• Fungi are a large and diverse group of eukaryotic, non-photosynthetic, spore-forming organisms. They have rigid cell walls. Respiration takes place in bodies called mitochondria in the cytoplasm. Fungal cells have an elaborate arrangement of internal membranes. Fungi can be divided into two broad groups: filamentous fungi (including moulds and macrofungi) and yeasts.
  
  
• Protozoa are a large group of eukaryotic, single celled organisms, which lack a rigid cell wall and usually chloroplasts. They vary widely in size, cell structure and form, ranging from Amoeba with its very fluid shape and simple internal organization and few specialised organelles through to Paramecium with its fixed shape, complex internal organisation and many specialised organelles.
  
  
• Algae are a diverse group of eukaryotic organisms that contain chlorophyll and carry out photosynthesis. Some contain other photosynthetic pigments which gives them their characteristic colour - most algae are green, but some are red. They occur in a wide range of forms from microscopic to macroscopic e.g. seaweeds, some of which are up to 30 metres long and are not considered to be micro-organisms. Microscopic algae exist either as single cells e.g. chlorella, in colonies e.g. Volvox or in filaments e.g. Spirogyra.
  
  
• Bacteria range in size from 0.1 to 15 micron, with some "giants" that may reach half a millimeter. They make up the most metabolically diverse group of living organisms. Although some are parasitic to animals and plants, the majority of bacteria are free-living, having either a neutral or beneficial relationship with humans and other animals and plants. Their metabolic versatility is incredible; while most are heterotrophs, using either light or chemical energy. One of their most remarkable characteristics is their ability to multiply rapidly, with generation times usually ranging between minutes to hours. Bacteria also include cyanobacteria, a specific group of microorganisms capable of oxygenic photosynthesis.
  
  
• Archaebacteria have a wide range of shapes: spheres, rods, spirals, lobed, flat rectangular or irregular. Some exist as single cells, other form filaments or clusters. Some are motile. They are often called extremophiles because they are found in extreme conditions in the environment, such as in hot springs, or salt crystallizing pans the depths of the ocean.
  
  
• Viruses are very small, ranging between 0.01 and 0.03 ?m, and only we visualized under electron microscope. They cannot live independently, and only multiply inside the cells of other organisms. However, their demand for a host is fairly specific. For example, it is unlikely that a crustacean virus will attack humans or fish. Viruses are also the simplest of all organisms and are made of nucleic acid (either DNA or RNA), frequently coated with a protein layer.

Back