Biomolecules

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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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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 in. plants and glycogen in animals Cell wall of bacteria and plants is. made up of cellulose We build furniture etc from cellulose in the form. 419 Biomolecules, of wood and clothe ourselves with cellulose in the form of cotton fibre. They provide raw materials for many important industries like textiles. paper lacquers and breweries, Two aldopentoses viz D ribose and 2 deoxy D ribose Section. 14 5 1 Class XII are present in nucleic acids Carbohydrates are found. in biosystem in combination with many proteins and lipids. Intext Questions, 14 1 Glucose or sucrose are soluble in water but cyclohexane or. benzene simple six membered ring compounds are insoluble in. water Explain, 14 2 What are the expected products of hydrolysis of lactose. 14 3 How do you explain the absence of aldehyde group in the. pentaacetate of D glucose, 14 2 Proteins Proteins are the most abundant biomolecules of the living system.
Chief sources of proteins are milk cheese pulses peanuts fish meat. etc They occur in every part of the body and form the fundamental. basis of structure and functions of life They are also required for. growth and maintenance of body The word protein is derived from. Greek word proteios which means primary or of prime importance. All proteins are polymers of amino acids, 14 2 1 Amino Amino acids contain amino NH2 and carboxyl COOH functional. Acids groups Depending upon the relative position of amino group with. respect to carboxyl group the amino acids can be,classified as and so on Only amino. acids are obtained on hydrolysis of proteins They NH2. may contain other functional groups also a amino acid. All amino acids have trivial names which R side chain. usually reflect the property of that compound or, its source Glycine is so named since it has sweet taste in Greek glykos. means sweet and tyrosine was first obtained from cheese in Greek tyros. means cheese Amino acids are generally represented by a three letter. symbol sometimes one letter symbol is also used Structures of some. commonly occurring amino acids along with their 3 letter and 1 letter. symbols are given in Table 14 2,Table 14 2 Natural Amino Acids H2N H. Name of the Characteristic feature Three letter One letter. amino acids of side chain R symbol code,1 Glycine H Gly G.
2 Alanine CH3 Ala A,3 Valine H3C 2CH Val V,4 Leucine H3C 2CH CH2 Leu L. Chemistry 420,5 Isoleucine H3C CH2 CH Ile I,6 Arginine HN C NH CH2 3 Arg R. 7 L ysine H2N CH2 4 L ys K,8 Glutamic acid HOOC CH2 CH2 Glu E. 9 Aspartic acid HOOC CH2 Asp D,10 Glutamine H2N C CH2 CH2 Gln Q. 11 Asparagine H2N C CH2 Asn N,12 Threonine H3C CHOH Thr T.
13 Serine HO CH2 Ser S,14 Cysteine HS CH2 Cys C,15 Methionine H3C S CH2 CH2 Met M. 16 Phenylalanine C6H5 CH2 Phe F,17 Tyrosine p HO C6H4 CH2 Tyr Y. 18 Tryptophan Trp W,19 Histidine His H,20 Proline Pro P. essential amino acid a entire structure, 14 2 2 Amino acids are classified as acidic basic or neutral depending upon. Classification of the relative number of amino and carboxyl groups in their molecule. Amino Acids Equal number of amino and carboxyl groups makes it neutral more. number of amino than carboxyl groups makes it basic and more. carboxyl groups as compared to amino groups makes it acidic The. amino acids which can be synthesised in the body are known as non. essential amino acids On the other hand those which cannot be. synthesised in the body and must be obtained through diet are known. as essential amino acids marked with asterisk in Table 14 2. 421 Biomolecules, Amino acids are usually colourless crystalline solids These are.
water soluble high melting solids and behave like salts rather than. simple amines or carboxylic acids This behaviour is due to the presence. of both acidic carboxyl group and basic amino,group groups in the same molecule In aqueous. solution the carboxyl group can lose a proton,and amino group can accept a proton giving rise. to a dipolar ion known as zwitter ion This is,neutral but contains both positive and negative. In zwitter ionic form amino acids show amphoteric behaviour as. they react both with acids and bases, Except glycine all other naturally occurring amino acids are. optically active since the carbon atom is asymmetric These exist. both in D and L forms Most naturally occurring amino acids have. L configuration L Aminoacids are represented by writing the NH2 group. on left hand side, 14 2 3 Structure You have already read that proteins are the polymers of amino acids.
of Proteins and they are connected to each other by peptide bond or peptide. linkage Chemically peptide linkage is an amide formed between. COOH group and NH2 group The reaction between two molecules of. similar or different amino acids proceeds through, the combination of the amino group of one molecule. with the carboxyl group of the other This results in. the elimination of a water molecule and formation of. a peptide bond CO NH The product of the reaction, is called a dipeptide because it is made up of two. amino acids For example when carboxyl group of,glycine combines with the amino group of alanine. we get a dipeptide glycylalanine, If a third amino acid combines to a dipeptide the product is called a. tripeptide A tripeptide contains three amino acids linked by two peptide. linkages Similarly when four five or six amino acids are linked the respective. products are known as tetrapeptide pentapeptide or hexapeptide. respectively When the number of such amino acids is more than ten then. the products are called polypeptides A polypeptide with more than hundred. amino acid residues having molecular mass higher than 10 000u is called. a protein However the distinction between a polypeptide and a protein is. not very sharp Polypeptides with fewer amino acids are likely to be called. proteins if they ordinarily have a well defined conformation of a protein such. as insulin which contains 51 amino acids, Proteins can be classified into two types on the basis of their.
