carbon #2. 74

75 Pentoses Ribulose is a ketose that is related to ribose. It is a biological intermediate used by cells to make other monosaccharides. 75

76 Your Turn! Write the aworth formula for β-d-ribofuranose. 76

77 Your Turn! Write the aworth formula for β-d-ribofuranose. 1 CO C 2 D-ribose 5 C 2 O D-ribofuranose 77

78 Disaccharides Disaccharides are carbohydrates consisting of two monosaccharide units. The two monosaccharides are connected by a glycosidic linkage as shown here for the disaccharide lactose. 78

79 Disaccharides Sucrose and lactose are important disaccharides found in the free state in nature. Sucrose is known as table sugar while lactose is known as milk sugar. Both undergo hydrolysis in the presence of an acid or the enzymes sucrase or lactase, respectively. 79

80 Disaccharides Maltose is not found in the free state but is the product when a polysaccharide is degraded during the sprouting of grain. Maltose undergoes hydrolysis in the presence of acid or maltase to produce two molecules of glucose. 80

81 Structures and Properties of Disaccharides Maltose is derived from two glucose molecules by the elimination of a molecule of water between the group on carbon 1 of one glucose unit and the group of carbon 4 on the other glucose unit. This is an α-1,4-glycosidic linkage since the glucose units have the α-configuration and are joined at carbons 1 and 4. The structure of maltose is on the following slide... 81

82 Structures and Properties of Disaccharides 82

83 Structures and Properties of Disaccharides Lactose consists of a β-d-galactopyranose unit linked to an α-d-glucopyranose unit. These are joined by a β-1,4-glycosidic linkage from carbon 1 on galactose to carbon 4 on glucose. There are two ways that the β-glycosidic linkage in lactose can be drawn (bent or stacked)... 83

84 Structures and Properties of Disaccharides Stacked structure Bent structure 84

85 Structures and Properties of Disaccharides Sucrose consists of an α-d-glucopyranose unit and a β-dfructofuranose unit. These monosaccharides are joined by an α-1,2-glycosidic linkage... 85

86 Structures and Properties of Disaccharides 86

87 Your Turn! Draw a disaccharide formed from two α-d-glucopyranose units with an α-1,4-glycosidic linkage. C 2 O -D-glucopyranose 87

88 Your Turn! Draw a disaccharide formed from two α-d-glucopyranose units with an α-1,4-glycosidic linkage. 4 6 C 2 5 O C 2 5 O C 2 5 O C 2 5 O D-glucopyranose -D-glucopyranose disaccharide with -glycosidic linkage. This disaccharide is maltose. O 88

89 Objectives for Today Distinguish between artificial and natural sweeteners Examine reactions of monosaccharides Compare the structure and function of important polysaccharides 89

90 SWEETENERS & DIET 90

91 Sucrose represents 40-60% of all sweeteners and is 20-30% of the average caloric intake in the United States. Fructose is also a common natural sweetener Sucrose is hydrolyzed to glucose and fructose to prevent crystallization in certain food preparations and in these cases it is known as invert sugar. 91

92 Artificial sweeteners Balance concerns for safety, relative sweetness, and aftertaste. May have a higher relative sweetness than the common sweeteners like sucrose, glucose or fructose as shown in Table 27.1 on the next slide... 92

93 93

94 The compounds shown below are examples of artificial sweeteners. 94

95 Your turn! What are two reasons you can think of why artificial sweeteners are widely used? Consumers perspective Food manufacturers perspective 2-95

96 Your turn! What are two reasons you can think of why artificial sweeteners are widely used? Consumers perspective Control of diabetes; prevention of tooth decay; weight control Food manufacturers perspective Reduce amount of sweeteners significantly and thereby reduce cost to produce; meet consumer demand 2-96

97 IMPORTANT REACTIONS OF MONOSACCARIDES Oxidation and Reduction Reactions Used for Testing 2-97

98 The redox chemistry of carbohydrates is fundamental to life. Glucose is the most important carbohydrate in biochemistry. Biologically, oxidation of glucose provides cells energy through Glycolysis Citric acid cycle Oxidative phosphorylation. 98

99 The monosaccharide aldehyde group can be oxidized to a monocarboxylic acid with a mild oxidizing agent. For example glucose is oxidized to gluconic acid in the presence of bromine and water. 99

100 Aldohexoses form dicarboxylic acids when treated with stronger oxidizing agents. For example glucose is oxidized to glucaric acid in the presence of nitric acid. 100

101 exahydric alcohols (containing six groups) are formed when aldohexoses are treated with reducing agents. For example glucose is reduced to glucitol (sorbitol) in the presence of 2 /Pt. 101

102 Your turn! Where have you seen sorbitol listed as an ingredient? 102

103 Your turn! Where have you seen sorbitol listed as an ingredient? Sorbitol is often listed as one of the sweeteners in chewing gum. It is more slowly metabolized by the body and does not encourage the growth of the bacteria that cause tooth decay. 103

104 Under prescribed conditions, some sugars reduce silver ions to free silver and copper(ii) ions to copper(i) ions. Such sugars are called reducing sugars. A reducing sugar will have one of the following groups. An aldehyde group (as in glyceraldehyde) A hydroxyketone (as in fructose) A cyclic hemiacetal group (as in glucose and maltose) 104

105 Chemical tests for monosaccharides The Benedict, Barfoed, and Fehling tests are based on the formation of a brick red copper(i) oxide precipitate The Barfoed test is more sensitive in that it can distinguish a reducing monosaccharide from a reducing disaccharide. The Tollens test is based on the formation of a silver mirror. The sugars are oxidized to carboxylic acids and the metal 105 ions are reduced...

