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CARBOHYDRATES

CARBOHYDRATES. Carbohydrates - the most abundant class of biological molecules on Earth. Carbohydrates - polyhydroxyaldehydes or polyhydroxyketones or substances that yield these compounds when hydrolyzed. Empirical formula - (CH 2 O) n , where n ≥ 3

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CARBOHYDRATES

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  1. CARBOHYDRATES

  2. Carbohydrates - the most abundant class of biological molecules on Earth Carbohydrates - polyhydroxyaldehydes or polyhydroxyketones or substances that yield these compounds when hydrolyzed Empirical formula -(CH2O)n, where n ≥ 3 -consist of three main elements:carbon, oxygen and hydrogen; -have aldehyde or ketone functional groups and multiple hydroxylgroups; -can also contain nitrogen, sulfur and phosphorus The simplest carbohydrates

  3. Classification of carbohydrates • Monosaccharides - one monomeric unit • Disaccharides – 2 monosaccharides (alike or different) • Monosaccharides and disaccharides have sweet taste. They are called sugars • Oligosaccharides - ~2-6 monosaccharides • Polysaccharides - > 6 monosaccharides (glycogen in animals; starch, cellulose in plants) • - Homopolysaccharides contain only one type of monosaccharides - Heteropolysaccharides consist of more than one type of monosaccharides • Glycoconjugates - linked to proteins or lipids

  4. Monosaccharides Monosaccharides- the simplest carbohydrates with a single aldehyde or ketone unit and multiple hydroxyl groups Classification of monosaccharides 1. According to the amount of carbon atoms (only monosaccharides of three to seven carbons are commonly found in the biosphere). -trioses – the smallest monosaccharide (C3H6O3); -tetroses (C4H8O4); -pentoses (C5H10O5); -hexoses (C6H12O6); -heptoses (C7H14O7)

  5. 2. Depending on whether an aldehyde or ketone group is present monosaccharides can bealdoses and ketoses Aldoses - polyhydroxy aldehydes Ketoses- polyhydroxy ketones Oxidized carbon: aldoses - C-1, ketoses - usually C-2 1 1 2 2 ketose aldose ketose aldose

  6. 3. Depending on the spatial orientation of the –H and –OH groups attached to the carbon atom adjacent to the terminal primary alcohol group monosaccharides can exist as a D- or L- isomers D-isomer - OH group is written to the right of this carbon in the projection formula L-isomer– OH group is written to the left of this carbon in the projection formula L and D are mirror image of each other -enantiomers

  7. 4. Depending on whether the monosaccharide rotates the plane of polarized light to the right (+) or to the left (-) monosaccharides are divided into (+) and (-) isomers or dextrorotatory and levorotatory isomers Plane-polarized light vibrating in single plane Ordinary light vibrating in all possible planes.

  8. 5. Monosaccharides can exist in cyclic structure Depending on whether the cyclic structure of monosaccharide is related to that of furan or pyran monosaccharide can be classified as furanose or pyranose 6. Depending on the orientation of –H and –OH groups about specific chiral carbon atom in the cyclic form monosaccharides can have alpha () or beta () configuration

  9. Importance of carbohydrates 1. Energetic role – carbohydrates are very effective energy-yielding nutrients 2. Structural role - carbohydrates are building material (cell wall in bacteria and plants; connective tissue in animals) Cellulose forms fibers of wood, cotton clothing, paper 3. Pentoses – components of nucleic acids 4. Glycoproteins – receptors, cell recognition. Many important macromolecules in living systems are glycoproteins

  10. Fisher projection formulas Aldehyde C-1 is drawn at the top of a Fischer projection The –H and –OH groups are written to the right or to the left (D and L isomers) Any two monosaccharides that differ only in the configuration around a single carbon atom are calledepimers

  11. Fisher projection formulas of glucose D-glucose – OH group is written to the right of the carbon atom adjacent to the terminal primary alcohol group L-glucose - OH group is written to the left of the carbon atom adjacent to the terminal primary alcohol group L and D glucoses are enantiomers (differ at every chiral carbon and are mirror image of each other)

  12. Family of D-aldoses D-Sugars predominate in nature • Aldoses shown in blue are most important in chemistry and biochemistry D-confi-guration -OH group is written to the right of the carbon atom adjacent to the terminal primary alcohol group

