ch22 - Chapter 22 Carbohydrates Introduction Classification...

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Chapter 22 Carbohydrates
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Chapter 22 2 Introduction Classification of Carbohydrates Carbohydrates have the general formula C x (H 2 O) y Carbohydrates are defined as polyhydroxy aldehydes or ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones Monosaccharides are carbohydrates that cannot be hydrolyzed to simpler carbohydrates Disaccharides can be hydrolyzed to two monosaccharides Oligosaccharides yield 2 to 10 monosaccharides Polysacccharides yield >10 monosaccharides
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Chapter 22 3 Photosynthesis and Carbohydrate Metabolism Carbohydrates are synthesized in plants by photosynthesis Light from the sun is absorbed by chlorophyll and this is converted to the energy necessary to biosynthesize carbohydrates Carbohydrates act as a repository of solar energy The energy is released when animals or plants metabolize carbohydrates Much of the energy released by oxidation of glucose is trapped in the molecule adenosine triphosphate (ATP) The phosphoric anhydride bond formed when adenosine triphosphate (ADP) is phosphorylated to make ATP is the repository of this energy This chemical energy is released when ATP is hydrolyzed or a new anhydride linkage is created
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Chapter 22 4
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Chapter 22 5 Monosaccharides Classification of Monosaccharides Monosaccharides are classified according to: (1) The number of carbon atoms present in the molecule and (2) whether they contain an aldehyde or ketone group D and L Designations of Monosaccharides The simplest carbohydrates are glyceraldehyde, which is chiral, and dihydroxyacetone, which is achiral Glyceraldehyde exists as two enantiomers
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Chapter 22 6 In the early 20th century (+)-glyceraldehyde was given the stereochemical designation (D) and (-)-glyceraldehyde was given the designation (L) A monosaccharide whose highest numbered stereogenic center has the same configuration as D-(+)-glyceraldehyde is a D sugar A monosaccharide whose highest numbered stereogenic center has the same configuration as L-(-)-glyceraldehyde is an L sugar
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Chapter 22 7 Structural Formulas for Monosaccharides Fischer projections are used to represent stereochemistry in carbohydrates In Fischer projections horizontal lines are understood to project out of the plane toward the reader and vertical lines are understood to project behind the plane A Fischer projection cannot be removed from the plane of the paper or turned 90 o and still represent the molecule accurately Glucose exists primarily in two cyclic hemiacetal forms that are diastereomers of each other The cyclic hemiacetal forms interconvert via the open-chain form The cyclic hemiacetals differ only in configuration at C1 and are called anomers The carbon at which their configurations differ is called the anomeric carbon The α -anomer has the C1 hydroxyl trans to the -CH 2 OH group The β -anomer has the C1 hydroxyl cis to the -CH
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ch22 - Chapter 22 Carbohydrates Introduction Classification...

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