415 Exam 2 Notes - CHAPTER 11 CARBOHYDRATES Carbohydrates...

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C HAPTER 11- C ARBOHYDRATES Carbohydrates are aldehydes or ketones with multiple hydroxyl groups that serve many purposes: o As energy stores, fuels, and metabolic intermediates o As the structural framework of RNA and DNA (ribose and deoxyribose) o As structural elements in the cell walls of bacteria and plants (polysaccharides) Cellulose is one of the most abundant organic compounds in the biosphere. o As links to many proteins and lipids; such linked carbohydrates play key roles in cell-cell communication and in interactions between cells and other elements in the cellular environment Monosaccharides, the simplest carbohydrates (3-9 carbons), are aldehydes or ketones with multiple hydroxyl groups. o They are important fuel molecules as well as building blocks for nucleic acids. o Linking together monosaccharides forms oligosaccharide structures; the sheer number of possible oligosaccharides makes this class of molecules information rich. The symbols D and L designate the absolute configurations of the asymmetric carbon atom farthest from the aldehydes or keto group. Unlike enantiomers, diastereoisomers are not mirror images of each other. Horizontal lines on Fischer projections indicate atoms in from of the plane, and those joined by vertical bonds are behind the page. Sugars differing in configuration at a single asymmetric center are called epimers. Ketoses have one fewer asymmetric center than do aldoses with the same number of carbons. Pentoses and hexoses cyclize to form furanose and pyranose rings. An aldehyde can react with an alcohol to form a hemiacetal. Similarly, a ketone can react with an alcohol to form a hemiketal. The designation α means that the hydroxyl group attached to C-1 is on the opposite side of the ring from the CH 2 OH at the carbon atom that determines whether the sugar is designated D or L (the chiral center); β means that the hydroxyl group is on the same side as the CH 2 OH at the chiral center. The C-1 carbon is called the anomeric carbon atom, and the α and β forms are called anomers; the β form is more prevalent in an equilibrium mixture of glucose. Pyranose and furanose rings can assume different conformations. Pyranose and furanose rings are not planar, because of the tetrahedral geometry of its saturated carbon atoms; in the chair form of pyranose, axial bonds are nearly perpendicular to the average plane of the ring, whereas equatorial bonds are nearly parallel to this plane. Axial substituents sterically hinder each other if they emerge on the same side of the ring. Monosaccharides are joined to alcohols and amines through glycoside bonds. Solutions of cupric ions (known as Fehling’s solution) provide a simple test for sugars, such as glucose, that can exist as a free aldehyde or ketone; sugars that react are called reducing sugars (they are nonspecific binders); those that do not are called nonreducing sugars.
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