Key difference - monosaccharides vs. disaccharides vs. polysaccharides
Carbohydrates are the main components of all living organisms. All carbohydrates are made up of carbon (C), hydrogen (H) and oxygen (O) atoms in various combinations. Sugars are carbohydrates. The most important types of sugar 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 made up of two monomers while 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 - overview of Common Features 5. What is the difference between monosaccharides, disaccharides and polysaccharides - comparison of the main differences
Key Terms: Aldoses, Carbohydrates, Disaccharides, Dietary Fiber, Ketoses, Monosaccharides, Polysaccharides, Starch, Sugar
What is a monosaccharide?
Monosaccharides are single sugar molecules that serve as the building blocks of disaccharides and polysaccharides. Monosaccharides are the simplest form of carbohydrate. These monosaccharides consist of C, H and O atoms. The general formula of monosaccharides is (CH 2 O) n . The letter “n” refers to the number of CH 2 O units present in the sugar molecule.
Monosaccharides are found either in the form of aldehydes or ketones . This means that monosaccharides essentially have one carbonyl group in their chemical structure. Monosaccharides with aldehyde groups are called aldoses and monosaccharides with ketone groups are called ketoses . In the case of aldoses, the carbonyl group is on the terminal carbon atom, while in ketoses the carbonyl group is 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 derivatives. Most monosaccharides can have the same molecular formula but different arrangements.
According to the confirmation of the monosaccharide, there are two kinds of isomers per monosaccharide. They are D-isomer and L-isomer. The position of the -OH group on the penultimate carbon determines whether a monosaccharide is a D-isomer or an L-isomer. The atomic arrangement of the monosaccharide can be most easily represented by the Fischer projection of the molecule. It's the 2-D structure of the molecule.
Figure 01: The Fischer projection of D-glucose
The picture above shows the Fischer projection of D-glucose. But the real structure of a monosaccharide is a 3-D structure. It is called stool confirmation and is a cyclical structure. The following picture shows the real structure of glucose.
Figure 02: Confirmation of D-glucose stool
Monosaccharides are soluble in water. 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 to the cyclic form due to its instability. All monosaccharides are reducing sugars.
Examples of monosaccharides
- Glyceraldehyde (3 carbon atoms)
- Erythrosis (4 carbon atoms)
- Pentose (5 carbon atoms)
- Glucose (6 carbon atoms)
What is a disaccharide?
Disaccharides are sugar molecules that are made up of two monosaccharides. Hence, each disaccharide consists of two chemical rings. The bond between two monosaccharides is called a glycosidic bond. Disaccharides are also simple sugars. Disaccharides are divided into two groups according to their reduction strength.
- Reducing sugars - can act as a reducing agent
- Non-reducing sugars - cannot act as reducing agents
Figure 03: Structure of a disaccharide
Hence, some disaccharides are reducing sugars and some are not. All disaccharides are soluble in water and colorless when dissolved in water. Some disaccharides taste sweet, but others don't.
Examples of disaccharides and their properties
Disaccharide | Monomers | Reduce the strength | taste |
Glucose and fructose | Not reducing | Sweet | |
lactose | Glucose and galactose | To reduce | Sweet |
Two glucose | To reduce | Sweet |
What is a polysaccharide?
A polysaccharide is a carbohydrate made up of a series of monosaccharides linked by glycosidic bonds. Polysaccharides are chains of monosaccharides. Hence, each polysaccharide is made up of a series of chemical rings. A polysaccharide is formed by condensation polymerization, since one water molecule is formed for each 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
Similarities Between Monosaccharides, Disaccharides, and Polysaccharides
- All three are carbohydrates.
- All are made up of C, H and O atoms.
Difference between monosaccharides disaccharides and polysaccharides
definition
Monosaccharides: Monosaccharides are single sugar molecules that serve as the building blocks of disaccharides and polysaccharides.
Disaccharides: Disaccharides are sugar molecules that consist of two monosaccharides.
Polysaccharides: Polysaccharides are carbohydrates that are made up of a series of monosaccharides linked by glycosidic bonds.
Water solubility
Monosaccharides: Monosaccharides are soluble in water.
Disaccharides: Most disaccharides are soluble in water.
Polysaccharides: Polysaccharides are insoluble in water.
taste
Monosaccharides: Monosaccharides taste sweet.
Disaccharides: Disaccharides taste sweet.
Polysaccharides: Polysaccharides do not taste sweet.
Reduce the strength
Monosaccharides: Monosaccharides are reducing sugars.
Disaccharides: Some disaccharides are reducing sugars, others are not.
Polysaccharides: Polysaccharides are non-reducing carbohydrates.
Number of monomers
Monosaccharides: Monosaccharides have a single monomer.
Disaccharides: Disaccharides are made up of two monomers.
Polysaccharides: Polysaccharides are made up 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.
diploma
Carbohydrates are a macronutrient found in foods. Monosaccharides, disaccharides, and polysaccharides are the main types of carbohydrates found in nature. The main difference between monosaccharides, disaccharides and polysaccharides is that monosaccharides are sugar monomers and disaccharides are made up of two monomers, while polysaccharides are made up of a large number of monomers.
References:
1. Szalay, Jessie. "What are carbohydrates?" LiveScience. Purchase, August 25, 2015. Web. Available here . June 28, 2017. 2. "Monosaccharide". Encyclopædia Britannica. Encyclopædia Britannica, inc. June 1, 2015. Web. Available here. June 28, 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
