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Chapter 5

Chapter 5

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McMaster University
Lovaye Kajiura

Chapter 5 –An Introduction to Carbohydrates  Introduction  - Carbohydrates are formed from monomer units of monosaccharaides that form small polymers (oligosaccharides), and then go to form large polymers (polysaccharides) - General formula (CH 2) nhere n is the number of carbon hydrate groups o Misleading because it claims carbohydrates have only carbon and water, but it has lot of hydroxyl and carbonyl groups 5.1 Sugars as Monomers - Figure 5.1 o Distinguishing feature of monosaccharide’s are the aldehydes(C=O) found either at end of molecule (aldose) or middle of molecule (ketone) o Presence of carbonyl group along with multiple hydroxyl groups provides an array of functional groups in sugar which is why they can participate in a variety of reactions o Number of carbon atom and arrangement of atoms varies in monosaccharide:  3 carbon sugar= triose, 5 carbon sugar= pentose (RNA), 6 carbon sugar= hexose (glucose)  Varying spatial arrangement of atoms: e.g. glucose and galactose are optical isomers (refer to figure 5.2) • Structure relates to function: glucose is used in storage of chemical energy, but galactose has to go through enzyme catyzyled activities to be converted to glucose and carry out the same function o IMPORTANT: monosaccharaides are unique with optical isomers, arrangement of hydroxyl groups, mirror image form variation, ring structures, number of carbons- this translates to function - Examples- ribose (RNA), and deocyribose( DNA), Glucose (starch, cellulose, glycogen), galactose(cartilage) How to distinguish between two ring formations of sugars? - In aqueous solutions, sugars tend to form ring structures- alpha glucose and beta glucose - Figure 5.3 o When cyclic structure forms in glucose, carbon number 1 in linear chain bonds to oxygen atom and with a hydroxyl group that has 2 orientations o Beta glucose more common than alpha form because more stable o Alpha glucose- the (C-OH) is below the plane o Beta glucose- the (C-OH) is above the plane 5.2 The Structure of Polysaccharides - Polysaccharides form when monosaccharaides are linked together.Also referred to as complex carbohydrates. - Simplest polysaccharide is 2 simple sugar known as a disaccharide: 2 linking sugars can be identical (maltose) or different (lactose) - Simple sugars polymerize using condensation reactions between 2 hydroxyl groups resulting in formation of covalent bond (called glycolytic linkages) o Similar to formation for peptide bonds between amino acids and Phosphodiester bonds formed to hold nucleotides together. o Key difference is hydroxyl group arrangement is different in each monosaccharide, so bonds form in different locations compared to other macromolecules where bonds form in same location/geometry - Disaccharide- figure 5.4 o Aglucose+ a glucose = maltose- a-1,4 glycosidic linkage between 2 alpha glucose o B glucose + b glucose = lactose- B-1,4 glycosidic linkage o Glucose + fructose- sucrose – a- 1,2 glycosidic linkage o Structural diagram of fructose: Real World Examples: - Lactose intolerance- adults do not produce sufficient amounts of lactase. Lactase is enzyme which splits la
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