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Chapter 20 Carbohydrates


Carbohydrates Polysaccharides Figure 20.3 Amylopectin, a branched polymer of approximately 10,000 units of D-glucose joined by -1,4-glycosidic bonds. – PowerPoint PPT presentation

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Title: Chapter 20 Carbohydrates

Chapter 20 Carbohydrates
  • Carbohydrates (or saccharides) consist of only
    carbon, hydrogen and oxygen.
  • Carbohydrates come primarily from plants, however
    animals can also biosynthesize them
  • The Carbon Cycle describes the processes by
    which carbon is recycled on our planet
  • - Energy from the sun is stored in plants, which
    use photosynthesis to convert carbon dioxide and
    water to glucose and oxygen
  • - In the reverse process, energy is produced
    when animals oxidize glucose during respiration

Simplified Carbon Cycle
  • Carbohydrate A polyhydroxyaldehyde or
    polyhydroxyketone, or a substance that gives
    these compounds on hydrolysis.
  • Monosaccharide A carbohydrate that cannot be
    hydrolyzed to a simpler carbohydrate. Simple
    sugars. Cant be split into smaller carbohydrate
  • - Examples glucose, fructose, galactose,
  • Monosaccharides have the general formula
    CnH2nOn, where n varies from 3 to 8.
  • Aldose A monosaccharide containing an aldehyde
  • Ketose A monosaccharide containing a ketone

  • Disaccharides are two monosaccharides bonded
  • - Can be split into two monosaccharides using an
    acid or enzyme catalyst
  • - Examples sucrose (table sugar), lactose
    (milk sugar)
  • Polysaccharides are polymers of monosaccharides
  • - Used for storage of carbohydrates
  • - Can be split into many monosaccharides with
    acid or enzymes
  • - Examples starch, cellulose, glycogen

Types of Carbs
  • Are chiral! The suffix -ose indicates that a
    molecule is a carbohydrate.
  • The prefixes tri-, tetra, penta, and so forth
    indicate the number of carbon atoms in the chain.
  • Those containing an aldehyde group are classified
    as aldoses.
  • Those containing a ketone group are classified as
  • There are only two trioses
  • Often aldo- and keto- are omitted and these
    compounds are referred to simply as trioses.
  • Although triose does not tell the nature of the
    carbonyl group, it at least tells the number of

  • Glyceraldehyde, the simplest aldose, contains
    one stereocenter and exists as a pair of

  • Fischer projection A two-dimensional
    representation for showing the configuration of
    tetrahedral stereocenters.
  • Horizontal lines represent bonds projecting
    forward from the stereocenter.
  • Vertical lines represent bonds projecting to the
  • Only the stereocenter is in the plane.

Terminal carbonyl groups (aldehydes CHO and
carboxylic acids COOH)
are written vertically on Fisher diagram
  • In 1891, Emil Fischer made the arbitrary
    assignments of D- and L- to the enantiomers of
  • D-monosaccharide the -OH on its penultimate
    carbon is on the right in a Fischer projection.
  • L-monosaccharide the -OH on its penultimate
    carbon is on the left in a Fischer projection.

Fischer got lucky. For sugars, DR and rotates
light clockwise () LS and counter (-). BUT,
does the Fisher projection for D appear to be R
Get a Model Kit
  • Before building the molecule, draw the D-
    Glyceraldehyde enantiomer (last slide) on your
    paper as both a wedge diagram and a Fisher
    diagram. Answer the following
  • 1- Based solely on the Fisher diag, assign
    priority to the groups (Ch 15 p.427). Reading the
    order of the groups, what direction would you
    assign,clockwise or counterclockwise?__
  • 2- Build the molecule, based on the Fisher. Is
    the carbonyl C group pointing toward you or away
    from you?____ What about the H and OH? _______
  • 3- Now, rotate the molecule, putting the lowest
    ranked group away from you. Approximately, how
    many degrees did you turn the molecule?____Where
    is the OH after the rotation?
  • 4- Is the direction now clockwise, or
  • 5- In terms of ONLY seeing a (similar) Fisher
    diagram, what will you have to do (in your head)
    before determining D or L?

