Carbohydrates: Disaccharides, Oligosaccharides & Polysaccharides PDF

CARBOHYDRATES  Disaccharides & Oligosaccharides  Disaccharides contain two monosaccharides joined together by a covalent bond called a glycosidic linkage  A glycosidic linkage is an ether linkage that forms between two monosaccharides through a condensation reaction ...

CARBOHYDRATES  Disaccharides & Oligosaccharides  Disaccharides contain two monosaccharides joined together by a covalent bond called a glycosidic linkage  A glycosidic linkage is an ether linkage that forms between two monosaccharides through a condensation reaction  Examples of disaccharides: maltose sucrose lactose  Oligosaccharides contain three to ten monosaccharides attached by glycosidic linkages Maltose  Found in grains  Used in the production of beer  The product of starch digestion  Composed of two glucose monosaccharides joined together by an α-1,4 glycosidic linkage  The glucose on the left must be α-glucose  The glucose on the right can be α-glucose or β-glucose α-1,4 glycosidic linkage: α is from the OH on the anomeric carbon of the first glucose (on the left) Glycosidic linkage is between the 1’ carbon of the glucose on the left and the 4’ carbon of the glucose on the right Sucrose  Also called table sugar  Composed of an α-glucose joined together with an α-fructose by an α-1,2 glycosidic linkage  The glucose on the left must be α-glucose  The fructose on the right must be α-fructose  The anomeric carbons of both monosaccharides face each other Both glucose and fructose are linked H together at their 1’ 2’ anomeric carbon. All of the groups in fructose are inverted. α-1,2 glycosidic linkage: α is from the OH on the anomeric carbon of the glucose (on the left) Glycosidic linkage is between the 1’ carbon of the glucose and the 2’ carbon of the fructose (inverted and linked at its anomeric carbon) Lactose  Found in milk  Composed of a β-galactose joined together with a glucose by a β-1,4 glycosidic linkage  The glucose on the left must be β-galactose  The glucose on the right can be α-glucose or β-glucose H H 1’ 4’ β-1,4 glycosidic linkage: β is from the OH on the anomeric carbon of the galactose (on the left) Glycosidic linkage is between the 1’ carbon of the galactose and the 4’ carbon of the glucose  Reducing vs. Non-Reducing Sugars  Reducing sugars have a free –OH (hydroxyl) group, that was Glucose is a reducing sugar originally the carbonyl group, at the anomeric carbon when the sugar is in ring formation  The sugar can act as a reducing agent, resulting in its carbonyl group (aldehyde or ketone) becoming a carboxyl group (– COOH)  Examples: glucose, maltose, lactose  Non-reducing sugars do NOT have a free –OH group at the anomeric carbon in the ring formation H  In a non-reducing sugar, the carbonyl group on the anomeric carbon is attached or linked  Example: sucrose  Identifying Reducing vs. Non-Reducing Sugars  Benedict’s solution can be used to identify reducing sugars  Benedict’s solution is a deep blue alkaline solution that contains copper (II) sulfate (CuSO4)  To test, add Benedict’s solution to a sugar solution (the mixture will be blue) and heat it up for about 6 minutes  In a negative test (non-reducing sugar), the solution will stay blue  If a reducing sugar is present, a precipitate will form and the colour indicates the amount of reducing sugar  What happens?  The cupric ion (Cu2+) in the Benedict’s solution is reduced to cuprous ion (Cu1+) by the carbonyl group (aldehyde or ketone) of the reducing sugar to form cuprous oxide (Cu2O), which is the precipitate  The carbonyl (aldehyde/ketone) group of the sugar is oxidized to carboxylic acid Polysaccharides  Polysaccharides are complex carbohydrates composed of several hundred to thousand monosaccharide subunits joined together by glycosidic linkages  Polysaccharides are composed of long chains which may be straight or branched  The function of polysaccharides is energy storage and structural support  Examples of polysaccharides: starch glycogen cellulose chitin  Starch Amylose  Starch is a polymer of glucose  Plants convert excess glucose into starch for storage  There are two main types of starch: 1. Amylose - linear, unbranched chains of α-glucose containing α-1,4 glycosidic linkages - more compact, resists digestion Amylopectin 2. Amylopectin - branched chains of glucose containing α-1,4 glycosidic linkages in the main branch and α-1,6 glycosidic linkages at branching points - easier to digest due to branching and larger surface area Glycogen  Animals store excess glucose as glycogen  Glycogen has a similar structure to amylopectin, but it has more frequent and shorter branches  Branched chains of glucose containing α-1,4 glycosidic linkages in the main branch and α-1,6 glycosidic linkages at branching points  Glycogen is stored in muscle and liver cells of animals  Cellulose  Cellulose is found in plants, as well as some bacteria and algae  Cellulose is a straight chain polymer of β-glucose held together by β-1,4 glycosidic linkages  The hydroxyl groups (-OH) at the 1 & 4 positions cause every other monomer to be inverted  This allows the hydroxyl groups of parallel molecules to form hydrogen bonds  This results in the formation of tight bundles called microfibrils, which are used in the cell walls of plants for structural support  Cellulose is difficult for animals to digest, therefore some animals (such as cows) have a symbiotic relationship with bacteria that are able to digest the cellulose  Cellulose is a source of dietary fibre for humans Polysaccharides  Chitin  Chitin forms the tough, resistant exoskeleton of insects and crustaceans, and is found in the cell walls of some fungi  Chitin is made up of repeating units of N-acetylglucosamine, a form of glucose containing functional groups with nitrogen atoms Polysaccharides