Introduction

Monosaccharides are the simplest form of carbohydrates that are present in nature. They act as building blocks of complex carbohydrates like oligosaccharides and polysaccharides. All the complex carbohydrates in our body are broken down to monosaccharides and then are absorbed in the blood. They provide energy to our body. They also form important components of other biologically important molecules. 

In this section of notes, we will discuss different aspects of monosaccharides like their structure, general formula, physical and chemical properties, and examples along with their functions and occurrence, etc. You should go through the entire notes in order to grasp the complete concept of monosaccharides. 

Structure

Chemically, monosaccharides are known as polyhydroxy aldehydes or ketones. It means they are the organic compound having;

• Aldehydic or ketonic functional group
• Multiple hydroxyl groups

Let us discuss these components of monosaccharides in some detail. 

Aldehydic group

The aldehydic functional group confers the properties of aldehyde to the monosaccharides. This functional group is represented by the molecular formula (R-CHO). Monosaccharides that have the aldehydic functional group are known as aldoses or aldose sugars. This functional group is always present at one end of the chain of carbon atoms in a monosaccharide. 

Ketonic group

The ketonic functional group confers the properties of ketones to the monosaccharides. The molecular formula to represent the ketonic group is written as (R-C(=O)R’). Monosaccharides having ketonic functional groups are known as keto sugars or ketoses. This functional group never occurs at the end of the chain of carbon atoms, rather it is present at second or second-last position in monosaccharides. 

Hydroxyl groups

Monosaccharides are polyhydroxy compounds having multiple hydroxyl groups. These hydroxyl groups confer the properties of alcohol to carbohydrates. All the carbon atoms in a monosaccharide are attached to a hydroxyl group except the one which is a part of the aldehydic or ketonic group. 

General Formula

The general molecular formula of all the monosaccharides is (CH₂O)_x. In the case of monosaccharides, the value of ‘x’ ranges from 3 to 7 or more. This means that monosaccharides must have at least 3 carbon atoms. 

Nomenclature

The nomenclature of monosaccharides is based on the number of carbon atoms present in their structure, and the type of functional group they carry. 

The scientific name of a monosaccharide begins with the name of the functional group present in it, followed by the number of carbon atoms in Latin. A suffix ‘ose’ is also added at the end of the name.

For example, the term ‘pentose’ means a monosaccharide having five carbon atoms. If this monosaccharide contains the aldehydic functional group, it will be called an ‘aldopentose’, while if it contains a ketonic functional group, it will be called a ‘ketopentose’.

The term ‘aldoses’ is used to refer to all the monosaccharides having an aldehydic group, while the term ‘ketoses’ is used to refer to all the monosaccharides having the ketonic functional group. 

Sometimes, specific names are also used for monosaccharides, such as Glucose, fructose, ribose, etc. 

Physical Properties 

In this section, we will discuss the common physical properties found in all monosaccharides. 

• They do not undergo Hydrolysis

Hydrolysis is the process in which a larger molecule is broken down into smaller ones using a water molecule. The bond is cleaved using a water molecule and the hydrogen and hydroxyl components of water get added to the new molecules. 

Monosaccharides are the simplest form of carbohydrates. They cannot be broken down into simpler compounds. Therefore, they do not undergo hydrolysis. 

• They are soluble in water

As we already know, monosaccharides have abundant hydroxyl groups in their structure. These hydroxyl groups can form hydrogen bonds with the water molecules. Because of these abundant hydroxyl groups, all monosaccharides can dissolve in water very easily. 

• They are sweet in taste

Monosaccharides are sweet in taste because of the particular orientation of hydroxyl groups. These hydroxyl groups in monosaccharides are arranged in such a way that they interact and activate the sweet receptors present on our tongue. This is the same reason why they are called sugars. 

• They are reducing sugars

A reducing sugar is the one that can act as a reducing agent. It can donate an electron and reduce the recipient compound and thus, itself gets oxidized. For a sugar to behave as a reducing agent, it must have a free aldehydic or ketonic group so that the oxygen atom of the free functional group can donate the electrons to the recipient compounds. 

