What you'll learn

• Why very different organisms share a similar biochemical basis.
• What monomers and polymers are, with key A-Level Biology examples.
• How condensation reactions join molecules by releasing water.
• How hydrolysis reactions break molecules by using water.

The big idea: life is diverse, but the chemistry is shared

Life is incredibly varied: bacteria, oak trees, mushrooms, insects, humans, and extinct organisms all look and function very differently. However, at the molecular level, living things are built from a relatively small set of similar chemical building blocks.

This matters because A-Level Biology often asks you to connect structure and function. If you understand how small biological molecules join together, you can explain larger topics later, such as carbohydrates, proteins, DNA, enzymes, digestion, and cell structure.

Starting from the basics

Before monomers and polymers, it helps to be clear about a few chemistry words.

An atom is the smallest part of an element that still has the properties of that element. An element is a substance made of only one type of atom, such as carbon, hydrogen, oxygen or nitrogen.

A molecule is made when two or more atoms are chemically bonded together. Biological molecules are the molecules found in living organisms, such as glucose, amino acids, proteins and DNA.

A chemical bond is an attraction that holds atoms or molecules together. In this topic, the important idea is that bonds can be formed to build larger molecules, or broken to split them into smaller molecules.

Monomers: the small units

The word monomer comes from ideas meaning “one part”. In biology, monomers are small molecules that can be joined to other similar molecules.

Important biological examples include:

• Monosaccharides: single sugar units, such as glucose.
• Amino acids: the monomers used to build polypeptides and proteins.
• Nucleotides: the monomers used to build nucleic acids, such as DNA and RNA.

Each type of monomer has its own detailed structure, which you will study later. For now, focus on the general pattern: small units can be joined to make larger molecules.

Polymers: large molecules made from many monomers

The word polymer comes from ideas meaning “many parts”. A polymer is not just any large molecule: it must be made from many repeating or similar smaller units joined by chemical bonds.

Examples you will meet throughout biological molecules include:

• Many monosaccharides joined together form a polysaccharide, such as starch, glycogen or cellulose.
• Many amino acids joined together form a polypeptide; one or more polypeptides can make a protein.
• Many nucleotides joined together form a polynucleotide, such as a DNA strand or RNA strand.

Condensation reactions: joining molecules

To build polymers, cells must join monomers together. This happens using condensation reactions.

“Elimination” simply means that the water molecule is removed or released as a product of the reaction.

A simple way to show this is:

$monomer+monomer\to dimer+water$

A dimer is a molecule made from two monomers joined together. If more monomers keep joining, the chain becomes a polymer.

The water molecule usually forms because one molecule contributes a hydrogen atom and the other contributes a hydroxyl group. These combine to form water, H2O. At the same time, a new chemical bond forms between the two molecules.

The diagram below shows the overall logic: condensation builds a polymer and releases water, while hydrolysis does the reverse.

Hydrolysis reactions: breaking molecules

The reverse of condensation is hydrolysis.

The word hydrolysis is useful: hydro refers to water, and lysis means splitting.

A simple way to show this is:

$dimer+water\to monomer+monomer$

In cells, hydrolysis is important in digestion. Large food molecules, such as starch and proteins, are too large to be absorbed directly, so they are hydrolysed into smaller molecules such as glucose and amino acids.

Counting bonds and water molecules

When monomers join in a straight, unbranched chain, each new bond forms between two neighbouring monomers. The first two monomers form one bond. Adding a third monomer forms one more bond, and so on.

For a straight chain of $n$ monomers:

$$number\; of\; bonds=n-1$$

Because each condensation reaction forms one bond and releases one water molecule:

$$water\; molecules\; released=n-1$$

For complete hydrolysis of that same chain:

$$water\; molecules\; used=n-1$$

The main biological polymer groups

Carbohydrates

Carbohydrates include sugars and polysaccharides. The monomers are monosaccharides, such as glucose. When many glucose molecules join, they can form polysaccharides such as starch, glycogen or cellulose.

You will later learn that the bonds between monosaccharides are called glycosidic bonds.

Proteins

Proteins are built from amino acids. When amino acids join, they form polypeptides. Proteins may contain one polypeptide chain or several polypeptide chains folded into a specific shape.

You will later learn that the bonds between amino acids are called peptide bonds.

Nucleic acids

Nucleic acids include DNA and RNA. Their monomers are nucleotides. A DNA molecule is made from two polynucleotide strands, while RNA is usually single-stranded.

You will later learn that the bonds within a polynucleotide strand are called phosphodiester bonds.

Why this topic matters later

This section is short, but it underpins a lot of the Biological molecules unit.

When you study carbohydrates, you will use condensation and hydrolysis to explain how disaccharides and polysaccharides form and break down. When you study proteins, you will apply the same reaction logic to peptide bonds. When you study DNA, you will see nucleotides joined into polynucleotide strands.

So the important skill is not just memorising the definitions. You need to be able to recognise whether a reaction is building a larger molecule or breaking it down, and then connect that to water being released or used.