Common Features: Prokaryotic Organisms (Edexcel IGCSE Biology: Double Science)

• All living organisms can be grouped or ‘classified’ using a classification system that consists of five kingdoms. These five kingdoms are:
  • Animals
  • Plants
  • Fungi
  • Protoctists
  • Prokaryotes
• The prokaryotes are different from the other four kingdoms (which are all eukaryotes) as prokaryotic organisms are always single-celled and do not contain a nucleus
• Instead, the nuclear material of prokaryotic cells is found in the cytoplasm
• Bacteria are prokaryotic organisms

Bacteria

• Bacteria, which have a wide variety of shapes and sizes, all share the following biological characteristics:
  • They are microscopic single-celled organisms
  • They have a cell wall (not made of cellulose), cell membrane, cytoplasm and plasmids
  • They lack a nucleus but contain a circular chromosome of DNA
  • They lack mitochondria and other membrane-bound organelles found in eukaryotic cells
• Examples of bacteria include:
  • Lactobacillus (a rod-shaped bacterium used in the production of yoghurt from milk)
  • Pneumococcus (a spherical bacterium that acts as the pathogen causing pneumonia)
• Bacteria feed in different ways:
  • Some bacteria can carry out photosynthesis despite having no chloroplasts. This is because they still possess chlorophyl and enzymes necessary to synthesize sugars from carbon dioxide.
  • Most feed on other living or dead organisms (if they feed on dead organic matter then they are known as saprobionts or decomposers)

A typical bacterial cell

Prokaryotic Cells Table