1
Carbohydrates:
A First Class of
Biochemicals
2
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.
3
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.
4
Fischer projection formulas are drawn with these
characteristics:
(1) 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 .
5
This is an example of a modified structural formula of
glucose written as a Fischer projection formula.
6
Classification
of
Carbohydrates
7
Types of Carbohydrates
The four major types of carbohydrates are……..
1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides
8
Monosaccharides
• A monosaccharide is a carbohydrate that
cannot be hydrolyzed to simpler
carbohydrate units.
• The monosaccharide is the basic
carbohydrate unit of cellular metabolism.
9
Types of Monosaccharides
Monosaccharides can be classified by the ….
(a) number of carbon atoms in the molecule
( e.g. a pentose versus a hexose)
(b) functional group ( aldoses versus ketoses)
(c) configuration ( D versus L isomers)
(d) optical activity [(+) versus (–) isomers]
(e) ring structure ( furanoses versus pyranoses)
(f) stereochemistry at an anomeric carbon ( versus 
isomers)
10
Types of Monosaccharides
Number of Carbons
The monosaccharides shown below are classified based on
the number of carbons in the molecule.
11
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
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
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
left are (–) isomers.
Note: The D and L designations
do not indicate the direction of
rotation, e.g., the D isomer of
glucose could be either the (+)
isomer or it can be the (–) isomer.
14
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
15
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
H
OH
H
OH
H
H
OH
CH2OH
L-Threose
HO
CH2OH
D-Fructose
16
Learning Check
Write the projection formula for a D-aldopentose.
17
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.
H
C
O
H
C
OH
HO
C
H
H
C
OH
D-configuration
CH 2OH
18
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.
19
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.
20
Disaccharides
• A disaccharide yields two
monosaccharides – either alike or
different – when hydrolyzed:
H+ or
disaccharide + water  2 monosaccharides
enzymes
21
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.
22
Oligosaccharides
• An oligosaccharide has two to six
monosaccharide units linked together.
23
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.
24
Importance of
Carbohydrates
25
Why are carbohydrates so important?
Carbohydrates are important because they are widely available
and because they have exceptional utility.
26
Monosaccharides
27
The most important monosaccharides are the pentoses
and hexoses as shown in the diagram below .
28
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.
29
• “Tree” formulas
• Shorthand formulas used in carbohydrate
configurations where the following symbols
are used:
= H
C
= H
= HO
Example
O
C
OH
C
= CH2OH
H
H
C
H
C
O
OH
is
CH2OH
30
• Glucose 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
or
31
Glucose
The concentration of glucose in blood is normally about
80-100 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).
32
• 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
or
OH
33
CH 2OH
• 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
It is the sweetest
common sugar
being about two
times sweeter
than glucose.
HO
H
H
OH
H
OH
CH 2OH
34
Structures of the Pentoses
(Ribose and Deoxyribose)
35
Structures of Glucose
and Other Aldoses
36
Epimers
• Any two monosaccharides that differ
only in the configuration around a
single carbon atom are called epimers.
• In the case of glyceraldehyde this
atom is carbon 2.
37
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
Learning Check
Draw the enantiomer of D-allose using the previous
slide.
39
CHO
CHO
The enantiomer of Dallose is L-allose.
These molecules are
mirror images.
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
CH 2OH
D-allose
CH 2OH
L-allose
40
Cyclic Structure of
Glucose; Mutarotation
41
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.
OH’s removed
42
OH
O
O
H

O
H
43
H
OH
H
HH
H
HH H
H
H
H
H
O
O
H

HH
H
H
O H
H
44
Most naturally occurring monosaccharides occur in
the chair conformation shown.
-D-glucopyranose
45
What is an anomer?
An anomer is the  or  form of a monosaccharide as shown here.
46
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.
Fischer projection formulas showing mutarotation of D-glucose.
Emil Fischer
1852-1919
47
Haworth formulas showing mutarotation of D-glucose.
