Monomers and Polymers

Definition of a polymer

Polymers are molecules made from monomers that join together.

• Polymers are very large molecules made of single, similar repeating units (monomers).

• Poly- means 'many' or 'multi-', meaning that a polymer consists of many monomers.

Differences between monomers and polymers

A monomer is a molecule that can chemically bond with other molecules to form a longer chain. While a polymer is the longer chain, made up usually of an unspecified number of monomers. These are the differences that set apart the smaller molecules - monomers, and larger molecules - polymers:

What are the three categories of monomers?

There are three categories of monomers: monosaccharides, amino acids, and nucleotides.

1. Monosaccharides

When monosaccharides join together, they form polymers that are polysaccharides (complex carbohydrates). For this reason, monosaccharides are monomers of complex carbohydrates, such as starch and cellulose.

Monosaccharides are organic molecules. They contain carbon, hydrogen, and oxygen atoms. Examples include glucose, fructose, galactose, ribose (found in RNA), and deoxyribose (found in DNA).

Figure 1 shows the ring structures of each of synthesis important monosaccharide.

Fig. 1 - Ring structures of monosaccharides glucose, fructose, galactose, deoxyribose, and ribose

2. Amino acids

Amino acids are the building blocks of polypeptides (proteins). For this reason, amino acids are monomers of proteins, such as haemoglobin and insulin.

Amino acids are also organic molecules. They contain carbon and hydrogen, but oxygen and nitrogen as well.

Amino acids consist of:

• a central carbon atom

• to amino group

• a carboxyl group and

• one organic R group that is unique to each amino acid.

Examples of amino acids include alanine and valine.

Fig. 2 - Structure of amino acid with its relevant groups

3. Nucleotides

Nucleotides join together to form polynucleotides (nucleic acids). Therefore, nucleotides are monomers of nucleic acids (DNA and RNA).

Nucleotides are also organic molecules, as they contain carbon and hydrogen. They also contain oxygen, hydrogen, and one to three phosphates.

Nucleotides have a pentose (a five-carbon sugar) as a base, which is attached to a nitrogenous base and a phosphate group.

Nucleotides in DNA have deoxyribose as a base, while the ones in RNA have ribose.

Figure 3 illustrates a simplified structure of a nucleotide. Note the phosphodiester bond on the third carbon atom, linking it to the next nucleotide in the chain.

Fig. 3 - Simplified structure of a nucleotide

Figure 3. Simplified structure of a nucleotide with a phosphodiester bond linking it to the next nucleotide.

Source: commons.wikimedia.org

What are the three categories of polymers?

Polymers are divided into three groups: polysaccharides, polypeptides, and polynucleotides.

They all have one clear property in common: Their long chains consist of repeating similar units - monomer.

1. Polysaccharides (Complex carbohydrates)

Polysaccharides are polymers composed of multiple monosaccharides. Complex carbohydrates are polysaccharides: starch, glycogen, and cellulose. All three are composed of repeating units of glucose. Figure 4 shows the complex branched structure of polysaccharide glycogen. Individual repeating circles are glucose molecules.

Fig. 4 - Complex branched structure of polysaccharide glycogen with individual glucose units joined together

2. Polypeptides (proteins)

Polypeptides are composed of monomers that are amino acids. Proteins are polypeptides. Examples of polypeptides include hemoglobin , insulin and keratin . Take a look at Figure 5, which illustrates the primary structure of a protein, a polypeptide. Similar to the image above, individual circles represent amino acids.

Fig. 5 - The primary structure of a protein, a polypeptide

3. Polynucleotides (Nucleic Acids)

Polynucleotides are composed of monomers that are nucleotides. Nucleic acids are polynucleotides. Biologically, the most essential polynucleotides are DNA and RNA .

Let's study the image below. It shows one part of the DNA structure. Can you spot individual nucleotides?

Fig. 6 - One part of DNA structure with nucleotides joined in a chain

How do monomers join together to form polymers?

Monomers bond together with chemical bonds to form polymers. This process is called polymerization.

Two different reactions form and break polymers: condensation reaction and hydrolysis reaction.

Let's have a look at what these two reactions look like on a diagram. Figure 7 illustrates a simplified diagram of how condensation and hydrolysis reactions make and break polymers. Note the addition and removal of water molecules.

Fig. 7 - Simplified diagram of condensation (dehydration synthesis) and hydrolysis of monomers and polymers

1. Condensation reaction

As seen in the diagram above (a), monomers need to join together for a polymer to form. Monomers join with chemical bonds called covalent bonds. These bonds form with the help of water, which is removed during the reaction (it is "lost").

Three covalent bonds form between various monomers: glycosidic, peptide, and phosphodiester bonds.

As a result, we can conclude that:

• Condensation of monosaccharides results in the formation of polysaccharides. The bond that forms between monosaccharides is a glycosidic bond.

• Condensation of amino acids results in the formation of polypeptides. The bond that forms between amino acids is a peptide bond.

• Condensation of nucleotides results in the formation of polynucleotides. The bond that forms between nucleotides is a phosphodiester bond.

2. Hydrolysis reaction

Above, in Figure 7 (b), you can see that polymers are broken down during the reaction of hydrolysis.

Hydrolysis is the opposite of condensation. Here, the covalent bonds between monomers are broken, not created, with the help of water. That is why we say that water is added to this reaction.

Similar to condensation, we can conclude that:

• Hydrolysis of polysaccharides results in the breaking down of the molecule into its monomers: monosaccharides. The covalent glycosidic bonds between monosaccharides break down.

• Hydrolysis of polypeptides results in the breaking down of the molecule into its monomers: amino acids. The covalent peptide bonds between amino acids break down.

• Hydrolysis of polynucleotides results in the breaking down of the molecule into its monomers: nucleotides. The covalent phosphodiester bonds between nucleotides break down.