1. Hein * Pattison * Arena * Best
Chapter 27 Carbohydrates
Hein * Pattison * Arena * Best
Jerry Poteat Science Department Georgia Perimeter College © John Wiley and Sons, Inc.
Version 1.0 1 1

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

3. Chapter Outline 27.9 Pentoses 27.10 Disaccharides
Structures and Properties of Disaccharides
Sweeteners and Diet
Redox Reactions of Monosaccharides
Polysaccharides Derived from Glucose
Complex Polysaccharides 3 3

4. 27.1 Carbohydrates: A First Class of Biochemicals4 4

5. What are carbohydrates?
Carbohydrates are energy-yielding macronutrients in the
same class of nutrients as fats and proteins.
These polyhydroxy aldehydes or ketones include simple
carbohydrates like glyceraldehyde and dihydroxyacetone.

6. 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.

7. 27.2 Classification of Carbohydrates

8. Types of Carbohydrates
The four major types of carbohydrates are……..
1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides

9. Monosaccharides
A monosaccharide is the smallest unit of a carbohydrate that
cannot be hydrolyzed to a simpler unit. It is the basic
carbohydrate unit of cellular metabolism
For example glucose is a monosaccharide.
Monosaccharides like glucose are important sources of
cellular energy. 9 9

10. Types of Monosaccharides
Monosaccharides can be classified by the ….
(a) number of carbon atoms in the molecule
( e.g. a pentose versus a hexose)
functional group ( aldoses versus ketoses)
configuration ( D versus L isomers)
(d) optical activity ( (+) versus (-) isomers )
ring structure ( furanoses versus pyranoses)
stereochemistry at an anomeric carbon ( versus  isomers) 10 10

11. Types of Monosaccharides
Number of Carbons
The monosaccharides shown below are classified based on the number of carbons in the molecule. 11 11

12. Types of Monosaccharides
Functional Group
Mononosaccharides with a –CHO (aldehyde) group are known as aldoses while those with a (keto) group are known as ketoses. 12 12

13. Types of Monosaccharides
Configuration
Monosaccharides with the –OH group on the right of the
carbon alpha to the terminal ROH carbon are D isomers
while those with the –OH group on the left are L isomers. 13 13

14. Types of Monosaccharides
Optical Activity
Monosaccharides that rotate plane-polarized light to the right
are known as (+) isomers while those that rotate it to the
right are (-) isomers.
Note: The D and L designations
indicate the direction of rotation,
e.g. the D isomer of glucose
could be either the (+) isomer or
it can be the (-) isomer. 14 14

15. Types of Monosaccharides
Ring Structure
The cyclic form of monosaccharides that have five atoms
in the ring are known as furanoses while those with six atoms
are known as pyranoses based on the corresponding
heterocyclic ring structures furan and pyran. 15 15

16. Types of Monosaccharides
Anomeric Configuration
Monosaccharides that have an –OH below the ring at the
anomeric carbon are known as  (alpha) isomers while
those with the –OH above the ring are  (beta) isomers. 16 16

17. Disaccharides
A disaccharide yields two monosaccharides when hydrolyzed
as shown below.

18. General Characteristics of Monosaccharides and Disaccharides
Disaccharides are often used by plants or animals to
transport monosaccharides from one cell to another.
Both monosaccharides and disaccharides generally have the
ending –ose .
For example, glucose, sucrose, and lactose.
Both are water-soluble carbohydrates that have a
sweet taste and are therefore called sugars.

19. Oligosaccharides
An oligosaccharide is a carbohydrate with at least two but not
more than six monosaccharide units linked together. 19 19

20. Polysaccharides
A polysaccharide is a macromolecule ( i.e. polymer) that
produces many monosaccharide units when hydrolyzed
Polysaccharides are important structural materials
in plants and animals.
These carbohydrates also serve as a storage depot for monosaccharides which cells use for energy.