Sir (Walter) Norman
Haworth
1883-1950
48
Comparison
of Formulas
49
H
OH
O
O
H

O
H
50
Conversion of Fischer
to Haworth Formulas
http://faculty.chemeketa.edu/lemme/CH%20123/Handouts/HaworthFormulas.pdf
51
Hemiacetals and
Acetals
52
Hemiacetals
Hemiacetals are structures that contain an alkoxy group and
a hydroxyl group on the same carbon atom.
53
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.
54
Acetals
Acetals are structures that contain two alkoxy groups on
the same carbon atom.
55
Glycosides
Cyclic acetals are known as glycosides and glycosides are
derivatives of hemiacetals.
56
Glycosides
Glycosides like the methyl isomers shown below are
less reactive than the corresponding monosaccharide.
The methyl isomers shown below will not undergo
mutarotation.
57
Structures of Galactose,
Fructose, Ribose, and
Deoxyribose
58
Structure of Galactose
Galactose has the same structure as glucose except the
configuration at carbon four is reversed as shown here.
59
Structure of Galactose
Galactose is an aldohexose like glucose and like glucose
it also exists in the alpha and beta cyclic pyranose forms.
60
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)
61
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.
62
Disaccharides
63
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
64
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.
65
Disaccharides
Maltose is not found in the free state but is the product
when a polysaccharide is degraded during the sprouting of
grain. Maltose is known as grain sugar.
Maltose undergoes hydrolysis in the presence of acid or
maltase to produce two molecules of glucose.
66
Structures and
Properties of
Disaccharides
67
Formation of Maltose
68
Structure of Lactose
Shown here are Haworth structures using the stacked
position convention and the bent structure convention.
stacked position
bent structure
69
This is a Haworth projection formula of the
disaccharide sucrose
70
This is an alternate Haworth projection
formulas of the disaccharide sucrose
1
6
2
5
3
4
71
Sweeteners
and Diet
72
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.
73
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 the table on the next slide.
74
Relative Sweetness of Sugars and Sugar Substitutes
based on fructose = 100
Sugars
Relative
sweetness
Sugar substitutes
Relative
sweetness
Fructose
100
Sucralose
(Splenda)
3.5 × 104
Invert Sugar
75
Saccharin
(Sweet ‘N Low)
1.7 × 104
Sucrose
58
Acesulfame
potassium
(Sweet One)
1.2 × 104
Glucose
43
Asparatame
(Equal)
1.0 × 104
Maltose
19
Rebiana
(Truvia, PureVia)
1.2 × 104
Galactose
19
Neotame
4.1 × 105
Lactose
9.2
Stevia
3.0 × 104
Xylitol
58
75
Sugars
Name
Calories /
Gram
Sweetness
Index
Glycemic
Index
Calories /
SpoonEquiv
Fructose
4
1.7
23
9
Sucrose
4
1
65
16
Glucose
4
0.75
100
21
Dextrose
4
0.75
100
21
Trehalose
4
0.45
70
36
Galactose
4
0.3
23
53
Maltose
4
0.3
105
53
Lactose
4
0.15
45
107
76
Sugar Alcohols
Name
Calories /
Gram
Sweetness
Index
Glycemic
Index
Calories /
SpoonEquiv
Erythritol
0.2
0.65
1
1
Xylitol
2.4
1
12
10
Maltitol
2.4
0.9
35
11
Mannitol
1.6
0.5
2
13
Isomalt
2.1
0.5
2
17
Sorbitol
2.6
0.55
4
19
Lactitol
2
0.4
3
20
HSH
3
0.4
36
30
77
Natural Caloric Sweeteners
Name
Calories /
Gram
Sweetness
Index
Glycemic
Index
Calories /
SpoonEquiv
Honey
4
1.1
50
14
Maple Syrup
4
1
54
15
Coconut
Palm Sugar
4
1
35
15
Sorghum
Syrup
4
1
50
15
78
Natural Zero Calorie Sweeteners
Name
Calories /
Gram
Sweetness
Index
Glycemic
Index
Calories /
Spoon-Equiv
Thaumatin
4
2,000
0
0
Monellin
4
1,500
0
0
Brazzein
4
1,000
0
0
Pentadin
4
500
0
0
LuoHanGuo
0
300
0
0
Stevia
0
300
0
0
79
Modified Sugars
Name
Calories /
Gram
Sweetness
Index
Glycemic
Index
Calories /
Spoon-Equiv
Tagatose
4
0.92
0
7
Agave Syrup
4
1.5
15
10
HFCS-90
4
1.6
31
10
HFCS-55
4
1.2
58
13
HFCS-42
