LICHENS

Definition & Basic Concept

Lichens are symbiotic associations between a fungus (mycobiont) and a photosynthetic partner (photobiont). The photobiont is either green algae (commonly Trebouxia) or cyanobacteria (commonly Nostoc).

Symbiotic Relationship

• Fungal component: Provides structure, protection, and absorbs moisture/minerals.
• Algal/cyanobacterial component: Photosynthesizes, providing organic nutrients (carbohydrates).
• This relationship is often considered mutualistic, though some biologists view it as a controlled parasitism by the fungus.

Structure of Lichen Thallus

Lichens have a stratified structure:

1. Upper cortex – Tightly packed fungal hyphae for protection.
2. Algal layer – Where photobiont cells are embedded.
3. Medulla – Loosely arranged hyphae for storage and gas exchange.
4. Lower cortex & rhizines – Attachment structures (not true roots).

Growth Forms (Morphological Types)

1. Crustose: Crust-like, tightly attached to substrate (e.g., rocks, bark).

• Example: Graphis, Lecanora

1. Foliose: Leaf-like, with lobes loosely attached.

• Example: Parmelia, Xanthoria

1. Fruticose: Shrub-like, branched, and erect or hanging.

• Example: Usnea (Old man’s beard), Cladonia (Reindeer moss)

Reproduction

• Asexual: Primarily through soredia (small clusters of algal cells wrapped in hyphae) or isidia (broken-off projections).
• Sexual: Only the fungal partner reproduces sexually, producing spores that must find an appropriate algal partner to form new lichen.

Ecological & Economic Importance

Ecological Roles:

• Pioneer species in primary succession – colonize bare rocks, initiating soil formation.
• Bioindicators of air pollution – especially sensitive to sulfur dioxide.
• Nitrogen fixation – cyanobacterial lichens add nitrogen to ecosystems.
• Food source for reindeer, caribou, and some invertebrates.

Economic Uses:

• Dyes – Traditional dyes for wool (e.g., orchil from Rocella).
• Perfumes – Some lichens produce aromatic compounds.
• Traditional medicine – Antibiotic properties (usnic acid).
• Litmus paper – From Roccella and Dendrographa.

Interesting Facts

• Extremely slow-growing (0.5–5 mm per year).
• Can survive extreme environments (arctic, deserts, high altitudes).
• Some species may live for hundreds to thousands of years.

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VIRUSES

Definition & Nature

Viruses are acellular, infectious agents that are obligate intracellular parasites. They exist at the boundary between living and non-living:

• Non-living outside host: Cannot metabolize, grow, or reproduce independently.
• Living inside host: Capable of replication and evolution.

Discovery

• Dmitri Ivanovsky (1892): Discovered Tobacco Mosaic Virus (TMV) – first evidence of “filterable agents.”
• Martinus Beijerinck (1898): Coined term “virus” (Latin for “poison”).
• Wendell Stanley (1935): Crystallized TMV, showing non-cellular nature.

Structure

1. Genetic Material: Either DNA or RNA (never both), single- or double-stranded.
2. Capsid: Protein coat made of subunits called capsomeres.
3. Envelope: Some viruses have a lipid bilayer membrane stolen from host cell, studded with viral glycoproteins (e.g., HIV, influenza).
4. Other components: Enzymes (reverse transcriptase in retroviruses).

Shapes/Symmetry

1. Helical – Rod-like (TMV)
2. Icosahedral – Spherical (Adenovirus, Poliovirus)
3. Complex – Combination (Bacteriophage T4 with head and tail)

Classification

Based on:

1. Nucleic acid type (DNA/RNA, single/double stranded)
2. Capsid symmetry
3. Presence/absence of envelope
4. Host range (animal, plant, bacterial)

Baltimore Classification (7 groups based on mRNA synthesis):

