Class 11 Biology Biological Classification Viruses, Viroids and Lichens

Viruses, Viroids and Lichens


  • The viruses are non-cellular obligate parasites that are characterized by having an inert crystalline structure outside the living cell.
  • Once the viruses infect a cell, they kill the host cell and take over the machinery of the host cell to replicate themselves.
  • The name virus that means venom or poisonous fluid was given by Pasteur. D.J. Ivanowsky.
  • J. Ivanowsky recognized certain microbes as causal organism of the mosaic disease of tobacco, which were found to be smaller than bacteria because they passed through bacteria-proof filters.
  • W. Beijerinek demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum.
  • M. Stanley showed that viruses could be crystallized and crystals consist largely of proteins, which are inert outside their specific host cell.
  • In addition to proteins viruses also contain genetic material that could be either RNA or DNA.
  • Viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA.
  • Bacteriophages are usually double stranded DNA viruses.
  • The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid.
  • Viruses cause diseases like mumps, small pox, herpes and influenza, AIDS.


       Fig. Different types of virus


  • Viroids are smaller than viruses, which was found to be a free RNA.
  • Virioids lacked the protein coat that is found in viruses, hence the name viroid.


  • Lichens are symbiotic associations between algae and fungi.
  • The algal component is known as phycobiont and fungal component as
  • Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.
  • Lichens are very good pollution indicators.


Fig. Lichens 

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