4
1.1
68
14
Golden
Syrup
4
1.1
60
15
Barley Malt
Syrup
4
0.5
42
32
Brown Rice
Syrup
4
0.5
25
32
80
Artificial Sweeteners
Name
Calories /
Gram
Sweetness
Index
Glycemic
Index
Calories /
Spoon-Equiv
Advantame
0
20,000
0
0
Neotame
0
8,000
0
0
Sucralose
0
600
0
0
Saccharin
0
300
0
0
AcesulfameK
0
200
0
0
Aspartame
4
180
0
0
Cyclamate
0
40
0
0
81
Discovered in 1976
82
Discovered in 1879
83
Discovered in 1967
84
Discovered in 1965
85
Agave nectar (sometimes called agave
syrup) is most often produced from the Blue
Agaves that thrive in the volcanic soils of
Southern Mexico. Agaves are large, spikey
plants that resemble cactus or yuccas in
both form and habitat, but they are actually
succulents similar to the familiar Aloe Vera.
To make the agave nectar, sap is extracted
from the pina, filtered, and heated at a low
temperature, which breaks down the
carbohydrates into sugars. Lighter and
darker varieties of agave nectar are made
from the same plants. Because of the low
temperatures used in processing many
varieties (under 118°F) raw foods
enthusiasts generally regard agave nectar
as a raw food.
http://www.allaboutagave.com/
The taste of agave nectar is comparable, though not
identical, to honey. Many people who do not like the taste of
honey find agave a more palatable choice. It also has none
of the bitter aftertaste associated with artificial sweeteners.
86
Stevia (sweetleaf, sweet
leaf or sugarleaf)
This sweetener is made from a crude
preparation (powder or liquid) of dried
stevia leaves. It may contain a mixture
of many substances,
only some of which are sweet.
87
Truvia™ natural sweetener is made
from rebiana, the best tasting part of
the stevia leaf, erythritol and natural
flavors.
Rebiana is the common or usual name for a food-grade high-purity extract
of the stevia leaf that is at least 97 percent rebaudioside-A, the best tasting
sweet substance found in the stevia leaf.
Chemically, erythritol is simply a four-carbon sugar alcohol.
Erythritol is made by fermenting glucose then separating and
purifying the resulting product.
88
Most fruits, berries and plants contain xylitol (also called wood
sugar), the richest natural sources being plums, strawberries,
raspberries, cauliflower and endives.
89
Xylitol is extremely toxic to dogs
The toxic dose of xylitol is 0.1 gm/kg
body weight, while liver failure results
from doses greater than 0.5 g/kg body
weight. Translating these numbers into
something usable in the every-day
world is a little harder to do, since the
amount of xylitol varies from one
product to another. Two sticks of gum is
enough to cause a serious drop in blood
sugar for a small (under 20 lb) dog,
while it might take 8 to 10 sticks to
affect a large (over 60 lb) dog, but these
amounts are only an estimate. As for
baked goods containing xylitol, again,
the amount in each cookie or muffin
will vary. In one case, a Standard
Poodle died after eating 5 or 6 cookies
sweetened with xylitol.
90
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.
Discovered in 1937
91
Redox Reactions of
Monosaccharides
92
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.
93
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.
94
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.
Sorbitol is in many
moisturizers
95
Learning Check
Write the products of the mild oxidation and reduction
of D-mannose.
O
C H
HO
H
HO
H
H
OH
H
OH
CH2OH
D-Mannose
96
Solution
Write the products of the mild oxidation and reduction of
D-mannose.
D-Mannitol
D-Mannose
D-Mannonic
acid
97
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;
(a) An aldehyde group ( e.g. glyceraldehyde)
(b) A hydroxyketone ( e.g. fructose)
(c) A cyclic hemiacetal group ( e.g. glucose or maltose)
98
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.