All monosaccharides have a free aldehydic or ketonic group and thus can behave as a reducing agent. That is why they are also called reducing sugars. 

•  Stereo-isomerism

Excluding the last carbon atom and the one that is a part of the functional group, all the other carbon atoms in the monosaccharides are attached to four different groups and thus, are called chiral carbons.  

Due to the chirality of the carbon atoms, every monosaccharide can have two structures that are the mirror images of each other. They are known as stereoisomers or enantiomers. 

The two enantiomers include an L-form and a D-form. In the L-isomer, all the hydroxyl groups are arranged on the left side of the carbon atoms while in the D-isomer, they are arranged to the right side. 

All monosaccharides have two enantiomers, a D-isomer and an L-isomer. For example, glucose exists in two forms, a D-glucose and L-glucose. The D-isomers are more common in nature. 

Ring structure

Monosaccharides that have more than three carbon atoms can exist in two structural forms, an open-chain or acyclic structure and a closed-ring or cyclic structure. 

Whenever a monosaccharide is dissolved in water, it undergoes a neutrophilic reaction between the carbonyl group (C=O) and one of the hydroxyl groups resulting in the formation of a ring that involves bridging of an oxygen atom. 

Chemical Properties

The chemical properties of monosaccharides are based on the functional groups present in them. As they have a hydroxyl group as well as an aldehydic group or ketonic group, monosaccharides show the following chemical properties.

Oxidation

Due to the presence of the aldehydic group, monosaccharides can undergo oxidation to form carboxylic acids. Monosaccharides that have a ketonic group undergo conversion to its aldehydic isomer in the solution before undergoing oxidation. This process in which the ketonic isomer is converted to the aldehydic isomer is known as Tautomerization.  

Examples of oxidation in living organisms include the process of glycolysis in which glucose is converted into two molecules of pyruvic acid.

Reduction

In the process of reduction, the aldehydic or ketonic functional group of monosaccharides is converted to the hydroxylic group resulting in the formation of a polyalcohol also known as ‘polyol’. In this process, one or more electrons are donated to monosaccharide in the form of hydrogen atoms by the reducing agent. 

An example of monosaccharide reduction in the human body is the conversion of glucose to sorbitol in the eyes and some other organs.   

Ester Formation

Esters are the compounds formed when an acid reacts with an alcohol releasing a water molecule. Monosaccharides can also form esters because they behave as alcohol due to the presence of the multiple hydroxyl groups. They react with carboxylic acids or other organic acids forming esters in our body. 

Fermentation

Fermentation is a catabolic process in which complex molecules are broken down into simpler ones in the absence of oxygen. Although monosaccharides do not undergo hydrolysis, they can be broken down into alcohol and carbon dioxide by the process of fermentation. The process of fermentation usually occurs in bacteria, yeast, and other micro-organisms. Fermentation also occurs in human bodies under anaerobic conditions. 

For example, glucose undergoes fermentation to form ethyl alcohol and carbon dioxide. 

Glycosidic Bonds

Monosaccharides are combined to form oligosaccharides or polymers known as polysaccharides. When two or more monosaccharides are combined, the bond between them is called a glycosidic bond. 

A glycosidic bond is formed when the hydroxyl group of one monosaccharide reacts with the aldehydic or ketonic group of another monosaccharide and a water molecule is released. It is a high energy covalent bond that releases a considerable amount of energy upon hydrolysis. 

It should be kept in mind that the term glycosidic bond is used for any covalent linkage that connects a monosaccharide to any other molecule or group that may or may not be a monosaccharide. 

Numbers are used to donate the carbon atoms participating in the glycosidic bond formation. For example, a 1-4 glycosidic bond means that the carbon 1 of the first monosaccharide is attached to the carbon 4 of the second amino acid. 

Let us now discuss some examples of monosaccharides.

Glucose

Glucose is an aldohexose having molecular formula C₆H₁₂O₆. It performs several important functions that are necessary to support all forms of life

Synthesis

Plants are the main source of glucose as they synthesize it in the process of photosynthesis. 

