Carbohydrates

Introduction of carbohydrates

Carbohydrates are the organic molecules that are composed of elements carbon, hydrogen and oxygen. These carbohydrates are referred to as saccharides. Carbohydrates are defined as polyhydroxy-aldehydes or polyhydroxy ketones or compounds, which produce them on hydrolysis. They supply energy and serve as structural constituents.
Carbohydrates have the general formula C_x(H₂O)_y. They are the ultimate source of most of our food. We clothe ourselves with cellulose in the form of cotton, linen and rayon. We build furniture and houses from cellulose in the form of wood.

Classification of carbohydrates

Carbohydrates are classified into three groups based on the number of sugar units and upon their behaviour towards hydrolysis. They are

• Monosaccharides

• Oligosaccharides and

• Polysaccharides.

Monosaccharides

These are simplest group of carbohydrates and are referred as simple sugars as they are sweet in taste. They cannot be further hydrolyzed to simpler compounds. They have the general formula C_n(H₂O)_n.

Examples: Glucose and fructose.

Depending upon the total number of carbon atoms in monosaccharides and aldehyde and ketone functional groups present they are classified using terms shown in the below table.

Oligosaccharides

These carbohydrates liberate two to ten monosaccharide molecules on hydrolysis. They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc. based on the number of monosaccharide units. For e.g., disaccharides like sucrose produce two molecules of monosaccharides on hydrolysis.

A trisaccharide like raffinose on hydrolysis gives glucose, fructose and galactose.

Polysaccharides

These carbohydrates liberate a large number of monosaccharide molecules on hydrolysis. They are usually amorphous, insoluble in water and tasteless and are called non-sugars. They are again sub-divided into two types. They are homopolysaccharides and heteropolysaccharides.

Homopolysaccharides

They possess only a single type of monosaccharide units.

Examples: Starch, cellulose and glycogen.

Heteropolysaccharides

They possess two or more types of monosaccharide units.

Examples: Heparin and chondroitin sulphate.

Carbohydrates may also be classified as either reducing or non-reducing sugars.

All those carbohydrates which contain free aldehyde or ketonic group and reduce Fehling's solution and Tollen's reagent referred to as reducing sugars.

All monosaccharides whether aldose or ketose are reducing sugars.

In disaccharides if the reducing group of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non reducing sugars e.g., sucrose, while others in which these functional groups are free are reducing sugars.

Examples: Maltose and lactose.

Monosaccharides

These are the simple carbohydrates that cannot be hydrolysed to simpler compounds. They are sweet to taste, crystalline and soluble in water. They are commonly known as sugars. They are further classified based on the functional group and number of carbon atoms. They are aldoses and ketoses.

Aldoses

Monosaccharides with an aldehyde

group as a functional group are known as aldoses.

Example: Glyceraldehyde

Ketoses

Monosaccharides with a keto

group as functional group are known as ketoses.

Example: Dihydroxy acetone

Based on the number of carbon atoms monosaccharides are classified as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C).

Structures of Pentoses

The empirical formula of pentoses is C₅H₁₀O₅. They are of two types

• Aldopentoses and

• Ketopentoses

Examples for Aldopentoses are D-Ribose and D-xylose.

Examples for Ketopentoses are D-Ribulose and D-xylulose.

Structures of Aldo pentoses

D-Ribose

Ribose is wide spread as a constituent of ribonucleic acid and nucleotides.

D-Xylose

Xylose is a constituent of glycoproteins and gums. It is involved in the function of glycoproteins.

Structures of Keto pentoses

D-Ribulose

It is produced during metabolism. It is an important metabolite in hexose monophosphate shunt.

D-Xylulose

It is an important intermediate in uronic acid pathway.

Structure of Hexoses

The empirical formula of hexoses is C₆H₁₂O₆. They are also of two types - Aldo hexoses and keto-hexoses.

Example for ketohexoses are D-fructose.

Examples for Aldohexoses are D-glucose and D-galactose.

Structures of Aldohexoses

D-Glucose

It is a constituent of polysaccharides and disaccharides. It is also found in fruits.

D-Galactose

It is a constituent of Lactose.

Structure of Ketohexoses

D-Fructose

It is a constituent of sucrose. It is also found in fruits and honey.

Glucose

Glucose occurs in nature in free as well as combined form. It is present in sweet fruits and honey. Ripe grapes contain ~ 20% of glucose.

Preparation of Glucose

1. From sucrose is boiled with dil. HCl or H₂SO₄ in alcoholic solution, glucose and fructose are obtained in equal amounts.

2. From Starch

Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H₂SO₄ at 393 K under pressure.

Properties of Glucose

Glucose has one aldehyde group, one primary and four secondary hydroxyl groups.

It gives the following reactions:

1. Acetyletion if glucose with acetic anhydride gives a pentaacetate confirming the presence of five hydroxyl groups in glucose.

