A-Level Biology (IGCSE CIE) – Introduction to biological molecules

2.1 - Introduction to Biological Molecules

1The different types of biological molecules
2The difference between monomers and polymers
3The difference between condensation and hydrolysis reactions

9 min

Types of biological molecules

The cells of all living organisms primarily consist of four types of molecules: carbohydrates, lipids, proteins, and nucleic acids. These biological molecules are organic, meaning they contain the element carbon.

These molecules also contain additional elements:

Carbohydrates - Carbon (C), hydrogen (H), and oxygen (O).
Lipids - Carbon (C), hydrogen (H), and oxygen (O).
Proteins - Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
Nucleic acids - Carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P).

Monomers and polymers

Most carbohydrates, proteins, and nucleic acids are polymers made up of small units known as monomers.

Monomer - Smaller units that combine to make a large molecule (polymer).
Polymer - Large molecule made up of many repeating units of monomers joined together by chemical bonds.

The process by which monomers join to form a polymer is known as polymerisation.

Diagram showing monomers combining to form a polymer through polymerisation.

Table showing biological molecules, monomers, and polymers. Carbohydrates with monosaccharides and polysaccharides, proteins with amino acids and polypeptides, nucleic acids with nucleotides and polynucleotides.

Condensation and hydrolysis reactions

Most polymers are synthesised via a condensation reaction and broken down via a hydrolysis reaction.

Diagram showing a condensation reaction where two molecules form a bond and release water.

Condensation - The removal of water to form a chemical bond between two molecules.

Diagram showing a hydrolysis reaction where a water molecule is added to break a chemical bond between two molecules.

Hydrolysis - The addition of water to break a chemical bond between two molecules.