Polysaccharides

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Polysaccharides

• To be able to explain
• The formation of the polysaccharides starch,
  glycogen and cellulose.
• Hydrolysis of disaccharides and polysaccharides.
• Relationship of structure to function in starch,
  glycogen and cellulose molecules.

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Polysaccharides

• Polysaccharides are long chains of many
  monosaccharides joined together by glycosidic
  bonds.
• There are three important polysaccharides
• Starch
• Glycogen
• Cellulose

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Starch

• This is the plant storage polysaccharide.
• It is insoluble and forms starch granules inside
  many plant cells.
• It is not a pure substance, but is a mixture of
  amylose and amylopectin

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Amylose and amylopectin

• Amylose is simply poly-(1-4) glucose, so is a
  straight chain. In fact the chain is floppy, and
  it tends to coil up into a helix.
• Amylopectin is poly(1-4) glucose with about 4
  (1-6) branches. This gives it a more open
  molecular structure than amylose. Because it has
  more ends, it can be broken more quickly than
  amylose by amylase enzymes.

Both amylose and amylopectin are broken down by
the enzyme amylase into maltose, though at
different rates. The enzyme can only break down
the 1-4 bond NOT the 1-6 so starch is broken down
into glucose and a disaccharide
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Glycogen

• This is similar in structure to amylopectin.
• It is poly (1-4) glucose with 9 (1-6) branches.
• It is made by animals as their storage
  polysaccharide, and is found mainly in muscle and
  liver.
• Because it is so highly branched, it can be
  mobilised (broken down to glucose for energy)
  very quickly.
• It is broken down to glucose by the enzyme
  glycogen phosphorylase.

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Cellulose

• This is only found in plants, where it is the
  main component of cell walls.
• It is poly (1-4) glucose, but with a different
  isomer of glucose.
• Starch and glycogen contain a-glucose, in which
  the hydroxyl group on carbon 1 sticks down from
  the ring, while cellulose contains ß-glucose, in
  which the hydroxyl group on carbon 1 sticks up.
• This means that in a chain alternate glucose
  molecules are inverted.

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Starch and Cellulose

• Whereas the a1-4 glucose polymer in starch coils
  up to form granules, the ß1-4 glucose polymer in
  cellulose forms straight chains.
• Hundreds of these chains are linked together by
  hydrogen bonds to form cellulose microfibrils.
• These microfibrils are very strong and rigid, and
  give strength to plant cells, and therefore to
  young plants and also to materials such as paper,
  cotton and sellotape.

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Other polysaccharides

• Chitin (poly glucose amine), found in fungal cell
  walls and the exoskeletons of insects.
• Callose (poly 1-3 glucose), found in the walls of
  phloem tubes.
• Dextran (poly 1-2, 1-3 and 1-4 glucose), the
  storage polysaccharide in fungi and bacteria.
• Inulin (poly fructose), a plant food store.
• Pectin (poly galactose uronate), found in plant
  cell walls.
• Agar (poly galactose sulphate), found in algae
  and used to make agar plates.
• Murein (a sugar-peptide polymer), found in
  bacterial cell walls.
• Lignin (a complex polymer), found in the walls of
  xylem cells, is the main component of wood.

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What can you remember?

• What is a hexose?
• Which of the following is a hexose sugar?
• Glucose
• Fructose
• Maltose
• Lactose
• Galactose
• Starch
• Cellulose
• Glycogen
• Give the general formula for a carbohydrate
• What is the name for the reaction that joins two
  monosaccharides together?

• Name a carbohydrate that is
• Stored in mammalian liver and muscle
• Stored in the cells of plants
• Found in cell walls
• Translocated in phloem
• Transported in the blood
• An isomer of glucose
• Found in DNA
• What is the name for the bond that is formed
  between them?
• What is the name for the reactionthat separates
  two monosaccharides joined together?

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What can you remember?

• What is a hexose?
• Which of the following is a hexose sugar?
• Glucose
• Fructose
• Maltose
• Lactose
• Galactose
• Starch
• Cellulose
• Glycogen
• Give the general formula for a carbohydrate
• What is the name for the reaction that joins two
  monosaccharides together?

• Name a carbohydrate that is
• Stored in mammalian liver and muscle
• Stored in the cells of plants
• Found in cell walls
• Translocated in phloem
• Transported in the blood
• An isomer of glucose
• Found in DNA
• What is the name for the bond that is formed
  between them?
• What is the name for the reactionthat separates
  two monosaccharides joined together?

Monosaccharide sugar containing 6 carbon atoms
glycogen
starch
?
cellulose
?
sucrose
?
glucose
Fructose/galactose
deoxyribose
Glycosidic bond
CH2O
condensation
Hydrolysis
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Carbohydrates worksheet 1.

• Structural isomers have the same molecular
  formula but their atoms are linked in different
  sequences.
• For example, fructose and glucose are structural
  isomers because, although they have the same
  molecular formula (C6H12O6), glucose contains an
  aldehyde group (it is an aldose) and fructose
  contains a keto group (it is a ketose).
• In contrast, optical isomers are identical in
  every way except that they are mirror images of
  eachother. The two ring forms of glucose, a and
  ß glucose, are optical isomers, being two mirror
  image forms.

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2

• Isomers wilt have different bonding properties
  and will form different disaccharides and
  macromolecules depending on the isomer involved,
  e.g. glucose and fructose are structural isomers
  glucose glucoseforms maltose, glucose
  fructose from sucrose.
• A polysaccharide of the a isomer of glucose forms
  starch whereas the ß isomer forms cellulose.

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3.

• Compound sugars are formed and broken down
  bycondensation and hydrolysis reactions
  respectively.
• Condensation reactions join two
  carbohydratemolecules by a glycosidic bond with
  the release of awater molecule.
• Hydrolysis reactions use water to split a
  carbohydrate molecule into two, where the
  watermolecule is used to provide a hydrogen atom
  and ahydroxyl group.

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4.

• Cellulose, starch, and glycogen are all polymers
  ofglucose, but differ in form and function
  because of theoptical isomer involved. The
  length of the polymers,and the degree of
  branching.
• Cellulose is an unbranched, long chain glucose
  polymer held by p-1,4 glycosidic bonos
• The straight, tightly packed chains give
  cellulose high tensile strength and resistance to
  hydrolysis
• Starch is a mixture of two polysaccharides

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