Classification of Carbohydrates And Its Structure - Definition, Types, Structure, Properties, FAQs

Carbohydrates are an excellent and very diverse class of biomolecules whose majority plays very important roles in almost all aspects of life. Besides their role as primary sources of energy, there are structural forms that act as the framework for cells and tissues. Some also form part of the signaling molecules in a number of biological processes. This work is going to focus on the classification of carbohydrates, in particular their cyclic structures, and will further develop concepts on anomers, epimers, and mutarotation.

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This Story also Contains

1. What are Carbohydrates?
2. Classification
3. Cyclic Structure of Glucose (Haworth Projection)
4. Cyclic Structure of Fructose: Haworth Projection
5. Anomers, Epimers, and Mutarotation
6. Relevance and Applications
7. Some Solved Examples
8. Summary

What are Carbohydrates?

Carbohydrates are composed of carbon, hydrogen, and oxygen; the general formula is $\mathrm{C}x(\mathrm{H}2\mathrm{O})\mathrm{y}$. They are categorized based on their degree of polymerization as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides consist of the simplest units of carbohydrates, which could not be further hydrolyzed into still smaller carbohydrate molecules. Two monosaccharides are then linked together via a condensation reaction to form a single disaccharide. Oligosaccharides and polysaccharides are chains of monosaccharide units.

The cyclic forms of monosaccharides, mainly glucose and fructose, have been of immense importance in understanding the rich properties and behaviors. The open-chain form of these monosaccharides undergoes intramolecular cyclization in an aqueous solution, leading to a six-membered ring in fructose. There are many ways of representing the structure of these cyclic forms; probably the most common is by a Haworth projection in which the ring is projected onto a plane with substituents arranged as indicated.

These are polyhydroxy aldehydes or ketones or substances that form these on hydrolysis and possess at least one chiral atom. The (-OH) group is available in the form of hemiacetals or hemiketals. The carbohydrates are stored in the animal body as glycogen which is also known as animal starch because its structure is highly branched like amylopectin. It is found in liver, muscles and brain as well as in fungi and yeast. When the body requires glucose, the enzymes break glycogen to glucose. Carbohydrates are indispensable for both plant and animal lives. These are utilised as the storage molecules in the form of starch in plants and glycogen in animals. The cell wall of bacteria and plants consists of cellulose. Carbohydrates are present in biosystem in combination with several proteins and lipids.

Related Topics,

• Structure of Glucose and Fructose
• Sucrose

Classification

The carbohydrates can be classified into three categories:

Monosaccharides:

They are the simplest carbohydrates which cannot be hydrolysed to smaller molecules. They are sweet and crystalline and are called sugars.

Oligosaccharides:

These carbohydrates on hydrolysis give two to nine molecules of monosaccharides classified as di-, tri, tetra-saccharides, etc. For example, sucrose, maltose, lactose and raffinose, etc. They are also called sugars.

Polysaccharides:

These carbohydrates, on hydrolysis, give a large number of monosaccharide, e.g., starch, cellulose, etc. They are also called non-sugars.

Reducing and Non-reducing Sugars

Those sugars which reduce Fehling's and Tollens solutions are called reducing sugars and those which do not reduce these reagents are called non-reducing sugars. All the monosaccharides and disaccharides, except sucrose, are reducing sugars, whereas all the polysaccharides are called non-reducing carbohydrates.

Cyclic Structure of Glucose (Haworth Projection)

Glucose represents a hexose monosaccharide that occurs in both forms: openchain and cyclic. The open-chain form of the glucose undergoes intramolecular cyclization in water solution to form a sixmembered ring—a pyranose. Haworth projection is a method of representing the cyclic form of glucose, exhibiting the ring in the plane configuration with substituents at specific positions on the ring. In a Haworth projection of glucose, the hydroxyls are positioned either above (a-Dglucopyranose) or below (b-D-glucopyranose) the plane of the ring with the position of the anomeric carbon attached to two oxygen atoms. This cyclic structure is responsible for quite a number of its properties with respect to the formation of glycosidic bonds and behavior in many chemical reactions.

The CHO group of glucose either reacts with$\mathrm{C}-5\mathrm{OH}$ group or $\mathrm{C}-4\mathrm{OH}$ group to give hemiacetalic linkage and forms stable six- and five- membered cyclic rings, respectively. The reaction ofCHO group with $\mathrm{C}-6\mathrm{OH}$ group or with $\mathrm{C}-3\mathrm{OH}$does not occur as the formation of seven- membered or four-membered ring respectively is not favourable due to angle strain theory.

Haworth representation:

Cyclic structure of glucose was established by English chemist W.N. Haworth. The cyclic structure of glucose are depicted below.

Cyclic Structure of Fructose: Haworth Projection

The other important hexose monosaccharide is that of fructose. Like glucose, this also occurs in cyclic forms. Unlike glucose, however, it forms a five-membered ring—a furanose. In its Haworth projection the ring of the fructose is planar with the substituents in an arrangement as in glucose. In the Haworth projection of fructose, the hydroxyl groups are above or below the plane of the ring. The anomeric carbon is C2, bonded to two oxygens. The cyclic form of fructose is important for its reactivity and its biological functions.

It also exists in two cyclic forms which are obtained by the addition of —-OH at C 5 to the $(\mathrm{C}=0)$ 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 below.

