Monomers

Four biological macromolecules are constantly present and necessary for life: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules have one thing in common: they are polymers made up of tiny identical monomers.

In the following, we will discuss what monomers are, how they form biological macromolecules, and what are other examples of monomers.

What is a Monomer?

Now, let's take a look at the definition of a monomer.

Figure 1 shows how monomers join together to form polymers.

Monomers link up in repetitive subunits similar to a train: each car represents a monomer, while the whole train which consists of many identical cars linked to one another represents a polymer.

Monomers and Biological Molecules

Many biologically essential molecules are macromolecules. Macromolecules are large molecules that are typically produced through the polymerization of smaller molecules. Polymerization is a process where a large molecule called polymer is made through the combination of smaller units called monomers.

Types of Monomers

Biological macromolecules are composed primarily of six elements in varying quantities and arrangements. These elements are sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen.

To form a polymer, monomers are linked together, and a water molecule is released as a by-product. Such a process is called dehydration synthesis.

On the other hand, polymers can be broken down by adding a water molecule. Such a process is called hydrolysis.

There are four basic types of macromolecules that are made up of corresponding monomers:

• Carbohydrates - monosaccharides

• Proteins - amino acids

• Nucleic acids - nucleotides

• Lipids - fatty acids and glycerol

In this section, we will go through each of these macromolecules and their monomers. We will also cite some pertinent examples.

Carbohydrates Consist Of Monosaccharides

First up, we have carbohydrates.

Carbohydrates are further subdivided into monosaccharides, disaccharides, and polysaccharides based on the number of monomers contained in the macromolecule.

• Monosaccharides are considered the monomers that make up carbohydrates. Examples of monosaccharides include glucose, galactose, and fructose.

• Disaccharides are composed of two monosaccharides. Examples of disaccharides include lactose and sucrose. Lactose is produced through the combination of monosaccharides glucose and galactose. It is typically found in milk. Sucrose is produced through the combination of glucose and fructose. Sucrose is also a fancy way of saying table sugar.

• Polysaccharides are composed of three or more monosaccharides. A polysaccharide chain can be made up of different types of monosaccharides.

Examples of polysaccharides include starch and glycogen.

Like starch, glycogen is also made up of monomers of glucose. You can consider glycogen to be the equivalent of starch that animals store in liver and muscle cells to provide energy.

Proteins Consist Of Amino Acids

The second type of macromolecule is called protein.

Proteins consist of monomers called amino acids. Amino acids are molecules made up of a carbon atom bonded to an amino group (NH₂), a carboxyl group (-COOH), a hydrogen atom, and another atom or group referred to as the R group.

There are 20 common amino acids, each having a different R group. Amino acids have varying chemistry (e.g., acidity, polarity, etc.) and structure (helices, zigzags, and other shapes). Variations in amino acids in protein sequences result in variation in the function and structure of proteins.

A polypeptide is a long chain of amino acids attached to each other via peptide bonds.

Nucleic Acids Consist Of Nucleotides

Next, we have nucleic acids.

The two main forms of nucleic acids are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).

Nucleotides are the monomers that make up nucleic acids: when nucleotides join together, they create polynucleotide chains, which then form segments of biological macromolecules known as nucleic acids. Each nucleotide has three major components: a nitrogenous base, a pentose sugar, and a phosphate group.

Nitrogenous bases are organic molecules with one or Two rings with nitrogen atoms. Both DNA and RNA contain four nitrogenous bases. Adenine, cytosine, and guanine can be found in both DNA and RNA. Thymine can be found only in DNA, while uracil can be found only in RNA.

A pentose sugar is a molecule with five carbon atoms. There are two types of pentose sugar found in nucleotides: ribose in RNA and deoxyribose in DNA. What distinguishes deoxyribose from ribose is the lack of hydroxyl group (-OH) on its 2’ carbon (hence, it is called “deoxyribose”).

Each nucleotide has one or more phosphate groups attached to the pentose sugar.

Lipids

Lastly, we have lipids. However, keep in mind that lipids are not considered "true polymers".

Some lipids are made up of fatty acids and glycerol. Fatty acids are long hydrocarbon chains with a carboxyl group at one end. Fatty acids react with glycerol to form glycerides.

• One fatty acid molecule attached to a glycerol molecule forms a monoglyceride.

• Two fatty acid molecules attached to a glycerol molecule form a diglyceride.

• Three fatty acid molecules attached to a glycerol molecule form a triglyceride, which are the main components of body fat in humans.

Examples of Monomers

There is a long list of monomers that can be used as examples to explain how monomers give way to polymers. Here are some examples of monomers that can help you understand how that process works:

1. Amino acids, like glutamate, tryptophan or alanine. Amino acids are the monomers that build proteins. There are 20 different types of amino acids, each with a unique chemical structure and side chain. Amino acids can bond together through peptide bonds to form polypeptide chains, which then fold into functional proteins.

2. Nucleotides (adenine (A), thymine (T), guanine (G), cytosine (C), and uracil (U)): nucleotides are the monomers that make up nucleic acids, including DNA and RNA. A nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base. Nucleotides can join together through phosphodiester bonds to form a single strand of DNA or RNA.

3. Monosaccharides: monosaccharides are the monomers that build carbohydrates, including sugars, starches, and cellulose. Monosaccharides are simple sugars that consist of a single ring of carbon atoms, with hydrogen and oxygen atoms attached. Glucose, fructose, and galactose are all examples of monosaccharides. Monosaccharides can join together through glycosidic bonds to form more complex carbohydrates.

Difference Between Monomers and Polymers

A monomer is a single unit of an organic molecule that when linked with other monomers can produce a polymer. This means that polymers are more complex molecules compared to monomers. A polymer consists of an unspecified number of monomers. Figure 2 below shows how monomers form polymer macromolecules.

It is also important to note that not all polymers are biological molecules. Humans have been creating and using artificial polymers since the 20^th century.

Examples of Artificial Polymers and their Monomers

Artificial polymers are materials created by humans by linking monomers. We’ll discuss two examples of popular artificial polymers: polyethylene and polyvinyl chloride.

Polyethylene

Polyethylene is a flexible, crystalline, and translucent material. You would see it used in packaging, containers, toys, and even wires. In fact, it is the most commonly used plastic today. Polyethylene is an artificial polymer made up of ethylene monomers. One polyethylene chain can have as many as 10,000 monomer units!

Polyvinyl chloride

Another commonly used artificial polymer is polyvinyl chloride (PVC). It is a material that is rigid and does not easily catch fire so it is used in pipes and coverings for windows and doors. As its name implies, polyvinyl chloride is a polymer made up of vinyl chloride monomers. Vinyl chloride is a gas produced by passing oxygen, hydrogen chloride and ethylene through copper which functions as a catalyst.