Chapter  20:   Carbohydrates
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Chapter 20: Carbohydrates. Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone , or a substance that gives these compounds on hydrolysis. Chapter Overview: Monosaccharides Fischer projections Haworth projections Hemiacetals and Acetals Oxidation and reduction Disaccharides

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Chapter 20: Carbohydrates

Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.

Chapter Overview:

  • Monosaccharides

    • Fischer projections

    • Haworth projections

    • Hemiacetals and Acetals

    • Oxidation and reduction

  • Disaccharides

  • Polysaccharides

Chapter 20: Monosaccharides

Monosaccharide:A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.

  • Monosaccharides have the general formula CnH2nOn, where n varies from 3 to 8.

  • Aldose: A monosaccharide containing an aldehyde group.

  • Ketose: A monosaccharide containing a ketone group.

  • The prefixes tri-, tetra, penta, and so forth indicate the number of carbon atoms in the chain.


( ____________ )

Chapter 20: Fischer projections

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 rear.

Chapter 20: Monosaccharides

In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

  • 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.

Chapter 20: Monosaccharides

The most common monosaccharides:


D- Glucose

D- Galactose

D- Fructose

Chapter 20: 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.

Chapter 20: Haworth Projections

  • D-Glucose forms these two cyclic hemiacetals.

Chapter 20: Anomers

  • The anomeric carbon of an aldose is C-1;

  • The anomeric carbon of most ketoses is C-2.

  • β means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH.

  • α means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH.

  • 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.

Chapter 20: Monosaccharides

The prefix “deoxy” means “without oxygen.”

Chapter 20: Monosaccharides

Fructose is a ketose and it also forms cyclic hemiacetals

Chapter 20: Monosaccharides

  • 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.

Chapter 20: Monosaccharides

Hemiacetal + Alcohol = Acetal

  • The acetal obtained from a monosaccharide is called glycoside

  • Mutarotation is not possible in glycosides

Chapter 20: Disaccharides

Disaccharide: a carbohydrate containing two monosaccharide units joined by a glycosidic bond.

Sucrose (table sugar) = Glucose + Fructose

  • Sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets.

  • Sucrose is a nonreducing sugar.

Chapter 20: Monosaccharides

Lactose = Galactose + Glucose

  • 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 β-1,4-glycosidic bond to carbon 4 of D-glucopyranose.

  • Lactose is a reducing sugar.

Chapter 20: Monosaccharides

Maltose = Glucose + Glucose

  • 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.

  • Maltose is a reducing sugar.

Chapter 20: Physical Properties

Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol.

Sweetness relative to sucrose:

Chapter 20: Polysaccharides

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 amylopectin.

  • Amylose is composed of unbranched chains of up to 4000 D-glucose units joined by α-1,4-glycosidic bonds.

  • Amylopectin contains chains up to 10,000 D-glucose units also joined by α-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by α-1,6-glycosidic bonds.

Chapter 20: Polysaccharides

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

Chapter 20: Polysaccharides

Glycogen is the energy-reserve carbohydrate for animals.

  • Glycogen is a branched polysaccharide of approximately 106 glucose units joined by α-1,4- and α-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.

Chapter 20: Polysaccharides

Cellulose is a linear polysaccharide of D-glucose units joined by β-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.

Chapter 20: Polysaccharides

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

  • Humans and other animals can not digest cellulose because their digestive systems do not contain β-glycosidases, enzymes that catalyze the hydrolysis of β-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 α-glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.

  • Many bacteria and microorganisms have β-glucosidases.