1.2 - Variety of Living Organisms

The features of bacteria and viruses

Bacteria

• Bacteria make up the prokaryote kingdom.
• A bacterium is a microscopic, single-celled organism.
• The cell has no nucleus - instead the genetic material is a single circular chromosome of DNA loose in the cell.
• Every bacterial cell has a cell wall, a cell membrane, and cytoplasm.
• Many also carry small separate rings of DNA called plasmids.
• Some bacteria can photosynthesise, but most feed off other organisms, living or dead.

Examples of bacteria:

• Lactobacillus bulgaricus - A rod-shaped bacterium used to turn milk into yoghurt
• Pneumococcus - A spherical bacterium that causes pneumonia

Viruses

• Viruses are not living organisms and are not placed in any kingdom.
• A virus is a particle, much smaller than a bacterium, with no cellular structure at all.
• It is simply a protein coat wrapped around a small amount of genetic material, which can be either DNA or RNA.
• Viruses come in a wide variety of shapes and sizes, and the one shown below is just one of them.

How viruses reproduce

• A virus cannot reproduce on its own.
• It can only reproduce inside the living cells of another organism, which it takes over.
• A virus therefore depends entirely on a host and acts as a parasite.
• Because of this, all viruses are pathogens - they cause disease.
• A pathogen is any organism that causes disease. Pathogens can be fungi, bacteria, protoctists, or viruses.
• Viruses aren't living organisms, but they still count as pathogens.

Examples of viruses:

• Tobacco mosaic virus - Discolours the leaves of tobacco plants by stopping them making chloroplasts
• Influenza virus - Causes flu
• HIV - Causes AIDS