Carbohydrates
Larry J Scheffler
Lincoln High School
2009
Version 1.11
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Carbohydrates
• Contain Carbon, Hydrogen and Oxygen
• Can be characterized as
– Monosaccharides
– Disaccharides
– Polysaccharides
• Includes sugars, starches, cellulose,
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Carbohydrates
• Carbohydrates are produced in green plants in the
presence of chlorophyll and sunlight in a process
known as photosynthesis.
• They serve as food sources for living organisms and
provide the structural support for plants.
• Many carbohydrates are large polymers composed of
repeating units of simple sugars.
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Types of Carbohydrates
Carbohydrates have the following basic composition:
I
(CH2O)n or H - C - OH
I
 Monosaccharides - simple sugars with multiple -OH
groups. Based on number of carbons (3, 4, 5, 6), a
monosaccharide is a triose, tetrose, pentose or
hexose.
 Disaccharides - Two monosaccharides linked by a
covalent bond.
 Oligosaccharides - a few monosaccharides linked by
covalent bonds
 Polysaccharides - polymers consisting of chains of
multiple monosaccharide or disaccharide units.
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Carbohydrates
Monosaccharides
• Single (simple) sugars
• Contain C, H, and O in a 1:2:1
ratio
• Quick energy sources
Examples:
Glucose C6H12O6
Fructose C6H12O6
Galactose C6H12O6
Glucose
glucose
Fructose
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Monosaccharides
• Empirical formula is CH2O
• Both open chain and ring structures are possible
• Mulitple structural isomers are possible
• Multiple chiral carbon atoms lead to optical isomers
• Monosaccharides generally have between 3 and 6
carbon atoms
• The most common monosaccharides are:
– Five carbons C5H10O5 - called pentoses
– Six carbons C6H12O6 - called hexoses
• Monosaccharide straight chains have at least one
carbonyl group C=O.
• If the carbonyl group is at the end it is an aldose
sugar. If it is within the chain it is a ketose sugar
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Monosaccharides
Aldoses (e.g., glucose)
have an aldehyde group
at one end.
H
Ketoses (e.g., fructose)
have a ketone group,
usually at C2.
O
CH2OH
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
D-glucose
CH2OH
D-fructose
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Optical Isomers: D and L Forms
D or dextrorotatory &
L or levorotatory are
designations for
optical isomers that
are based on the
configuration about
the single asymmetric
C in glyceraldehyde.
The lower
representations are
Fischer Projections.
CHO
CHO
H
C
OH
HO
L-glyceraldehyde
CHO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
CH2OH
D-glyceraldehyde
C
CHO
HO
C
H
CH2OH
L-glyceraldehyde
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Sugar Nomenclature
For sugars with
more than one chiral
center, D and L refer
to the asymmetric C
farthest from the
aldehyde or keto
group.
Most naturally
occurring sugars are
D isomers.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
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Steroisomers
D & L sugars are mirror
images of one another.
They have the same
name, e.g., D-glucose
& L-glucose.
Other stereoisomers
have unique names,
e.g., glucose, mannose,
galactose, etc.
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
The number of stereoisomers is 2n, where n is the
number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers. Thus
there are 16 possible stereoisomers (8 D-sugars and 8
L-sugars).
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Ring Structures
Pentoses and
hexoses can form
ring structures as
the ketone or
aldehyde reacts
with a distal OH.
Glucose forms an
intra-molecular
hemiacetal, as the
C1 aldehyde & C5
OH react, to form
a 6-member ring
known as a
pyranose ring,
1
H
HO
H
H
2
3
4
5
6
CHO
C
OH
C
H
C
OH (linear form)
C
OH
D-glucose
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
These representations of the cyclic sugars are called Haworth projections.
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Fructose Ring Structures
Fructose may form either
 a 6-member
pyranose ring,
by reaction of
the C2 keto
group with the
OH on C6, or
 a 5-member
furanose ring,
by reaction of
the C2 keto
group with the
OH on C5.
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
6
HOH2C 6
CH2OH
D-fructose (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
-D-fructofuranose
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Monosaccharides
Some examples
of pyranose ring
structures for
hexose sugars.
The ring is not
actually planar
but exists in boat
and chair
conformers
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Sugar Derivatives
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
OH
OH
H
NH2
O
H
H
H
O OH
OH
H
N
C
CH3
H
-D-glucosamine
-D-N-acetylglucosamine
An Amino sugar is a sugar in which an amino group
substitutes for a hydroxyl. An example is glucosamine.
The amino group may be converted to an amide, as in
N-acetylglucosamine.
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Anomers of Glucose
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
Cyclization of glucose produces a new asymmetric center
at C1. The 2 stereoisomers are called anomers,  & .
Haworth projections represent the cyclic sugars as having
essentially planar rings, with the OH at the anomeric C1:
  (OH below the ring)
  (OH above the ring).
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Glycosidic Bonds
The anomeric hydroxyl groups of two sugars can join
together, splitting out water to form a glycosidic bond.
Two glucose molecules combine to form a disaccharide
known as maltose.