  13. Epimers- sugars that differ at only one of several chiral centers (e.g., D-galactose is an epimer of D-glucose at C-4)

  14. Family of D-ketoses D-confi-guration -OH group is written to the right of the carbon atom adjacent to the terminal primary alcohol group

  15. Glucose, Galactose and Fructose Glucose, galactose and fructose - the most important carbohydrates in nature • Glucose (grape sugar, dextrose) • Aldohexose • Exist in free state in animal and plant tissue • Component of sucrose, maltose, lactose, starch, cellulose,glycogen • Key sugar for body (blood sugar) • 3.3-5.5 mmol/l in blood plasma • Oxidation in body – important source of energy

  16. Galactose • Aldohexose • Component of lactose, pectin, glycolipids and glycoproteins playing the structural role (cell membranes, connective tissue) • Abundant in milk • Can be converted to glucose in liver • Fructose (levulose) • Ketohexose • Constituent of sucrose • Abundant in fruit juices and honey • Verysweet (about twice as sweet as glucose) • Can be converted to glucose in liver

  17. Cyclic structure of mono-saccharides • (a) Six-membered sugar ring is a “pyranose” • (b) Five-membered sugar ring is a “furanose”

  18. Hemiacetals and acetals Compounds derived from aldehydes that contain an alkoxy and a hydroxy group on the same carbon atom are called hemiacetals Compounds derived from aldehydes that contain two alkoxy groups on the same carbon atom are called acetals Hemiacetals are unstable compounds Acitals are stable in alkaline solutions but unstable in acidic solutions

  19. Mutarotation In solution aldehyde group reacts with hydroxyl group attached to carbon 5 and cyclic form is formed D-glucose gives two cyclic forms - -D-glucopyranose and -D-gluco-pyranose(different orientation of –H and –OH groups about carbon 1) There is equilibrium between open chain, -D-glucopyranose (36 %) and -D-glucopyranose (64 %)in solution (they can be interconverted) Process of interconversion is called mutarotation Anomers - two cyclic isomers differ only in their stereo arrangement about the carbon involved in mutarotation

  20. Cyclic structures of monosaccharides are intramolecular hemiacetals When monosaccharide hemiacetal reacts with an alcohol the product is an acetal Acetal structure is calledglycoside

  21. When -D-glucopyranose react with alcohol (CH3OH) two optically active isomers (glycosides) are formed – methyl -D-glucopyranoside and methyl -D-glucopyranoside Methyl -D-glucopyranoside and methyl -D-glucopyranoside are acetals (stable) All carbohydrates other than monosaccharides are glycosides

  22. Structure of Galactose and Fructose Galactose is epimer of glucose (differ from each other at only one of several chiral centers) Galactose:aldohexose; exists in open chain and two cyclic pyranose forms Fructose: ketohexose; exists in open chain and cyclic pyranose forms

  23. Pentoses The most important pentoses are ribose and deoxyribose (constituents of nucleic acids) Ribulose - important ketopentose -precursor in synthesis of ribose in organism -captures carbon dioxide in photosynthesis

  24. Disaccharides Disaccharides are carbohydrates composed of two monosaccharides residues united byglycosidic linkage Disaccharides contain acetal structure and some also contain a hemiacetal structure Maltose Glucose is linked via its 1-st carbon atom hydroxyl group to the hydrohyl group on C4 of the second glucose by -1,4-glycosidic bond Enzymemaltase hydrolyses maltose into two glucose molecules

  25. Lactose Consists of -D-galactose and -D-glucose joined by -1,4-glycosidic bond Lactose (milk sugar) - the main sugar of milk In the intestine it is decomposed to galactose and glucose by the enzyme lactase

  26. Intolerance to Milk Many people are unable to metabolize the milk sugar lactose and experience gastro-intestinal disturbances if they drink milk Lactose intolerance (hypolactasia) - deficiency of the enzyme lactase, which cleaves lactose into glucose and galactose Microorganisms in the colon ferment undigested lactose to lactic acid generating methane (CH4) and hydrogen gas (H2) Symptoms: gut distention; annoying problem of flatulence Undigested lactose and lactic acid are osmotically active and draws water into the intestine resulting in diarrhea The gas and diarrhea hinder the absorption of other nutrients (fats and proteins) Treatment:- to avoid the products containing lactose; - the enzyme lactase can be ingested