molecular shape,a Fibrous proteins, When the polypeptide chains run parallel and are held together by. hydrogen and disulphide bonds then fibre like structure is formed Such. proteins are generally insoluble in water Some common examples are. keratin present in hair wool silk and myosin present in muscles etc. Chemistry 422,b Globular proteins, This structure results when the chains of polypeptides coil around. to give a spherical shape These are usually soluble in water Insulin. and albumins are the common examples of globular proteins. Structure and shape of proteins can be studied at four different. levels i e primary secondary tertiary and quaternary each level. being more complex than the previous one,i Primary structure of proteins Proteins may have. one or more polypeptide chains Each polypeptide in a. protein has amino acids linked with each other in a. specific sequence and it is this sequence of amino acids. that is said to be the primary structure of that protein. Any change in this primary structure i e the sequence. of amino acids creates a different protein,ii Secondary structure of proteins The secondary. structure of protein refers to the shape in which a long. polypeptide chain can exist They are found to exist in. two different types of structures viz helix and, pleated sheet structure These structures arise due.
to the regular folding of the backbone of the polypeptide. chain due to hydrogen bonding between and,NH groups of the peptide bond. Helix is one of the most common ways in which, a polypeptide chain forms all possible hydrogen bonds. by twisting into a right handed screw helix with the. Fig 14 1 Helix NH group of each amino acid residue hydrogen bonded to the. structure of proteins C O of an adjacent turn of the helix as shown in Fig 14 1. In pleated sheet structure all peptide chains are, stretched out to nearly maximum extension and then. laid side by side which are held together by, intermolecular hydrogen bonds The structure resembles. the pleated folds of drapery and therefore is known as. pleated sheet,iii Tertiary structure of proteins The tertiary.
structure of proteins represents overall folding of the. polypeptide chains i e further folding of the secondary. structure It gives rise to two major molecular shapes. viz fibrous and globular The main forces which,stabilise the 2 and 3 structures of proteins are. hydrogen bonds disulphide linkages van der Waals,and electrostatic forces of attraction. Fig 14 2 Pleated sheet structure of iv Quaternary structure of proteins Some of the. proteins proteins are composed of two or more polypeptide. chains referred to as sub units The spatial, arrangement of these subunits with respect to each. other is known as quaternary structure,423 Biomolecules. A diagrammatic representation of all these four structures is. given in Figure 14 3 where each coloured ball represents an. amino acid, Fig 14 3 Diagrammatic representation of protein structure two sub units.
of two types in quaternary structure,Fig 14 4 Primary. secondary tertiary,and quaternary,structures of,haemoglobin. 14 2 4 Protein found in a biological system with a unique three dimensional. Denaturation of structure and biological activity is called a native protein When a. Proteins protein in its native form is subjected to physical change like change. in temperature or chemical change like change in pH the hydrogen. bonds are disturbed Due to this globules unfold and helix get uncoiled. and protein loses its biological activity This is called denaturation of. Chemistry 424, protein During denaturation secondary and tertiary structures are. destroyed but primary structure remains intact The coagulation of. egg white on boiling is a common example of denaturation Another. example is curdling of milk which is caused due to the formation of. lactic acid by the bacteria present in milk,Intext Questions. 14 4 The melting points and solubility in water of amino acids are generally. higher than that of the corresponding halo acids Explain. 14 5 Where does the water present in the egg go after boiling the egg. 14 3 Enzymes Life is possible due to the coordination of various chemical reactions in. living organisms An example is the digestion of food absorption of. appropriate molecules and ultimately production of energy This process. involves a sequence of reactions and all these reactions occur in the. body under very mild conditions This occurs with the help of certain. biocatalysts called enzymes Almost all the enzymes are globular. proteins Enzymes are very specific for a particular reaction and for a. particular substrate They are generally named after the compound or. class of compounds upon which they work For example the enzyme. that catalyses hydrolysis of maltose into glucose is named as maltase. C12 H22 O11 2 C6 H12 O6,Maltose G lucose, Sometimes enzymes are also named after the reaction where they.
are used For example the enzymes which catalyse the oxidation of. one substrate with simultaneous reduction of another substrate are. named as oxidoreductase enzymes The ending of the name of an. enzyme is ase, 14 3 1 Mechanism Enzymes are needed only in small quantities for the progress of a reaction. of Enzyme Similar to the action of chemical catalysts enzymes are said to reduce. Action the magnitude of activation energy For example activation energy for. acid hydrolysis of sucrose is 6 22 kJ mol 1 while the activation energy is. only 2 15 kJ mol 1 when hydrolysed by the enzyme sucrase Mechanism. for the enzyme action has been discussed in Unit 5. 14 4 Vitamins It has been observed that certain organic compounds are required in. small amounts in our diet but their deficiency causes specific diseases. These compounds are called vitamins Most of the vitamins cannot be. synthesised in our body but plants can synthesise almost all of them. so they are considered as essential food factors However the bacteria. of the gut can produce some of the vitamins required by us All the. vitamins are generally available in our diet Different vitamins belong. to various chemical classes and it is difficult to define them on the. basis of structure They are generally regarded as organic compounds. required in the diet in small amounts to perform specific. biological functions for normal maintenance of optimum growth.

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