Introduction

Monomers can be defined as small molecules that join together to form larger molecules. In order to completely understand the concept of monomers, let us first revise our definition of molecules. Molecules are defined as the stable pure particles formed by the chemical combination of two or more atoms. They can be either macro-molecules or macro-molecules. 

Monomers belong to the category of micro-molecules. They are the smallest form of stable pure substance that can be joined together to form giant molecules or macromolecules. The identical monomers join together via different types of chemical bonding to form giant-molecules called polymers. In this article, we will discuss monomers in terms of their size, classification, structures, chemical combinations, their occurrence, and several other facts. 

How to identify Monomers?

No specific size range of monomers exist in the literature because monomers of different categories or even within the same category can vary in size. The simplest way to identify a monomer is to look at its structure. It always contains different combinations of atoms that together form a unique molecule having a molecular formula in accordance with the general formula of that class. For example, the general formula for monomers of carbohydrates is (CH₂O)_x. We call glucose a monomer of carbohydrates because its molecular formula (CH₂O)₆ follows the general formula of carbohydrate monomers. 

Classification of Monomers

Monomers are classified into two broad categories, natural monomers and synthetic monomers. 

• Natural monomers are the organic molecules that exist in nature and join together to form larger biological molecules. These molecules are responsible for all forms of life on our planet.
• Synthetic monomers are artificially made by combining different atoms for the welfare of mankind. These synthetic monomers are then reacted together to form larger molecules used in industries for several beneficial purposes. 

 Natural Monomers

As mentioned earlier, natural monomers are the bio-molecules that already exist in nature and are the building blocks of life on earth. They join together to form larger molecules that then result in the formation of complex structures of living beings. 

Natural monomers or biological monomers are further divided into four categories.

• Monosaccharides
• Amino acids
• Nucleotides
• Fatty Acids and Alcohols
• Isoprene

All these monomers are discussed in detail below. 

Monosaccharides

These are the monomers or building blocks of complex carbohydrates. Monosaccharides undergo chemical combinations to form complex carbohydrate molecules such as starch, cellulose, and glycogen.

Properties

Monosaccharides are known to have the following properties;

• They contain only one molecule of sugar.
• They have three to seven carbon atoms. 
• They are polyhydroxy aldehydes or ketones. 
• They cannot undergo hydrolysis. 
• They are sweet in taste.
• They are completely soluble in water. 
• Some of them can have a ring structure. 

Structure

Chemically, all the monosaccharides are either polyhydroxy aldehydes or ketones. This means that they have multiple hydroxyl groups(-OH) and have either an aldehyde group(-CHO) or a ketonic group (-CO-) in their structure. All the carbon atoms in a monosaccharide are attached to a hydroxyl group, except the one atom which is a part of either an aldehydic group or a ketonic group. 

Types

Based on the number of carbon atoms in their structure, monosaccharides have following types;

• Trioses such as glyceraldehyde and dihydroxyacetone. 
• Tetroses such as erythrose and erythrulose. 
• Pentoses such as ribose and ribulose. 
• Hexoses include glucose, fructose, and galactose. 
• Heptoses such sudoheptulose. 

Biologically important monosaccharides are trioses, pentoses, and hexoses. 

General Formula 

Monosaccharides have general formula (CH₂O)_x where x=number of carbon atoms. It shows that the number of water molecules in a monosaccharide is equal to the number of carbon atoms in it. 

Chemical Combinations

Monosaccharides combine via the glycosidic bonds to form larger molecules such as disaccharides, trisaccharides, and polysaccharides. In a glycosidic bond, the aldehyde or ketonic group of a sugar reacts with the hydroxyl group of another sugar and a water molecule is released. Two monosaccharides combine to form a disaccharide, three to form a trisaccharide, and so on. 

Occurrence

In combined form, they are present in all the complex biological structures. Glucose in combined form is present in starch, cellulose, glycogen, etc. In combined form, Ribose is present in DNA. In free form, monosaccharides are found in fruits and some of the body fluids. For example, glucose is present in figs, dates, grapes, etc. It is also present in free form in human blood. Fructose is present in human semen. 

