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CHAPTER 3 CARBOHYDRATES Introduction occurrence, classification, biochemical functions and physiological importance of carbohydrates. INTRODUCTION Carbohydrates occur widely in nature. Cellulose of wood and paper, starches of cereals, roots and tubers, sugars of fruits and milk are examples of carbohydrates. Animal tissues contain glycogen (animal starch). They are the major sources of energy and play key role in many biochemical processes including energy generation. DEFINITION Chemically all the carbohydrates are aldehyde or ketone derivatives of polyhydroxy alcohol or their condensation products. They contain C, H and O. The general (empirical) formula is Cn (H2O)n . However, there are many exception to this general formula such as deoxy ribose ( C5 H10 O4 ), formaldehyde (HCHO), acetic acid (C2 H4O2 ) (not carbohydrates). CLASSIFICATION The carbohydrates are classified in four different groups. 1. Monosaccharides: Those sugars which can't be hydrolysed to smaller units. 2. Disaccharides: Those sugars which are composed of two mono saccharides. 3. Oligosaccharides: Upon hydrolysis can yield 3 to 10 monosaccharides. 4. Polysaccharides:Simple polysaccharides and Complex polysaccharides PROPERTIES : (A) PHYSICAL PROPERTIES OF MONOSACCHRIDIES: 1. Colourless, odourless, neutral, sweet in taste 2. Soluble in water, insoluble in ether 25 3. Optically active 4. Melting point indefinite 5. Heating effect: Melt on heating, on more heating, they are converted to black carbon. CARBOHYDRATES ▼ SUGARS Mono Saccharides Disaccharide (C12H22 O11) NON SUGAR Oligo Simple poly Saccharides Saccharide Complex Poly Saccharides Pentose----Hexose (C5H10O5) (C6H12O6) Sucrose * Trisaccharides Pentosan Hemicellulose (Glu+Fru) Pectic substances Maltose Raffinose Gum Arabinose Glucose (Glu+Glu) (Glu+Fru+Gala) # Araban Mucilage Xylose Galactose Lactose *Tetrasaccharide # Xylan Lignin Ribose Fructose (Glu+Gala) Stachyose # Riban Glycosides (2Gala+Glu+Fru) Hexosans 1. Glucosan (Starch, dextrin, cellulose, Glycogen) 2. Fructosan (Inulin) 3. Galactan and 4. Mannan (B) CHEMICAL PROPERTIES : 1. Oxidation with mild oxidising agent: In presence of weak oxidising agent such as alkaline CuSO4, Ammonical AgNO3 or bromine water give acid (Gluconic acid), Glucosaccharic acid, Tri-hydroxy butyric acid. R-CHO + CuSO4 ------------ R-COOH Oxidation Red ppt. 26 2. Reduction: When monosaccharides are heated in presence of catalyst with hydrous solution, they give sorbitol or mannitol. R-CHO + Reducing agent ------------- R-OH 3. Reaction with HCN : Upon reaction with HCN, they produce corresponding cyanohydrins 4. Reaction with hydrazine : Monosaccharides produce corresponding hydrazone with phenyl hydrazine 5. Reducing properties : Some monosaccharides are strong reducing agents and they reduce oxidising agents such as CuSO4 and they are converted to acid OTHER PROPERTIES OF MONOSACCHARIDES: 1. ACETYLATION : When reacted with acetyl chloride, give acetyl derivative 2. FERMENTATION : Fermentation take place with the help of enzymes of microorgs to yield alcohol, acetic acid, putrefaction of meat, sourness of milk etc a. FERMENTATION OF SUCROSE FROM MOLASSES: C12H22O11 + H2O Sucrose ---------------> Invertase C6H12O6 + Glucose C6H12O6 Fructose from yeast C6H12O6 ---------------------------------> 2C2H5OH + 2CO2 Zymase from yeast ethanol b) MALTING OF BARLEY (BEER FERMENTATION): n ( C6H12O5 ) n + H2O Starch from barley -----------------------> Hydrolysis by amylase n ( C12H22O11 ) Maltose 27 C12H22O11 + H2O --------> Maltose C6H12O6 + Maltase Glucose C6H12O6 Glucose C6H12O6 ---------------------------------> 2C2H5OH + 2CO2 Glucose Zymase from yeast ethanol 3. ESTERIFICATION : When monosaccharides react with acid they yield Esters. 