2. Glucose reacts with hydroxylamine to give monoxime.

Glucose adds a molecule of hydrogen cyamide to give a cyanohydrin.

These reactions confirm the presence of a carbonyl group in glucose.

3. Glucose reduces ammoniacal silver nitrate solution (Tollens reagent) to metallic silver and also Fehlings solution to reddish brown cuprous oxide and itself gets oxidized to gluconic acid. This confirms the presence of an aldehydic group in Glucose.

4. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid saccharic acid. This indicates that presence of a primary alcoholic group in glucose.

5. Glucose on prolonged heating with HI forms n-hexane suggesting that all the 6 carbon atoms in glucose are linked linearly.

6. D-glucose reacts with phenyl hydrazine to give glucose phenyl hydrazine which is soluble. If excess od phenyl hydrazine is used, a dihydrazone, known as osazone is obtained.

7. On heating with conc. solution of NaOH, glucose first turns yellow, then brown and finally resinifies. However, with dilute NaOH, glucose undergoes a reversible isomerisation and is converted into a mixture of D-glucose, D-maltose and D-fructose. This reaction is known as Lobry de Bruyn-van Ekenstein rearrangement. Same results are obtained if maltose. or fructose are treated with alkali. It is probably on account of this isomerisation that fructose reduces Fehling's and Tollen's reagent in alkaline medium although it does not contain a -CHO group.

8. Dehydration

When treated with concentrated sulphuric acid glucose undergoes dehydration and results in the formation of hydroxy methyl furfural.

Cyclic Structure of D-Glucose

The open chain structure of glucose proposed by Baeyer explained most of its properties. However, it could not explain the following:

1. Despite having an aldehydic group, glucose does not gives Schiff's test and it does not react with sodium bi-sulphite and ammonia.

2. The penta acetate of glucose does not react with hydroxylamine indicating absence of -CHO group.

3. Mutarotation.

When glucose was crystallized from a concentrated solution at 30^oC it gave a form of glucose (Melting point 146^oC) whose optical rotation is 111^o. The b form (Melting point 150^o) obtained on crystallization of glucose from a hot saturated aqueous solution at a temperature above 98^oC has an optical rotation of 19.2^o. These two forms of glucose are called anomers.

Disaccharides

A disaccharide upon hydrolysis liberates two monosaccharide units. These two molecules are held together by a glycosidic bond. The monosaccharides liberated due to hydrolysis may be of similar or different molecules. The disaccharides are of two types. They are - Reducing sugars and non-reducing sugars.

Reducing sugars

The carbohydrates, which reduce Fehling's and Benedict's reagents are called as reducing sugars. They have a free aldehyde or keto group.

Examples: Maltose, lactose

Non-reducing sugars

The carbohydrates, which do not reduce Fehling's and Benedict's reagents are called as non-reducing sugars. They have no free aldehyde or keto group.

Examples: Sucrose, trehalose

The most common disaccharides are:

• Sucrose (cane sugar) made up of glucose + fructose

• Maltose (Malt sugar) made up of glucose + glucose

• Lactose (milk sugar) made up of glucose + galactose.

Sucrose

Sucrose is made up of a-D-Glucose and b-D-fructose held together by a glycosidic bond, between C₁ ofa-glucose and C₂ of b-fructose. The reducing groups of glucose and fructose are involved in glycosidic bond, so it is a non-reducing sugar.

Haworth's Representation of Sucrose

Sucrose is most abundant among all the naturally occurring sugars. It is used as sweetening agent in food industry. It is sweeter than some common sugars like glucose, lactose and maltose. It is a colourless, crystalline and sweet substance soluble in water. Sucrase is the enzyme that can hydrolyze sucrose in the body. As sucrose is a non-reducing sugar, it cannot form osazones.

Inversion of cane sugar

On hydrolysis with dilute acids or enzyme invertose, canesugar 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^o ) is more than dextrotation of glucose (+52.5) the mixture is laevorotatory.

Thus hydrolysis of sucrose brings a change in the sign of rotation from dextro(+) is laevo(-) and is known as inversion and the mixture is known as invert sugar.

Sucrose solution is fermented by yeast when the enzyme invertase hydrolyses sucrose to glucose and fructose.

Enzyme zymase converts these monosaccarides to ethyl alcohal.

Maltose

Maltose is made up of two a-D-glucose (in pyranose form) units held together by a(14) glycosidic bond. As there is a free aldehyde group at C-1 position of the second glucose molecule, maltose is known as reducing sugar. Maltose forms osazones. The enzyme that hydrolyses maltose is maltase.

or

The structure of cellobiose, a disaccharide is similar to maltose. They differ in the glycosidic linkage. The linkage of cellobiose is b(14).

