Viruses, Viroids and lichens:
In the five kingdoms classification of Whittaker there is no mention of some acellular organisms like virus, viroids and lichens.
· Viruses did not find a place in classification since they are not true living, if we understand living as those organisms that have a cell structure.
· The viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell.
· The name virus means venom or poisonous fluid, uses by Pasteur.
· Viruses are obligate parasite; once they infect a cell they take the machinery of host cell to replicate them by killing the host.
· In addition to proteins, viruses also contain genetic material that ould be either RNA or DNA. No virus contains both DNA and RNA.
· Generally viruses that infect plants have single stranded RNA and viruses that infect animals have either single stranded or double stranded RNA or double stranded DNA.
· Bacterial viruses are bacteriophage (virus infect bacteria) are usually dsDNA viruses.
· The protein coat called capsid which is made up of small subunit called capsomeres, it protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.
· Viruses cause diseases like mumps, small pox, herpes, influenza, AIDS and many more.
· In plants symptoms of disease may appear in the form of mosaic formation, leaf rolling and curling and vein clearing, dwarfing and stunted growth.
· Viroids are infectious agents that are smaller than viruses.
· A viroid was found to be free RNA; it locked the protein coat that is found in viruses, hence the name viroid.
· The RNA of the viroid was of low molecular weight.
· Viroid caused potato spindle tuber disease.
· Lichens belong to the kingdom fungi, they have the ability to reproduce asexually and produce their own DNA through the process of photosynthesis.
· Lichens are symbiotic association i.e. mutually useful association between algae and fungi. The algal component is called phycobiont and fungal component is mycobiont, which are autotrophic and heterotrophic respectively. Algae prepare food for the fungi and in return fungi provide shelter and absorb minerals, nutrients and water for its partner.
· Some lichens take the form of algal cells fall into the kingdom protista while other lichens comprises of cyanobacteria and fall into the kingdom Monera.
· Lichens grow in many habitats and tolerate extreme conditions such as desert climates and artic tundras. Lichens grow in soil and on decomposing plants and trees as well as rocks, leaves, tree bark and even on other lichens.
· Lichens are very good pollution indicator. They do not grow in polluted areas.
· They reproduce sexually and asexually and spawn from pores and seeds.
Classification of organisms can be explained by following categories like two kingdom, three kingdom, four kingdom, five kingdom and six kingdom.
Five kingdom classification was given by R.H. Whittaker. He placed monera, protista, fungi,plantae and fungi in separate groups. Monera includes bacteria, protista includes eukaryotic organisms, fungi includes eukaryotic and heterotrophic organisms, plantae includes eukaryotic and multicellular organisms, animalia includes multicellular eukaryotic heterotrophic organisms.