21. 27.3 Importance of Carbohydrates21 21

22. Why are carbohydrates so important?
Carbohydrates are important because they are widely available and because they have exceptional utility. 22 22

23. 27.4 Monosaccharides 23 23

24. The most important monosaccharides are the pentoses
and hexoses as shown in the diagram below . 24 24

25. Glucose Glucose and galactose are both aldohexoses. An aldohexose
is a hexose with an aldehyde group.
Glucose is the most important monosaccharide because of its
role in cellular metabolism.

26. Glucose The concentration of glucose in blood is normally about
mg/100 mL
Glucose is known by either of the following names dextrose
(from dextrorotatory), grape sugar (found in grapes) , or blood
sugar (because it is transported in the blood). 26 26

27. Galactose
Galactose, like glucose, is an aldohexose and is a component of the disaccharide lactose.
Fructose
Fructose is a ketohexose (i.e. it is a hexose with a keto group).
Fructose is known as levulose and is found in fruit juices and
honey.
It is the sweetest common sugar being about two times sweeter
than glucose. 27 27

28. Structures of the Hexoses
(Glucose, Galactose, and Fructose)
These structures are shown as Fischer projection formulas
where each line intersection represents a carbon atom. 28 28

29. Structures of the Pentoses
(Ribose and Deoxyribose) 29 29

30. 27.5 Structure of Glucose and Other Aldoses30 30 30 30

31. Glucose Enantiomers
The enantiomers of glucose can be represented by Fischer projection formulas. 31 31 31

32. Fischer projection formulas are drawn with these characteristics:
The keto or aldehyde group is placed at the top of the
projection.
(2) Each interior carbon atom is shown as an intersection
point between two lines ( )
(3) The H atom and –OH group are written to left or right
of the projection . 32 32 32 32

33. This is an example of a modified structural formula of
D-glucose written as a Fischer projection formula. 33 33 33 33

34. 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 this atom is carbon 2. 34 34

35. Fischer projections can also be represented by a three-dimensional representation.35 35

36. (continued from previous slide)
Fischer projections are shown for the family of D-aldoses in Figure 27.1 and the Kiliani-Fischer synthesis is shown in Figure 27.2. 36 36

37. Figure 27. 1 The D-family of aldoses
Figure The D-family of aldoses. The red –OH group indicate the new chiral carbon added in case from top to bottom of the diagram.

38. Figure 27.2 An example of the Kiliani-Fischer synthesis in which two aldotetrose molecules are formed from an aldotriose molecule.

39. 27.6 Cyclic Structure of Glucose; Mutarotation39 39 39 39

40. What is mutarotation?
Mutarotation is the change in specific rotation of an anomer as it is
converted into an equilibrium mixture of the  and  forms as shown
in Figure 27.3.
Figure Fischer projections and Haworth formulas showing mutarotation of D-glucose.
Fischer projections 40 40

41. Figure ( continued)
Haworth formulas 41 41

42. What is an anomer?
An anomer is the  or  form of a monosaccharide as shown here. 42 42

43. Haworth Formulas
Haworth formulas are structural formulas that represent cyclic
sugars.
In the case of glucose the formula is drawn as a flat hexagon with
H and –OH written above and below the plane of the ring.
Haworth formulas are sometimes shown in a abbreviated form as shown here.

44. Most naturally occurring monosaccharides however occur in the
chair conformation as shown in the ball-and-stick model below.
The spacefilling model is a more accurate representation of a sugar molecule.
Ball-and-stick model
Spacefilling model
Figure Three-dimensional representations of the chair form of
a-D-glucopyranose 44 44

45. 27.7 Hemiacetals and Acetals45 45

46. Hemiacetals
Hemiacetals are structures that contain an alkoxy group and
a hydroxyl group on the same carbon atom. 46 46

47. The cyclic structures of monosaccharides are intramolecular
hemiacetals.
Five and six -membered ring hemiacetals are stable but these
rings can open in aqueous solution to the straight-chain
aldehyde. 47 47

48. Acetals Acetals are structures that contain two alkoxy groups on
the same carbon atom. 48 48

49. Glycosides Cyclic acetals are known as glycosides and glycosides are
derivatives of hemiacetals. 49 49

50. Glycosides
Glycosides like the methyl isomers shown below are less reactive than the corresponding monosaccharide.
The methyl isomers shown below will not undergo mutarotation. 50 50

51. 27.8 Structures of Galactose and Fructose51 51

52. Structure of Galactose
Galactose has the same structure as glucose except the
configuration at carbon four is reversed as shown here. 52 52

53. Structure of Galactose
Galactose is an aldohexose like glucose and like glucose it also exists in the alpha and beta cyclic pyranose forms.