4. METHYLATION : Methylating agents such as CH3I + Ag2O or CH3OH + HCl, CH3Cl to yield glycosides. C6H11 O5 OH + CH3OH ------ HCl C6H12O5-OCH3 + H2O Methyl glycoside 5. OPTICAL ACTIVITY: Many of the soluble carbohydrates are optically active when present in solution. Compounds which in solution, rotate the plane of polarised light to the right are called dextro rotatory (clock wise) and to left side are called leavo rotatory (anti-clock wise). e.g. 1. Glucose = (+) 52.5 0 (dextro-rotatory) 2. Fructose = (-) 133.5 0 (Leavo-rotatory) Glucose exists in two isomeric forms α - d-glucose and β -d-glucose. 6. ISOMERISM: It is a phenomenon in which substances have same molecular formula but different structural formulas are called isomerism. There are two types of isomerism: a. CHEMICAL ISOMERISM: Where the molecular formula is same but the atomic groupings are different.e.g. Glucose (aldehyde) and fructose (keto). Both have molecular formula = C6H12O6 . Glucose ------------------ Fructose (Aldehyde) --------------- (Keto) -CHO -------------------- C = 0 C6 H12 O6 --------------- GLUCOSE C6 H12 O6 FRUCTOSE 28 CHO | H – C - OH | OH – C – H | H – C – OH | H – C – OH | CH2OH CH2OH | C=O | OH-C-H | H-C-OH | H-C-OH | CH2OH b. STEREOISOMERISM: Molecules that have exactly the same atomic grouping but arranged in different patterns. Glucose D-GLUCOSE ↔ Mannose D-MANNOSE CHO | H – C - OH | OH –C – H | H – C – OH | H– C – OH | CH2OH CHO | H - C- H | OH-C-H | H-C-OH | H-C-OH | CH2OH (7) RING STRUCTURE: Haworth proposed a simple ring or cyclic hemi acetal structure for glucose, which is called Pyranose structure H | OH –C--------------| H – C – OH | OH—C – H O | H – C – OH | H –C --------------| CH2OH 29 CH2OH | H H | OH OH | H O O H | H OH | OH CH2OH | H OH | | OHCH OH 2 H | OH Pyranose structure | H Furanose structure (8) Polymerisation and Condensation: a) Polymerisation: It is a process in which two or more same monomers are united together to form a complex polymer without evolution of water, NH3 or other compound. The total number of elements remains unaltered. e.g. CO2 + H2O --------------- HCHO + Photosynthesis 6(HCHO) b) Condensation: H2O Formaldyhyde ------------------- C6H12O6 (glucose) It is a process in which two or more same or different compounds are united together to form a complex polymer with or without evolution of water, NH3 or other compound. The total number of elements may not remain same because of loss of H2O,NH3 etc. E.g. Synthesis of Starch: n(C6H12O6) ----------- n (C6H10O5 ) + n (H2O) Glucose Starch PENTOSES There are 5 carbon sugars such as xylose, arabinose and ribose in a complex polymeric form which has importance is agriculture. 1. D-Xylose : Wood gum (xylan) Wheat bran oat hulls, cotton seed hull. 2. L-Arabinose : Vegetable gum- - It has an industrial importance Arabic, cherry tree gum 3. D- Ribose : Nucleic acid ------- Part of DNA and RNA structure. 30 HEXOSES GLUCOSE FRUCTOSE GALACTOSE 1. Aldehyde sugar Keto sugar Aldehyde sugar 2. C6 H12 O6 C6 H12 O6 C6 H12 O6 3. Grape sugar Fruit sugar Brain sugar 4. Structure (5) Reducing sugar Reducing sugar Reducing sugar (6) Osazone +Ve Osazone +Ve Osazone +Ve (7) Can occur freely Occur in fruit and honey Doesn’t occur freely in nature. Can be obtained only from lactose. GLUCOSE CHO | H – C - OH | OH – C – H | H – C – OH | H – C – OH | CH2OH CH2OH | H H | OH OH | H FRUCTOSE CH2OH | C=O | OH-C-H | H-C-OH | H-C-OH | CH2OH O O H | H OH | OH CH2OH | H H | OH OH | | OH CH2OH | H GALACTOSE CHO | H-C-OH | OH-C-H | OH-C-H | H-C-OH | CH2OH CH2OH | H OH | H OH | H O H | OH H | OH 31 Pyranose structure Furanose structure Pyranose structure DISACCHARIDES The disaccharides are group of compounds having two Monosaccharides linked with a glycosidic bond with elimination of water. Molecular formula: C12H22O11 C6 H12 O6 + C6 H12 O6 --------- C12 H22 O11 + H2O Two types of disaccharides: 1. Reducing sugars. 2. Non reducing sugar. REDUCING SUGARS: Those sugars which have either free aldehyde or keto group for the reaction. NON REDUCING SUGAR: Those sugars which do not have either free aldehyde or keto group for the reaction. 