Difference Between Monosaccharides Disaccharides and Polysaccharides

Main Difference – Monosaccharides vs Disaccharides vs Polysaccharides

Carbohydrates are the major components of all living organisms. All the carbohydrates are composed of Carbon (C), Hydrogen (H) and Oxygen (O) atoms in different combinations. Sugars are carbohydrates. The major types of sugars include Monosaccharides and disaccharides. Polysaccharides are complex carbohydrates. The main difference between Monosaccharides Disaccharides and Polysaccharides is that monosaccharides are monomers of sugars and disaccharides are composed of two monomers whereas polysaccharides are composed of a large number of monomers.

Key Areas Covered

1. What is a Monosaccharide
      – Definition, Properties, Examples
2. What is a Disaccharide
      – Definition, Properties, Examples
3. What is a Polysaccharide
      – Definition, Properties, Examples
4. What are the similarities between Monosaccharides Disaccharides and Polysaccharides
      – Outline of Common Features
5. What is the difference between Monosaccharides Disaccharides and Polysaccharides
      – Comparison of Key Differences

Key Terms: Aldoses, Carbohydrates, Disaccharides, Fiber, Ketoses, Monosaccharides, Polysaccharides, Starch, Sugars

What is a Monosaccharide

Monosaccharides are single sugar molecules which act as the building blocks of disaccharides and polysaccharides. Monosaccharides are the simplest form of carbohydrates. These monosaccharides are composed of C, H and O atoms. The general formula of Monosaccharides is (CH₂O)_n. The letter “n” refers to the number of CH₂O units present in the sugar molecule.

Monosaccharides are found in either aldehyde form or ketone form. This means, monosaccharides essentially have a carbonyl group in their chemical structure. Monosaccharides with aldehyde groups are called aldoses and monosaccharides with ketone groups are called ketoses. In the aldoses, the carbonyl group is located on the terminal carbon atom whereas, in ketoses, the carbonyl group is located on the second carbon atom.

There are different types of monosaccharides, depending on the number of carbon atoms present in the sugar molecule. They are categorized according to their isomerism and the derivatives. Most of the monosaccharides may have the same molecular formula but different arrangements.

According to the confirmation of the monosaccharide, there are two types of isomers per each monosaccharide. They are D-isomer and L-isomer. The position of the –OH group of the penultimate carbon determines whether a monosaccharide is a D-isomer or an L-isomer. The easiest way of showing the atomic arrangement of the monosaccharide is the Fischer projection of the molecule. It is the 2-D structure of the molecule.

Figure 01: The Fischer Projection of D-Glucose

The above image shows the Fischer projection of D-Glucose. But the real structure of a Monosaccharide is a 3-D structure. It is called the chair confirmation and is a cyclic structure. The following image shows the real structure of glucose.

Figure 02: D-Glucose Chair Confirmation

Monosaccharides are water soluble. Almost all monosaccharides taste sweet and are colorless when dissolved in water. Simple monosaccharides have a linear, unbranched structure but the acyclic form is typically converted into the cyclic form due to its instability. All monosaccharides are reducing sugars.

Examples for Monosaccharides

• Glyceraldehyde (3 carbon atoms)
• Erythrose (4 carbon atoms)
• Pentose (5 carbon atoms)
• Glucose (6 carbon atoms)

What is a Disaccharide

Disaccharides are sugar molecules composed of two monosaccharides. Therefore every disaccharide is composed of two chemical rings. The bond between two monosaccharides is called a glycosidic bond. Disaccharides are also simple sugars. Disaccharides are classified into two groups according to their reducing strength.

• Reducing sugars – can act as a reducing agent
• Non-reducing sugars – cannot act as a reducing agent

Figure 03: Structure of a Disaccharide

Therefore, some disaccharides are reducing sugars and some are not. All disaccharides are water soluble and colorless when dissolved in water. Some disaccharides are sweet tasting but some are not.

Examples for Disaccharides and their Properties

What is a Polysaccharide

A polysaccharide is a carbohydrate made out of a number of monosaccharides linked via glycosidic bonds. Polysaccharides are chains of monosaccharides. Therefore, every polysaccharide is composed of a number of chemical rings. The formation of a polysaccharide occurs via condensation polymerization since a water molecule is formed per every glycosidic bond.

Figure 04: Structure of a Polysaccharide

Most polysaccharides are insoluble in water and do not have a sweet taste. Almost all polysaccharides are non-reducing agents due to their complex structure.

Examples of Polysaccharides

• Starch – glucose monomers               
• Cellulose – glucose monomers

Similarities Between Monosaccharides Disaccharides and Polysaccharides

• All three are carbohydrates.
• All are composed of C, H and O atoms.

Difference Between Monosaccharides Disaccharides and Polysaccharides

Definition

Monosaccharides: Monosaccharides are single sugar molecules which act as the building blocks of disaccharides and polysaccharides.

Disaccharides: Disaccharides are sugar molecules composed of two monosaccharides.

Polysaccharides: Polysaccharides are carbohydrates made out of a number of monosaccharides linked via glycosidic bonds.

Water Solubility

Monosaccharides: Monosaccharides are water soluble.

Disaccharides: Most of the Disaccharides are water soluble.

Polysaccharides: Polysaccharides are insoluble in water.

Taste

Monosaccharides: Monosaccharides taste sweet.

Disaccharides: Disaccharides taste sweet.

Polysaccharides: Polysaccharides do not taste sweet.

Reducing Strength

Monosaccharides: Monosaccharides are reducing sugars.

Disaccharides: Some disaccharides are reducing sugars whereas some are not.

Polysaccharides: Polysaccharides are non-reducing carbohydrates.

Number of Monomers

Monosaccharides: Monosaccharides have a single monomer.

Disaccharides: Disaccharides are composed of two monomers.

Polysaccharides: Polysaccharides are composed of a large number of monomers.

Chemical Structure

Monosaccharides: Monosaccharides have simple, linear, unbranched structures.

Disaccharides: Disaccharides have simple, linear, unbranched or branched structures.

Polysaccharides: Polysaccharides have complex,  branched structures.

Chemical Rings

Monosaccharides: Monosaccharides have a single ring structure.

Disaccharides: Disaccharides have two ring structures.

Polysaccharides: Polysaccharides have a number of ring structures.

Conclusion

Carbohydrates are a macronutrient found in food. Monosaccharides, disaccharides, and polysaccharides are the major types of carbohydrates found in nature. The main difference between monosaccharides disaccharides and polysaccharides is that monosaccharides are monomers of sugars and disaccharides are composed of two monomers whereas polysaccharides are composed of a large number of monomers.

References:

1. Szalay, Jessie. “What Are Carbohydrates?” LiveScience. Purch, 25 Aug. 2015. Web. Available here. 28 June 2017. 
2. “Monosaccharide.” Encyclopædia Britannica. Encyclopædia Britannica, inc., 01 June 2015. Web. Available here. 28 June 2017. 

Image Courtesy:

1. “DGlucose Fischer” By Christopher King – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Beta-D-Glucose” By Yikrazuul – Own work (Public Domain) via Commons Wikimedia 
3. “Sucrose-inkscape” By Don A. Carlson – (CC BY-SA 3.0) via Commons Wikimedia
4. “Amylose 3Dprojection.corrected” By glycoform – Own work (Public Domain)  via Commons Wikimedia