Carbohydrates: Disaccharides, Oligosaccharide, Polysaccharides, Practice Problems and FAQs

All of us consume a lot of carbohydrates on a daily basis in different food items such as table sugar, rice, milk, meat, bread, right! Do you know which type of carbohydrate is present in these food items? Do all of them possess the same type of carbohydrate? The answer is no. Some of them contain disaccharides, others oligosaccharide and some others polysaccharides. 

You must be wondering what are these carbohydrates different from each other? Well they differ in terms of the number or type of monosaccharide units that they are composed of. Do you know how these monosaccharides are joined together? Which bond is present in between them? Well, they are joined together by a glycosidic bond. Let’s take a deeper look at these topics.

List of Contents

• Disaccharides
• Oligosaccharide
• Polysaccharides
• Practice Problems of Carbohydrates
• FAQs of Carbohydrates

Disaccharides

The word Disaccharides means two sugars (Di = Two; Saccharide = Sugar unit). A glycosidic bond connects two monosaccharide units.

Glycosidic Bond

Individual monosaccharides join together by the elimination of water to form a glycosidic bond between carbon of two adjacent monosaccharides.

Examples of Disaccharide

Name of disaccharide sugar	Monomeric units	Structure	Reducing or Non-reducing sugar	Source
Sucrose (table sugar or cane sugar)	Glucose + Fructose		Non-Reducing sugar	Sugar cane
Maltose (malt sugar)	Glucose + Glucose		Reducing Sugar	Malted cereal grains, corn syrup, sweet potatoes
Lactose (milk sugar)	Glucose + Galactose		Reducing sugar	Milk and milk derived products

Oligosaccharide

The word oligosaccharide means two sugars (‘Oligo’ = Few; ‘Saccharide’ = Sugar). It contains 3-9 monosaccharide units which are linked together by glycosidic linkage.

Examples of Oligosaccharide 

Polysaccharides

They are polymers of repeating units of monosaccharides and are also known as ‘glycans’. They are biomacromolecules having high molecular weight. They are present in the acid insoluble fraction of the cellular extract. They have one non-reducing end at the left hand end and a reducing end on the extreme right end.

On the basis of the kind of monosaccharide units present, polysaccharides are classified as follows: Homopolysaccharide and Heteropolysaccharide.

Homopolysaccharide

They consist of the same kind of monosaccharide units. They can be branched or unbranched.

On the basis of function performed, homopolysaccharides are classified as follows:

• Structural Polysaccharides - Example: Cellulose and chitin
• Storage Polysaccharides - Example: Starch, glycogen and inulin

Cellulose 

It forms the structural part of the living organisms. The monomeric unit of cellulose is glucose arranged in a straight chain and in an unbranched manner. It is the most abundant biomolecule on the planet and is found in plants and protists as a component of cell walls.

Chitin 

It is the second most common organic compound. It is a structural component of insect exoskeletons and fungal cell walls. It is made up of repeated units of N-acetyl glucosamine, a nitrogen-containing glucose derivative ( NAG).

Starch

It is formed by the polymerisation of repeating units of glucose. Plants store food in the form of starch.

It is a branched structure consisting of amylose and amylopectin.

Iodine Test

Iodine test is a chemical test used to check the presence or absence of starch among other polysaccharides in an analyte. When iodine solution (brown colour) is mixed with a substance having starch, its colour changes to blue-black, indicating positive iodine test. If the colour of iodine solution does not change, it indicates a negative iodine test. The starch helices can hold iodine and it turns blue-black due to formation of starch iodide. Other polysaccharides such as glycogen and cellulose do not exhibit positive iodine tests.

Glycogen

It is formed by the polymerisation of repeating units of glucose. Animals store food in the form of glycogen in their liver and muscles. It is a highly branched polysaccharide in which the branch points have α-1,4 glycosidic bonds and the regular straight chain has α-1,6 glycosidic bonds.

