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CARBOHYDRATES

CARBOHYDRATES. MALIK ALQUB MD. PhD. Summary. Structure of carbohydrates Monosaccharides Disaccharides Polysaccharides Functions of carbohydrates. Carbohydrate Structure. Monomer – monosaccharide Chemical formula: (CH 2 O) n

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CARBOHYDRATES

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  1. CARBOHYDRATES MALIK ALQUB MD. PhD.

  2. Summary • Structure of carbohydrates • Monosaccharides • Disaccharides • Polysaccharides • Functions of carbohydrates

  3. Carbohydrate Structure • Monomer – monosaccharide • Chemical formula: (CH2O)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.

  4. 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

  5. Isomers and epimers

  6. Enantiomers

  7. 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

  8. Monosaccharides can form ring structures

  9. Carbohydrate Structure • Common monosaccharides • 6 Carbon: Glucose, fructose, galactose • 5 Carbon: Ribose and deoxyribose

  10. 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)

  11. 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)

  12. 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

  13. 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. 2cm3 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

  14. 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

  15. 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 allmonosaccharides are reducing sugars • All monosaccharides reduce Benedict’s reagent

  16. 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

  17. 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

  18. 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

  19. Oxidation/reduction glucose sorbitol

  20. Oxidation/reduction

  21. Disaccharides – two linked sugar units • 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

  22. 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

  23. 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

  24. 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

  25. 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.

  26. 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.

  27. 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.

  28. 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.

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