54. Structure of Fructose
Fructose is a ketohexose and like glucose it also exists in the
open-chain and cyclic forms as shown here.
( open-chain form)
( cyclic form)

55. 27.9 Pentoses 55 55 55 55

56. Structure of Ribose and Deoxyribose
D-Ribose and its derivative
D-2-dexoyribose are pentoses
found in nuclei acids RNA and DNA.
Notice that the 2-deoxy in
D-2-deoxyribose means an
oxygen is omitted from the
D-ribose molecule at carbon two. 56 56

57. Structure of Ribulose Ribulose is a ketose that is related to ribose.
It is a biological intermediate used
by cells to make other monosaccharides. 57 57

58. 27.10 Disaccharides 58 58 58 58

59. Disaccharides Disaccharides are carbohydrates consisting of two
monosaccharides.
The two monosaccharides are connected by a glycosidic
linkage as shown here for the disaccharide lactose .
lactose 59 59

60. 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. 60 60

61. 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. 61 61

62. 27.11 Structures and Properties of Disaccharides62 62 62 62

63. Formation of Maltose

64. Structure of Lactose
Shown here are Haworth structures using the stacked position convention and the bent structure convention.
stacked position
bent structure 64 64

65. This is a Haworth projection formula of the disaccharide sucrose65 65

66. 27.12 Sweeteners and Diet 66 66 66 66

67. Importance of Sucrose as a Sweetener
Sucrose represents 40-60% of all sweeteners and is 20-30%
of the average caloric intake in the United States because of
its low price and sweet taste.
Sucrose is hydrolyzed to prevent crystallization in certain
food preparations and in these cases it is known as invert
sugar. 67 67

68. Artificial Sweeteners
Artificial sweeteners have been developed with the intent to
balance the concerns for safety, relative sweetness, and
aftertaste.
Many artificial sweeteners have a higher relative sweetness
than the common sweeteners like sucrose, glucose or fructose
as shown in Table 27.1 68 68

69. 69 69

70. Artificial Sweeteners
The compounds shown below are examples of artificial
sweeteners. 70 70

71. The history of sodium cyclamate illustrates the difficulty
in balancing consumer safety with the needs of the
consumer market.
This sweetener was banned in 1970 because of research that
indicated risks of cancer from consuming the sweetener.
Natural sweeteners being developed as substitutes for sucrose include sweeteners derived from citrus fruits and stevoside. 71 71

72. 27.13 Redox Reactions of Monosaccharides72 72 72 72

73. Oxidation of Aldohexoses
The aldehyde group in monosaccharides can be oxidized to
monocarboxylic acids with a mild oxidizing agent.
For example glucose is oxidized to gluconic acid in the
presence of bromine water.

74. Oxidation of Aldohexoses
Dicarboxylic acids are formed when aldohexoses are treated
with stronger oxidizing agents.
For example glucose is oxidized to glucaric acid in the
presence of nitric acid. 74 74

75. Reduction of Aldohexoses
Hexahydric alcohols ( six –OH groups) are formed when
aldohexoses are treated with reducing agents.
For example glucose is reduced to glucitol (sorbitol) in the
presence of H2/Pt. 75 75

76. Redox Tests for Carbohydrates
A reducing sugar is a compound that will reduce
Ag + → Ag or Cu2+ → Cu+.
A reducing sugar will have one of the following groups;
An aldehyde group ( e.g. glyceraldehyde)
A hydroxyketone ( e.g. fructose)
A cyclic hemiacetal group ( e.g. glucose or maltose) 76 76

