There is a great diversity of living organisms on earth, but the closer you look at them, the more similar they become.

The Cell Theory states that:
  1. All organisms are composed of cells and the products of cells
  2. All cells come from pre-existing cells
  3. The cell is the smallest living organisational unit

You should remember that there are two basic types of cells – prokaryotic and eukaryotic. Also recall some of the basic components of cells (including the plasma membrane, cytoplasm and nucleus) and the differences between plant and animal cells (cell wall, chloroplasts and large vacuole). In Unit 3 Biology we look closer at the structure and function of cellular organelles and the molecular composition of cells. Inorganic molecules (water, carbon dioxide, oxygen and nitrogen) are important substances for cellular function. Organic compounds include carbohydrates, lipids, proteins and nucleic acids. We will study these biochemicals in more detail this term. This site, Organic Chemistry, provides an excellent summary of biochemistry, including diagrams of the various molecules and bonds between atoms.

The following elements make up the majority of all living organisms:
  • Carbon (12-19% by mass)
  • Hydrogen (10% by mass)
  • Oxygen (63-77% by mass)
  • Phosphoros
  • Nitrogen
  • Calcium

These elements are organised into four types of biological macromolecules;