Animals mainly obtain glucose from plant sources. However, it can also be synthesized within the bodies of animals from non-carbohydrate sources by a process known as gluconeogenesis. This process occurs in the liver and to some extent in the kidneys of human beings. 

Occurrence

Glucose exists in nature in two forms, free form and combined form. 

In free form, it is present in fruits like dates, figs, grapes etc. Glucose in its free form is also present in different fluids present in the human body like blood, CSF, etc. 

In combined form, it occurs in nature as a part of oligosaccharides and polysaccharides. Sucrose and maltose are the examples of disaccharides that contain oxygen. Starch, cellulose, and glycogen are the examples of polysaccharides that are purely made up of glucose molecules.

Functions

Glucose is the main energy fuel in the human body. Almost all the cells in our body (except heart cells) rely on glucose as the sole source of energy for various metabolic processes. It undergoes oxidation in living cells to release a large amount of energy which is in turn used in the formation of ATP. 

Ribose

Ribose is an aldose sugar made up of four carbon atoms. Its molecular formula is C₅H₁₀O₅. It is the most important pentose sugar present in our body. It is an essential component of nucleotides. Ribose and its deoxygenated form, deoxyribose, are present in RNA and DNA, respectively.

It is synthesized in our body from glucose in the pentose phosphate pathway. 

Fructose

Fructose is a hexose containing a ketonic functional group. It has the same molecular formula as of glucose (C₆H₁₂O₆) but differs in the configuration of atoms and is thus an isomer of glucose. It is known to be sweeter than glucose. 

Fructose is present in berries and other fruits like apple. It is the most abundant sugar present in honey that contains around 40% fructose. Sucrose or table sugar is also a source of fructose as it is a disaccharide made up of glucose and fructose. Besides, fructose is present in human semen and provides nutrition to the sperms. 

Summary

Monosaccharides are the simplest sugars that act as building blocks of complex carbohydrates. 

Chemically,  monosaccharides are polyhydroxy aldehydes or ketones meaning that they are made up of;

• Multiple hydroxyl groups
• An aldehydic or ketonic functional group

They are represented by a general formula (CH₂O)_x where x is equal to or greater than three.  

The scientific name of monosaccharides begins with the name of the functional group followed by the number of carbon atoms written in Latin and a suffix “ose’.

All monosaccharides have the following physical properties;

• They can easily dissolve in water
• They cannot be broken down into simpler sugars
• They are sweet in taste
• They are reducing in nature
• They show the phenomenon of stereo-isomerism

The following chemical properties are also found in monosaccharides;

• They undergo oxidation to from carboxylic acids
• They can be reduced to form polyalcohol or polyol
• They react with acids to form esters
• They can be fermented in the absence of oxygen to yield alcohol and carbon dioxide

Monosaccharides combine via the glycosidic bonds in which two carbon atoms are attached using an oxygen bridge. 

Examples of important monosaccharides include glucose, fructose, and ribose. 

Frequently Asked Questions

What are monosaccharides?

Monosaccharides are simpler sugars. They are the building blocks of carbohydrates.

Which bond is present between two adjacent monosaccharides in carbohydrates?

In carbohydrates or complex sugars, two adjacent monosaccharides are attached to each other by a glycosidic bond.

Where are free monosaccharides present in nature?

Free monosaccharides are present in honey, fresh fruits like dates, cherries, grapes, etc

What are the properties of monosaccharides?

Monosaccharides are the simplest sugars that are crystalline, colorless, and water-soluble. Some of the monosaccharides are sweet in taste like fructose.

References

1.  Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) Biochemistry. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6
2. N.A.Campbell (1996) Biology (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3
3. Kwan, Lam Peng (2000). Biology- A course for O Level. p. 59. ISBN 9810190964.
4. “Carbohydrates”. Chemistry for Biologists. Royal Society of Chemistry. Retrieved 10 March 2017.
5. Pigman, William Ward; Anet, E. F. L. J. (1972). “Chapter 4: Mutarotations and Actions of Acids and Bases”. In Pigman and Horton (ed.). The Carbohydrates: Chemistry and Biochemistry. 1A (2nd ed.). San Diego: Academic Press. pp. 165–194.