  • The most common D-tetroses and D-pentoses are
  • The three most common D-hexoses are

You do NOT have
To memorize these!
See table 20.1 and 20.2
Sugars (Monosaccharides)
  • All other sugars are classified based on the
    position of the hydroxyl group farthest away from
    the carbonyl, but not the one on the end.
  • Penultimate carbon- Point of reference that
    refers to the next to last C atom in the chain.
  • See Table 20.1 and 20.2 (penultimate C is in red)

Three Important Monosaccharides
  • D-Glucose (aldose) is the most common
  • - Primary fuel for our cells, required for many
  • - Main sources are fruits, vegetables, corn
    syrup and honey
  • - Blood glucose is maintained within a fairly
    small range
  • - Some glucose is stored as glycogen, excess is
    stored as fat
  • D-Galactose (aldose) comes from hydrolysis of the
    disaccharide lactose
  • - Used in cell membranes of central nervous
  • - Converted by an enzyme into glucose for
    respiration (lack of this enzyme causes
    galactosemia, which can cause retardation in
    infants if not treated by complete removal from
  • D-Fructose (ketose) is the sweetest carbohydrate
  • - Converted by an enzyme into glucose for
  • - Main sources are fruits and honey
  • - Also obtained from hydrolysis of the
    disaccharide sucrose

Structures of Glucose, D-Galactose and D-Fructose
  • Note that in nature, only the D enantiomers of
    sugars are used
  • What is the relationship between D-glucose and
  • What is the relationship between D-glucose and
  • What is the relationship between D-glucose and
  • (use these choices for each question
    enantiomers, diastereomers or constitutional

Amino Sugars
  • Amino sugars contain an -NH2 group in place of
    an -OH group.
  • Only three amino sugars are common in nature
    D-glucosamine, D-mannosamine, and D-galactosamine.

Cyclic Structure
  • Aldehydes and ketones react with alcohols to form
    hemiacetals (Chapter 17).
  • Cyclic hemiacetals form readily when the hydroxyl
    and carbonyl groups are part of the same molecule
    and their interaction can form a five- or
    six-membered ring.

AlcoholAldehyde (or alcoholketone)?Hemiacetal
Cyclic Structures of Monosaccharides
  • Recall that an alcohol can react with an aldehyde
    or ketone to form a hemiacetal
  • If the alcohol and aldehyde or ketone are in the
    same molecule, a cyclic hemiacetal is formed
  • Monosaccharides in solution are in equilibrium
    between the open-chain and ring forms, and exist
    primarily in the ring form

Haworth Projection
  • A Haworth projection is a common way of
    representing the cyclic structure of
    monosaccharides with a simple three-dimensional

Drawing Haworth Structures for Cyclic Forms
  • Step 1 Number the carbons in the chain and turn
    the Fischer projection of the open-chain form
    clockwise 90 degrees
  • - Hydroxyl groups that were on the right are now
    on the bottom, and hydroxyl groups that were on
    the left are now on the top (they will stay on
    bottom or top in the Haworth structure)
  • Step 2 Rotate around so that C-6 sticks up from
    C-5, and the hydroxyl group on C-5 points towards
  • Step 3 Form the cyclic hemiacetal by bonding
    the hydroxyl O to the carbonyl C and moving the
    hydroxyl H to the carbonyl O
  • Note For C-6 aldose sugars, the cyclic
    hemiacetal has a new chiral carbon at C-1
  • - The two possible stereoisomers are called
  • - The alpha anomer has the hydroxyl group down
  • - The beta anomer has the hydroxyl group up

Haworth Projections
  • D-Glucose forms these two cyclic hemiacetals.

Haworth Projections
  • Groups bonded to the carbons of the ring then lie
    either above or below the plane of the ring.
    (Remember your stereoisomer lab!)
  • Stereoisomers that differ in configuration only
    at the anomeric carbon are called anomers.
  • In a ring, the OH on the alpha anomer is down
    and axial Beta is up and equatorial. Most
    Glucose in our bodies is Beta because its
    farther away and more stable
  • The anomeric carbon of an aldose is C-1 that of
    the most common ketoses is C-2 (because there is
    a branch off of the first C and the C in the
    branch is 1). See next slide.

Haworth Projections
  • D-Fructose (a 2-ketohexose) also forms a
    five-membered cyclic hemiacetal.

For test, know that 6 C ketose sugar (fructose)
has anomeric C at 2
Haworth Projections
  • A six-membered hemiacetal ring is called a
    pyranose, and a five-membered hemiacetal ring is
    called a furanose because these ring sizes
    correspond to the heterocyclic compounds furan
    and pyran. (DO NOT have to know furanose for

Chair Conformations
  • For pyranoses, the six-membered ring is more
    accurately represented as a chair conformation.

