Carbohydrates
Chapter 27
Hein * Best * Pattison * Arena
Version 1.0
Colleen Kelley
Chemistry Department
1 College
Pima Community
© John Wiley and Sons, Inc.
Chapter Outline
27.1 Carbohydrates: A
First Class of Biochemicals
27.2 Classification of
Carbohydrates
27.4 Monosaccharides
27.5 Structure of Glucose and
Other Aldoses
27.3 Importance of
Carbohydrates
2
Chapter Outline (continued)
27.6 Cyclic Structure of
Glucose; Mutarotation
27.7 Hemiacetals and Acetals
27.8 Structures of Galactose
and Fructose
27. 9 Pentoses
27.10 Disaccharides
27.11 Structures and
Properties of
Disaccharides
27.12 Sweeteners and Diet
27.13 Redox Reactions of
Monosaccharides
27.14 Polysaccharides
Derived from Glucose
3
Carbohydrates:
A First Class of
Biochemicals
4
• Carbohydrates are generally defined as
polyhydroxy aldehydes or ketones or
substances that yield these compounds
when hydrolyzed.
H
CHO
H
H
C
OH
H
C
OH
H
glyceraldehyde
H
C
OH
C
O
C
OH
H
dihydroxyacetone 5
Classification
of
Carbohydrates
6
A carbohydrate can be classified
as:
1.monosaccharide
2.disaccharide
3.oligosaccharide
4.polysaccharide
7
Monosaccharides
• A monosaccharide is a carbohydrate that
cannot be hydrolyzed to simpler
carbohydrate units.
• The monosaccharide is the basic
carbohydrate unit of cellular metabolism.
8
Disaccharides
• A disaccharide yields two
monosaccharides – either alike or
different – when hydrolyzed:
H+ or
disaccharide + water  2 monosaccharides
enzymes
9
Monosaccharides & Disaccharides
• Disaccharides are often used by plants or
animals to transport monosaccharides from
one cell to another.
• The monosaccharides and disaccharides
generally have the ending –ose – for example,
glucose, sucrose, and lactose.
• These are water-soluble carbohydrates, which
have a characteristically sweet taste and are
called sugars.
10
Oligosaccharides
• An oligosaccharide has two to six
monosaccharide units linked together.
11
Polysaccharides
• A polysaccharide is a macromolecular substance
that can be hydrolyzed to yield many
monosaccharide units:
polysaccharide + water  monosaccharides
H+ or
enzymes
• Polysaccharides are important structural
supports, particularly in plants, and also serve
as a storage depot for monosaccharides, which
cells use for energy.
12
Other Ways to Classify Carbohydrates
• As a triose, tetrose, pentose, hexose, or
heptose
• As an aldose or ketose
• As a D or L isomer
• As a (+) or (-) isomer
• As a furanose or a pyranose
• As having an alpha (a) or beta (b)
configuration
13
Importance of
Carbohydrates
14
Importance of Carbohydrates
1. Carbohydrates are very effective
energy-yielding nutrients.
2. Carbohydrates can serve as very
effective building materials.
3. Carbohydrates are important watersoluble molecules.
15
Monosaccharides
16
Monosaccharides
• The hexose monosaccharides are the most
important carbohydrate sources of cellular
energy.
• Three hexoses – glucose, galactose, and
fructose – are of major significance in
nutrition.
– All three have the same formula, C6H12O6, and
thus deliver the same amount of cellular energy.
– They differ in structure, but are biologically
interconvertible.
17
• Glucose (dextrose) is the most important of
the monosaccharides.
• It is an aldohexose and is found in the free
state in plant and animal tissue.
CHO
H
HO
OH
H
H
OH
H
OH
CH 2OH
18
• Galactose is also an aldohexose and
occurs, along with glucose, in lactose and
in many oligo- and polysaccharides such
as pectin and gums.
CHO
H
OH
HO
H
HO
H
H
OH
CH 2OH
19
• Fructose, also know as levulose, is a ketohexose
that occurs in fruit juices, honey, and along with
glucose, as a constituent of sucrose.
CH 2OH
O
HO
H
H
OH
H
OH
CH 2OH
20
Structures of Glucose
and Other Aldoses
21
Epimers
• Any two monosaccharides that differ
only in the configuration around a
single carbon atom are called epimers.
• D- and L-glyceraldehyde are epimers.
CHO
CHO
H
C
OH
HO
C
H
CH2OH
CH2OH
D-glyceraldehyde
L-glyceraldehyde
22
Figure 27. 1 Configurations of the D-family of
aldoses. The hydroxyl group on the new chiral
carbon atom, added in going from triose to tetrose
to pentose to hexose, is shown in red.
