Biological Classification

Viruses, Viroids and Lichens

After going through all the classification groups of the plant and animal kingdom, don’t you feel we missed something out? What about the viruses and lichens? We hear so much about viruses but where are they on the classification list? Whittaker, in his classification of the five kingdoms, did not mention any acellular organisms like viruses and viroids, and lichens. Therefore, we will briefly introduce those to you in this chapter.

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Almost all of us have suffered the ill effects of common cold or ‘flu’. Therefore, we know what effects viruses can have on us, even if we do not associate it with our condition. Viruses did not find a place in classification since they are not truly ‘living’. This is based on the understanding that living organisms that have a cell structure.

The viruses are non-cellular organisms. They, in fact, have an inert crystalline structure outside the living cell. Once they infect a cell, they take over the machinery of the host cell to replicate themselves, killing the host. Would you call viruses living or non-living?


Pasteur. D.J. Ivanowsky (1892) gave the name virus. It means venom or poisonous fluid. According to his research, certain microbes caused the mosaic disease of tobacco.

These organisms were smaller than bacteria because they passed through bacteria-proof filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants. He named the fluid as Contagium vivum fluidum (infectious living fluid).

W.M. Stanley (1935) discovered that viruses could be crystallised. These virus crystals are composed largely of proteins. They are inert outside their specific host cell. Viruses are nothing but obligate parasites.

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Viruses, Viroids and Lichens

Genetic Material of Viruses

In addition to proteins, viruses also contain genetic material, that could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious. Speaking in strictly general terms, viruses infecting plants have single-stranded RNA. On the other hand, viruses that infect animals have either single or double-stranded RNA or they might have double-stranded DNA.

Bacterial viruses or bacteriophages usually have a double-stranded DNA structure. By bacteriophages, we mean viruses that infect the bacteria. The protein coat, capsid made of small subunits (capsomeres) protects the nucleic acid. They have these capsomeres arranged in various geometric forms like helical or polyhedral forms.


In 1971 T.O. Diener discovered a new infectious agent. This agent was more minute than the viruses. it was responsible for causing the potato spindle tuber disease. He found a free RNA. It lacked the protein coat that is found in viruses. Hence, the name viroid. The RNA of the viroid was of low molecular weight.


Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi. The algal component called phycobiont. And, the fungal component is what we call as mycobiont. These components are autotrophic and heterotrophic, respectively.

Algae prepare food for fungi. On the other hand, fungi provide shelter and absorb mineral nutrients and water for its partner. Their association is so close that if one saw a lichen in nature one would never imagine that they had two different organisms within them. Lichens are very good pollution indicators – they do not grow in polluted areas.

Solved Examples for You

Question: Mention some diseases caused by a virus.

Answer: Viruses cause diseases like mumps, smallpox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

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2 responses to “Introduction to Biological Classification”

  1. Danladi Solomon says:

    What is an example of liverwort

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