Anomers, Epimers, and Mutarotation

Anomers are stereoisomers that differ at the anomeric carbon. All the α-anomer and β-anomer physical and chemical properties of a monosaccharide differ, for instance, melting points and optical rotations. Epimers are stereoisomers that differ in configuration at a single chiral center. Glucose and galactose are, therefore, C4 epimers because they differ only in the configuration of the hydroxyl group at C4. Mutarotation is a process in which one anomeric form of a monosaccharide spontaneously changes into the other anomeric form in solution. This process continues until an equilibrium mixture is reached, whereby the relative proportions of the two anomers at that time are dependent both on the particular monosaccharide and on the conditions of the solution. Understanding anomers, epimers, and mutarotation is critical for both the interpretation of experimental data and the prediction of carbohydrate behavior in a host of chemical and biological contexts.

Also read -

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Anomers

Anomers are diastereomers that differ in the configuration at the acetal or hemiacetal C atom of a sugar in its cyclic form. In other words, anomers are epimers whose conformations differ only about C-1. For example, α-D(+) and β-D(+) glucose are anomers.

Epimers

Diastereomers with more than one stereocentre that differ in the configuration about only one stereocentre are called epimers.

1. D-Glyceraldehyde and L-glyceraldehyde are epimers
2. D-Erythrose and L-threose are epimers.
3. Epimerisation of glucose at C-2 gives mannose.
4. Epimerisation of glucose at C-3 gives allose.
5. Epimerisation of glucose at C-4 gives galactose.

Mutarotation

The change in specific rotation of an optically active compound in solution with time, to an equilibrium value, is called mutarotation, or it is the change in the optical rotation occuring by epimerisation, i.e., the change in the equilibrium between two epimers, when the corresponding stereocentres interconvert. During mutarotaion, the ring opens and then ring 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 undergo mutarotation in aqueous solution. Mutarotation proves the existence of anomers and cyclic structures.

Also read :

• NCERT notes Class 12 Chemistry Chapter 14 Biomolecules
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Relevance and Applications

The cyclic structures of glucose and fructose, together with such concepts as anomers, epimers, and mutarotation, find applications in many diverse areas—from biochemistry and molecular biology through organic chemistry and food science to analytical chemistry. In the fields of biochemistry and molecular biology, knowledge of the ring forms of monosaccharides is absolutely necessary for an understanding of the chemistry by which glycosidic linkages give disaccharides and polysaccharides. Both of these are important classes of biomolecules. Moreover, such principles about anomers and epimers can be elaborated taking into consideration the specificity displayed by carbohydrate-binding proteins including lectins and antibodies. Theoretical applications, other than ring forms of monosaccharides, include organic chemistry in synthesis of carbohydrate derivatives and glycoside synthesis. As far as the synthesis and characterization of carbohydrates, the ideas evolved from the anomers and mutarotation. In food science and nutrition, application will need knowledge on the physical properties like sweetness, digestibility of carbohydrates in the development of food products, and assessing nutritional values. One of the major fields of application of concepts of anomers and epimers in relation to the next analytical techniques is in Nuclear Magnetic Resonance spectroscopy and Mass Spectrometry for carbohydrate characterization and identification.

Recommended topic video on (Classification of Carbohydrates and its Structure )

Some Solved Examples

Example 1
Question: Which of the following will not react with Tollen's reagent?
1. Glucose
2. Sucrose
3. Fructose
4. Galactose

Solution:
Reducing sugars contain free aldehyde (-CHO) or ketone (-C=O) groups and are capable of reducing Tollen's reagent. Examples of reducing sugars include glucose, fructose, and galactose. Sucrose is not a reducing sugar as it does not have a free aldehyde or ketone group. Hence, the answer is option 2.

Example 2
Question: Which of the following can be classified as polysaccharides?
a) Cellulose
b) Starch
c) Maltose
d) Aldohexose

1. a, b, c
2. b, c, d
3. a, b
4. c, d

Solution:
Polysaccharides are carbohydrates that yield a large number of monosaccharide units upon hydrolysis. Cellulose and starch are polysaccharides. Maltose is a disaccharide, and aldohexose is a monosaccharide. Hence, the answer is option 3.

Example 3
Question: Which of the following can be classified as an oligosaccharide?
1. Galactose
2. Glyceraldehyde
3. Sucrose
4. Fructose

Solution:
Oligosaccharides are carbohydrates that yield 2-10 molecules of monosaccharides upon hydrolysis. Galactose, glyceraldehyde, and fructose are monosaccharides, while sucrose is an oligosaccharide as it gives glucose and fructose upon hydrolysis. Hence, the answer is option 3.

Example 4
Question: Which of the following can be classified as a monosaccharide?
1. Sucrose
2. Lactose
3. Amylose
4. Glucose

Solution:
Sucrose and lactose are disaccharides, while amylose is a polysaccharide. Only glucose is a monosaccharide among the given carbohydrates. Hence, the answer is option 4.

Example 5
Question: Which of the following are one of the macromolecules making up the living organism?
a) Carbohydrates
b) Amino acids and Proteins
c) Nucleic acid

1. a, b
2. a, b, c
3. c
4. b, c

Solution:
Biomolecules are complex macromolecules that build up living organisms and are required for their growth and maintenance. The four kinds of macromolecules making up living organisms are carbohydrates, lipids, proteins, and nucleic acids. Hence, the answer is option 2.

NCERT Chemistry Notes:

• NCERT Notes Class 11th Chemistry
• NCERT Notes Class 12th Chemistry
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Summary

In this chapter, we have explained the classification of carbohydrates and their structures, covering the cyclic structure of glucose and fructose, also anomers, epimers, and mutarotation. Since carbohydrates are a very important group of biomolecules, their roles in different living organisms are many, so their classification and structure become very important in understanding behavior and interactions.

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