Bio-molecules Carbohydrates Preparation and Properties
→ Carbohydrate is derived by combination of two words ‘carbo’ and ‘hydrate’. Carbo means carbon and hydrate means water i.e, water hydrated carbon are known as carbohydrates. Carbohydrates are naturally occurring carbon compounds and are formed in plants by the process of photosynthesis.
Definition of Carbohydrates :
→ Carbohydrates are primarily produced by plants and for a very large group of naturally occurring organic compounds. Some common examples are canesugar, glucose, starch, etc. Most of them have a general formula.
→ Cx (H2O)y, and were considered as hydrates of carbon from where the name carbohydrate was derived.
→ For example, the molecular formula of glucose (C6H12O6) fits into this general formula, C6(H2O)6. But all the compounds which fit into this formula may not be classified as carbohydrates.
→ Acetic acid (CH3 COOH) fits into this general formula, C2(H2O)2 but is not a carbohydrate. Similarly, rhamnose, (C6O12O5) is a carbohydrate but does not fit in this definition.
→ Hence, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
Functions of Carbohydrates :
- The main function of carbohydrates is to provide energy and heat to the body.
- Carbohydrates are used as storage molecules as starch in plants and as glycogen in animals.
Classification of Carbohydrates :
- Non-Sugar Carbohydrates.
Type 1 :
→ Sugars : Sugars are sweet in taste, soluble in water, and crystalline solid, e.g. glucose, fructose, sucrose, lactose etc.
- Non-Sugar Carbohydrates : Non sugars are tasteless, insoluble in water, used for making
- colloidal solution, e.g. starch, cellulose, glycogen etc.
→ On the basis of chemical structure carbohydrates are divided into three groups
- Poly saccharides.
Type 2 :
→ Reducing Sugars : Those carbohydrates which are reduced by Fehiing solution and Tollen’s reagent (ammonical silver nitrate solution) are known as reducing sugars.
Example : Glucose and Fructose
→ Non-reducing Sugars : Those carbohydrates which are not reduced by Fehling solution and Tollen’s reagent (Ammonical Silver Nitrate solution) are known as non-reducing sugars.
Example : Sucrose etc.
→ Monosaccharides : A carbohydrate that can not be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called s monosaccharide. Example, glucose and fructose etc.
General Properties :
→ Monosaccharides are also called simple sugars. These are made up of 3 to 7 carbon Stoma.
→ These are generally cystalline. solid, sweet and soluble in water. It is soluble in water due to the presence of OH group and hydrogen bonding between its molecules.
→ About 20 monosaccharides are known to occur in nature, Oligosaccharid.s s Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaucharides.
Example : Sucrose or cat sugar on hydrolysis gives glucose and fructose. Maltose on hydrolysis gives two molecules of gLucose etc.
General Properties :
- In oligossocharides. monosaccharide units are joined by glycosidic bond.
- Monosaccharide unit obtained on hydrolysis of oligoseocharides may be same or different.
- They are further classified as disaccharides, trisaccharides, tetrasaccharids, etc.. depending upon the number of monosaccharides, they give on hydrolysis.
→ Disaccharides : It is main unit of oligosaccharidea, e.g. Sucrose, maltose etc. Two monosaccharides are joined together by oxide bonds. Which is called glycoside bond.
→ Trianechisrides : It contains three units of monosaccharides, e.g. Raffinoge
→ Tetrasnecharidea : It contains four units of monosaccharides,eg stahytisc (C24H42O21).
→ Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharidea. Some common examples are starch. cellulose, glycogen. gums. etc.
General propertIes :
- These are the polymers of monosaccharides and tlwirgrncnd formula is (C6H10O5)n.
- These arc tasteless and insoluble in water. They give monosaccharides on hydrolysis.
- Polyssocharides are not Sweet in taste, hence they are also called nonsugars.
- In polysaccharides, monosaccharide units are joined by glycosidic bond.
Their general properties are studied earlier.
