Summary Structure of carbohydrates Monosaccharides Disaccharides Polysaccharides Functions of carbohydrates
Carbohydrate Structure Monomer – monosaccharide Chemical formula: (CH 2 O) n Carbon chains or rings with H’s, OH groups and a C=O or carbonyl group. Depending on the placement of the carbonyl group they may be aldoses or ketoses.
Carbohydrate Structure Most monosaccharides have 3, 5, or 6 carbons. 3 carbons = triose 5 carbons = pentose 6 carbons = ? Different placement of the OH groups creates several different monosaccharides with the same chemical formula. Isomers, fructose, glucose, mannose, and galactose = C6H12O6 Epimers, configuration around one specific carbon atom
Carbohydrate Structure Often monosaccharides form a carbon ring, an oxygen combining with another C in the chain. A new OH group is formed. The OH group may be above or below the plane of the ring, Anomeric carbon
Carbohydrate Structure Disaccharides – 2 sugars joined by a condensation reaction to form a glycosidic bond. Common disaccharides Sucrose (glucose + fructose) Lactose (galactose + glucose) Maltose (glucose + glucose)
Carbohydrate Structure Oligosaccharides from 3 to 12 monosaccharides glycoproteins Polysaccharides consist of many monosaccharides joined together by glycosidic bonds. Common polysaccharides. Starch Glycogen Cellulose Homopolysaccarides (glycogen) vs. Heteropolysaccarides (glycosaminoglycan)
Reducing and Non-Reducing Sugars Reduction is the chemist’s term for electron gain A molecule that gains an electron is thus…… “reduced” A molecule that donates electrons is called a…… “reducing agent” A sugar that donates electrons is called a…… “reducing sugar” The electron is donated by the carbonyl group Benedict’s reagent changes colour when exposed to a reducing agent
Benedict’s Test For Reducing Sugars All monosaccharide’s and most disaccharide's will reduce copper (II) sulphate, producing a precipitate of copper (I) oxide on heating, so they are called reducing sugars. Benedict’s reagent is an aqueous solution of copper (II) sulphate, sodium carbonate and sodium citrate. Grind up sample To approx. 2cm 3 of test solution add equal quantity of Benedict’s reagent. Shake, and heat for a few minutes at 95 C in a water bath A precipitate indicates reducing sugars Original Pale Blue = no reducing sugar Brown/Red = reducing sugar
Benedict’s Test Benedict’s reagent undergoes a complex colour change when it is reduced The intensity of the colour change is proportional to the concentration of reducing sugar present The colour change sequence is: Blue… green… yellow… orange… brick red
The carbonyl group - monosaccharides The carbonyl group is “free” in the straight chain form But not free in the ring form BUT remember – the ring form and the straight chain form are interchangeable So all monosaccharides are reducing sugars All monosaccharides reduce Benedict’s reagent
The carbonyl group – disaccharides - maltose In some disaccharides e.g. maltose one of the carbonyl groups is still “free” Such disaccharides are reducing sugars They reduce Benedict’s reagent
The carbonyl group – disaccharides - sucrose In some disaccharides e.g. sucrose both of the carbonyl groups are involved in the glycosidic bond So there are no free carbonyl groups Such sugars are called non- reducing sugars They do NOT reduce Benedict’s reagent
The carbonyl group – disaccharides - sucrose The subunits of sucrose (glucose and fructose) are reducing sugars If sucrose is hydrolysed the subunit can then act as reducing sugars This is done in the lab by acid hydrolysis After acid hydrolysis sucrose will reduce Benedict’s reagent
Sucrose: glucose + fructose “table sugar” Made from sugar cane and sugar beets Lactose: glucose + galactose “milk sugar” Found in milk and dairy products Maltose: glucose + glucose Found in germinating cereal grains Product of starch breakdown Disaccharides – two linked sugar units
Complex Carbohydrates Chains of more than two sugar molecules Oligosaccharides contain 3-12 sugar molecules Polysaccharides contain 100’s or 1000’s of monosaccharide units starch-digestible fiber-indigestible
Complex Carbohydrates Starch Long chains of glucose units Amylose – straight chains Amylopectin – branched chains; Found in grains, vegetables, legumes Glycogen Highly branched chains of glucose units Body’s storage form of carbohydrate
Carbohydrate Digestion and Absorption Mouth Salivary amylase begins digestion of starch Small intestine Pancreatic amylase completes starch digestion Brush border enzymes digest disaccharides End products of carbohydrate digestion Glucose, fructose, galactose Absorbed into bloodstream Fibers are not digested, excreted in feces
Digestion and Absorption of Carbohydrates The small intestine can only absorb monosaccharides. Starch digestion begins in the mouth with and enzyme, amylase, that breaks it down into disaccharides – continues in storage section of the stomach. Small intestine – amylase from the pancreas continues starch digestion.
Digestion and Absorption of Carbohydrates Disaccharides – are broken down into monosaccharides small intestine by enzymes produced in the walls of the small intestine. Sucrase Maltase Lactase Monosaccharides are absorbed in small intestine and enter the blood stream.
Digestion and Absorption of Carbohydrates Blood carries monosaccharides to the liver. All are converted to glucose. Glucose travels to other cells via the blood. Extra glucose is stored as glycogen in the liver and skeletal muscles.
Digestion and Absorption of Carbohydrates Lactose intolerance – lack of the enzyme lactase causes inability to digest lactose in milk products. There are few with complete intolerance, most can ingest small quantities of milk products. Some people are really allergic to milk. This allergy seems to be increasing in infants.