Inulin

It is formed by the polymerisation of repeating units of fructose. It is stored in the roots of Dahlia, Dandelion and Artichoke. It cannot be metabolised by the human body and so cannot be filtered by the kidney. It's also used to check the rate of filtration in the kidneys, as well as other related functions.

Heteropolysaccharide

They consist of more than one type of monosaccharide units. They can be branched or unbranched. Two examples of heteropolysaccharides are peptidoglycan and agar.

Peptidoglycan

It is a component of the bacterial cell wall. It is made up of two different kinds of repeating units, that is, N- Acetyl glucosamine (NAG) and N-Acetyl muramic acid (NAM).

Agar

It is obtained from the red algae - Gelidium and Gracilaria. Its monomeric units are Galactose and 3,6-anhydro-L-galactopyranose.

Practice Problems of Carbohydrates

Ques:- Which of the following descriptions is applicable for chitin.

A. It forms the exoskeleton of insects.
B. It acts as the storage food for animals.
C. It is extracted from the red algae.
D. It is a component of the bacterial cell wall.

Solution: Chitin is a polysaccharide found in the exoskeleton of insects.

Glycogen is the storage carbohydrate in animals.

Agar is extracted from seaweeds, generally red algae.

Peptidoglycan is a polysaccharide, the main component of bacterial cell wall.

Thus, the correct option is a.

Ques:- Read the given assertion and reason and select the correct option.

Assertion: Disaccharides such as lactose and maltose are reducing sugars.

Reason: Reducing sugars are those which do not have free aldehyde or ketone group at C-1 to react with other molecules.

A. Both assertion and reason are correct, reason is the correct explanation of assertion.
B. Both assertion and reason are correct but reason is not the correct explanation of assertion.
C. Assertion is correct, reason is incorrect.
D. Both assertion and reason are incorrect.

Solution: Reducing sugars have free aldehyde or ketone groups at C-1 which can react with other molecules. Thus, the reason is incorrect. Examples of reducing sugars include monosaccharides (glucose, fructose, galactose), disaccharides (lactose, maltose), some oligosaccharides and polysaccharides. Hence, option c is correct.

Ques:- Select the correct statement about cellulose.

A. It is a branched polysaccharide having an alpha 1,4-glycosidic bond between glucose molecules. 
B. It is an unbranched polysaccharide having an alpha 1,4-glycosidic bond between glucose molecules. 
C. It is an branched polysaccharide having a beta 1,4-glycosidic bond in the linear chains and alpha 1,6- glycosidic bond at the sites of branching 
D. It is an unbranched polysaccharide having an beta 1,4-glycosidic bond between glucose molecules.

Solution: Cellulose is an unbranched homopolymer made up of repeated glucose units connected by a beta 1,4-glycosidic bond. 

Thus, the correct option is d.

FAQs of Carbohydrates

Ques:- What is the structure of the carbohydrate stored by plants during photosynthesis? 

Answer: Plants synthesise glucose during photosynthesis and store it in the form of starch. 

Starch is a branched homopolymer made up of repeating glucose units. The linear branched chain of glucose units linked by alpha 1,4 glycosidic bond, is called amylose. The branched chain of glucose units in which the branch points have α-1,6 glycosidic bonds and the regular straight chain has α-1,4 glycosidic bonds is called amylopectin.

Ques:- Define glycosidic bonds.

Answer: Glycosidic bond is formed in between the adjacent carbon of two monosaccharide units by the elimination of water.

Ques:- Mention the major difference between glycogen and cellulose.

Answer:

Ques:- Mention the disaccharide present in the milk.

Answer: Lactose is a disaccharide made up of glucose and galactose and is found in the milk.

Ques:- Write a short note on polysaccharides.

Answer: Polysaccharides are the polymers of repeating monosaccharide units. They are also known as glycans. They are found in the acid insoluble fraction and have high molecular weight.

They have one non-reducing end at the left hand end and a reducing end on the extreme right end.

Other Related Topics

Other Related Topic Of Biology

• Carbohydrates
• Amino acids
• Dynamic state of body constituents