8. Acellular Organisms and Lichens

These entities are not included in the Five Kingdom classification because they are not considered truly living in the cellular sense.

• Viruses:
  • Non-cellular organisms with an inert, crystalline structure outside a living cell.
  • Are obligate parasites; they take over the host cell’s machinery to replicate.
  • The name “virus” means venom or poisonous fluid.
  • Composed of a protein coat (capsid) made of subunits (capsomeres) and a core of genetic material (either RNA or DNA, but never both).
  • Plant-infecting viruses often have single-stranded RNA.
  • Animal-infecting viruses have single or double-stranded RNA or double-stranded DNA.
  • Bacteriophages (viruses that infect bacteria) are usually double-stranded DNA viruses.
  • Diseases: Mumps, smallpox, herpes, influenza, AIDS.
• Viroids:
  • Discovered by T.O. Diener in 1971.
  • They are smaller than viruses and consist only of free RNA with a low molecular weight.
  • They lack a protein coat.
  • Cause diseases like potato spindle tuber disease.
• Prions:
  • Infectious agents consisting of abnormally folded proteins.
  • Similar in size to viruses.
  • Cause infectious neurological diseases like bovine spongiform encephalopathy (BSE or mad cow disease) in cattle and Creutzfeldt-Jakob disease (CJD) in humans.
• Lichens:
  • Symbiotic, mutually useful associations between algae and fungi.
  • The algal component (phycobiont) is autotrophic and prepares food.
  • The fungal component (mycobiont) is heterotrophic and provides shelter, absorbs minerals, and provides water.
  • They are excellent pollution indicators, as they do not grow in polluted areas.