Monosaccharides can be classified in two groups :
→ Aldoses : If a thonosaccharide contains an aldehyde group (—CHO), it is known as an aldose. Aldehydic group is present at one end of the carbon chain i.e., at C – 1 carbon.
→ Ketoses: If a monosaccharide contains a keto group (>CO), it is known as a ketose. Here, ketonic group can be present anywhere except at the last carbon.
On the basis of number of carbon stoms, these are classified as triose, tetrose, pentose, hexose etc.
|No. of Carbon atoms
|Glycerose (or) Glyceraldehyde Erythrose
|Dihydroxy acetone Erythrulose
|Ribose, Xylose Arabinose Glucose, Galactose Mannose
|Ribulose Xylulose Fructose
Glucose and Fructose :
The general formula of both glucose and fructose is C6H12O6. Glucose is aldohexose and fructose is 184.108.40.206 ketohexose.
- Introduction : Glucose is the most abundant naturally occurring carbon compound, It is the monomer of many carbohydrates like cellulose, starch etc. Glucose occurs freely in nature as well as in the combined state.
- It is present in sweet fruits and honey. Ripe grapes also contain glucose in large amount. Human blood contains almost 0.1% of glucose in combined state. Glucose is found in many oligosaccharides and polysaccharides. Glucose is obtained by their hydrolysis.
- Preparation of glucose : Two main sources of glucose are starch and sucrose.
- From sucrose : Sucrose is a disaccharide. Its general formula is C12H22O11. If sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution then glucose and fructose are obtained in equal amounts :
From starch : Starch is a polysaccharide. Commercially glucose is obtained by hydrolysis of starch when it is boiled with dilute H2SO4 at 393 K under pressure.
Physical Properties :
- Glucose is an aldohexose and it is also known as dextrose. It is white crystalline solid and sweet in taste.
- Its melting point is 419 K and in the form of monohydrate, its melting point is 319 K.
- Glucose is easily soluble in water, partially soluble in water and insoluble in ether.
- It is dextrorotatory in nature.
Structure and Configuration : Structure :
→ Experimentally it was found that chemical structure of glucose in CH2OH(CHOH)4CHO. A molecule of glucose contains and aldehydic group (—CHO), a primary alcoholic group (—CH2OH) an four secondary alcoholic groups. General chemical reactions of glucose are given below which are shown by these functional groups.
→ These reactions confirm the structure of glucose.
→ On heating with HI. it forms n-hexane, suggesting that all the six carbon atoms are linked in a straight chain
→ When glucose is reduced with NaIHg amalgam and water, sorbitol is formed which shows the presence of aldehydic group in glucose.
→ Glucose gets oxidised to six carbon containing carboxylic cid i.e., (gluconic acid) on reaction with a mild oxidising agent like bromine water. This indicates that the carbonyl group is present as an aldehydic group.
→ 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.
→ Reaction with Hydroxyl amine Glucose reacts with hydroxyl amine to form an oxime. These reactions confirm the presence of a carbonyl group
(>C = O) in glucose.
→ Reaction with Hydrogen Cyanide (HCN) Glucose adds a molecule of hydrogen cyanide to give glucose cyanohydrin.
→ Reaction with Fehling’s solution and Tollen’s reagent Glucose reduces Tollen’s reagent and fehling’s solution and itself gets oxidised to form gluconic acid. Above two reactions shows the presence of carbonyl group in glucose molecule and this reaction shows that carbonyl group is present as aldehyde group.
→ Tollen’s reagent is ammoniacal silver nitrate solution.
AgNO3 + NH4 OH → AgOH + NH4NO3
→ When glucose is heated with Tollen’s reagent in a test tube, a silvery mirror is obtained at the bottom of the test tube. Similarly, when glucose heated with Fehling’s solution, gluconic acid and a red precipitate (Cu2O) is obtained.
