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Transcript of 1 Chapter 17: Carbohydrates. 2 IMPORTANT FUNCTIONS OF CARBOHYDRATES To provide energy through their...
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Chapter 17:Carbohydrates
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IMPORTANT FUNCTIONS OF CARBOHYDRATES• 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 monosaccharideSometimes called dextrose or blood sugar
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IMPORTANT MONOSACCHARIDES, cont.• Galactose
A component of lactose (milk sugar)
• FructoseThe sweetest monosaccharideSometimes 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 carbonatom joined by the linkage
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IMPORTANT DISACCHARIDES• Maltose
Two glucose units linked (14)Formed during the digestion of starch to glucoseFound in germinating grainHemiacetal means maltose is a reducing sugar
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IMPORTANT DISACCHARIDES, cont.• Lactose
Galactose and glucose units linked (14)Found in milkHemiacetal means lactose is a reducing sugar
• SucroseFructose 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 unitsUsed to store glucose, especially in the liver and musclesStructurally similar to amylopectin with (14) and (16) linkages, but more highly branched
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POLYSACCHARIDES, cont.• Cellulose
A polymer of glucose unitsThe most important structural polysaccharideFound in plant cell wallsLinear polymer like amylose, but has (14) glycosidic
linkagesNot easily digested, a constituent of dietary fiber
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CELLULOSE STRUCTURE