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Chapter 17:
Carbohydrates
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IMPORTANT FUNCTIONS OF CARBOHYDRATES
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To provide energy through their oxidation
To supply carbon for the synthesis of cell components
To serve as a stored form of chemical energy
To form a part of the structural elements of some cells and
tissues
• Biomolecule – a general term referring to organic
compounds essential to life
• Biochemistry – a study of the compounds and processes
associated with living organisms
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CARBOHYDRATES
• Carbohydrates are polyhydroxy aldehydes or ketones, or
substances that yield such compounds upon hydrolysis.
Example:
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CLASSIFICATION OF CARBOHYDRATES
• Carbohydrates are classified according to size:
Monosaccharide – a single polyhydroxy aldehyde or
ketone unit
Disaccharide – composed of two monosaccharide units
Polysaccharide – very long chains of linked
monosaccharide units
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STEREOCHEMISTRY
• Many carbohydrates exist as enantiomers –
stereoisomers that are mirror images.
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• A chiral object cannot be superimposed on its mirror
image.
• A chiral carbon is one that has four different groups
attached to it.
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• The presence of a single chiral carbon gives rise to
stereoisomerism.
If a carbon atom is attached to four different groups, it
is chiral.
If any two groups are identical, it is not chiral.
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• Compounds can have more than one chiral carbon:
• The maximum number of stereoisomers is 2n where n=
number of chiral carbon atoms.
• Therefore, this compound with two chiral carbon atoms
has 22 or 4 stereoisomers.
• The compound on the previous slide with four chiral
carbon atoms has 24 or 16 stereoisomers.
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FISCHER PROJECTIONS
• Fischer projections depict three-dimensional shapes for
chiral molecules, with the chiral carbon represented by the
intersection of two lines.
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• Fischer projections of carbohydrates have the carbonyl
(C=O) at the top. It is projecting away from the viewer
behind the plane in which it is drawn.
• The hydroxyl group on the chiral carbon farthest from the
C=O group determines whether the carbohydrate is D (OH
on right) or L (OH on left). The two horizontal bonds are
coming toward the viewer out of the plane in which they
are drawn.
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• D and L enantiomers rotate polarized light in opposite
directions.
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• The enantiomer that rotates polarized light to the left is the
levorotatory or (-) enantiomer.
• The enantiomer that rotates it to the right is the
dextrorotatory or (+) enantiomer.
• The D and L designations do not represent dextrorotatory
and levorotatory.
• In some instances only the D or L enantiomers are found
in nature. They are rarely found together in the same
biological system.
• For example, humans can only metabolize the D-isomers
of monosaccharides.
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MONOSACCHARIDE CLASSIFICATION
• Is the monosaccharide an aldehyde (aldose) or ketone
(ketose)?
• How many carbon atoms are in the monosaccharide?
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COMBINING MONOSACCHARIDE CLASSIFICATIONS
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PHYSICAL PROPERTIES OF MONOSACCHARIDES
• Most are called sugars because they taste sweet.
• Because of the many –OH groups, they form hydrogen
bonds with water molecules and are extremely water
soluble.
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MONOSACCHARIDE REACTIONS
• All monosaccharides with at least five carbon atoms exist
predominantly as cyclic hemiacetals and hemiketals.
• A Haworth structure can be used to depict the  (-OH on
the anomeric carbon pointing down) and  (-OH on the
anomeric carbon pointing up) anomers of a
monosaccharide.
• Anomers are stereoisomers that differ in the 3-D
arrangement of groups at the anomeric carbon of an
acetal, ketal, hemiacetal, or hemiketal group.
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SIX-MEMBERED PYRANOSE RING SYSTEM
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FIVE-MEMBERED FURANOSE RING SYSTEM
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MONOSACCHARIDE REACTIONS, cont.
• The –OH groups of monosaccharides can behave as
alcohols and react with acids (especially phosphoric acid)
to form esters.
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MONOSACCHARIDE REACTIONS, cont.
• Cyclic monosaccharide hemiacetals and hemiketals react
with alcohols to form acetals and ketals, referred to as
glycosides.
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IMPORTANT MONOSACCHARIDES
• Ribose and Deoxyribose
Used in the synthesis of DNA and RNA
• Glucose
Most nutritionally important monosaccharide
Sometimes called dextrose or blood sugar
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IMPORTANT MONOSACCHARIDES, cont.
• Galactose
A component of lactose (milk sugar)
• Fructose
The sweetest monosaccharide
Sometimes called levulose or
fruit sugar
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DISACCHARIDES
• Two monosaccharide units linked together by acetal or
ketal glycosidic linkages
• A glycosidic linkage is identified by:
• the numbers associated with the carbon atoms joined ﾊ
together by the linkage
• the configuration of the linkage for any anomeric
carbon atom joined by the linkage
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IMPORTANT DISACCHARIDES
• Maltose
Two glucose units linked (14)
Formed during the digestion of starch to glucose
Found in germinating grain
Hemiacetal means maltose is a reducing sugar
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IMPORTANT DISACCHARIDES, cont.
• Lactose
Galactose and glucose units linked (14)
Found in milk
Hemiacetal means lactose is a reducing sugar
• Sucrose
Fructose and glucose units
Found in many plants
(especially sugar cane, sugar beets)
Not a reducing sugar
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POLYSACCHARIDES
• Starch
A polymer consisting of glucose units
Has two forms
•Unbranched amylose (10-20%)
•Branched amylopectin (80-90%)
Amylose complexes with iodine to form a dark blue color
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THE STRUCTURE OF AMYLOSE
The molecular conformation of starch and the
starch-iodine complex: (a) the helical
conformation of the amylose chain and
(b) the starch-iodine complex.
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AMYLOPECTIN STRUCTURE
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POLYSACCHARIDES, cont.
• Glycogen (animal starch)
A polymer of glucose units
Used to store glucose, especially in the liver and muscles
Structurally similar to amylopectin with (14) and
(16) linkages, but more highly branched
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POLYSACCHARIDES, cont.
• Cellulose
A polymer of glucose units
The most important structural polysaccharide
Found in plant cell walls
Linear polymer like amylose, but has  (14) glycosidic
linkages
Not easily digested, a constituent of dietary fiber
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CELLULOSE STRUCTURE
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