→ Fehiing solution A (aqueous CuSO4 solution) and Fehiing solution B (aqueous NaOH solution) mixed with a small amount of salt), when mixed in equal proportion gives blue solution in the test tube. This solution is known as Fehiing solution.
→ Reaction with Phenyl hydrazine Glucose reacts with phenyl hydrazine to give glucose phenyl hydrazone which is soluble. If excess phenyl hydrazine is used, a dihydrazone. known as osazone is obtained.
→ Acetylatlon Glucose when heated with acetic anhydride or acetyl chloride gives pentaacetyl derivative (Pentaacetate). This reaction sh5ws presence of five hydroxy groups in glucose.
→ Methylation Glucose when reacted with methyl alcohol in the presence of HCl, gives, methyl glucoside.
→ Configuration of Glucose : The exact spatial arrangement of different —OH groups was given by Fischer after studying many other properties.
→ In glucose, tetrahedral chiral carbon is surrounded by hydrogen and hydroxy groups. Glucose is correctly named as D(+)-glucose.
→ Meaning of ‘D’ and ‘L’ sign : D’ before the name of glucose represents the configuration whereas ‘(+)‘ represents dextrorotatory nature of the molecule. It may be remembered that ‘D’ and ‘L’ have no relation with the optical activity of the compound.
→ The letters D or ‘L’ before the name of any compound indicate the relative configuration of a particular stereo isomer, This refers to their relation with a particular isomer of glyceraldehyde.
→ Glyceraldehyde is an aldotriose containing chiral carbon. Glyceraldehyde contains one asymmetrk carbon atom exists in two enantiomeric forms as shown below.
→ All those compounds which can be chemically correlated with (+) isomer of glyceraldehyde are said to have D-configuration whereas those which can be correlated with (—) isomer of glyceraldehyde are said to have L-configuration. For assigning 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 it is assigened as D-configuration. For this comparison, the structure of glucose can be written in a way that most oxidised carbon is at the top.
Cyclic structure of D(+) glucose :
→ The structure (I) of glucose explained most of its properties but the following reactions and tacts could not be explained by this structure :
→ Despite having the aldehyde group, glucose does not give 2,4-DNP test. Schiffs test and it does not form the hydrogen suiphate addition product with NaHSO3.
→ The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free -CHO group.
→ Glucose exist in two different crystalline forms which are named as α and β. The α-form of glucose (m.p. 419 K) is obtained by crystallisation from concentrated solution of glucose at 303 K while the β-form (mp. 423 K) is obtained by crystallisation from hot and saturated aqueous solution at 371 K. This means that both forms are actually different
→ Keeping all the above points in mind, Tollen gives a cyclic structure for glucose which is known as Hemi-acetal structure. He 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 six membered cyclic structure of glucose is called pyranose structure (α or β), 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.
→ Anomeric Carbon : The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at C1, called anomeric carbon (the aldehyde carbon before cyclisation). Such isomers, i.e., α-form and β-form. are called anomers.
→ In monosaccharides, cyclisation structure is formed due to formation of acetal and hemiacetal and in which carbon atom is asymmetric. This carbon atom is called anomeric carbon and the optical isomers formed are called anomers.
→ It should be noted that anomers which have different configuration of H and OH only at C – 1 carbon are not mirror images of each other.
→ Above cyclic structure of glucose explains most of its properties. It is assumed that when glucose reacts with HCN, NH2OH, Tollen’s reagent and Fehiing solution, aldehydic group is free to reacts with these reagents. But with sodium bisulphite and 2,4-dinitrophenyl hydrazine etc, reagents attack cyclic structure where aldehyde group is not free and thus, these reagents do not react with aldehydic group.
→ Mutarotatlon : Multarotation is the change of the sign of specific rotation when the anomeric forms are dissolved in water. This property is found in carbohydrates such as glucose.
→ The monosaccharides D-glucose exists in two cyclic forms, α-D-glucose ([α]D25 = + 112). and β-D-g1ucose ([α]D25 = + 18.7). which are epimers and are available as pure compounds.