106 106

107 Many clinical tests that monitor color change are based on the oxidation reaction shown here. Note that it is the same chemistry as the Fehling s and related tests. 107

108 Sugars with the hemiacetal structure can be reduced under alkaline conditions because the ring opens as shown below forming an aldehyde group. They are referred to as reducing sugars. Remember this is the same reaction that can happen with non-cyclic hemiacetals. 108

109 Your turn! Glucose, lactose, and maltose have the hemiacetal structure and are reducing sugars Sucrose is not a reducing sugar because it does not have the hemiacetal structure. What happens if you run the Benedict test on sucrose? 109

110 Your turn! Glucose, lactose, and maltose have the hemiacetal structure and are reducing sugars Sucrose is not a reducing sugar because it does not have the hemiacetal structure. What happens if you run the Benedict test on sucrose? Since sucrose does not have a hemiacetal structure that can open into an aldehyde, there will be no reaction. The solution stays blue. 110

111 Your Turn! Draw the products formed when D-ribose reacts with Br 2 and water? CO C 2 D-ribose 111

112 Your Turn! Draw the products formed when D-ribose reacts with Br 2 and water? CO CO + Br O + 2 Br C 2 C 2 D-ribose Notice that the product is a carboxylic acid. 112

113 POLYSACCARIDES 113

114 There are three types of naturally occurring polysaccharides. They are cellulose, glycogen, and starch that are of major importance. 114

115 Starch, glycogen, and cellulose all yield D-glucose when hydrolyzed. 115

116 Starch is a polysaccharide found in plants composed of amylose and amylopectin. Amylose is a large molecule consisting of unbranched chains composed of about a- D-glucose units joined by a-1,4-glycosidic linkages. Amylopectin is a large molecule with branched chains composed of a-1,4-glycosidic linkages in the main chain and a-1,6-glycosidic linkages at branch points. 116

117 The unbranched structure of amylose. 117

118 The branched structure of amylopectin. 118

119 Digestion - involves hydrolysis of starchy foods as shown below. Starch is not soluble in cold water due to its large size and will form a colloidal dispersion in hot water. Starch solutions form a blue-black color in the presence of free iodine. 119

120 Glycogen = animal starch Stored in the liver and muscle tissues in animals. Energy-storage carbohydrate in animals. Structure similar to amylopectin More highly branched with the -1,6-glycosidic linkages occurring more frequently along the polymer chain. 120

121 Cellulose Most abundant organic substance found in nature Chief structural component of plants and wood. 121

122 Cellulose Also a glucose-based polymer. The glucose units in cellulose are join by -1,4- glycosidic linkages instead of -1,4-glycosidic linkages. This difference in stereochemistry at the anomeric carbon allows for extensive hydrogen bonding in cellulose. 122

123 aworth formula Three-dimensional model of cellulose Figure 27.9 Two representations of cellulose. In the threedimensional model note the hydrogen bonding that links the extended cellulose polymers to form cellulose fibers. 123

124 Your turn! Since cellulose is composed of repeating glucose units, why can t humans derive energy by digesting cellulose? 124

125 Your turn! Since cellulose is composed of repeating glucose units, why can t humans derive energy by digesting cellulose? umans do not have a cellulase enzyme that can break apart the beta-glycosidic bonds between the glucose units. Since we can t extract the glucose from cellulose, we can t derive energy from cellulose. 125

126 Complex polysaccharides are found in animal tissue including glycosaminoglycans and antigens. Glycosaminoglycans are part of the connective tissue found in joints such as the knee. These complex polysaccharides act as shock absorbers between bones. 126

127 Antigens are even more complex polysaccharides Act as labels to help the immune system differentiate an animal s cells from invading bacteria. Antigens are found on red blood cells and are used in the ABO classification system as shown in Figure on the next slide

128 Figure These are antigens that are used in the ABO blood group classification system. 128

129 Objectives for Today Distinguish between artificial and natural sweeteners Examine reactions of monosaccharides Compare the structure and function of important polysaccharides 129

130 Chapter 27 Summary Carbohydrates are among the most important classes of biochemicals. Carbohydrates are generally defined as polyhydroxy aldehydes or polyhydroxy ketones. Carbohydrates are classified based on the number of monosaccharide units linked to form the molecule: monosaccharides, disaccharides, oligosaccharides and polysaccharides. 130

131 Chapter 27 Summary Carbohydrates are very effective energy-yielding nutrients and very effective building materials. Important pentoses are ribose and deoxyribose. Important hexoses are glucose, galactose, and fructose. Fischer projection formulas are commonly used to represent the structures of carbohydrates. 131

132 Chapter 27 Summary Mutarotation is the process by which monosaccharide anomers are interconverted. Sucrose (table sugar), lactose (milk sugar) and maltose are important disaccharides. The aldehyde groups of monosaccharides can be oxidized to monocarboxylic acids by mild oxidizing agents. Reducing sugars will reduce copper(ii) ions to copper(i) ions or silver ions to silver metal. 132

133 Chapter 27 Summary Important polysaccharides derived from glucose are amylose, amylopectin and glycogen. Glycosaminoglycans are complex carbohydrates that are part of animal connective tissue. Even more complex carbohydrates are found on the surface of cells and act as labels, or antigens, to identify the cells. 133