Biological monomers and polymers

Living things contain large biological molecules built from smaller molecular units. A monomer is a small molecular unit that can join with others. A polymer is a molecule made of a large number of monomer units joined together.

The monomers in a polymer form a connected chain. They do not all have to be identical: different sequences of monomer units can form different polymers.

Examples of biological monomers include:

• Monosaccharides, which can join to form polysaccharides.
• Amino acids, which join to form proteins.
• Nucleotides, which join to form nucleic acids such as DNA.

Living things have been highly diverse throughout the past and present, but they share a similar biochemical basis: their larger biological molecules are built from smaller molecular units.

Condensation and hydrolysis reactions

Two molecules can be joined by forming a chemical bond between them, or separated by breaking that bond. Condensation and hydrolysis describe these opposite changes. They differ in what happens to both the bond and water.

Condensation: forming a bond

In a condensation reaction, two molecules join as a chemical bond forms between them. A molecule of water, $$\ce{H2O}$$, is eliminated in the process.

Small molecules called monomers can be joined by repeated condensation reactions. Each reaction forms a bond between monomers and eliminates water. A chain of repeating monomers formed in this way is a polymer.

Hydrolysis: breaking a bond

In a hydrolysis reaction, a water molecule is used to break a chemical bond between two joined molecules. The bond is broken, separating the molecules. Hydrolysis can therefore break the bonds linking monomers in a polymer.

Condensation forms a bond and eliminates water; hydrolysis breaks a bond and uses water.