1. SUCROSE: It is also called cane sugar composed of glucose and fructose. Glucose + Fructose ------- Sucrose + H2O CH2OH | H H | OH OH | H O O H | H OH | OH + CH2OH | H OH | | OH CH OH 2 H | OH α Glucose ------ | H Fructose CH2OH | H OH | H OH | H O H H | OH SUCROSE O O CH2OH | H H | OH OH CH2OH | H 32 The linkage in the sucrose involves the C1 of glucose with the C2 of fructose with an oxygen bond. During the formation of sucrose, the aldehyde group of glucose and keto group of fructose is lost so the sucrose becomes non-reducing. It is hydrolysed to monosaccharide (glucose and fructose with the enzyme invertase (sucrase). It gives Osazone negative reaction. It does not show mutarotation and can be crystallised. It is also called invert sugar. 2. MALTOSE: Malt sugar is composed of glucose + glucose with α 1, 4 linkage. It is a reducing sugar. Molecular formula: C12 H22 O11 C6 H12 O6 + Glucose C6 H12 O6 --------- C12 H22 O11 + H2O Glucose Maltose It is a product formed from starch during malting process. The hydrolysis of starch by diastase enzyme yields maltose. Maltose is hydrolysed to two glucose by maltase enzyme. It is also called glucose glucoside. Specific rotation is 138.5 0 and forms characteristic osazone. STRUCTURE: CH2OH | H H | OH OH | H O H H | OH H O CH2OH | H OH | H O H | OH H | OH 3. LACTOSE: It is called milk sugar and composed of glucose + galactose Molecular formula: C12 H22 O11 It is joined with the β 1, 4 linkage. 33 It is a reducing sugar giving a characteristic osazone positive reaction. It does not ferment easily hence ideal constituent of milk. It furnishes galactose (brain sugar) requirement for brain function. It is hydrolysed by βgalactosidase (old name lactase) to Glucose + galactose. It is also hydrolysed by emulsion enzyme. Structure of Lactose: CH2OH | H OH | H OH | H O H H | OH O H CH2OH | H OH | H O H | OH H | OH POLYSACCHARIDES: The polysaccharides are complex carbohydrates, which are polymerised anhydride of a large but undetermined number of simple sugar. Empirical formula: (C6 H10 O5) n PROPERTIES OF POLYSACCHARIDES: High molecular weight, unreactive, form colloidal solution, insoluble in water. Upon hydrolysis, it yields their constituent monosaccharides, they are most important nutrients of foods of plant origin, and their main function is storage of reserve energy and structure. STARCH: It’s a glucose polymer with α 1, 4, and α 1, 6 linkage (branching). It occurs in leaves, seeds, fruits and tubers. It is found in leaf chloroplast wherein they are accumulated in daytime and hydrolysed and trans-located in night. It consists of amylose and amylopectin. Amylose occurs inside the starch granule while amylopectin is found in outer layer of starch grain. There are basic differences 34 between Amylose and Amylopectin. It is a white amorphous powder, insoluble in water. AMYLOSE AMYLOPECTIN 1. Inner layer of starch 1. Outer layer of starch 2. Straight chain 2. Branched 3. Alpha 1, 4, linkage 3. Alpha 1, 6 linkages 4. Mol.wt. 1 K – 50 K 4. mol.wt. 500 K – 1000 K 5. Blue colour with I2 5. Purple colour wilth I2 6. Forms gel 6. Doesn’t form gel 7. 20% in starch 7. 80% in starch PROPERTIES OF STARCH: a) Action of enzyme: Starch ------------ Amylodextrin ----------- Arithrodextrin Diastase V Glucose-------- Maltose ------------Achrodextrin Maltase b) ACTION OF ACIDS: When boiled with dilute acid, it is hydrolysed to dextrin, maltose and finally to glucose. When treated with concentrated H2SO4 + Conc. HNO3, it gives nitro starch which is used as blasting explosive. c) GEL FORMATION: It is a characteristic used in a dry cell and sizing operations. d) It reacts with iodine to give blue colour. USES: 1. It is used as food. It provides glucose upon hydrolysis to give the energy from the food of plant origin such as cereals and tubers. 