| 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 |
|
| 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. |