Amino Acids

Amino acids are the monomers or building blocks of proteins. All the structural and functional proteins in our body are made up of amino acids. The simplest molecules obtained after the complete hydrolysis of proteins are called amino acids.

Structure

All the amino acids are made up of an amino group (-NH₃) and a carboxylic group (-COOH) attached to the central carbon atom known as the alpha carbon. Besides, a hydrogen atom and a side chain (-R) is also attached to the alpha carbon. All the amino acids differ from one another based on the structure of the side chain (-R). 

Classification of Amino Acids

There are several criteria to classify amino acids. 

Based on the availability of amino acids, they are classified as;

• Essential amino acids: They are not formed in the human body and must be taken in the form of a diet to fulfill the requirement of the body. 
• Non-essential amino acids: They are naturally formed in the human body and cause no harm if not present in the diet. 

Based on the structure of the side chain, some of the classes of amino acids are as follows;

• Hydrophilic Amino Acids: They are soluble in water.
• Hydrophobic Amino Acids: They are insoluble in water. 
• Polar Amino Acid: They have a polar structure. 
• Non-Polar Amino Acids: They have a non-polar structure. 
• Sulphur Amino Acids: They contain a Sulphur atom in their side chain. 

and so on. 

Chemical Combinations

Amino acids combine to form larger molecules via peptide bonds. A peptide bond is formed between two amino acids when the amino group (-NH₃) of one amino acid reacts with the carboxylic group (-COOH) of another amino acid and a water molecule is released as a by-product. All the amino acids in the complex protein structure are joined together via several peptide bonds. The structure formed by the joining together of two amino acids is called a dipeptide.

A dipeptide molecule also has an amino group (-NH₃) on one end while a carboxylic group (-COOH) on the other end that can react with other amino acids to form longer structures. In this way, the chain length continues to increase and the complex protein containing several thousands of amino acids are formed. 

Occurrence

Amino acids always exist in nature in combined form. They are present in animal and plant proteins. Humans consume amino acids in the form of meat and milk, etc. Proteins present in these diet forms are broken down in the stomach to release individual amino acids that get absorbed into the blood in the intestine. 

Nucleotides

These are the building blocks of nucleic acids like DNA and RNA. The entire genetic material of an organism is made up of these monomers called nucleotides. The nucleic acids carry information for all types of activities performed by the cell. They are also essential for cell division and passing the information to the next generation of cells. 

Structure

The structure of these monomers is not as simple as that of amino acids and monosaccharides. Rather, these monomers are made up of three different molecules that are;

1. A pentose sugar molecule (that might be ribose or de-oxy ribose)
2. A nitrogen-containing base 
3. One or more phosphate groups

The sugar molecule and the phosphate group/s are attached to one single nitrogen-containing base to form a nucleotide. 

Types of Nucleotides

Based on the type of pentose sugar, nucleotides can be;

1. Ribonucleotide (containing ribose sugar)
2. De-oxy ribonucleotide (containing deoxygenated ribose sugar)

Nucleotides also vary depending on the nature of the nitrogenous base present in their structure. The nitrogenous bases that could be present in nucleotides are of two types

1. Purine (they have two rings in their structure)
2. Pyrimidines (they have only one ring in their structure)

Chemical Bonding

The phosphate group of one nucleotide reacts with the hydroxyl group present on the pentose sugar of another nucleotide to form a chemical bond known as phosphodiester bond. The resultant compound is called a dinucleotide. This dinucleotide also contains a free phosphate group at one end and a free hydroxyl group at the other end. Both these ends are ready to react with other nucleotides forming phosphodiester bonds and extending the chain of nucleotides. The long chains of these monomers i.e. nucleotides then form the nucleic acids. 

