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
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.
I (CH2O)n or H - C - OH
I
Carbohydrates have the following basic composition:
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fructose
Monosaccharides• Single (simple) sugars• Contain C, H, and O in a 1:2:1
ratio • Quick energy sources
Examples:Examples: Glucose C6H12O6
Fructose C6H12O6
Galactose C6H12O6
glucose
Carbohydrates
5Glucose Fructose
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 6
Monosaccharides
Aldoses (e.g., glucose) have an aldehyde group at one end.
Ketoses (e.g., fructose) have a ketone group, usually at C2.
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose7
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
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehydeD-glyceraldehyde
L-glyceraldehydeD-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 O H C C H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
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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.
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).
O H O H C C H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
Steroisomers
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Ring StructuresPentoses 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,
These representations of the cyclic sugars are called Haworth projections.
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
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4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose (linear form)
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Fructose Ring Structures
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
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
HOH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
Fructose may form either
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Monosaccharides
Some examplesof 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
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.
H O
OH
H
OH
H
NH2H
OH
CH2OH
H
-D-glucosamine
H O
OH
H
OH
H
NH
OH
CH2OH
H
-D-N-acetylglucosamine
C CH3
O
H
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Anomers of 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).
H O
OH
H
OHH
OH
CH2OH
H
-D-glucose
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose
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4
5
6
1 1
6
5
4
3 2
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Glycosidic BondsThe 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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H H
•Double sugars•Good source of energy•Break down into simple sugars
Sucrose (glucose + fructose)Lactose (glucose + galactose)
Disaccharides
Other disaccharides include: -- 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 (12). The full name of sucrose is -D-glucopyranosyl-(12)-D-fructopyranose.)
-- Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric OH of galactose. Its full name is -D-galactopyranosyl-(1 4)--D- glucopyranose
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H H
•Compare the structures of these three common disaccharides
Disaccharides
•Sucrose is an (1-4) link between D-Glucose and D-Fructose•Lactose is an (1-4) link between two D glucose
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Examples:Starch- (plants) found in leaves, tubers…Glycogen- (animals) found in the liver and
musclesCellulose- (plants) make up cell walls
•3 or more sugars linked together•Complex sugars•Important for energy storage
Starch
Polysaccharides
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• Plants store glucose as amylose or amylopectin. Both are glucose polymers collectively called starch.
Amylose is a glucose polymer with (14) 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.
Polysaccharides - Starches
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Reducing end
Amylose
Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) 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.
H O
OH
H
OHH
OH
CH2OH
HO H
H
OHH
OH
CH2OH
H
O
HH H O
OH
OHH
OH
CH2
HH H O
H
OHH
OH
CH2OH
H
OH
HH O
OH
OHH
OH
CH2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
CH2OH
HH H O
H
OHH
OH
CH2OH
HH
O1
OH
3
4
5
2
amylopectin
Amylopectin
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Amylopectin
• Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin found in plants
• Glycogen has more (16) 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.
H O
OH
H
OHH
OH
CH 2OH
HO H
H
OHH
OH
CH 2OH
H
O
HH H O
OH
OHH
OH
CH 2
HH H O
H
OHH
OH
CH 2OH
H
OH
HH O
OH
OHH
OH
CH 2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
CH 2OH
HH H O
H
OHH
OH
CH 2OH
HH
O1
OH
3
4
5
2
glycogen
Glycogen
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Glycogen
The essential difference between amylose starch and cellulose is in the glycosidic link between the saccharide units. Amylose has links. Cellulose has links.
Starch and Cellulose
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Cellulose
Amylose
• 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
Cellulose
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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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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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