NCERT-NEET-In the five kingdom classification of Whittaker there is no mention of lichens and some acellular organisms like viruses, viroids and prions. These are briefly introduced here.

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Type	Characteristics
Viruses	– Non-cellular organisms with inert crystalline structure outside the living cell. – Infect host cells and take over their machinery to replicate, killing the host. – Recognized by Dmitri Ivanowsky (1892) as causal organisms of tobacco mosaic disease. – Named “virus” by M.W. Beijerinek (1898). – W.M. Stanley (1935) showed viruses could be crystallized and consist largely of proteins. – Contain either RNA or DNA as genetic material, but not both. – Obligate parasites causing diseases like mumps, smallpox, herpes, influenza, and AIDS in humans. – Symptoms in plants include mosaic formation, leaf rolling and curling, yellowing, vein clearing, dwarfing, and stunted growth.
Viroids	– Discovered by T.O. Diener (1971) as causal agents of potato spindle tuber disease. – Smaller than viruses, consist of free RNA without a protein coat. – RNA is of low molecular weight.
Prions	– Infectious agents consisting of abnormally folded proteins. – Cause neurological diseases such as bovine spongiform encephalopathy (BSE) in cattle and Creutzfeldt-Jakob disease (CJD) in humans. – Similar in size to viruses.
Lichens	– Symbiotic associations between algae and fungi. – Algal component (phycobiont) is autotrophic and prepares food. – Fungal component (mycobiont) is heterotrophic, providing shelter and absorbing mineral nutrients and water. – Excellent pollution indicators as they do not grow in polluted areas.

Feature	Viroids	Prions
Discovery	1971 by T.O. Diener	Modern medicine
Disease Caused	Potato spindle tuber disease	Neurological diseases (e.g., BSE, CJD)
Infectious Agent	Free RNA	Abnormally folded protein
Size Comparison	Smaller than viruses	Similar in size to viruses
Molecular Characteristics	RNA of low molecular weight	Protein
Notable Diseases	Potato spindle tuber disease	Bovine spongiform encephalopathy (mad cow disease) and Creutzfeldt–Jakob disease (CJD) in humans
Presence of Protein Coat	Lacks protein coat	Not applicable (protein-based agent)

Feature	Details
Symbiotic Association	Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont). The algae are autotrophic and prepare food, while fungi are heterotrophic and provide shelter and absorb nutrients and water.
Role of Algae (Phycobiont)	Prepare food through photosynthesis for the fungi.
Role of Fungi (Mycobiont)	Provide shelter, absorb mineral nutrients and water for the algae.
Pollution Indicator	Lichens are very good pollution indicators of SO2; they do not grow in polluted areas.
Types of Lichens	Crustose Foliose Fruticose
Rocella tinctoria	Contributes to the production of litmus, a dye used to test acidity.
Reindeer Lichen	Commonly found in Arctic and subarctic regions, it is a significant food source for reindeer.
Growth Rate	Lichens typically grow at a rate of 0.5 to 5 mm (1mm) per year.