  27. Sucrose Sucrose consists of -D-glucose and  -D-fructose joined by -1,2-glycosidic bond Sucrose (table sugar) Abundent in sugar cane and sugar beets In the intestine it is decomposed to fructose and glucose by the enzyme sucrase

  28. Cellobiose Consists of two -D-glucose joined by -1,4-glycosidic bond Cellobiose is the component of cellulose There is no enzyme in the human intestin to split -1,4-glycosidic bond

  29. Reactions of monosaccharides Oxidation Aldehyde group in monosaccharides is oxidized to monocarboxylic acid (suffix –onic) by mild oxidizing agent (bromine water) The stronger oxidizing agent, nitric acid, oxidizes both carbonone and carbon six to form dicarboxylic acid (suffix –aric)

  30. Reduction Monosaccharides are reduced to polyhydroxy alcohols by reducing agents such as H2/Pt or sodium amalgam (Na(Hg)) Glucose is reduced to glucitol (sorbitol),galactose - to galactitol, mannose - to mannitol To name the alcohol the suffix –itol have to be added

  31. Redox tests for carbohydrates Aldehydes can be oxidized by Ag+ and Cu2+ (Ag+ and Cu2+ are reduced) Sugars containing aldehyde group (glucose, galactose) reduce Ag+ and Cu2+ -reducing sugars Tollens, Fehling, Benedict tests can be used to detect reducing sugars Tollens test Fehling test

  32. A carbohydrate need not to have a free aldehyde group to be a reducing sugar A hemiacetal structure also react Disaccharides maltose and lactose have the hemiacetal structures and are therefore reducing sugars In alkaline conditions the ring open to form aldehyde group

  33. Sucrosedoes not have hemiacetal structure and therefore it is not a reducing sugar

  34. Color reactions (Fehling, Benedict) based on the reductive properties of sugars are used to monitor the glucose concentration in blood and urine Ketose test Seliwanow’s reagent (resorcimol in HCl) is used to detect the ketoses (fructose) Seliwanow’s reagent produces a red color within 90 sec for a ketose

  35. Polysaccharides • Homopolysaccharides - contain only one type of monosaccharide • Heteropolysaccharides - contain residues of more than one type of monosaccharide • Storage polysaccharides – depot of the energy molecule, glucose • Structuralpolysaccharides – provide a protective wall or lubricative coating to cells

  36. Storage polysaccharides Starch and Glycogen • D-Glucose is stored intracellularly in polymeric forms • Plants and fungi – starch. Animals – glycogen • Starch is especially abundant in potatoes, corn and wheat • Glycogen is present primarily in liver and muscles • Starch and glycogen are stored in the cell in cytoplasmic packages called granules

  37. Starch Starch is a mixture of amylose (unbranched) and amylopectin (branched) Amylose- a linear unbranched polymer of D-glucose units linked by -1,4-glycosidic bond Molecular weight from several thousands to 500000 Enzyme -amylase cleaves -1,4-glycosidic bonds

  38. Amylopectin: • a main backbone composed of glucose units linked by -1,4-glycosidic bond(like amylose); • branches connected to the backbone via -1,6-glycosidic bonds(about every 25 glucose residues – 1 branch)

  39. Molecular structures of amylose and amylo-pectin

  40. Glycogen Depot of glucose in animal cells Present in liver (10 % of the wet weight) and muscles (about 1 %) The structure is identical to amylopectin but has more numerous -1,6-glycosidic branches and much higher molecular weight (several millions)

  41. Cellulose • Major structural component of wood and plant fibers • Abundant in nature (50 % of the organic matter in biosphere) • Polymer of glucose (glucose molecules are connected by -1,4-glycosidic bonds) • Important in textile and paper industry • Unbranched chain • Chains of cellulose can associate into bundles called fibrils (very strong) • Animals and humans can’t digest cellulose (amilase doesn’t split -1,4-bonds) • Wood-rot fungi and some bacteria synthesize cellulase • Ruminant animals are able to use cellulose (stomach contain bacteria producing cellulase)

  42. OH-groups of cellulose can react with nitric acid or acetic acid or acetic anhydride - nitrocellulose (celluloid) and cellulose acetate are formed • Nitrocellulose and cellulose acetate: • important materials in textile industry • production of photographic films • celluloid shirt collars • billiard balls and many other articles

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