Occurrence

Nucleotides are present in every living cell. They are present in free as well as combined forms. Adenosine Triphosphate (ATP) is an example of the free nucleotide present in every metabolically active cell. In combined form, nucleotides are part of the DNA present in the nucleus and nucleolus, and RNA present both in the nucleus and the cytoplasm. 

Fatty Acids and Alcohol

Although not applicable universally, fatty acids and alcohols can be regarded as the monomers of lipids. The fatty acids react with alcohol forming an ester linkage and a lipid is formed. 

Triglycerides and phospholipids are most important in this regard.

As the name indicates, triglycerides are made up of three fatty acids attached to a single molecule of glycerol alcohol. The glycerol and fatty acids are known as monomers of triglycerides. 

The monomers that combine to form phospholipids include the following:

1. Two fatty acid molecules
2. Glycerol alcohol
3. A nitrogenous base
4. A phosphate group

The fatty acids and the phosphate group form ester linkage with the glycerol. The nitrogenous base is attached to the phosphate group and a phospholipid molecule is formed. 

Isoprene

Isoprene are the monomers of natural rubbers. Isoprene is an organic compound that is a volatile colorless liquid in its purest form. Several molecules of isoprene combine to form natural rubber.  Natural rubber is a polymer formed by several repeating units of isoprene. 

Summary

Monomers are the small molecules that combine using different forms of chemical linkages to form larger molecules. 

Thousands of monomers can join to form giant molecules called polymers.

There is no specific size range of the monomers.

It is also not necessary that monomers always contain a single molecule. 

The easiest way to identify a monomer is to look at its chemical formula that will always correspond to the general formula of that class of compounds. 

Two broad categories of monomers are present; 

• Natural monomers are naturally present in all living organisms.
• Artificial monomers are made artificially for use in the industry for the welfare of human beings. 

Natural monomers are all organic compounds responsible for all forms of life on our planet. They combine to form giant molecules of life that later form complex living structures and carry out functions of living bodies. 

Natural monomer include;

• Monosaccharides
• Amino acids
• Nucleotides
• Fatty acids and Alcohol
• Isoprene

Monosaccharides are the building blocks of complex carbohydrates like cellulose, starch, and glycogen. 

Monosaccharides are the simplest sugars that are soluble in water and cannot undergo hydrolysis. 

Two or monosaccharides are attached in the form of chain by forming glycosidic bonds. 

Polymers of monosaccharides i.e. polysaccharides are formed when thousands of monosaccharides are attached to form long chains. 

Amino acids are the monomers of proteins. 

Each amino acid is made up of an amino group, a carboxylic group, a hydrogen atom and a side-chain (-R) attached to the alpha carbon. 

Two or more amino acids are linked together via peptide bonds to form long chains or peptides. 

When several thousands of amino acids are attached in the form of a chain, polypeptides are formed. These polypeptides then join to form large proteins. 

Nucleotides are the monomers of nucleic acids. 

They join together via phosphodiester bonds to form long chains known as nucleic acids. 

Fatty acids and alcohol are the monomers of several lipids such as fats, oils, triglycerides, and phospholipids, etc.

Isoprene are the monomers of natural rubber produced by several plants. 

Frequently Asked Questions

What are monomers?

Monomers are small molecules that combine together to form polymers. They are the simplest representatives of their class. They have a basic structure consisting of a few atoms only. 

What are the monomers of proteins?

Amino acids are the monomers of proteins. Individual aminal acids are combined together during translation to form long polypeptide chains. These polypeptide chains undergo structural changes to form complex proteins. 

What are the monomers of nucleic acids?

Nucleotides are the monomers of nucleic acids like DNA and RNA. The nucleotides are joined by polymerase enzymes to make polymers. 

What are the monomers of cellulose?

Glucose molecules are monomers of cellulose. These molecules are joined in linear chains to make cellulose that is found in the cell walls of plant cells. 

References

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3. International Union of Pure and Applied Chemistry, et al. (2000) IUPAC Gold Book, Polymerization
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