Carbohydrates

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Carbohydrates

• Larry J Scheffler
• Lincoln High School
• 2009

Version 1.10
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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

• 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 121 ratio
• Quick energy sources

Examples Glucose C6H12O6 Fructose
C6H12O6 Galactose C6H12O6
Fructose
Glucose
glucose
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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 CO.
• 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.

Ketoses (e.g., fructose) have a ketone group,
usually at C2.
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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.

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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.

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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.
• 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,

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.

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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

• 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

• Cyclization of glucose produces a new asymmetric
  center at C1. The 2 stereoisomers are called
  anomers, a b.
• Haworth projections represent the cyclic sugars
  as having essentially planar rings, with the OH
  at the anomeric C1
• a (OH below the ring)
• b (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) Lactose (glucose
galactose)
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 a (O points down from ring),
the linkage is a(1?2). The full name of
sucrose is a-D-glucopyranosyl-(1?2)-b-
D-fructopyranose.) -- Lactose, milk sugar, is
composed of galactose glucose, with b(1?4)
linkage from the anomeric OH of galactose. Its
full name is b-D-galactopyranosyl-(1? 4)-a-D-
glucopyranose
H
H
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Disaccharides

• Compare the structures of these three common
  disaccharides

H
H

• Sucrose is an a (1-4) link between D-Glucose and
  D-Fructose
• Lactose is an a (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 a (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
Amylopectin

• Amylopectin is a glucose polymer with mainly
  a(1?4) linkages, but it also has branches formed
  by a (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
Glycogen

• Glycogen, the glucose storage polymer in animals,
  is similar in structure to amylopectin found in
  plants
• Glycogen has more a (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
  successive saccharide units. Cellulose has
  alternating a and b 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, Crohns
  disease, hemorrhoids and diabetes mellitus.

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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 b
  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 b-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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