35 2. Used in cloth washing 3. Used as explosive 4. Used as indicator in the lab. 5. Used in the preparation of gum 6. Used as electrolyte medium in dry cell. CELLULOSE: Cellulose is the most abundant organic substance found in nature. It is the principal constituent of cell walls in higher plants. It occurs in almost pure form (98%) in cotton fibres and to a lessor extent in flax (80%), jute (60-70%), wood (40-50%) and cereal straws (30-43%). It is linear, unbranched homoglycan of 10,000 to 15,000 D-glucose units joined by -14 linkages. The structure of cellulose can be represented as a series of glucopyranose rings in the chair conformation. The most stable conformation for the polymer is the chair turned 180 relative to the adjacent glucose residues yielding a straight extended chain. Celluose molecules within the plant cell walls are organized into biological units of structure known as microfibrils. A microfibril consists of a bundle of cellulose molecules arranged with its long axis parallel to that of the others. This arrangement permits the formation of intramolecular hydrogen bonding between the hydroxyl group of C-3 of one glucose residue and the pyranose ring oxygen atom of the next glucose residue. This hydrogen bond impart a double bond character to the glycosidic bond and impedes the rotation of adjacent glucose residues around the glycosidic bond. Within the microfibril, the adjacent cellulose molecules are linked by intermolecular hydrogen bond between C-6 hydroxyl group of one molecule and the glycosidic bond oxygen atom of adjacent cellulose molecule. 40 - 52% Wool and brans 30 - 40% Wood Flax, Hemp, Cotton 70 – 90% Hemicellulose: Mixed polysaccharide containing uronic acid, hexose and pentose units which are soluble in alkali and digested by enzymes. Act as cementing materials of plant cellulose also utilised in compost making. 36 PROPERTIES: It is a colourless, tasteless, fibrous compound, which is insoluble in water and organic solvents but soluble in cupric amino.hydroxide prepared in cold HCl and ZnCl2 Mol. Wt.: 20.000 ----50.000 CHEMICAL PROPERTIES: 1. HYDROLYSIS: It is converted to glucose when either boiled in conc. H2SO4 for a long time or treated with enzyme cellulase which breaks β 1, 4 linkage. Cellulase Cellulose ------------- Cellobiose (Disaccharide) ------------ Glucose Glucose + Glucose with β 1, 4 linkage 2. HYDRATION: When treated with H2O, it gives hydro- cellulose which is a characteristics of plant sap and important for physiological functions of plants. 3. NITRATION: When heated with mixture of H2SO4 + HNO3, it is converted to nitro cellulose. 4. ACTION OF ALKALI: No reaction with dilute alkali but with conc. alkali, the cellulose occupy the shape of smooth bright fibre which is translucent. This process is called mercerisation. 5. REACTION WITH LIGNIN : When it reacts with lignin, it gives lignocellulose which is a crusting material in the wood bark. The lingo-cellulose is not easily broken down. The amylase enzyme cannot act upon cellulose. The animal gut (Cows, buffalo etc.) and microorgs such as fungi and bacteria, which secrete cellulase enzyme which breaks the β 1,4 linkage to yield glucose. Human cannot digest cellulose because we do not have cellulase enzyme. 37 IMPORTANCE 1. Cotton and textile industry: Production of wrapping, gunny bags. Ropes and netting are made from cellulose. The cotton cloth which involve the use of pure cellulose. 2. Paper manufacture: The papers of all types are prepared either from low grade or pure Cellulose, which are used as newspaper wrappers or better quality papers. 3. Explosive: Fully nitrated cellulose (gum coated) in form of compressed blocks are used as explosives. When gun cotton + nitro-glycerine + Vaseline are prepared, it produces smokeless explosive which is called cordite. 4. Used in making of celluloid: Toys, bangles, combs, knife, garlands etc. 5. Used in preparing celluloid paper for motorcar. 6. Used in artificial silk and in bamboo furniture 7. Used in photographic films. GLYCOGEN The storage of carbohydrate in animal is in form of glycogen (animal starch). It is similar to amylopectin in its structure except that the mol.wt is lower and chains are shorter. The branching is extensive. It is found in liver and muscle. It dissolves in water to yield opalescent solution. It is easy hydrolysed to glucose. It is non-reducing and gives red colour with iodine. OTHER POLYSACCHARIDES : Inulin : Inulin is a non-digestible fructosyl oligosaccharide found naturally in more than 36000 types of plants. It is a storage polysaccharide found in onion,garlic,chicory,artichoke,asparagus,banana,wheat