Inorganic molecules are simple molecules that do not contain both carbon and hydrogen e.g. water (H₂O) and carbon dioxide (CO₂). They can be represented using molecular formulae which give the number of each type of atom present in a molecule. Organic molecules contain both carbon and hydrogen and usually have a carbon backbone. Complex organic molecules are represented by their structural formula which show how the different atoms in a molecule are joined together. These help distinguish between isomers. Isomers are molecules that have the same molecular formula but different structural formula.

Metabolism is all the chemical reactions that take place inside cells or organisms. Anabolic reactions are those that build large molecules from smaller ones e.g. protein synthesis. Catabolic reactions are those that breakdown large molecules into smaller ones e.g. Digestion and respiration.

Many biological molecules are polymers. These are macromolecules (large molecules) made by joining together many small molecules of the same type, called monomers. Joining monomers is called polymerisation and involves condensation reactions. Breaking down polymers into its constituent monomers involves hydrolysis reactions. Condensation and hydrolysis reactions in living organisms are both catalysed by specific enzymes. In a condensation reaction a water molecule is removed and a covalent bond is formed. In a hydrolysis reaction a water molecule is added and a covalent bond is broken.

Examples of biological polymers:

• Carbohydrates

  • Polymer: Polysaccharides

  • Monomer: Monosaccharides

  • Elements: Carbon, Hydrogen, Oxygen

• Proteins

  • Polymer: Polypeptides

  • Monomer: Amino acids

  • Elements: Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur

• Nucleic acids

  • Polymer: Polynucleotides

  • Monomer: Nucleotides

  • Elements: Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus

Triglycerides contain the elements carbon, hydrogen and oxygen but are not polymers as they are composed of fatty acids and glycerol which are different types of small molecules.