→ When one of the cyclic forms of D-glucose is added to water, it undergoes reversible epimerization to the other via the open-chain form, during which the specific rotaion of the solution changes gradually until it reaches the equilibrium value + 52.7°.
→ The two stereoisomeric forms of glucose i.e, α-D-glucose and β-D-glucose exist in separate crystalline forms and thus have different melting points and specific rotations.
→ For example α-D-glucose has a m.p. of 419 K with a specific rotation of +112° while β-D-glucose has a m.p. of 424 K and has a specific rotation of +19°.
→ However, when either of these two forms is dissolved in water and allowed to stand, it gets converted into an equilibrium mixture of a-and β-forms through a small amount of the open chain form
→ As a result of this equilibrium, the specific rotation of a freshly prepared solution of α-D-glucose gradually decreases from of +112° to +52.7° and that of -D-glucose gradually increases from +19° to +52.7°.
→ During mutarotation, the ring opens and then recloses either in the inverted position or in the original position giving a mixture of α-and β-forms. All reducing carbohydrates, i’e., monosaccharides and disaccharides (maltose, lactose etc.) undergo mutarotation in aqueous solution. At equilibrium, α-form is 36%, β-form 63.5% and open chain is 0.5%
→ Introduction : Fructose is a monosaccharide. Its general formula is C6H12O6. Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose.
→ Structure of Fructose : Fructose also has the molecular formula C6H12O6 and on the basis of its reactions it was found to contain a ketonic functional group at carbon number 2 and six carbons in straight chain as in case of glucose.
→ Open chain structure of fructose also does not explain some of its properties. It also exists in two cyclic forms which are obtained by the addition of—OH at C-5 to the (CO) 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.
→ It belongs to D.series and is a laevorotatory compound. It means that it rotates the plane polarised light to the left. It is appropriately written as D-(—)-fructose. All the structures of glucose can be written as ;
Physical Properties of Fructose :
- Its melting point is 102°C.
- Unhydrated fructose is white crystalline substance.
- It is soluble in water and insoluble in benzene and ether.
- It shows mutarotation like glucose.
- In all sugars fructose in sweetest.
The cyclic structure of two anomers of fructose are represented by Haworth structure as:
Oligosaccharides or Disaccharides :
→ Disaccharide is a oligosacchande which on hydrolysis with dilute 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.
→ All disaccharides are crystalline solid, sweet in taste and soluble in water. Some important disaccharides are sucrose, maltose, lactose etc.
These are of two types :
→ ReducIng : Those disaccharides in which carbonyl group of one monosaccharide does not take part in bond formation are called reducing. Examples are maltose and lactose.
→ Non-reducing : Those disaccharides in which carbonyl group of both the monosaccharides form glycoside bond with each other are called non-reducing. Example is sucrose.
→ Sucrose is an important disaccharide. It is also known as cane sugar. It is found in nature in juices, fruits, seeds and in many plants. The main source of sucrose is sugar cane and sugar beet. Sugar cane contains 15—20% sucrose and sugar beet contains 10—17% sucrose.
Properties of Sucrose :
- The formula of sucrose is C12H22O11. It is white crystalline, sweet in taste and soluble in water.
- Its melting point is 180°C and when heat above its melting point changes to brown substance which is known, as caramel
- It is a non-reducing sugar.
- It is dextro-rotatory and it does not show mutarotatioçi.
Structure of Sucrose :
Sucrose is a disaccharide and its molecular formula is C12H22O11.
Its structure is based on following facts :
→ On hydrolysis with dilute acids or enzyme, cane sugar gives equimolar mixture of D(+) glucose and D(—) fructose.
→ Sucrose is dextrorotatory but after hydrolysis gives dextrorotary glucose and laevorotatory fructose. Since the laevorotation of fructose (—92.4°) is more than dextrotation of glucose (+52.5) the mixture is laevorotatory.