Maltose is obtained by partial hydrolysis of starch by diastase an enzyme present in malt (sprouted barley seeds).

lactose

Lactose is made up of b-D-galactose and b-D-glucose held together by b(14) glycosidic bond. As the aldehyde group at C-1 position of glucose is free, lactose is known as reducing sugar. Lactose forms osazones.

or

Lactose known as milk sugar is most important carbohydrate present in milk. It is an important nutrition for young mammals. Lactase is the enzyme that hydrolyses lactose. It gets hydrolyzed by emulsin an enzyme which specifically hydrolyses b - glycoside linkages.

Polysaccharides

They consist of repeat units of monosaccharides or their derivatives. These units are held by glycosidic bonds. These carbohydrates liberate large number of monosaccharide molecules on hydrolysis. They are colorless and tasteless. So, they are called non-sugars. They are concerned with two important functions - structural and storage of energy. Some examples of polysaccharides are starch, cellulose, glycogen and dextrins. However starch and cellulose are the most important of these.

Polysaccharides are linear as well as branched polymers. The general formula is (C₆H₁₀O₅)n, where 'n' stands for a very large number. The occurrence of branches in polysaccharides is due to the glycosidic linkages formed at any one of the hydroxyl groups of a monosaccharide.

Classification of polysaccharides

Polysaccharides are divided into two types:

Homopolysaccharides and Heteropolysaccharides.

Homopolysaccharides

These are composed of only one type of monosaccharide molecules. Some e.g., of these are: starch, cellulose, glycogen, insulin and chitin.

Heteropolysaccharides

These are composed of different types of monosaccharide molecules. They are also called as heteroglycans.

Mucopolysaccharides are the heteroglycans made of repeating units of sugar derivatives like amino sugars and uronic acids. These are known as glycosamino glycans (GAG). Important mucopolysaccharides are hyaluronic acid, chondroitin sulphate and heparin.

Structure of starch and cellulose

Starch

Starch occurs in all plants, particularly in their seeds. The main sources are wheat, maize, rice, potatoes, barley and sorghum.

Starch is a white amorphous powder, insoluble in cold water. It solution in water gives a blue color with iodine solution. The blue color disappears on heating and reappears on cooling. On hydrolysis with dilute acids or enzyme, starch breaks down into molecules of variable complexity and finally D-Glucose.

Starch does not reduce Fehlingss solution or Tollens reagent and does not form an osazone indicating that all the hemiacetal hydroxyl group of glucose units (C-1) are linked with glycosidic linkages.

Starch consists of two polysaccharide components. They are amylose (20% - 80%) and amylopectin (80% - 90%).

Amylose is water soluble, long unbranched (linear) chain with 200-1000 D-glucose units. These units are joined together by a(14) glycosidic linkage involving C-1 of one glucose unit and C-4 of the other glucose unit. Its molecular weight can range from 10,000 to 500,000. Amylose gives blue color with iodine.

Amylopectin

Amylopectin is water insoluble, branched chain with 20-30 glucose units per branch. These units are held with two types of glycosidic bonds, a(16) glycosidic bonds at branching points and a(14) bonds in the linear chain. Amylopectin does not give blue color with iodine.

Amylase (present in saliva), is the enzyme that hydrolyses starch. It acts specifically on a(14) linkages. The end product of hydrolysis of starch is glucose which is an essential nutrient.

Cellulose

It is the chief constituent of the cell walls of plants wood contains 45-50% while cotton contains 90-95% cellulose. It is a colourless amorphous solid which decomposes on heating. It is largely linear and its individual strands align with each other through multiple hydrogen bonds. This lends rigidity to its structure. It is thus used effectively as a cell wall material. Cellulose does not reduce Tollens reagent or Fehlings solution. It does not from osazone and is not fermented by yeast. It is not hydrolyzed so easily as starch but on heating with dilute H₂SO₄ under pressure yields only D-glucose.

Cellulose is composed of b-D-glucose units linked by b(14) glycosidic bonds. It is a linear chain cellulose on hydrolysis yields a disaccharide cellobcose and then produces b-D-glucose. Due to the lack of an enzyme that can cleave b-glycosidic bonds, all mammals cannot digest cellulose. Large population of cellulolytic bacteria present in the stomach of ruminant mammals like cattle, sheep etc., breaks down the cellulose with the help of enzyme cellulose. It is then digested and converted into glucose.

Structure of Cellulose

Functions of carbohydrates

Carbohydrates participate in a wide range of functions:

• Carbohydrates are most abundant dietary source of energy for all organisms.

• They supply energy and serve as storage form of energy.

• Carbohydrates such as glucose, fructose, starch, glycogen, etc. provide energy for functioning of living organisms.

• Carbohydrates are utilized as raw materials for several industries. For e.g., paper, plastics, textiles etc.

• Polysaccharides like cellulose act as chief structural material for cell walls in plants.

• Carbohydrates participate in cellular functions such as cell growth, adhesion and fertilization.