Carbohydrates: Monosaccharides, Disaccharides and Polysaccharides

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Carbohydrates Monosaccharides, Disaccharides
and Polysaccharides
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Carbohydrates

• Contain carbon, hydrogen and oxygen
• Ratio hydrogen oxygen 21
• Monosaccharides (CH2O)n n 3 to 9
• Disaccharides 2(CH2O)n H2O
• Polysaccharides (C6H10O5)n n gt 300

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Monosaccharides

• Formula (CH2O)n
• C H O 121 ratio
• Contain carbonyl group CO
• Contain at least 2 hydroxyl groups OH
• Carbonyl and hydroxyl groups are the reactive
  groups in the molecule

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Trioses
H
O
H
Alkanal
C
C
OH
H
Alkanone
OH
H
H
C
C
O
H
H
H
OH
C
C
OH
H
Glyceraldehyde (aldose sugar)
Dihydroxyacetone (ketose sugar)
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Pentoses
H
O
CH2OH
OH
C
O
C
OH
H
H
H
H
H
OH
H
C
OH
H
C
OH
OH
OH
H
C
H
Ribose - ring form
Ribose - chain form
6
Hexoses
1
6
CH2OH
H C O H C OH HO C H H C
OH H C OH H C OH
O
5
2
H
H
H
1
4
3
OH
H
OH
HO
4
2
3
H
OH
5
Glucose ring form
6
Glucose chain form
Aldose sugar
H
7
1
6
CH2OH
H C O H C OH HO C H HO C
H H C OH H C OH
O
5
2
H
HO
H
1
4
3
OH
H
OH
H
4
2
3
H
OH
5
Galactose Aldose sugar
6
H
8
H
1
H C OH C O HO C H H C
OH H C OH H C OH
CH2OH
O
CH2OH
2
3
OH
H
H
HO
4
5
OH
H
6
Fructose ketose sugar
H
9
CH2OH
CH2OH
O
O
OH
H
H
H
H
H
OH
OH
H
H
OH
H
HO
HO
H
OH
H
OH
a-glucose
ß-glucose
Hydroxyl group on C1 below the ring
Hydroxyl group on C1 above the ring
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Disaccharides

• Formed by condensation reaction of two
  monosaccharides
• Water is removed
• Glycosidic bond is formed

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maltose
glucose
glucose
CH2OH
CH2OH
CH2OH
CH2OH
O
O
H
H
H
H
O
O
H
H
H
H
H
H
H
H
H2O
OH
H
H
OH
H
OH
H
OH
O
OH
HO
HO
OH
HO
OH
OH
H
OH
H
H
H
OH
OH
a1,4-glycosidic bond
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Disaccharide Constituent monosaccharide Type of glycosidic bond Occurrence
Lactose ß-galactose a-glucose ß1,4 glycosidic Found in mammalian milk
Maltose a-glucose a 1,4 glycosidic Formed by action of amylase on starch
Sucrose a-glucose ß-fructose a 1,2 glycosidic Found in sugar cane and sugar beet
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Reducing and Non-Reducing Sugars

• Sugars that are oxidised on heating are known as
  reducing sugars
• Turn Benedicts reagent from blue (copper sulphate
  to red (copper oxide)
• All Monosaccharides are reducing sugars
• Disaccharides Lactose and Maltose are reducing
  sugars
• Sucrose is non-reducing

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Polysaccharides

• Chains of monosaccharides
• Formed from condensation polymerisation
• Examples include
• Starch
• Cellulose
• Glycogen

All polymers of glucose
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Polysaccharide Constituent Monosaccharide Type of Glycosidic Bond Shape of Molecule
Starch a-glucose (amylose) a1,4 glycosidic Unbranched chain wound into helix
Starch a-glucose (amylopectin) a 1,4 with some a 1,6 Tightly packed branched chain
Glycogen a-glucose a 1,4 with more a 1,6 than amylopectin Very branched compact molecule
Cellulose ß-glucose ß1,4 glycosidic Unbranched straight chains
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Starch

• Polymer of a-glucose
• Mixture of amylose and amylopectin.
• 30 Amylose
• unbranched chains, a 1,4 glycosidic linkages.
• helical shape with 6 glucose residues per turn.
• 70 Amylopectin
• a 1,4 glycosidic linkages
• branches of a 1,6 glycosidic linkages
• usually every 20 to 30 residues.

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Amylopectin
a 1,6 glycosidic linkage
O
O
O
O
a 1,4 glycosidic linkage
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Glycogen

• Polymer of a-glucose monomers
• a 1,4 glycosidic linkages
• Branches a 1,6 glycosidic linkages every 8 to 12
  residues along the chain.
• Results in a very compact structure.
• Glycogen acts as an energy store in the liver and
  muscles of animals.

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Cellulose

• Cellulose is a polymer of ß-glucose
• ß 1,4 glycosidic linkages form straight
  unbranched chains.
• The chains do not coil into helices.
• Cellulose is an important structural component of
  plant cell walls.
• Only a few organisms possess the enzyme cellulase
  that can hydrolyse cellulose to glucose.

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Cellulose
O
O
O
O
O
O
O
O
O
O
O
O
ß 1,4 glycosidic linkage