→ Thus hydrolysis of sucrose bring a change in the sign of rotation from dextro (+) to laevo (—) and is known as inversion and the mixture is known as invert sugar.
Glucose and fructose are reducing sugars but sucrose is a non-reducing sugar. Because sucrose:
- do not form oxime with hydroxyla mine
- it does not form osazone with phenyihydrazine.
- does not reduce Fehling’s solution and Tollen’s reagent. Also it does not show mutarotation.
→ Above facts shows that in sucrose, aldehyde group of glucose and ketonic group of fructose are not free. Also it shows that both the functional groups (—CHO and > CO) are joined by glycosidic bond.
→ Apart from this, sucrose is hydrolysed in the presence of enzyme maltase (specific to aipha-glycosidic furansidic). Form this, it can be inferred that in sucrose, aldehydic carbon of a-glucose and ketonic group of p-fructose are joined by glycosidic linkage. The structure of sucrose on the basis of above facts can be given as :
→ Maltose is a disaccharide which is obtained by reaction of malt on starch. Therefore, it is known as Malt sugar. It is present in sprouted seeds especially in wheat in the form of starch. Maltose is obtained by partial hydrolysis of starch by diastase an enzyme present in malt (sprouted barley seeds) or beta-amylase.
Properties of Maltose :
- The molecular formula of Maltose is C12H22O11.
- It is a white crystalline solid, soluble in water and insoluble in alcohol and ether.
- Its melting point is 160°C — 165°C.
- It is a reducing sugar.
- It is also dextro rotatory and shows mutarotation.
- Its specific rotation of alpha form is +160°, and that β-form is +112° and general specific rotation is +136°.
Structure of Maltose :
Maltose is a disaccharide and its molecular formula is C12H22O11. its structure is based on following facts.:
→ Maltose on hydrolysis by dilute acids or by enzyme maltase gives two molecules of glucose.
- Forms oxides with hydroxyl amine.
- Forms osazones with phenyl hydrazine.
- Reduces Fehling’s solution i.e., it is a reducing sugar.
- It is dextro rotatory and does not show mutarotation.
→ From the above facts, it is clear that two glucose units in maltose are joined in such a way that aldehydic group (—CHO) of one glucose molecule is free. For this reason, it shows reducing properties. It is also clear that C-4 of reducing glucose forms glycosidic linkage with C-1 of non-reducing glucose.
Thus, maltose can be represented by the following structure:
→ Haworth structure can be represented as:
It is also a disaccharide and known as milk sugar. It is present in milk of all mammals. Commercially, lactose is obtained as by product in the process of formation of butter from milk. After the formation of butter, lactose is obtained by evaporation of aqueous solution.
Properties of Lactose
- It is white crystalline substance, soluble in water and soluble in ether and alcohol.
- Its melting point is 203°C, it is also decomposes at this temperature.
- It is a non-reducing sugar.
- it is also dextro-rotatory and shows mutarotation.
Structure of Lactose :
The molecular formula of lactose is C12H22O11. Its structure is based on following facts:
- Lactose on hydrolysis by dilute acids or enzyme lactase gives mixture of D-(+) glucose and D-(+) galactose.
- Lactose forms oxides with hydroxylamine.
- Forms osazones with phenyl hydrazine.
- Reduces Fehling’s solution i.e., it is a reducing sugar. It also shows mutarotation.
→ Above facts shows that in lactose, reducing part is glucose unit whose C-4 carbon forms glycosidic linkage with C-1 of galactose.
Structure of Lactose can be represented as:
- Polysaccharides are natural polymers. Its monomer units are monosaccharides and oligosaccharides.
- Molecular weight of plysaccharides may to upto many thousands.
→ Polysaccharides are linear as well as branched polymers. Their general formula is (C6H11O5) n, where ‘n’ stands for a very large number.
→ They are non-crystalline, colourless, tasteless, and insoluble in water. So, they are called non-sugars.
These are of two types:
→ Homopolysaccharides : These are composed of only one type of monosaccharide molecules. They are also known as homkoglycanes. Main example of homopolysaccharide insulin.
