Carbohydrates: What do you need to know?
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Characteristics of Carbohydrates
– Structural & functional
Chirality & Isomerism
– Fischer projections
– Stereoisomers:
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Monosaccharides
– Important ones to know
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Enantiomers vs. Diastereomers
(trioses-hexoses)
– Haworth projections
– Reactions
Disaccharides
Polysaccharides
– Characterisitics & types
Biological Issues
– Cells
– Diet
Carbohydrates
• Carbo (carbon) hydrate (water) - Cn(H2O)n
• Carbohydrates are produced in plants through Photosynthesis
–
CO2 + H2O +
--> carbohydrates + O2
cellulose (structural components)
starch (energy reserve)
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Carbs in Humans
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Carbohydrate oxidation --> chemical & heat energy
Carbohydrate storage = glycogen (energy reserve)
Carbohydrates provide C atoms for synthesis of proteins, nucleic acids & lipids
Carbohydrates are important in structure of DNA & RNA
Carbohydrates can be linked with lipids & proteins
Types of Carbohydrates
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All carbohydrates are polyhydroxy aldehydes (PA - ex: glucose) or
ketones (PK - ex: fructose) or compounds that hydrolyze to produce
them.
• Monosaccharides
– Composed of ONE PA or PK unit
– Monomers for larger carbohydrates that are formed by condensation
reactions
– Simplest- Trioses: 2,3-dihydroxypropanal & dihydroxypropanone
• Disaccharides - 2 monomer units
• Oligosaccharides
– Composed of 3 - 10 monosaccharide units
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Humans can’t digest most of these; bacteria in colon do (producing large
quantities of CO2 gas)
• Polysaccharides
– Composed of “many” monosaccharide units
aldohexose
ketohexose
Dietary Carbs
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Dietary Carbs: glucose (m), fructose (m), sucrose (d), starch (p)
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Simple (sugars) vs. Complex (starch)
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1900
eat less
eat more
relatively pure
quickly digested
obtained from nutrient rich foods
gradually digested
vs.
2000
cal = 2 starch : 1 sugar
commercial food = 25% sugar eaten
cal = 1 starch : 1 sugar
commercial food = 70% sugar eaten
Monosaccharides
• Most common in nature have 3-7 C atoms
– Trioses - Heptoses
– Aldoses vs. Ketoses
• Sugars - many are sweet-tasting
• Stereoisomers
• Nearly all naturally occurring are D
• L isomers cannot be used by the body.
– # of isomers/aldose - dependent on # of chiral
centers
– # of isomers/ketose - 1/2 the number possible for
aldoses (= # of C atoms)
Fischer projections and common names for D-aldoses three, four,
five, and six carbon atoms.
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Fischer
projections
and common
names for
ketoses
containing
three, four,
five, and six
carbon atoms.
SIX biochemically important monosaccharides
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All are D enantiomers
Two Trioses
One pentose
Three hexoses
• Know their structures!!
Trioses
• D-glyceraldehyde
• Dihydroxyacetone
– 2,3-dihydroxypropanal
– Dihyroxypropanone
– Intermediate in
glycolysis
– Chiral molecule
– Intermediate in
glycolysis
– Achiral molecule
Hexoses
D-glucose
aldohexose
Dextrose, Blood sugar, Grape sugar
D-galactose
aldohexose
glucose + galactose --> lactose
D-fructose
ketohexose
Levulose, Fruit sugar
Pentose
• D-ribose
– An important component of nucleic acids
– Found in energy rich compounds (ie. ATP)
– D-ribose: in RNA
– 2-deoxy-D-ribose: in DNA
Cyclic Monosaccharides
• Common in pentoses & hexoses
– Open chain <==> cyclic
• Method of cyclization
– Carbonyl group reacts with hydroxyl group
– Hemiacetal formation
– Ring to Chain conversion
cyclic
hemiacetal
forms of
D-glucose:
intramolecular
reaction between
carbonyl
group &
hydroxyl
group on
carbon #5.
Haworth projection formulas:
•Cyclic monosaccharide
•Count C # clockwise from O atom in ring
•Highest # C shows D or L form
(D form sticks up from the ring)
•Alpha or beta shown by #1C’s -OH group
 will be across D or L CH2OH group
 will be beside D or L CH2OH group
Haworth Projection Formulas
• Specifications
– D vs. L: determined by position of terminal CH2OH group
• Up = “D”
Down = “L”
 
determined by position of -OH on C#1 relative to CH2OH
 opposite directions
 : both same direction
– If  &  doesn’t matter, attach -OH with wavy line
– Specific identification of compound is determined by positions of
other -OH groups
• Fischer
Haworth
• -OH on right = -OH down
• -OH on left = -OH up
Reactions of Monosaccharides
• Oxidation (product is acidic sugar)
– Aldose --> aldonic acid
• Weak oxidizing agent (ex.:Fehling’s or Tollens) causes
oxidation at the aldehyde end
• Ketoses turn into aldoses due to basic solution
– Aldose --> aldaric acid
• Strong oxidizing agent causes oxidation at both ends
– Aldose --> alduronic acid
• Enzymes at certain lab conditions can cause oxidation at
the 1˚ alcohol end only
The glucose content of urine can be
determined by dipping a plastic strip
treated with oxidizing agents.
Blue Benedict’s solution makes a red
precipitate when reducing sugar
(glucose) reacts
Reactions of Monosaccharides
• Reduction (product is sugar alcohol)
– Reaction takes place at the carbonyl group
Glycoside Formation
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Remember: hemiacetal + alcohol --> acetal
– Acetals have two –OR groups attached to same C atom
• Cyclic Aldose/ketose + alcohol --> glycoside
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Naming:
Indicate  or  form
List alkyl or aryl group on O
Then monosaccharide name
Finally add -ide suffix
(monosaccharide acetal)
Giant Hogweed (Heracleum mantegazzianum)
•Native to Caucasus Mountains in Asia
•A problem weed recently found in Oregon.
•Clear, watery sap contains a glucoside,
which causes phyto-dermatitis.
•Skin contact with sap, then exposure to sunlight
produces painful, burning blisters which
may develop into purplish or blackened scars