Which is equatorial and which is axial?
Chair Conformations
  • In both Haworth projections and chair
    conformations, the orientations of groups on
    carbons 1- 5 of b-D-glucopyranose are up, down,
    up, down, and up.

  • Mutarotation The change in specific rotation
    that accompanies the equilibration of a- and
    b-anomers in aqueous solution.
  • Example When either a-D-glucose or b-D-glucose
    is dissolved in water, the specific rotation of
    the solution gradually changes to an equilibrium
    value of 52.7, which corresponds to 64 beta
    and 36 alpha forms.

ONLY have to know that most glucose in body is
  • Formation occurs by dehydration between 2 OHs
    of 2 monosaccharide monomers. One monomer removes
    OH and the other removes H. The O that was not
    removed forms a bond between the anomeric Cs.
    The new bond is called a Glycosidic bond. The
    process is commonly called dehydration synthesis
    or condensation.
  • Maltose is glucoseglucose
  • Sucrose is glucosefructose
  • Lactose is glucosegalactose

  • Disaccharide a carbohydrate containing two
    monosaccharide units joined by a glycosidic bond.
  • Sucrose (table sugar)
  • Sucrose is the most abundant disaccharide in the
    biological world it is obtained principally from
    the juice of sugar cane and sugar beets. A water
    molecule is removed (not shown)

  • Maltose
  • Present in malt, the juice from sprouted barley
    and other cereal grains.
  • Maltose consists of two units of D-glucopyranose
    joined by an a-1,4-glycosidic bond.

  • Lactose
  • Lactose is the principal sugar present in milk
    it makes up about 5 to 8 percent of human milk
    and 4 to 6 percent of cow's milk.
  • It consists of D-galactopyranose bonded by a
    b-1,4-glycosidic bond to carbon 4 of

Physical Properties
  • Monosaccharides are colorless crystalline
    solids, very soluble in water, but only slightly
    soluble in ethanol.
  • Sweetness relative to sucrose

  • Polysaccharide A carbohydrate consisting of
    large numbers of monosaccharide units joined by
    glycosidic bonds.
  • Starch A polymer of D-glucose.
  • Starch can be separated into amylose and
  • Amylose is composed of unbranched chains of up to
    4000 D-glucose units joined by a-1,4-glycosidic
  • Amylopectin contains chains up to 10,000
    D-glucose units also joined by a-1,4-glycosidic
    bonds at branch points, new chains of 24 to 30
    units are started by a-1,6-glycosidic bonds.

  • Figure 20.3 Amylopectin, a branched polymer of
    approximately 10,000 units of D-glucose joined by
    ?-1,4-glycosidic bonds.

  • Glycogen is the energy-reserve carbohydrate for
  • Glycogen is a branched polysaccharide of
    approximately 106 glucose units joined by a-1,4-
    and a-1,6-glycosidic bonds.
  • The total amount of glycogen in the body of a
    well-nourished adult human is about 350 g,
    divided almost equally between liver and muscle.

  • Cellulose is a linear polysaccharide of
    D-glucose units joined by b-1,4-glycosidic bonds.
  • It has an average molecular weight of 400,000
    g/mol, corresponding to approximately 2200
    glucose units per molecule.
  • Cellulose molecules act like stiff rods and align
    themselves side by side into well-organized
    water-insoluble fibers in which the OH groups
    form numerous intermolecular hydrogen bonds.
  • This arrangement of parallel chains in bundles
    gives cellulose fibers their high mechanical
  • It is also the reason why cellulose is insoluble
    in water.

  • Figure 20.4 Cellulose is a linear polymer
    containing as many as 3000 units of D-glucose
    joined by b-1,4-glycosidic bonds.

  • Cellulose (contd)
  • Humans and other animals can not digest cellulose
    because their digestive systems do not contain
    b-glycosidases, enzymes that catalyze the
    hydrolysis of b-glycosidic bonds.
  • Termites have such bacteria in their intestines
    and can use wood as their principal food.
  • Ruminants (cud-chewing animals) and horses can
    also digest grasses and hay.
  • Instead, we have only a-glucosidases hence, the
    polysaccharides we use as sources of glucose are
    starch and glycogen.
  • Many bacteria and microorganisms have

Chapter 20 Carbohydrates
  • End
  • Chapter 20