🦠 Introduction

In R.H. Whittaker's five-kingdom classification, acellular organisms like viruses, viroids, and prions, as well as symbiotic associations like lichens, are not included. They are not considered truly 'living' in the same sense as cells, but they are important biological entities. They exhibit characteristics of both living and non-living entities and play crucial ecological and medical roles.

🧬 1️⃣. Viruses

  • Definition: Viruses are non-cellular, obligate parasites. They have an inert, crystalline structure outside a living host cell. Once they infect a host cell, they take over the cell's machinery to replicate themselves, killing the host.
  • Discovery:
  • Dmitri Ivanowsky (1892) recognized them as the causal organism of the mosaic disease of tobacco, noting they were smaller than bacteria (passed through bacterial filters).
  • M.W. Beijerinck (1898) demonstrated their infectious nature, calling the fluid Contagium vivum fluidum (infectious living fluid).
  • W.M. Stanley (1935) showed that viruses could be crystallized, and these crystals consist largely of proteins.
  • Structure: A virus is a nucleoprotein, meaning it consists of genetic material and a protein coat.
  • Genetic Material: This can be either DNA or RNA, but never both. Viruses that infect plants typically have single-stranded RNA. Viruses that infect animals may have single or double-stranded RNA, or double-stranded DNA.
  • Bacteriophages: These are viruses that infect bacteria. They usually have double-stranded DNA.
  • Capsid: This is the protective protein coat. It's made of small subunits called capsomeres, which are arranged in helical or polyhedral geometric forms.
  • Diseases: Viruses cause many common diseases, including mumps, smallpox, herpes, influenza, and AIDS in humans. In plants, symptoms can include mosaic formation, leaf rolling and curling, yellowing, and stunted growth.

🧫 2️⃣. Bacteriophages (Bacteria-eating viruses)

  • Infect bacterial cells and multiply inside them.
  • Structure: Head (contains DNA), tail sheath, base plate, tail fibers.
  • Life cycles:
  1. Lytic Cycle: Virus replicates and lyses host cell.
  2. Lysogenic Cycle: Viral DNA integrates into host genome (prophage).

🧫 3️⃣. Viroids

  • Discovery: Discovered by T.O. Diener in 1971. He found they caused potato spindle tuber disease.
  • Structure: A viroid is simply a free RNA molecule of low molecular weight. It is infectious and lacks the protein coat (capsid) that is found in viruses.
  • Genome: Small, circular, single-stranded RNA.
  • Infects: Plants only.
  • Example: Potato spindle tuber disease.

🌿 4️⃣. Lichens

  • Definition: Lichens are symbiotic associations (mutually useful associations) between an alga and a fungus.
  • Components:
  • Phycobiont: The algal component, which is autotrophic and prepares food(performs photosynthesis) for the fungus.
  • Mycobiont: The fungal component, which is heterotrophic and provides shelter, absorbs mineral nutrients, and water for its algal partner.
  • Ecological Importance: Lichens are very good pollution indicators as they are highly sensitive to sulfur dioxide and do not grow in polluted areas.
  • Types of Lichens:
  • Crustose: Crust-like, tightly attached to substrate.
  • Foliose: Leaf-like, flat structures.
  • Fruticose: Shrub-like, branched form.

🌿 5️⃣. Prions

  • Structure: Prions are infectious agents consisting of abnormally folded proteins. They are similar in size to viruses but lack any genetic material (DNA or RNA).
  • Diseases: They cause notable fatal neurological diseases, such as bovine spongiform encephalopathy (BSE), or 'mad cow disease', in cattle, and its analogous variant, Creutzfeldt-Jakob disease (CJD), in humans.

⚖️ Economic and Ecological Importance

Organism Importance
Viruses Gene transfer (vectors), vaccines, bacteriophages used in genetic engineering
Viroids Study of plant RNA replication and infection
Lichens Bioindicators of air pollution, dyes, food (Reindeer moss), soil formation

🧠 Summary

  • Viruses: Acellular, require host, contain either DNA or RNA.
  • Viroids: Naked infectious RNA molecules without capsid.
  • Lichens: Symbiotic relationship between algae and fungi.

💡 Solved Examples

Q1. Who discovered viruses and when?
A1. Dmitri Ivanowsky discovered viruses in 1892 while studying tobacco mosaic disease.

Q2. What are bacteriophages?
A2. Viruses that infect bacteria; example – T4 bacteriophage.

Q3. Why are viruses called obligate parasites?
A3. Because they can multiply only inside living host cells.

Q4. What are viroids?
A4. Infectious RNA molecules without a protein coat, discovered by T.O. Diener in 1971.

Q5. What are lichens composed of?
A5. A symbiotic association between algae (phycobiont) and fungi (mycobiont).