carbohydrates
TRANSCRIPT
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Chapter 20: Carbohydrates
K.Dunlap Chem 104
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Carbohydrates• Composed of carbon, oxygen, hydrogen
• two-third of the human diet is composed of carbohydrates
• most carbohydrates are produced by the photosynthesis in green plants
• the three elements that make up all carbohydrates are arranged as alcohols, aldehydes, or ketones
•The three main classes of carbohydrates are: monosaccharides, disaccharides, polysaccharides
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Carbohydrates
• Monosaccharide:Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.
– Monosaccharides have the general formula CCnnHH2n2nOOnn, where nn varies from 3 to 8.
– Aldose:Aldose: a monosaccharide containing an aldehyde group.
– Ketose:Ketose: a monosaccharide containing a ketone group.
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1) Monosaccharides• also known as simple sugars can not be broken down into smaller CHO
• glucose, galactose, fructose, ribose, and deoxyribose
• hexoses and pentoses
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aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal forming a ring with an oxygen bridge between 2 carbon atoms. Rings with five carbons are called furanoses and rings with 6 carbons are call pyranoses
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Smallest monosaccharides
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Monosaccharides have chiral carbons
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The red atoms highlight the aldehyde group, and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this –OH is on the right of the Fischer projection, this is a
D sugar D-glucose
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Glucose
• the most biologically important monosaccharide
• used directly by the body for energy
• oxidized to carbon dioxide and water
• Excess stored as adipose
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Galactose is in many plant gums and pectins
• component of the disaccharide lactose
Fructose is the sweetest of all the naturally occurring sugars
• honey, fruits
• component of the disaccharide sucrose
Monosaccharides
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Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH group bound to the carbon 5: they are either on the opposite sides (α), or the same side (β) The α and β anomers of
glucose.
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Other monosaccarides
-components of DNA and RNA
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Disaccharides
consist of 2 monosaccharide Sucrose• glucose and fructose• table sugar • 1/4 of total calories
Lactose• glucose and galactose• milk
Maltose• glucose and glucose• germinating grains
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Disaccharides• 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.
– Glucose and fructose linked together with a -1,2-glycosidic bond
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Disaccharides• 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.
– Contains glucose and galactose linked together with a -1,4-glycosidic bond
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Disaccharides• Maltose
– Present in malt, the juice from sprouted barley and other cereal grains.
– Maltose consists of two units of glucose joined by an -1,4-glycosidic bond.
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Physical Properties• Monosaccharides are colorless crystalline
solids, very soluble in water, but only slightly soluble in ethanol– Sweetness relative to sucrose:
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Polysaccharides
- a carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.
Starch -2/3 of the human diet-Potatoes, rice, wheat, cereal grains-mixture of amylose and amylopectin
Glycogen-only storage for glucose in the body-liver and muscle-similar in structure to amylopectin but, more branched
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Cellulose - forms the structural component of the cell walls of plants
-cotton, paper, linen, rayon
-long unbranched chains of glucose (100- 10,000 glucose molecules)
- linkages instead of linkages that are present in starch
-many herbivores have the enzyme to breakdown linkages
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Polysaccharides• Starch:Starch: a polymer of D-glucose.
– Starch can be separated into amylose and amylopectin.– Amylose is composed of unbranched chains of up to 4000
glucose units joined by -1,4-glycosidic bonds.– Amylopectin contains chains up to 10,000 D-glucose units
also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds.
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Polysaccharides• Figure 20.3 Amylopectin.
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Polysaccharides
• GlycogenGlycogen is the energy-reserve carbohydrate for animals.
– Glycogen is a branched polysaccharide of approximately 106 glucose units joined by -1,4- and -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.
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Polysaccharides
• CelluloseCellulose is a linear polysaccharide of D-glucose units joined by -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 strength.
– It is also the reason why cellulose is insoluble in water.
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Polysaccharides• Cellulose (cont’d)
– Humans and other animals cannot use cellulose as food because our digestive systems do not contain -glucosidases, enzymes that catalyze hydrolysis of -glucosidic bonds.
– Instead, we have only -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.
– Many bacteria and microorganisms have -glucosidases and can digest cellulose.
– 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.
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• Figure 20.4 Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by -1,4-glycosidic bonds.
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Starch And Cellulose
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1. What are the three main classes of carbohydrates?
2. Cellulose is made from glucose, but humans can not digest cellulose. Explain.
3. How does your body store carbohydrates?
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4. Indicate whether each of the following is a monosaccharide, disaccharide, or polysaccharide.a) sucroseb) cellulosec) glucosed) lactose
5. Starch is a mixture of two types of polysaccharides. Name them.
6. Name the 2 monosaccharides in each of the following.a) sucroseb) maltosec) lactose