and rye.It consists of mainly,if not exclusively, of - 21 fructosyl-fructose links.A starting glucose moiety can be present,but is not necessary.Inulin is asoluble fibre that helps maintain normal bowel function,decreases constipation,lowers cholesrerol and 38 triglycerides.It is used for fat replacement and fibre enrichment in processed foods. Chitin : Structural polysaccharides in invertebrates (Insect wings). pectin substances. Pectin substances are natural components of plants and their fruits. They occur in plants in connection with cellulose and such substances are called protopectin. Protopectin is the binder of cell walls. Especially large amounts of pectin substances are present in fruit such as: currant, gooseberry, citrus fruits and apples. Pectin is a preparation obtained in industrial conditions, containing pectin substances isolated from plant material and soluble in water. Those preparations are used as food and medicine additives and they have the ability to make gels in proper conditions. Raw material for our pectin is dried apple pomace, containing 8-12 % pectin substances, and dried lemon peel, containing 18-25 % pectin substances, from where they are extracted by diluted acid solution and subsequently precipitated by alcohol, purified, dried and crumbled. Being the substance of plant origin, it is the best gelling agent for jams and fruit jellies production. Being the naturally compound of fruit, it makes products manufactured with its addition retain fully organoleptic characteristics Pectin classification Depending on the applied raw material the following pectins can be distinguished: Apple pectins Citrus-apple pectins Citrus pectins Depending on degree of esterification (DE) they are divided into : High esterificated pectins (DE above 50 %) Low esterificated pectins (DE below 50 %) Agar Agar is a heteropolysaccharide obtained from red algae.It is composed of agarose –neutral gelling fraction Agaropectin – sulfated non-gelling fraction 39 It is the most effective gelling agents known andis soluble in hot water.It melts in the temperature range of 60 –90oC and sets between 32 and 39oC to form gel. Uses As Solidifying agent, emulsifier Pharmaceuticals, cosmetics and food Laxative Sizing material in tentile industry Emulsifierin dairy products Microbial lab. Seaweed Polysaccharides Structurally they are highly branched and composed of many different monosaccharides. Alginic acid is obtained from seawood principally from brown algae. It is composed of 1 4 linked D-mannuronic acid and 4 linked L-guluronic acid – random or alternating sequence Uses: 1. Sauces, frozen deserts, fruit pies 2. Tabilize emulsions 3. Soft drinks (gum Arabic) 4. Beer making to stabilize foam (gum arabic0 5. Cosmetics and lotions, paints, ink 6. Adhesive, laxative, foods paper industry (karaya) 7. Food industry-ice creams, salad dressings, pie fillings (locust gum) 8. Ice cream, paper industry, fire hoses, medicines (guar) 9. Salad dressing, cheese (improves sprcading) lower cholesterol, helps diabetics to control sugar. METABOLISM OF CARBOHYDRATES: Metabolism is comprised of two processes : a) Anabolism : The process of formation of complex polymeric substances from simple monomeric units involving the energy is known as anabolism. 40 Sun light CO2 + H2O ------> HCHO -------> C6H12O6 -----> OLIGOSACCHARIDE photosynthesis -------------> POLYSACCHARIDE b) Catabolism: The process of breakdown of complex polymeric substances into simple monomeric units with release of energy. Carbohydrate (Polysaccharide) -----> oligosaccharide ------>monosaccharide Hydrolysis Glycolysis --------------> Pyruvic acid --------------> CO2 + H2O + ENERGY TCA cycle BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATE : 1. Structural component of cells : Cellulose, chitin etc. 2. Major source of energy 3. Key role in metabolism : Starch, glycogen. : Metabolism of amino acids and fatty acids. 4. Specific functions : Ribose ------------- Nucleic acid and nucleoprotein Galactose---------- Cerebroside - brain lipid Lactose ------------ Milk 41