Biomolecules

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in milk is known as lactose Carbohydrates are also called saccharides. Greek sakcharon means sugar, Carbohydrates are classified on the basis of their behaviour on. hydrolysis They have been broadly divided into following three groups. 14 1 1 i Monosaccharides A carbohydrate that cannot be hydrolysed further. Classification of to give simpler unit of polyhydroxy aldehyde or ketone is called a. Carbohydrates monosaccharide About 20 monosaccharides are known to occur in. nature Some common examples are glucose fructose ribose etc. ii Oligosaccharides Carbohydrates that yield two to ten. monosaccharide units on hydrolysis are called oligosaccharides They. are further classified as disaccharides trisaccharides tetrasaccharides. etc depending upon the number of monosaccharides they provide. on hydrolysis Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide. may be same or different For example one molecule of sucrose on. hydrolysis gives one molecule of glucose and one molecule of fructose. whereas maltose gives two molecules of only glucose. iii Polysaccharides Carbohydrates which yield a large number of. monosaccharide units on hydrolysis are called polysaccharides. Some common examples are starch cellulose glycogen gums. etc Polysaccharides are not sweet in taste hence they are also. called non sugars, The carbohydrates may also be classified as either reducing or non. reducing sugars All those carbohydrates which reduce Fehling s. solution and Tollens reagent are referred to as reducing sugars All. monosaccharides whether aldose or ketose are reducing sugars. 14 1 2 Monosaccharides are further classified on the basis of number of carbon. Monosaccharides atoms and the functional group present in them If a monosaccharide. contains an aldehyde group it is known as an aldose and if it contains. a keto group it is known as a ketose Number of carbon atoms. constituting the monosaccharide is also introduced in the name as is. evident from the examples given in Table 14 1,Table 14 1 Different Types of Monosaccharides. Carbon atoms General term Aldehyde Ketone,3 Triose Aldotriose Ketotriose. 4 Tetrose Aldotetrose Ketotetrose,5 Pentose Aldopentose Ketopentose.
6 Hexose Aldohexose Ketohexose,7 Heptose Aldoheptose Ketoheptose. 14 1 2 1 Glucose Glucose occurs freely in nature as well as in the combined form It is. present in sweet fruits and honey Ripe grapes also contain glucose. in large amounts It is prepared as follows, Preparation of 1 From sucrose Cane sugar If sucrose is boiled with dilute HCl or. Glucose H2SO4 in alcoholic solution glucose and fructose are obtained in. equal amounts,Chemistry 412,C12 H22O11 H2O C6 H12O6 C6 H12O6. Sucrose Glucose Fructose, 2 From starch Commercially glucose is obtained by hydrolysis of. starch by boiling it with dilute H2SO4 at 393 K under pressure. C6 H10 O5 n nH 2 O,393 K 2 3 atm,nC6 H12 O6,Starch or cellulose Glucose.
Structure of Glucose is an aldohexose and is also known as dextrose It is the. Glucose monomer of many of the larger carbohydrates namely starch cellulose. It is probably the most abundant organic compound on earth It was. assigned the structure given below on the basis of the following. CHO 1 Its molecular formula was found to be C6H12O6. CHOH 4 2 On prolonged heating with HI it forms n hexane suggesting that all. the six carbon atoms are linked in a straight chain. 3 Glucose reacts with hydroxylamine to form an oxime and adds a. molecule of hydrogen cyanide to give cyanohydrin These reactions. confirm the presence of a carbonyl group C O in glucose. 4 Glucose gets oxidised to six carbon carboxylic acid gluconic acid. on reaction with a mild oxidising agent like bromine water This. indicates that the carbonyl group is present as an aldehydic group. Br2 water CHOH 4,CH2OH CH2OH,Gluconic acid, 5 Acetylation of glucose with acetic anhydride gives glucose. pentaacetate which confirms the presence of five OH groups Since. it exists as a stable compound five OH groups should be attached. to different carbon atoms,413 Biomolecules, 6 On oxidation with nitric acid glucose as well as gluconic acid both. yield a dicarboxylic acid saccharic acid This indicates the presence. of a primary alcoholic OH group in glucose,CHO COOH COOH. Oxidation Oxidation,CHOH 4 CHOH 4 CHOH 4,CH2OH COOH CH2OH. Saccharic Gluconic, The exact spatial arrangement of different OH groups was given.
