A level Biology exam revision resources written by A level Examiners


Fundamentally living organisms are mostly water. Water is present not as a building block but as a medium. It is the solution which fills living cells and most of their volume is made up from water. The other chemicals that make up living organisms are called organic macromolecules. Most of the dry mass of living organisms is made up from four different types of macromolecule. Macromolecule is the term for large biologically important organic molecules.   Exam Advice

These are summarised in the table below.


Fatty acids and glycerol

C,H,O,N, (S)
Amino acids

Nucleic Acids
C,H,O,N, P

The three macromolecules that can form polymers all do so in the same way. The single unit of a large polymer is called a monomer (1 unit), these can join to each other to form a dimer (2 units). When lots of them join together a polymer is formed (many units). To join monomers together bonds need to be formed, and to break down polymers or dimers - bonds need to be broken. The process of bonds forming is by condensation and the process of bonds breaking is by hydrolysis. This is shown in the diagram below.


The exact nature of the chemical groups involved may change between molecules but the principal is the same every time.

Forms bonds between monomers, and removes a molecule of H2O.

With the addition of a molecule of H2O this breaks the bonds between monomers.

  • polymers : Long chain molecules made from many single units (monomers)
  • monomer : A single unit which can be joined to another single unit to for a dimer, or joined to lots of units to form a polymer
  • dimer : A molecule made from two monomers joined together
  • condensation : A reaction which joins molecules together and releases water
  • hydrolysis : A reaction where hydrogen (H) and hydroxyl (OH) [from a molecule of H2O] are added to a molecule causing it to split into smaller molecules

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