77. Redox Tests for Carbohydrates
The Benedict, Barfoed, and Fehling tests are based on the
formation of a brick red copper(I) oxide precipitate as
a positive result while the Tollens test is based on the
formation of a silver mirror.
The Barfoed test is more sensitive in that it can distinguish
a reducing monosaccharide from a reducing disaccharide. 77 77

78. Summary of Redox Tests 78 78

79. Redox Tests for Carbohydrates
Many clinical tests that monitor glucose are based on the reaction shown here. 79 79

80. Reduction of Hemiacetals
Sugars with the hemiacetal structure can be reduced under
alkaline conditions because the ring opens as shown below
forming an aldehyde group.
Therefore glucose, lactose, and maltose have the hemiacetal
structure and are reducing sugars but the disaccharide sucrose
is not a reducing sugar because it does not have the hemiacetal
structure. 80 80

81. 27.14 Polysaccharides Derived from Glucose81 81

82. Glucose Based Polysaccharides
There are three types of naturally occurring polysaccharides;
cellulose, glycogen, and starch as shown below. 82 82

83. Starch, glycogen, and cellulose all yield D-glucose when hydrolyzed as shown here.83 83

84. Starch Starch is a polysaccharide 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 as seen
in Figure 27.7.

85. Figure 27.7 Molecular Structure of Amylose
No Branching
Glucose units
in Amylose
Amylose
Figure 27.7 Molecular Structure of Amylose

86. Figure 27.7 (continued) Molecular Structure of amylopectin.
Branching
Glucose units
in Amylopectin
Amylopectin
Figure 27.7 (continued) Molecular Structure of amylopectin.

87. Hydrolysis of Starch
An important reaction during digestion is the hydrolysis of starchy foods as shown below.
Starch is not soluble in cold water and will form a colloidal dispersion in hot water.
Starch solutions form a blue-black color in the presence of free iodine. 87 87

88. Glycogen
Glycogen is a carbohydrate polymer that is stored in the liver
and muscle tissues in animals.
Glycogen has a structure similar to amylopectin except it
is more highly branched with the a-1,6-glycosidic linkages
occurring more frequently along the polymer chain.

89. Cellulose Cellulose, like starch and glycogen, is a glucose based
polymer.
However the glucose units in cellulose are join by
-1,4-glycosidic linkages instead of -1,4-glycosidic
linkages.

90. Cellulose This change in stereochemistry at the anomeric carbon
allows extensive hydrogen bonding in cellulose as shown
in Figure 27.9.
Cellulose is the most abundant organic substance found
in nature and it is the chief structural component of plants
and wood. 90 90

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

92. 27.15 Complex Polysaccharides92 92

93. Polysaccharides Complex polysaccharides are found in animal tissue to
include 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. 93 93 93 93

94. Antigens Antigens are even more complex polysaccharides and act as
labels to help the immune system differentiate an animal own cells from invading bacteria.
Antigens are found on red blood cells and are used in the
ABO classification system as shown in Figure 94 94 94 94

95. Figure 27.10 These are antigens that are used in the ABO blood
group classification system. 95 95 95 95

96. Chapter 27 Summary
Carbohydrates are polyhydroxy substances that are classified as monosaccharides, disaccharides, oligosaccharides or polysaccharides.
Important monosaccharides include ribose, deoxyribose, glucose, and fructose.
Important disaccharides include sucrose, lactose, and maltose.
Important polysaccharides include glycogen, starch, and cellulose.
Monosaccharides that differ in the configuration at a single carbon
atom are called epimers.
A glycoside is a cyclic acetal derived from a monosaccharide. 96 96

97. Chapter 27 Summary
Glucose exists in the straight-chain and cyclic forms.
The isomers formed when glucose forms a ring structure are known as anomers.
The interconversion between glucose anomers is known as mutarotation.
The hydrolysis of sucrose ( table sugar) produces invert sugar which is a mixture of glucose and fructose.
Reducing sugars like glucose can reduce copper(II) to copper(I) or silver ions to the silver metal.
Reducing sugars will generally have an aldehyde group, an -hydroxy ketone group, or a hemiacetal group. 97 97