by Fischer after studying many other properties Its configuration is. correctly represented as I So gluconic acid is represented as II and. saccharic acid as III,CHO COOH COOH,H OH H OH H OH. HO H HO H HO H,H OH H OH H OH,H OH H OH H OH,CH2OH CH2OH COOH. Glucose is correctly named as D glucose D before the name. of glucose represents the configuration whereas represents. dextrorotatory nature of the molecule It should be remembered that. D and L have no relation with the optical activity of the compound. They are also not related to letter d and l see Unit 10 The meaning. of D and L notations is as follows, The letters D or L before the name of any compound indicate the. relative configuration of a particular stereoisomer of a compound with. respect to configuration of some other compound configuration of. which is known In the case of carbohydrates this refers to their. relation with a particular isomer of glyceraldehyde Glyceraldehyde. contains one asymmetric carbon atom and exists in two enantiomeric. forms as shown below, Isomer of glyceraldehyde has D configuration It means that when. its structural formula is written on paper following specific conventions. which you will study in higher classes the OH group lies on right hand. side in the structure All those compounds which can be chemically. correlated to D isomer of glyceraldehyde are said to have D. configuration whereas those which can be correlated to L isomer of. glyceraldehyde are said to have L configuration In L isomer OH. group is on left hand side as you can see in the structure For assigning. Chemistry 414, the configuration of monosaccharides it is the lowest asymmetric carbon.
atom as shown below which is compared As in glucose OH on. the lowest asymmetric carbon is on the right side which is comparable. to glyceraldehyde so glucose is assigned D configuration Other. asymmetric carbon atoms of glucose are not considered for this. comparison Also the structure of glucose and glyceraldehyde is written. in a way that most oxidised carbon in this case CHO is at the top. CH2OH CH2OH,D Glyceraldehyde D Glucose, Cyclic The structure I of glucose explained most of its properties but the. Structure following reactions and facts could not be explained by this structure. of Glucose 1 Despite having the aldehyde group glucose does not give Schiff s. test and it does not form the hydrogensulphite addition product with. 2 The pentaacetate of glucose does not react with hydroxylamine. indicating the absence of free CHO group, 3 Glucose is found to exist in two different crystalline forms which are. named as and The form of glucose m p 419 K is obtained by. crystallisation from concentrated solution of glucose at 303 K while. the form m p 423 K is obtained by crystallisation from hot and. saturated aqueous solution at 371 K, This behaviour could not be explained by the open chain structure. I for glucose It was proposed that one of the OH groups may add. to the CHO group and form a cyclic hemiacetal structure It was. found that glucose forms a six membered ring in which OH at C 5. is involved in ring formation This explains the absence of CHO. group and also existence of glucose in two forms as shown below. These two cyclic forms exist in equilibrium with open chain structure. The two cyclic hemiacetal forms of glucose differ only in the. configuration of the hydroxyl group at C1 called anomeric carbon. 415 Biomolecules, the aldehyde carbon before cyclisation Such isomers i e form. and form are called anomers The six membered cyclic structure. of glucose is called pyranose structure or in analogy with. pyran Pyran is a cyclic organic compound with one oxygen atom. and five carbon atoms in the ring The cyclic structure of glucose is. more correctly represented by Haworth structure as given below. 14 1 2 2 Fructose Fructose is an important ketohexose It is obtained along with glucose. by the hydrolysis of disaccharide sucrose It is a natural. monosaccharide found in fruits honey and vegetables In its pure. form it is used as a sweetner It is also an important ketohexose. Fructose also has the molecular formula C6H12O6 and. Structure on the basis of its reactions it was found to contain a. of Fructose ketonic functional group at carbon number 2 and six. carbons in straight chain as in the case of glucose It. belongs to D series and is a laevorotatory compound. It is appropriately written as D fructose Its open. chain structure is as shown, It also exists in two cyclic forms which are obtained.
