MediumBiologyHigh School (11–12)Diversity of Living Organisms › Lichens and SymbiosisDiversity of Living Organisms › Viruses, Viroids and PrionsConceptual
Why does Whittaker's system leave out the viruses, viroids and prions, and also the lichens?
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Why does Whittaker's system leave out the viruses, viroids and prions, and also the lichens? What does this tell you about a system of classification?
Need a reminder? The key ideaShow
Viruses, viroids and prions are not in the five kingdoms. A virus is an obligate parasite: DNA or RNA, never both, inside a protein capsid. Ivanowsky (1892) found that the cause of tobacco mosaic passed through a bacterial filter; Beijerinck (1898) called it Contagium vivum fluidum; Stanley (1935) crystallised the virus. Diener (1971) found the viroid, free RNA with no coat. A prion is an abnormally folded protein (BSE, CJD). A lichen is a symbiotic association of an alga (the phycobiont), which makes food, and a fungus (the mycobiont), which gives shelter and takes in water and minerals. Lichens do not grow in polluted air, so they are pollution indicators.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Viruses, viroids and prions. The five kingdoms are for cellular organisms. These agents are not cells.
Step 2. Lichens. A lichen is not one organism but a partnership of an alga and a fungus, from two different kingdoms.
Step 3. The lesson. A system of classification is a tool made by people. It is improved when new facts are found, and other systems exist.
Answer: they are not cells, or not single organisms; a classification is a tool that changes with new knowledge.
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