23
Figure 27. 1 Configurations of the D-family of
aldoses. The hydroxyl group on the new chiral
carbon atom, added in going from triose to
tetrose to pentose to hexose, is shown in red.
24
Figure 27. 1 Configurations of the D-family of
aldoses. The hydroxyl group on the new chiral
carbon atom, added in going from triose to tetrose
to pentose to hexose, is shown in red.
25
Figure 27.2 An example of the Kilani-Fischer synthesis in
which two aldotetrose molecules are formed from an
aldotriose molecule.
26
Cyclic Structure of
Glucose; Mutarotation
27
Figure 27.3
Mutarotation
of D-glucose
28
Anomers
•
When two cyclic isomers differ only
in their stereo arrangement about the
carbon involved in mutarotation, they
are called anomers.
•
Mutarotation is the process by
which anomers are interconverted.
29
Figure 27. 4 Three-dimensional representations of
the chair form of a-D-glucopyranose
30
Hemiacetals and
Acetals
31
• Cyclic structures of monosaccharides are
intramolecular hemiacetals.
• Five- or six-membered rings are especially
stable.
CH2OH
O
hemiacetal structure
in a-D-glucopyranose
OH
HO
HO
HO
32
Glycoside
• When a monosaccharide hemiacetal reacts with
an alcohol, the product is an acetal.
• In carbohydrate terminology, this acetal
structure is called a glycoside.
CH2OH
acetal structure
O
OH
HO
RO
HO
glycosidic linkage
33
Structures of Galactose
and Fructose
34
Galactose
CHO
H
HO
CH2OH
CH2OH
OH
OH
OH
O
b
H
OH
HO
H
O OH
OH
H
OH
CH 2OH
D-galactose
a
HO
OH
a-D-galactopyranose
HO
b-D-galactopyranose
35
Fructose
CH 2OH
O
CH 2OH
HO
OH
H
OH
b
O
H
H
OH
OH
CH 2OH
CH 2OH
OH
36
Disaccharides
37
Disaccharides
• Disaccharides are carbohydrates composed
of two monosaccharide residues united by a
glycosidic linkage.
or glucose + fructose
• sucrose + water H+

sucrase
• lactose + water  galactose + glucose
H+ or
lactase
• maltose + water H+
or glucose + glucose
maltase
38
Structures and
Properties of
Disaccharides
39
• Disaccharides contain an acetal
structure (glycosidic linkage), and
some also contain a hemiacetal
CH2OH
CH2OH
structure.
• Maltose:
O
O
HO
OH
O
OH
OH
HO
HO
a-1,4-glycosidic linkage
40
Sweeteners
and Diet
41
42
Redox Reactions of
Monosaccharides
43
Oxidation
• The aldehyde groups in monosaccharides can
be oxidized to monocarboxylic acids by mild
oxidizing agents such as bromine water.
CHO
H
HO
COOH
OH
H
H
HO
OH
H
+ Br2 + H2O
+ 2HBr
H
OH
H
OH
H
OH
H
OH
44
CH 2OH
CH 2OH
Reduction
• Monosaccharides can be reduced to their
corresponding polyhydroxy alcohols by
reducing agents such as H2/Pt or sodium
amalgam, Na(Hg).
CHO
H
HO
CH 2OH
OH
H
H
HO
OH
H
+ H2 / Pt
H
OH
H
OH
H
OH
H
OH
CH 2OH
CH 2OH
45
Redox Test for Carbohydrates
• 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.
46
Polysaccharides
Derived from
Glucose
47
Starch
• Starch is found in plants, mainly in the seeds,
roots, or tubers.
• Corn, wheat potatoes, rice and cassava are the
chief sources of dietary starch.
• The two main components of starch are
amylose and amylopectin.
– Amylose molecules are unbranched chains
composed of about 25-1300 a-D-glucose units
joined by a-1,4-glycosidic linkages.
48
Figure 27.7 Representation of amylose.
49
50
51
Glycogen
• Glycogen is the energy-storage carbohydrate of
the animal kingdom.
• It is formed by the polymerization of glucose
and is stored in the liver and in muscle tissues.
• Structurally, it is very similar to the
amylopectin fraction of starch, except that it is
more highly branched.
• The a-1,6-glycosidic linkages occur on one of
every 12-18 glucose units.
52
Cellulose
• Cellulose is the most abundant organic
substance found in nature.
• It is the chief structural component of
plants and wood.
53
Figure 27.9 Two
representations of
cellulose. In the threedimensional drawing,
note the hydrogen
bonding that links the
extended cellulose
polymers to form
cellulose fibers.
54
55