by the addition of OH at C5 to the group The ring thus formed. is a five membered ring and is named as furanose with analogy to the. compound furan Furan is a five membered cyclic compound with one. oxygen and four carbon atoms, The cyclic structures of two anomers of fructose are represented by. Haworth structures as given,Chemistry 416, 14 1 3 You have already read that disaccharides on hydrolysis with dilute. Disaccharides acids or enzymes yield two molecules of either the same or different. monosaccharides The two monosaccharides are joined together by an. oxide linkage formed by the loss of a water molecule Such a linkage. between two monosaccharide units through oxygen atom is called. glycosidic linkage, In disaccharides if the reducing groups of monosaccharides i e. aldehydic or ketonic groups are bonded these are non reducing sugars. e g sucrose On the other hand sugars in which these functional groups. are free are called reducing sugars for example maltose and lactose. i Sucrose One of the common disaccharides is sucrose which on. hydrolysis gives equimolar mixture of D glucose and D fructose. These two monosaccharides are held together by a glycosidic. linkage between C1 of D glucose and C2 of D fructose Since. the reducing groups of glucose and fructose are involved in. glycosidic bond formation sucrose is a non reducing sugar. Sucrose is dextrorotatory but after hydrolysis gives. dextrorotatory glucose and laevorotatory fructose Since the. laevorotation of fructose 92 4 is more than dextrorotation of. glucose 52 5 the mixture is laevorotatory Thus hydrolysis of. sucrose brings about a change in the sign of rotation from dextro. to laevo and the product is named as invert sugar, ii Maltose Another disaccharide maltose is composed of two. D glucose units in which C1 of one glucose I is linked to C4. of another glucose unit II The free aldehyde group can be. produced at C1 of second glucose in solution and it shows reducing. properties so it is a reducing sugar,417 Biomolecules.
iii Lactose It is more commonly known as milk sugar since this. disaccharide is found in milk It is composed of D galactose and. D glucose The linkage is between C1 of galactose and C4 of. glucose Free aldehyde group may be produced at C 1 of glucose. unit hence it is also a reducing sugar, Polysaccharides Polysaccharides contain a large number of monosaccharide units joined. together by glycosidic linkages These are the most commonly. encountered carbohydrates in nature They mainly act as the food. storage or structural materials, i Starch Starch is the main storage polysaccharide of plants It is. the most important dietary source for human beings High content. of starch is found in cereals roots tubers and some vegetables It. is a polymer of glucose and consists of two components. Amylose and Amylopectin Amylose is water soluble component. which constitutes about 15 20 of starch Chemically amylose is. a long unbranched chain with 200 1000 D glucose units. held together by C1 C4 glycosidic linkage, Amylopectin is insoluble in water and constitutes about 80. 85 of starch It is a branched chain polymer of D glucose. units in which chain is formed by C1 C4 glycosidic linkage whereas. branching occurs by C1 C6 glycosidic linkage,Chemistry 418. ii Cellulose Cellulose occurs exclusively in plants and it is the most. abundant organic substance in plant kingdom It is a predominant. constituent of cell wall of plant cells Cellulose is a straight chain. polysaccharide composed only of D glucose units which are. joined by glycosidic linkage between C1 of one glucose unit and. C4 of the next glucose unit, iii Glycogen The carbohydrates are stored in animal body as glycogen.
It is also known as animal starch because its structure is similar to. amylopectin and is rather more highly branched It is present in liver. muscles and brain When the body needs glucose enzymes break the. glycogen down to glucose Glycogen is also found in yeast and fungi. 14 1 5 Carbohydrates are essential for life in both plants and animals They. Importance of form a major portion of our food Honey has been used for a long time. Carbohydrates as an instant source of energy by Vaids in ayurvedic system of. medicine Carbohydrates are used as storage molecules as starch. a living system falls in the domain of biochemistry Living systems are made up of various complex biomolecules like carbohydrates proteins nucleic acids lipids etc Proteins and carbohydrates are essential constituents of our food These biomolecules interact with each other and constitute the molecular logic of life processes In

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