99
The Barfoed test (a solution of cupric acetate and acetic
acid) is more sensitive in that it can distinguish a
reducing monosaccharide from a reducing disaccharide.
Monosaccharides form a precipitate within 3 minutes and
Disaccharides take a bit longer.
–
+
100
Redox Test for Carbohydrates
Benedict test for
reducing sugars. The
tube on the right
contains Benedict
reagent. The tube on
the left shows the
brick-red precipitate
of Cu2O when
glucose is added.
101
102
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.
103
Osazone Formation
104
Phenylhydrazine (C6H5NHNH2) reacts
with carbons #1 and #2 of reducing
sugars to form derivatives called
osazones. The formation of these
distinctive crystalline derivatives is
useful for comparing the structures of
sugars. Glucose and fructose react as
shown on next slide:
105
H
C
O
H
C
NNHC6H5
H
C
OH
HO
C
H
H
C
NNHC6H5
C
O
HO
C
H
H
C
OH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
C6H5NHNH2
CH2OH
CH2OH
C6H5NHNH2
CH2OH
D-glucose
106
H
C6H5NHNH2
C
NNHC6H5
C
NNHC6H5
HO
C
H
H
C
OH
H
C
OH
CH2OH
osazone
107
H
CH2OH
CH2OH
C
O
C
NNHC6H5
HO
C
H
HO
C
H
H
C
OH
H
C
H
C
OH
H
C
C6H5NHNH2
CH2OH
C
O
C
NNHC6H5
HO
C
H
OH
H
C
OH
OH
H
C
OH
CH2OH
C6H5NHNH2
CH2OH
D-fructose
108
H
C6H5NHNH2
C
NNHC6H5
C
NNHC6H5
HO
C
H
H
C
OH
H
C
OH
CH2OH
osazone
109
Identical osazones are obtained from D-glucose
and D-fructose. This demonstrates that carbons #3
through #6 of D-glucose and D-fructose molecules
are identical. The same osazone is also obtained
from D-mannose. This indicates that carbons #3
through #6 of the D-mannose molecule are the
same as those of D-glucose and D-fructose
molecules. In fact, D-mannose differs from Dglucose only in the configuration of the –H and
–OH groups on carbon #2.
110
111
112
113
114
Polysaccharides
Derived from
Glucose
115
Glucose Based Polysaccharides
There are three types of naturally occurring polysaccharides;
cellulose, glycogen, and starch as shown below.
116
Starch, glycogen, and cellulose all yield D-glucose
when hydrolyzed as shown here.
117
Starch
Starch is a polysaccharide composed of amylose and
amylopectin.
Amylose is a large molecule consisting of unbranched
chains composed of about 25-1300 -D-glucose units
joined by -1,4-glycosidic linkages.
Amylopectin is a large molecule with branched chains
composed of -1,4-glycosidic linkages in the main chain
and -1,6-glycosidic linkages at branch points as seen
in the next slide.
118
No Branching
Glucose units
Amylose
in Amylose
Molecular Structure of Amylose
119
Branching
Amylopectin
Glucose units
in Amylopectin
Molecular Structure of amylopectin.
120
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.
121
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 -1,6-glycosidic linkages
occurring more frequently along the polymer chain.
122
Cellulose
Cellulose, like starch and glycogen, is a glucose based
polymer.
Cellulose is the most abundant organic substance found
in nature and it is the chief structural component of plants
and wood.
123
Cellulose
However the glucose units in cellulose are join by
-1,4-glycosidic linkages instead of -1,4-glycosidic
linkages.
This change in stereochemistry at the anomeric carbon
allows extensive hydrogen bonding in cellulose as shown
in the next slide.
124
Haworth formula “Bent”
Two representations of
cellulose. In the threedimensional model note
the hydrogen bonding
that links the extended
cellulose polymers to
form cellulose fibers.
Three-dimensional model of cellulose
125
CH2OH
Cellulose:
stacked
structure
O O
CH2OH
OH
O O
CH2OH
OH
OH
O O
CH2OH
OH
OH
O O
OH
OH
O
OH
126
127