• I: dsDNA viruses (Adenovirus, Herpes)
• II: ssDNA viruses (Parvovirus)
• III: dsRNA viruses (Rotavirus)
• IV: (+)ssRNA viruses (Polio, SARS-CoV-2)
• V: (-)ssRNA viruses (Influenza, Rabies)
• VI: ssRNA-RT viruses (Retroviruses – HIV)
• VII: dsDNA-RT viruses (Hepatitis B)

Replication Cycle (Lytic Cycle in Bacteriophage)

1. Attachment: To specific receptor sites on host cell.
2. Penetration/Entry: Viral nucleic acid enters host.
3. Uncoating: Release of viral genome.
4. Replication & Synthesis: Host machinery hijacked to produce viral components.
5. Assembly: New viral particles assembled.
6. Release: Lysis of host cell (lytic) or budding (enveloped viruses).

Lysogenic cycle: Viral DNA integrates into host genome (prophage) and replicates with host until induced to enter lytic cycle.

Viral Diseases

• Animals: Influenza, HIV/AIDS, Rabies, COVID-19, Herpes
• Plants: Tobacco Mosaic Virus, Tomato Spotted Wilt Virus
• Bacteria: Bacteriophages

Special Categories

• Oncogenic viruses: Cause cancer (HPV → cervical cancer; EBV → Burkitt’s lymphoma).
• Retroviruses: RNA viruses using reverse transcriptase (HIV).
• Bacteriophages: Viruses infecting bacteria.

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VIROIDS

Definition & Discovery

• Discovered by T.O. Diener (1971) while studying Potato Spindle Tuber Disease.
• Simpler than viruses: Consist only of small, circular, single-stranded RNA without protein coat.
• Smallest known infectious agents (246–401 nucleotides).

Structure & Characteristics

• No capsid, no envelope – naked RNA molecules.
• RNA is highly self-complementary, forming double-stranded rod-like structure.
• No genes for proteins – rely entirely on host machinery.
• No metabolic activity of their own.

Mechanism of Pathogenicity

• RNA silencing interference: Viroid RNA may interfere with host gene expression.
• Activation of host defense responses causing symptoms.
• Interaction with host proteins (e.g., RNA polymerase).
• Some viroids have ribozyme activity – can self-cleave during replication.

Transmission

• Mostly through mechanical means (contaminated tools).
• Some through seeds and pollen (vertical transmission).
• No insect vectors typically involved.

Diseases Caused

Exclusively plant pathogens:

1. Potato Spindle Tuber Viroid (PSTVd) – stunted, deformed potatoes
2. Citrus Exocortis Viroid – bark scaling in citrus trees
3. Coconut Cadang-Cadang Viroid – fatal to coconut palms
4. Avocado Sunblotch Viroid – discolored, deformed avocados

Significance

• Model for studying RNA structure and function.
• Raise questions about the minimal requirements for biological replication.
• Important agricultural pathogens causing significant crop losses.

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COMPARATIVE SUMMARY

Feature	Lichens	Viruses	Viroids
Nature	Symbiotic organism	Acellular, obligate parasite	Acellular, obligate parasite
Cellularity	Multicellular (composite)	Non-cellular	Non-cellular
Components	Fungus + alga/cyanobacterium	Nucleic acid + protein coat	Only RNA
Genetic Material	DNA in both partners	DNA or RNA	Only ssRNA
Metabolism	Present (through photobiont)	Absent	Absent
Reproduction	Sexual/asexual components	Replication using host	Replication using host
Host Dependency	Not parasitic	Absolute	Absolute
Economic Impact	Beneficial mostly	Mostly harmful	Harmful to plants
Size Range	Macroscopic	20-400 nm	246-401 nucleotides

Scientific & Evolutionary Significance

• Lichens: Model for symbiosis, ecosystem pioneers, atmospheric indicators.
• Viruses: Gene transfer agents, tools in biotechnology (gene therapy, vaccines), drivers of evolution.
• Viroids: Minimal replicating agents, provide clues to RNA world hypothesis and origin of life.

These entities challenge traditional definitions of life and demonstrate the diversity of biological organization beyond conventional cellular life forms.