→ Heteropolysaccharides : These are composed of different types of monosaccharide molecules. They are also called as heteroglycans. For example
→ Some examplee of polysaccharidea are atarvh. celluloae. glvcogen and dextrins. However atarch and cellulose are the moat important of theee.
→ Starch je the main source of carbohydrates and a stored polysarchande in planta It is the moat important dietary source for human beings and gives energy to them.
Its molecular formula is (C6H10O5),,. Starch curs in all plants, particularly in their seeds, The mom sources are wheat, maize, rice. potatoes. barley and sorghum etc.
Strucutre of Starch :
On hydrolysis with dilute acids or enzyme, starch breaks down into molecules of variable complexity and finally D-Glucose.
Starch consists of two po”’wchitriile components. They are:
Amylose (15% – 20%) soluble in water and
Amylopect.in iS)% 90%) insoluble in water,
→ Amylose : Amylose is a water soluble component which constitutes about 15— 20% of starch. It contains a long undranched (linear) chain with 200-1000 D-glucose units. These unit are joined together by a (1 → 4) glycosidic linkage 500,000. Amylose gives blue colour with iodine, its structure can be given as:
→ Amylopectin : Amylopectin is insoluble in water and constitutes about 80-90% of starch. It is branched chain with 20-30 glucose unit per branch. rrhese units are held with two types of glycosidic bonds, a (1 → 6) glycosidic bonds at branching points and a (1 → 4) bonds in the linear chain.
→ Amylopectin does not give blue colour with iodine. Amylase (present in saliva), is the enzyme that hydrolyses starch. it acts specifically on a (1 → 4) linkages. The end product of hydrolysis of starch is glucose which is an essential nutrient.
Uses of Starch
- Manufacturing of Glucose and dextrin.
- In the formation of ethyl alcohol.
- In the form of main source of carbohydrate.
- As a reagent in laboratory.
→ Cellulose is also a polysaccharide and its molecular formula is (C6H10O5)n,. 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 also important for animals but it is used as source for clothes etc. whereas starch is used as food.
→ It is present in wood, cotton clothes, jute, cotton etc., In wood it is present 50%, in dry grass. it is 40—45% in jute 60—65%, in cotton 90—95%, in cotton clothes, 90% is cellulose and rest is fats and wax.
Structure of Cellulose :
→ Cellulose does not reduce Tollen’s reagent or Fehling’s solution. It does not form osazone and is not fermented by yeast. It is not hydrolysed so easily as starch but on heating with dilute H2SO4 under pressure yields only D-glucose. Cellulose is composed of β-D-glucose unit linked by b (1 → 4) glycosidic bonds.
→ The end product of hydrolysis of cellulose is β-D-glucose. Due to lack of an enzyme that can cleave β-glycosidic bonds, all mammals cannot digest cellulose. Large population of cellulolytic bacteria present in the stomach of ruminant mammals like cattle, sheep etc., break down the cellulose with the help of enzyme cellulose. It is then digested and converted into glucose.
→ The name Protein’ is given by Berzelius in 1938. The word protein is derived from Greek word, “proteios” which means primary or of prime importance because proteins are the most important chemical substances which are necessary for growth, repair and development of life.
→ Proteins are found in all living cells. Main sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc. They occur in every part of the body and form the fundamental basf of structure and functions of life. In humans, hair, skin, haemoglobin, nails, skin, haemoglobin, nails, enzymes and cells etc. are made of proteins.
Properties of Cellulose :
- Cellulose is white, uncrystalline, tasteless, fibre substance.
- It is insoluble in water and organic solvent.
→ In human body carbohydrate is stored in the form of glycogen. Its structure is similar with amylopectin so it is also called human starch. It has niore branches than amylopectin. It is present in liver, muscles and mind.
→ When glucose is needed in body enzymes breaks glycogen into Glucose glycogen is also present in yeast and algae.