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Disaccharides
•Double sugars
•Good source of energy
•Break down into simple sugars Sucrose (glucose + fructose)
Other disaccharides include: Lactose (glucose + galactose)
-- Sucrose, common table sugar, has a glycosidic bond
linking the anomeric hydroxyls of glucose & fructose.
-- Because the configuration at the anomeric C of glucose is
 (O points down from ring), the linkage is (12).
The full name of sucrose is -D-glucopyranosyl-(12)-D-fructopyranose.)
-- Lactose, milk sugar, is composed of galactose
& glucose,
H H
with (14) linkage from the anomeric OH of galactose. Its
full name is -D-galactopyranosyl-(1 4)--Dglucopyranose
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Disaccharides
•Compare the structures of these three common
disaccharides
H H
•Sucrose is an  (1-4) link between D-Glucose and DFructose
•Lactose is an  (1-4) link between two D glucose
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Polysaccharides
•3 or more sugars linked together
•Complex sugars
•Important for energy storage
Examples:
Starch- (plants) found in leaves, tubers…
Glycogen- (animals) found in the liver and muscles
Cellulose- (plants) make up cell walls
Starch
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Polysaccharides - Starches
Reducing
end
Amylose
• Plants store glucose as amylose or amylopectin. Both are
glucose polymers collectively called starch.
 Amylose is a glucose polymer with  (14) linkages.
 The end of the polysaccharide with an anomeric C1 that is not
involved in a glycosidic bond is called the reducing end.
• Glucose storage in polymer form minimizes osmotic
effects.
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Amylopectin
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
O
amylopectin
Amylopectin
H
1
O
6 CH2
H 5
H
OH
4
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
 Amylopectin is a glucose polymer with mainly (14)
linkages, but it also has branches formed by  (16)
linkages. Branches are generally longer than those shown in
the diagram above.
• The branches produce a compact structure & provide
multiple chain ends at which enzyme activity can occur.
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Glycogen
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
Glycogen
H
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
• Glycogen, the glucose storage polymer in animals, is similar
in structure to amylopectin found in plants
• Glycogen has more  (16) branches than amylopectin
• The ability to rapidly mobilize glucose is more essential to
animals than to plants.
• The highly branched structure permits rapid glucose release
from glycogen stores, e.g., in muscle during exercise.
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Starch and Cellulose
Amylose
Cellulose
The essential difference between amylose starch and
cellulose is in the glycosidic link between the saccharide
units. Amylose has (1-4) links. Cellulose has (1-4) links.
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Cellulose
• Cellulose is the major building component of plant cell
walls
• Long chain of glucose molecules would be expected to
be a great source of energy, but humans lack the
necessary enzyme to digest cellulose
• The Endosymbiotic Protist in cow guts DOES have the
enzyme
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Dietary Fiber
• Dietary fiber is mainly plant material that is not
hydrolyzed by enzymes secreted by the human
digestive tract but may be digested by
microflora in the gut.
• Examples of dietary fiber include cellulose,
hemicellulose, lignin and pectin.
• Dietary fiber may be helpful in the prevention of
conditions such as diverticulosis, irritable bowel
syndrome, constipation, obesity, Crohn’s
disease, hemorrhoids and diabetes mellitus.
Carbohydrate Functions:
Energy Sources
• During metabolism animals break down
carbohydrates to carbon dioxide and water
vapor.
• Monosaccharides and dissaccharides break
down quickly and provide quick energy
sources.
• Starches take longer to metabolize but the
end products are the same.
• Human beings cannot break down cellulose,
since we lack the appropriate enzyme to
breakdown the  1-4 linkage
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Carbohydrate Functions:
Storage
• The main storage polysaccharides are starches and
glycogen. While plants use starch as their storage
polysaccharides, animals use glycogen.
• When the body has a high glucose concentration, the
pancreas releases insulin, which converts glucose into
glycogen and stores it in the liver.
• When the glucose concentration is low, the hormone
glucagon converts glycogen back into glucose.
• Glycogen is the primary energy reserve in human beings .
Metabolism of glucose provides the energy necessary for
our bodies to function and carry out daily activities.
• When it is broken down into glucose and oxidized,
ultimately to CO2 and H2O, through cellular respiration,
large amounts of energy are released.
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•
•
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Carbohydrate Functions:
Structure
Cellulose is a major component of plant cell
walls. It is a polymer of -D-glucose and forms
a very strong fiber, which is excellent building
material in plants.
Cows and other ruminants have enzymes that
break down cellulose. In humans it is primarily
bulk or roughage.
Chitin is a structural polysaccharide found in the
exoskeletons of some insects.
Chitin is a leather like structural substance that
eventually hardens when it is shed.
Chitin is often used in medicine for sutures
because it is both strong and flexible, but it also
decomposes over time.
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Carbohydrate Functions:
Precursor Molecules
•
•
•
•
Carbohydrates are precursors for the synthesis of
certain biomolecules.
Carbohydrates (ribose) form part of the skeletons of
nucleic acids, DNA and RNA.
The carbon skeletons of carbohydrates serve as raw
material for the synthesis of other small organic
molecules, such as amino acids and fatty acids.
Disaccharides provide building material for structures
that protect the cell or whole organism.
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The End
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